Your Inner Fish - Weebly · precision and predictability. We work at home to maximize our chances...

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Transcript of Your Inner Fish - Weebly · precision and predictability. We work at home to maximize our chances...

Page 1: Your Inner Fish - Weebly · precision and predictability. We work at home to maximize our chances of success in the field. Then we let luck take over. The paradoxical relationship
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CONTENTS

TitlePageDedicationPreface

ONEFindingYourInnerFish

TWOGettingaGripTHREEHandyGenes

FOURTeethEverywhereFIVEGettingAhead

SIXTheBest-Laid(Body)PlansSEVENAdventuresinBodybuilding

EIGHTMakingScentsNINEVisionTENEars

ELEVENTheMeaningofItAll

EpilogueNotes,References,andFurtherReading

AcknowledgmentsCopyright

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TOMICHELE

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PREFACE

Thisbookgrewoutofanextraordinarycircumstanceinmylife.Onaccountoffacultydepartures,IendedupdirectingthehumananatomycourseatthemedicalschooloftheUniversityofChicago.Anatomyisthecourseduringwhichnervousfirst-yearmedicalstudentsdissecthumancadaverswhilelearningthenamesandorganizationofmostoftheorgans,holes,nerves,andvesselsinthebody.Thisistheirgrandentrancetotheworldofmedicine,aformativeexperienceontheirpathtobecomingphysicians.Atfirstglance,youcouldn’thaveimaginedaworsecandidateforthejoboftrainingthenextgenerationofdoctors:I’mapaleontologistwhohasspentmostofhiscareerworkingonfish.Itturnsoutthatbeingapaleontologistisahuge

advantageinteachinghumananatomy.Why?Thebestroadmapstohumanbodieslieinthebodiesofotheranimals.Thesimplestwaytoteachstudentsthenervesinthehumanheadistoshowthemthestateofaffairsinsharks.Theeasiestroadmaptotheirlimbsliesinfish.Reptilesarearealhelpwiththestructureofthebrain.Thereasonisthat

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thebodiesofthesecreaturesareoftensimplerversionsofours.Duringthesummerofmysecondyearleadingthecourse,

workingintheArctic,mycolleaguesandIdiscoveredfossilfishthatgaveuspowerfulnewinsightsintotheinvasionoflandbyfishover375millionyearsago.Thatdiscoveryandmyforayintoteachinghumananatomyledmetoexploreaprofoundconnection.Thatexplorationbecamethisbook.

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CHAPTERONE

FINDINGYOURINNERFISH

Typicalsummersofmyadultlifearespentinsnowandsleet,crackingrocksoncliffswellnorthoftheArcticCircle.MostofthetimeIfreeze,getblisters,andfindabsolutelynothing.ButifIhaveanyluck,Ifindancientfishbones.Thatmaynotsoundlikeburiedtreasuretomostpeople,buttomeitismorevaluablethangold.Ancientfishbonescanbeapathtoknowledgeaboutwho

weareandhowwegotthatway.Welearnaboutourownbodiesinseeminglybizarreplaces,rangingfromthefossilsofwormsandfishrecoveredfromrocksfromaroundtheworldtotheDNAinvirtuallyeveryanimalaliveonearthtoday.Butthatdoesnotexplainmyconfidenceaboutwhyskeletalremainsfromthepast—andtheremainsoffish,noless—offercluesaboutthefundamentalstructureofourbodies.Howcanwevisualizeeventsthathappenedmillionsand,

inmanycases,billionsofyearsago?Unfortunately,therewerenoeyewitnesses;noneofuswasaround.Infact,nothingthattalksorhasamouthorevenaheadwas

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aroundformostofthistime.Evenworse,theanimalsthatexistedbackthenhavebeendeadandburiedforsolongtheirbodiesareonlyrarelypreserved.Ifyouconsiderthatover99percentofallspeciesthateverlivedarenowextinct,thatonlyaverysmallfractionarepreservedasfossils,andthatanevensmallerfractionstillareeverfound,thenanyattempttoseeourpastseemsdoomedfromthestart.

DIGGINGFOSSILS—SEEINGOURSELVES

IfirstsawoneofourinnerfishonasnowyJulyafternoonwhilestudying375-million-year-oldrocksonEllesmereIsland,atalatitudeabout80degreesnorth.MycolleaguesandIhadtraveleduptothisdesolatepartoftheworldtotrytodiscoveroneofthekeystagesintheshiftfromfishtoland-livinganimals.Stickingoutoftherockswasthesnoutofafish.Andnotjustanyfish:afishwithaflathead.Oncewesawtheflatheadweknewwewereontosomething.Ifmoreofthisskeletonwerefoundinsidethecliff,itwouldrevealtheearlystagesinthehistoryofourskull,ourneck,evenourlimbs.Whatdidaflatheadtellmeabouttheshiftfromseato

land?Morerelevanttomypersonalsafetyandcomfort,whywasIintheArcticandnotinHawaii?Theanswerstothesequestionslieinthestoryofhowwefindfossilsandhowweusethemtodecipherourownpast.

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Fossilsareoneofthemajorlinesofevidencethatweusetounderstandourselves.(Genesandembryosareothers,whichIwilldiscusslater.)Mostpeopledonotknowthatfindingfossilsissomethingwecanoftendowithsurprisingprecisionandpredictability.Weworkathometomaximizeourchancesofsuccessinthefield.Thenweletlucktakeover.Theparadoxicalrelationshipbetweenplanningand

chanceisbestdescribedbyDwightD.Eisenhower’sfamousremarkaboutwarfare:“Inpreparingforbattle,Ihavefoundthatplanningisessential,butplansareuseless.”Thiscapturesfieldpaleontologyinanutshell.Wemakeallkindsofplanstogetustopromisingfossilsites.Oncewe’rethere,theentirefieldplanmaybethrownoutthewindow.Factsonthegroundcanchangeourbest-laidplans.Yetwecandesignexpeditionstoanswerspecific

scientificquestions.Usingafewsimpleideas,whichI’lltalkaboutbelow,wecanpredictwhereimportantfossilsmightbefound.Ofcourse,wearenotsuccessful100percentofthetime,butwestrikeitrichoftenenoughtomakethingsinteresting.Ihavemadeacareeroutofdoingjustthat:findingearlymammalstoanswerquestionsofmammalorigins,theearliestfrogstoanswerquestionsoffrogorigins,andsomeoftheearliestlimbedanimalstounderstandtheoriginsofland-livinganimals.Inmanyways,fieldpaleontologistshaveasignificantly

easiertimefindingnewsitestodaythanweeverdidbefore.Weknowmoreaboutthegeologyoflocalareas,thanksto

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thegeologicalexplorationundertakenbylocalgovernmentsandoilandgascompanies.TheInternetgivesusrapidaccesstomaps,surveyinformation,andaerialphotos.Icanevenscanyourbackyardforpromisingfossilsitesrightfrommylaptop.Totopitoff,imagingandradiographicdevicescanseethroughsomekindsofrockandallowustovisualizethebonesinside.Despitetheseadvances,thehuntfortheimportant

fossilsismuchwhatitwasahundredyearsago.Paleontologistsstillneedtolookatrock—literallytocrawloverit—andthefossilswithinmustoftenberemovedbyhand.Somanydecisionsneedtobemadewhenprospectingforandremovingfossilbonethattheseprocessesaredifficulttoautomate.Besides,lookingatamonitorscreentofindfossilswouldneverbenearlyasmuchfunasactuallydiggingforthem.Whatmakesthistrickyisthatfossilsitesarerare.To

maximizeouroddsofsuccess,welookfortheconvergenceofthreethings.Welookforplacesthathaverocksoftherightage,rocksoftherighttypetopreservefossils,androcksthatareexposedatthesurface.Thereisanotherfactor:serendipity.ThatIwillshowbyexample.Ourexamplewillshowusoneofthegreattransitionsin

thehistoryoflife:theinvasionoflandbyfish.Forbillionsofyears,alllifelivedonlyinwater.Then,asofabout365millionyearsago,creaturesalsoinhabitedland.Lifeinthesetwoenvironmentsisradicallydifferent.Breathinginwaterrequiresverydifferentorgansthanbreathinginair.

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Thesameistrueforexcretion,feeding,andmovingabout.Awholenewkindofbodyhadtoarise.Atfirstglance,thedividebetweenthetwoenvironmentsappearsalmostunbridgeable.Buteverythingchangeswhenwelookattheevidence;whatlooksimpossibleactuallyhappened.Inseekingrocksoftherightage,wehavearemarkable

factonourside.Thefossilsintherocksoftheworldarenotarrangedatrandom.Wheretheysit,andwhatliesinsidethem,ismostdefinitelyordered,andwecanusethisordertodesignourexpeditions.Billionsofyearsofchangehaveleftlayeruponlayerofdifferentkindsofrockintheearth.Theworkingassumption,whichiseasytotest,isthatrocksonthetopareyoungerthanrocksonthebottom;thisisusuallytrueinareasthathaveastraightforward,layer-cakearrangement(thinktheGrandCanyon).Butmovementsoftheearth’scrustcancausefaultsthatshiftthepositionofthelayers,puttingolderrocksontopofyoungerones.Fortunately,oncethepositionsofthesefaultsarerecognized,wecanoftenpiecetheoriginalsequenceoflayersbacktogether.Thefossilsinsidetheserocklayersalsofollowa

progression,withlowerlayerscontainingspeciesentirelydifferentfromthoseinthelayersabove.Ifwecouldquarryasinglecolumnofrockthatcontainedtheentirehistoryoflife,wewouldfindanextraordinaryrangeoffossils.Thelowestlayerswouldcontainlittlevisibleevidenceoflife.Layersabovethemwouldcontainimpressionsofadiversesetofjellyfish-likethings.Layersstillhigherwouldhave

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creatureswithskeletons,appendages,andvariousorgans,suchaseyes.Abovethosewouldbelayerswiththefirstanimalstohavebackbones.Andsoon.Thelayerswiththefirstpeoplewouldbefoundhigherstill.Ofcourse,asinglecolumncontainingtheentiretyofearthhistorydoesnotexist.Rather,therocksineachlocationonearthrepresentonlyasmallsliveroftime.Togetthewholepicture,weneedtoputthepiecestogetherbycomparingtherocksthemselvesandthefossilsinsidethem,muchasifworkingagiantjigsawpuzzle.Thatacolumnofrockshasaprogressionoffossilspecies

probablycomesasnosurprise.Lessobviousisthatwecanmakedetailedpredictionsaboutwhatthespeciesineachlayermightactuallylooklikebycomparingthemwithspeciesofanimalsthatarealivetoday;thisinformationhelpsustopredictthekindsoffossilswewillfindinancientrocklayers.Infact,thefossilsequencesintheworld’srockscanbepredictedbycomparingourselveswiththeanimalsatourlocalzoooraquarium.Howcanawalkthroughthezoohelpuspredictwherewe

shouldlookintherockstofindimportantfossils?Azoooffersagreatvarietyofcreaturesthatarealldistinctinmanyways.Butlet’snotfocusonwhatmakesthemdistinct;topulloffourprediction,weneedtofocusonwhatdifferentcreaturesshare.Wecanthenusethefeaturescommontoallspeciestoidentifygroupsofcreatureswithsimilartraits.AllthelivingthingscanbeorganizedandarrangedlikeasetofRussiannestingdolls,withsmaller

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groupsofanimalscomprisedinbiggergroupsofanimals.Whenwedothis,wediscoversomethingveryfundamentalaboutnature.Everyspeciesinthezooandtheaquariumhasaheadand

twoeyes.Callthesespecies“Everythings.”Asubsetofthecreatureswithaheadandtwoeyeshaslimbs.Callthelimbedspecies“Everythingswithlimbs.”Asubsetoftheseheadedandlimbedcreatureshasahugebrain,walksontwofeet,andspeaks.Thatsubsetisus,humans.Wecould,ofcourse,usethiswayofcategorizingthingstomakemanymoresubsets,buteventhisthreefolddivisionhaspredictivepower.Thefossilsinsidetherocksoftheworldgenerallyfollow

thisorder,andwecanputittouseindesigningnewexpeditions.Tousetheexampleabove,thefirstmemberofthegroup“Everythings,”acreaturewithaheadandtwoeyes,isfoundinthefossilrecordwellbeforethefirst“Everythingwithlimbs.”Moreprecisely,thefirstfish(acard-carryingmemberofthe“Everythings”)appearsbeforethefirstamphibian(an“Everythingwithlimbs”).Obviously,werefinethisbylookingatmorekindsofanimalsandmanymorecharacteristicsthatgroupsofthemshare,aswellasbyassessingtheactualageoftherocksthemselves.Inourlabs,wedoexactlythistypeofanalysiswith

thousandsuponthousandsofcharacteristicsandspecies.Welookateverybitofanatomywecan,andoftenatlargechunksofDNA.Thereissomuchdatathatweoftenneed

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powerfulcomputerstoshowusthegroupswithingroups.Thisapproachisthefoundationofbiology,becauseitenablesustomakehypothesesabouthowcreaturesarerelatedtooneanother.Besideshelpingusrefinethegroupingsoflife,hundreds

ofyearsoffossilcollectionhaveproducedavastlibrary,orcatalogue,oftheagesoftheearthandthelifeonit.Wecannowidentifygeneraltimeperiodswhenmajorchangesoccurred.Interestedintheoriginofmammals?GotorocksfromtheperiodcalledtheEarlyMesozoic;geochemistrytellsusthattheserocksarelikelyabout210millionyearsold.Interestedintheoriginofprimates?Gohigherintherockcolumn,totheCretaceousperiod,whererocksareabout80millionyearsold.Theorderoffossilsintheworld’srocksispowerful

evidenceofourconnectionstotherestoflife.If,diggingin600-million-year-oldrocks,wefoundtheearliestjellyfishlyingnexttotheskeletonofawoodchuck,thenwewouldhavetorewriteourtexts.Thatwoodchuckwouldhaveappearedearlierinthefossilrecordthanthefirstmammal,reptile,orevenfish—beforeeventhefirstworm.Moreover,ourancientwoodchuckwouldtellusthatmuchofwhatwethinkweknowaboutthehistoryoftheearthandlifeonitiswrong.Despitemorethan150yearsofpeoplelookingforfossils—oneverycontinentofearthandinvirtuallyeveryrocklayerthatisaccessible—thisobservationhasneverbeenmade.

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Whatwediscoveronourwalkthroughthezoomirrorshowfossilsarelaidoutintherocksoftheworld.

Let’snowreturntoourproblemofhowtofindrelatives

ofthefirstfishtowalkonland.Inourgroupingscheme,thesecreaturesaresomewherebetweenthe“Everythings”andthe“Everythingswithlimbs.”Mapthistowhatweknowoftherocks,andthereisstronggeologicalevidencethattheperiodfrom380millionto365millionyearsagois

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thecriticaltime.Theyoungerrocksinthatrange,thoseabout360millionyearsold,includediversekindsoffossilizedanimalsthatwewouldallrecognizeasamphibiansorreptiles.MycolleagueJennyClackatCambridgeUniversityandothershaveuncoveredamphibiansfromrocksinGreenlandthatareabout365millionyearsold.Withtheirnecks,theirears,andtheirfourlegs,theydonotlooklikefish.Butinrocksthatareabout385millionyearsold,wefindwholefishthatlooklike,well,fish.Theyhavefins,conicalheads,andscales;andtheyhavenonecks.Giventhis,itisprobablynogreatsurprisethatweshouldfocusonrocksabout375millionyearsoldtofindevidenceofthetransitionbetweenfishandland-livinganimals.Wehavesettledonatimeperiodtoresearch,andsohave

identifiedthelayersofthegeologicalcolumnwewishtoinvestigate.Nowthechallengeistofindrocksthatwereformedunderconditionscapableofpreservingfossils.Rocksformindifferentkindsofenvironmentsandtheseinitialsettingsleavedistinctsignaturesontherocklayers.Volcanicrocksaremostlyout.Nofishthatweknowofcanliveinlava.Andevenifsuchafishexisted,itsfossilizedboneswouldnotsurvivethesuperheatedconditionsinwhichbasalts,rhyolites,granites,andotherigneousrocksareformed.Wecanalsoignoremetamorphicrocks,suchasschistandmarble,fortheyhaveundergoneeithersuperheatingorextremepressuresincetheirinitialformation.Whateverfossilsmighthavebeenpreservedin

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themhavelongsincedisappeared.Idealtopreservefossilsaresedimentaryrocks:limestones,sandstones,silt-stones,andshales.Comparedwithvolcanicandmetamorphicrocks,theseareformedbymoregentleprocesses,includingtheactionofrivers,lakes,andseas.Notonlyareanimalslikelytoliveinsuchenvironments,butthesedimentaryprocessesmaketheserocksmorelikelyplacestopreservefossils.Forexample,inanoceanorlake,particlesconstantlysettleoutofthewaterandaredepositedonthebottom.Overtime,astheseparticlesaccumulate,theyarecompressedbynew,overridinglayers.Thegradualcompression,coupledwithchemicalprocesseshappeninginsidetherocksoverlongperiodsoftime,meansthatanyskeletonscontainedintherocksstandadecentchanceoffossilizing.Similarprocesseshappeninandalongstreams.Thegeneralruleisthatthegentlertheflowofthestreamorriver,thebetterpreservedthefossils.Everyrocksittingonthegroundhasastorytotell:the

storyofwhattheworldlookedlikeasthatparticularrockformed.Insidetherockisevidenceofpastclimatesandsurroundingsoftenvastlydifferentfromthoseoftoday.Sometimes,thedisconnectbetweenpresentandpastcouldnotbesharper.TaketheextremeexampleofMountEverest,nearwhosetop,atanaltitudeofoverfivemiles,lierocksfromanancientseafloor.GototheNorthFacealmostwithinsightofthefamousHillaryStep,andyoucanfindfossilizedseashells.Similarly,whereweworkintheArctic,temperaturescanreachminus40degreesFahrenheitinthe

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winter.Yetinsidesomeoftheregion’srocksareremnantsofanancienttropicaldelta,almostliketheAmazon:fossilizedplantsandfishthatcouldhavethrivedonlyinwarm,humidlocales.Thepresenceofwarm-adaptedspeciesatwhattodayareextremealtitudesandlatitudesatteststohowmuchourplanetcanchange:mountainsriseandfall,climateswarmandcool,andcontinentsmoveabout.Oncewecometogripswiththevastnessoftimeandtheextraordinarywaysourplanethaschanged,wewillbeinapositiontoputthisinformationtouseindesigningnewfossil-huntingexpeditions.Ifweareinterestedinunderstandingtheoriginoflimbed

animals,wecannowrestrictoursearchtorocksthatareroughly375millionto380millionyearsoldandthatwereformedinoceans,lakes,orstreams.Ruleoutvolcanicrocksandmetamorphicrocks,andoursearchimageforpromisingsitescomesintobetterfocus.Weareonlypartlyonthewaytodesigninganew

expedition,however.Itdoesusnogoodifourpromisingsedimentaryrocksoftherightageareburieddeepinsidetheearth,oriftheyarecoveredwithgrass,orshoppingmalls,orcities.We’dbediggingblindly.Asyoucanimagine,drillingawellholetofindafossiloffersalowprobabilityofsuccess,ratherlikethrowingdartsatadartboardhiddenbehindaclosetdoor.Thebestplacestolookarethosewherewecanwalkfor

milesovertherocktodiscoverareaswherebonesare“weatheringout.”Fossilbonesareoftenharderthanthe

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surroundingrockandsoerodeataslightlyslowerrateandpresentaraisedprofileontherocksurface.Consequently,weliketowalkoverbarebedrock,findasmatteringofbonesonthesurface,thendigin.Sohereisthetricktodesigninganewfossilexpedition:

findrocksthatareoftherightage,oftherighttype(sedimentary),andwellexposed,andweareinbusiness.Idealfossil-huntingsiteshavelittlesoilcoverandlittlevegetation,andhavebeensubjecttofewhumandisturbances.Isitanysurprisethatasignificantfractionofdiscoverieshappenindesertareas?IntheGobiDesert.IntheSahara.InUtah.InArcticdeserts,suchasGreenland.Thisallsoundsverylogical,butlet’snotforget

serendipity.Infact,itwasserendipitythatputourteamontothetrailofourinnerfish.Ourfirstimportantdiscoveriesdidn’thappeninadesert,butalongaroadsideincentralPennsylvaniawheretheexposurescouldhardlyhavebeenworse.Totopitoff,wewerelookingthereonlybecausewedidnothavemuchmoney.IttakesalotofmoneyandtimetogotoGreenlandorthe

SaharaDesert.Incontrast,alocalprojectdoesn’trequirebigresearchgrants,onlymoneyforgasandturnpiketolls.Thesearecriticalvariablesforayounggraduatestudentoranewlyhiredcollegeteacher.WhenIstartedmyfirstjobinPhiladelphia,thelurewasagroupofrockscollectivelyknownastheCatskillFormationofPennsylvania.Thisformationhasbeenextensivelystudiedforover150years.ItsagewaswellknownandspannedtheLateDevonian.In

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addition,itsrockswereperfecttopreserveearlylimbedanimalsandtheirclosestrelatives.Tounderstandthis,itisbesttohaveanimageofwhatPennsylvanialookedlikebackintheDevonian.Removetheimageofpresent-dayPhiladelphia,Pittsburgh,orHarrisburgfromyourmindandthinkoftheAmazonRiverdelta.Therewerehighlandsintheeasternpartofthestate.Aseriesofstreamsrunningeasttowestdrainedthesemountains,endinginalargeseawherePittsburghistoday.Itishardtoimaginebetterconditionstofindfossils,

exceptthatcentralPennsylvaniaiscoveredintowns,forests,andfields.Asfortheexposures,theyaremostlywherethePennsylvaniaDepartmentofTransportation(PennDOT)hasdecidedtoputbigroads.WhenPennDOTbuildsahighway,itblasts.Whenitblasts,itexposesrock.It’snotalwaysthebestexposure,butwetakewhatwecanget.Withcheapscience,yougetwhatyoupayfor.Andthenthereisalsoserendipityofadifferentorder:in

1993,TedDaeschlerarrivedtostudypaleontologyundermysupervision.Thispartnershipwastochangebothourlives.Ourdifferenttemperamentsareperfectlymatched:Ihaveantsinmypantsandamalwaysthinkingofthenextplacetolook;Tedispatientandknowswhentositonasitetomineitforitsriches.TedandIbeganasurveyoftheDevonianrocksofPennsylvaniainhopesoffindingnewevidenceontheoriginoflimbs.Webeganbydrivingtovirtuallyeverylargeroadcutintheeasternpartofthestate.Toourgreatsurprise,shortlyafterwebeganthesurvey,

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Tedfoundamarvelousshoulderbone.WenameditsownerHynerpeton,anamethattranslatesfromGreekas“littlecreepinganimalfromHyner.”Hyner,Pennsylvania,isthenearesttown.Hynerpetonhadaveryrobustshoulder,whichindicatesacreaturethatlikelyhadverypowerfulappendages.Unfortunately,wewereneverabletofindthewholeskeletonoftheanimal.Theexposuresweretoolimited.By?Youguessedit:vegetation,houses,andshoppingmalls.

AlongtheroadsinPennsylvania,wewerelookingatanancientriverdelta,muchliketheAmazontoday.ThestateofPennsylvania(bottom)withtheDevoniantopographymappedaboveit.

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AfterthediscoveryofHynerpetonandotherfossilsfromtheserocks,TedandIwerechampingatthebitforbetter-exposedrock.Ifourentirescientificenterprisewasgoingtobebasedonrecoveringbitsandpieces,thenwecouldaddressonlyverylimitedquestions.Sowetooka“textbook”approach,lookingforwell-exposedrocksoftherightageandtherighttypeindesertregions,meaningthatwewouldn’thavemadethebiggestdiscoveryofourcareersifnotforanintroductorygeologytextbook.OriginallywewerelookingatAlaskaandtheYukonas

potentialvenuesforanewexpedition,largelybecauseofrelevantdiscoveriesmadebyotherteams.Weendedupgettingintoabitofanargument/debateaboutsomegeologicalesoterica,andintheheatofthemoment,oneofuspulledtheluckygeologytextbookfromadesk.Whilerifflingthroughthepagestofindoutwhichoneofuswasright,wefoundadiagram.Thediagramtookourbreathaway;itshowedeverythingwewerelookingfor.Theargumentstopped,andplanningforanewfield

expeditionbegan.Onthebasisofpreviousdiscoveriesmadeinslightly

youngerrocks,webelievedthatancientfreshwaterstreamswerethebestenvironmentinwhichtobeginourhunt.ThisdiagramshowedthreeareaswithDevonianfreshwaterrocks,eachwithariverdeltasystem.First,thereistheeastcoastofGreenland.ThisishometoJennyClack’sfossil,averyearlycreaturewithlimbsandoneoftheearliestknowntetrapods.ThenthereiseasternNorthAmerica,

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wherewehadalreadyworked,hometoHynerpeton.Andthereisathirdarea,largeandrunningeast–westacrosstheCanadianArctic.Therearenotrees,dirt,orcitiesintheArctic.Thechancesweregoodthatrocksoftherightageandtypewouldbeextremelywellexposed.TheCanadianArcticexposureswerewellknown,

particularlytotheCanadiangeologistsandpaleobotanistswhohadalreadymappedthem.Infact,AshtonEmbry,theleaderoftheteamsthatdidmuchofthiswork,haddescribedthegeologyoftheDevonianCanadianrocksasidenticalinmanywaystothegeologyofPennsylvania’s.TedandIwerereadytopackourbagstheminutewereadthisphrase.ThelessonswehadlearnedonthehighwaysofPennsylvaniacouldhelpusintheHighArcticofCanada.Remarkably,theArcticrocksareevenolderthanthe

fossilbedsofGreenlandandPennsylvania.Sotheareaperfectlyfitallthreeofourcriteria:age,type,andexposure.Evenbetter,itwasunknowntovertebratepaleontologists,andthereforeun-prospectedforfossils.

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Themapthatstarteditall.ThismapofNorthAmericacaptureswhatwelookforinanutshell.ThedifferentkindsofshadingreflectwhereDevonianagerocks,whethermarineorfreshwater,areexposed.Threeareasthatwereonceriverdeltasarelabeled.Modifiedfromfigure13.1,R.H.DottandR.L.Batten,EvolutionoftheEarth(NewYork:McGraw-Hill,1988).ReproducedwiththepermissionofTheMcGraw-HillCompanies.

Ournewchallengesweretotallydifferentfromthosewe

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facedinPennsylvania.AlongthehighwaysinPennsylvania,weriskedbeinghitbythetrucksthatwhizzedbyaswelookedforfossils.IntheArcticweriskedbeingeatenbypolarbears,runningoutoffood,orbeingmaroonedbybadweather.Nolongercouldwepacksandwichesinthecaranddrivetothefossilbeds.Wenowhadtospendatleasteightdaysplanningforeverysingledayspentinthefield,becausetherockswereaccessibleonlybyairandthenearestsupplybasewas250milesaway.Wecouldflyinonlyenoughfoodandsuppliesforourcrew,plusaslendersafetymargin.And,mostimportant,theplane’sstrictweightlimitsmeantthatwecouldtakeoutonlyasmallfractionofthefossilsthatwefound.CouplethoselimitationswiththeshortwindowoftimeduringwhichwecanactuallyworkintheArcticeveryyear,andyoucanseethatthefrustrationswefacedwerecompletelynewanddaunting.Entermygraduateadviser,Dr.FarishA.Jenkins,Jr.,from

Harvard.FarishhadledexpeditionstoGreenlandforyearsandhadtheexperiencenecessarytopullthisventureoff.Theteamwasset.Threeacademicgenerations:Ted,myformerstudent;Farish,mygraduateadviser;andIweregoingtomarchuptotheArctictotrytodiscoverevidenceoftheshiftfromfishtoland-livinganimal.ThereisnofieldmanualforArcticpaleontology.We

receivedgearrecommendationsfromfriendsandcolleagues,andwereadbooks—onlytorealizethatnothingcouldprepareusfortheexperienceitself.Atnotimeisthis

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moresharplyfeltthanwhenthehelicopterdropsoneoffforthefirsttimeinsomegodforsakenpartoftheArctictotallyalone.Thefirstthoughtisofpolarbears.Ican’ttellyouhowmanytimesI’vescannedthelandscapelookingforwhitespecksthatmove.Thisanxietycanmakeyouseethings.InourfirstweekintheArctic,oneofthecrewsawamovingwhitespeck.Itlookedlikeapolarbearaboutaquartermileaway.WescrambledlikeKeystoneKopsforourguns,flares,andwhistlesuntilwediscoveredthatourbearwasawhiteArcticharetwohundredfeetaway.Withnotreesorhousesbywhichtojudgedistance,youloseperspectiveintheArctic.TheArcticisabig,emptyplace.Therockswewere

interestedinareexposedoveranareaabout1,500kilometerswide.Thecreatureswewerelookingforwereaboutfourfeetlong.Somehow,weneededtohomeinonasmallpatchofrockthathadpreservedourfossils.Reviewersofgrantproposalscanbeaferociouslot;theylightonthiskindofdifficultyallthetime.AreviewerforoneofFarish’searlyArcticgrantproposalsputitbest.Asthisrefereewroteinhisreviewoftheproposal(notcordially,Imightadd),theoddsoffindingnewfossilsintheArcticwere“worsethanfindingtheproverbialneedleinthehaystack.”IttookusfourexpeditionstoEllesmereIslandoversix

yearstofindourneedle.Somuchforserendipity.Wefoundwhatwewerelookingforbytrying,failing,and

learningfromourfailures.Ourfirstsites,inthe1999field

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season,werewayoutinthewesternpartoftheArctic,onMelvilleIsland.Wedidnotknowit,butwehadbeendroppedoffontheedgeofanancientocean.Therockswereloadedwithfossils,andwefoundmanydifferentkindsoffish.Theproblemwasthattheyallseemedtobedeep-watercreatures,notthekindwewouldexpecttofindintheshallowstreamsorlakesthatgaverisetoland-livinganimals.UsingAshtonEmbry’sgeologicalanalysis,in2000wedecidedtomovetheexpeditioneasttoEllesmereIsland,becausetheretherockswouldcontainancientstreambeds.Itdidnottakelongforustobeginfindingpiecesoffishbonesaboutthesizeofaquarterpreservedasfossils.

Ourcamp(top)lookstinyinthevastnessofthelandscape.Mysummerhome(bottom)isasmalltent,

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usuallysurroundedbypilesofrockstoprotectitfromfifty-mile-per-hourwinds.Photographsbytheauthor.

Therealbreakthroughcametowardtheendofthefield

seasonin2000.Itwasjustbeforedinner,aboutaweekbeforeourscheduledpickuptoreturnhome.Thecrewhadcomebacktocamp,andwewereinvolvedinourearly-eveningactivities:organizingtheday’scollections,preparingfieldnotes,andbeginningtoassembledinner.JasonDowns,thenacollegeundergraduateeagertolearnpaleontology,hadn’treturnedtocampontime.Thisisacauseforworry,aswetypicallygooutinteams;orifweseparate,wegiveeachotheradefinitescheduleofwhenwewillmakecontactagain.Withpolarbearsintheareaandfiercestormsthatcanrollinunexpectedly,wedonottakeanychances.Iremembersittinginthemaintentwiththecrew,theworryaboutJasonbuildingwitheachpassingmoment.Aswebegantoconcoctasearchplan,Iheardthezipperonthetentopen.AtfirstallIsawwasJason’shead.Hehadawild-eyedexpressiononhisfaceandwasoutofbreath.AsJasonenteredthetent,weknewwewerenotdealingwithapolarbearemergency;hisshotgunwasstillshouldered.Thecauseofhisdelaybecameclearashisstillshakinghandpulledouthandfulafterhandfuloffossilbonesthathadbeenstuffedintoeverypocket:hiscoat,pants,innershirt,anddaypack.Iimaginehewouldhavestuffedhissocksandshoesifhecouldhavewalkedhomethatway.Alloftheselittlefossilboneswereonthesurfaceofasmallsite,nobiggerthanaparkingspotforacompact

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car,aboutamileawayfromcamp.Dinnercouldwait.Withtwenty-fourhoursofdaylightintheArcticsummer,

wedidnothavetoworryaboutthesettingsun,sowegrabbedchocolatebarsandsetoffforJason’ssite.Itwasonthesideofahillbetweentwobeautifulrivervalleysand,asJasonhaddiscovered,wascoveredinacarpetoffossilfishbones.Wespentafewhourspickingupthefragments,takingphotos,andmakingplans.Thissitehadallthemakingsofpreciselywhatwewerelookingfor.Wereturnedthenextdaywithanewgoal:tofindtheexactlayerofrockthatcontainedthebones.ThetrickwastoidentifythesourceofJason’smessof

bonefragments—ouronlyhopeoffindingintactskeletons.TheproblemwastheArcticenvironment.Eachwinter,thetemperaturesinkstominus40degreesFahrenheit.Inthesummer,whenthesunneversets,thetemperaturerisestonearly50degrees.Theresultingfreeze-thawcyclecrumblesthesurfacerocksandfossils.Eachwintertheycoolandshrink;eachsummertheyheatandexpand.Astheyshrinkandswellwitheachseasonoverthousandsofyearsatthesurface,thebonesfallapart.Confrontedbyajumbledmassofbonespreadacrossthehill,wecouldnotidentifyanyobviousrocklayerastheirsource.Wespentseveraldaysfollowingthefragmenttrails,diggingtestpits,practicallyusingourgeologicalhammersasdiviningrodstoseewhereintheclifftheboneswereemerging.Afterfourdays,weexposedthelayerandeventuallyfoundskeletonuponskeletonoffossilfish,oftenlyingoneontopofanother.We

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spentpartsoftwosummersexposingthesefish.

Thisiswherewework:southernEllesmereIsland,inNunavutTerritory,Canada,1,000milesfromtheNorthPole.

Failureagain:allthefishwewerefindingwerewell-

knownspeciesthathadbeencollectedinsitesofasimilarageinEasternEurope.Totopitoff,thesefishweren’tverycloselyrelatedtoland-livinganimals.In2004,wedecidedtogiveitonemoretry.Thiswasado-or-diesituation.TheArcticexpeditionswereprohibitivelyexpensiveand,short

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ofaremarkablediscovery,wewouldhavetocallitquits.Everythingchangedoveraperiodoffourdaysinearly

July2004.Iwasflippingrockatthebottomofthequarry,crackingicemoreoftenthanrock.IcrackedtheiceandsawsomethingthatIwillneverforget:apatchofscalesunlikeanythingelsewehadyetseeninthequarry.Thispatchledtoanotherblobcoveredbyice.Itlookedlikeasetofjaws.Theywere,however,unlikethejawsofanyfishIhadeverseen.Theylookedasiftheymighthaveconnectedtoaflathead.Onedaylater,mycolleagueSteveGatesywasflipping

rocksatthetopofthequarry.Steveremovedafist-sizerocktorevealthesnoutofananimallookingrightoutathim.Likemyice-coveredfishatthebottomofthepit,ithadaflathead.Itwasnewandimportant.Butunlikemyfish,Steve’shadrealpotential.Wewerelookingatthefrontend,andwithlucktherestoftheskeletonmightbesafelysittinginthecliff.Stevespenttherestofthesummerremovingrockfromitbitbybitsothatwecouldbringtheentireskeletonbacktothelabandcleanitup.Steve’smasterfulworkwiththisspecimenledtotherecoveryofoneofthefinestfossilsdiscoveredtodateatthewater–landtransition.Thespecimenswebroughtbacktothelabathomewere

littlemorethanboulderswithfossilsinside.Overthecourseoftwomonths,therockwasremovedpiecebypiece,oftenmanuallywithdentaltoolsorsmallpicksbythepreparatorsinthelab.Everydayanewpieceofthefossil

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creature’sanatomywasrevealed.Almosteverytimealargesectionwasexposed,welearnedsomethingnewabouttheoriginofland-livinganimals.Whatwesawgraduallyemergefromtheserocksduring

thefallof2004wasabeautifulintermediatebetweenfishandland-livinganimals.Fishandland-livinganimalsdifferinmanyrespects.Fishhaveconicalheads,whereastheearliestland-livinganimalshavealmostcrocodile-likeheads—flat,withtheeyesontop.Fishdonothavenecks:theirshouldersareattachedtotheirheadsbyaseriesofbonyplates.Earlyland-livinganimals,likealltheirdescendants,dohavenecks,meaningtheirheadscanbendindependentlyoftheirshoulders.Thereareotherbigdifferences.Fishhavescalesallover

theirbodies;land-livinganimalsdonot.Also,importantly,fishhavefins,whereasland-livinganimalshavelimbswithfingers,toes,wrists,andankles.Wecancontinuethesecomparisonsandmakeaverylonglistofthewaysthatfishdifferfromland-livinganimals.

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Theprocessoffindingfossilsbeginswithamassinarockthatisgraduallyremovedovertime.HereIshowafossilasittravelsfromthefieldtothelabandiscarefullypreparedasaspecimen:theskeletonofthenewanimal.Photographinupperleftbyauthor;otherphotographscourtesyofTedDaeschler,AcademyofNaturalSciencesofPhiladelphia.

Butournewcreaturebrokedownthedistinction

betweenthesetwodifferentkindsofanimal.Likeafish,ithasscalesonitsbackandfinswithfinwebbing.But,likeearlyland-livinganimals,ithasaflatheadandaneck.And,whenwelookinsidethefin,weseebonesthatcorrespondtotheupperarm,theforearm,evenpartsofthewrist.Thejointsarethere,too:thisisafishwithshoulder,elbow,andwristjoints.Allinsideafinwithwebbing.Virtuallyallofthefeaturesthatthiscreatureshareswith

land-livingcreatureslookveryprimitive.Forexample,the

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shapeandvariousridgesonthefish’supper“arm”bone,thehumerus,lookpartfishandpartamphibian.Thesameistrueoftheshapeoftheskullandtheshoulder.Ittookussixyearstofindit,butthisfossilconfirmeda

predictionofpaleontology:notonlywasthenewfishanintermediatebetweentwodifferentkindsofanimal,butwehadfounditalsointherighttimeperiodinearth’shistoryandintherightancientenvironment.Theanswercamefrom375-million-year-oldrocks,formedinancientstreams.

Thisfiguresaysitall.Tiktaalikisintermediatebetweenfishandprimitiveland-livinganimal.

Asthediscoverersofthecreature,Ted,Farish,andIhad

theprivilegeofgivingitaformalscientificname.Wewantedthenametoreflectthefish’sprovenanceintheNunavutTerritoryoftheArcticandthedebtweowedtothe

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Inuitpeopleforpermissiontoworkthere.WeengagedtheNunavutCouncilofElders,formallyknownastheInuitQaujimajatuqangitKatimajiit,tocomeupwithanameintheInuktitutlanguage.MyobviousconcernwasthatacommitteenamedInuitQaujimajatuqangitKatimajiitmightnotproposeascientificnamewecouldpronounce.Isentthemapictureofthefossil,andtheelderscameupwithtwosuggestions,SiksagiaqandTiktaalik.WewentwithTiktaalikforitsrelativeeaseofpronunciationforthenon-Inuktitut-speakingtongueandbecauseofitsmeaninginInuktitut:“largefreshwaterfish.”Tiktaalikwastheleadstoryinanumberofnewspapers

thedayafterthefindwasannouncedinApril2006,includingabove-the-foldheadlinesinsuchplacesasTheNewYorkTimes.Thisattentionusheredinaweekunlikeanyotherinmynormallyquietlife.Thoughformethegreatestmomentofthewholemediablitzwasnotseeingthepoliticalcartoonsorreadingtheeditorialcoverageandtheheateddiscussionsontheblogs.Ittookplaceatmyson’spreschool.Inthemidstofthepresshubbub,myson’spreschool

teacheraskedmetobringinthefossilanddescribeit.IdutifullybroughtacastofTiktaalikintoNathaniel’sclass,bracingmyselfforthechaosthatwouldensue.Thetwentyfour-andfive-year-oldsweresurprisinglywellbehavedasIdescribedhowwehadworkedintheArctictofindthefossilandshowedthemtheanimal’ssharpteeth.ThenIaskedwhattheythoughtitwas.Handsshotup.Thefirstchildsaid

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itwasacrocodileoranalligator.Whenqueriedwhy,hesaidthatlikeacrocodileorlizardithasaflatheadwitheyesontop.Bigteeth,too.Otherchildrenstartedtovoicetheirdissent.Choosingtheraisedhandofoneofthesekids,Iheard:No,no,itisn’tacrocodile,itisafish,becauseithasscalesandfins.Yetanotherchildshouted,“Maybeitisboth.”Tiktaalik’smessageissostraightforwardevenpreschoolerscanseeit.Forourpurposes,thereisanevenmoreprofoundtakeon

Tiktaalik.Thisfishdoesn’tjusttellusaboutfish;italsocontainsapieceofus.ThesearchforthisconnectioniswhatledmetotheArcticinthefirstplace.HowcanIbesosurethatthisfossilsayssomething

aboutmyownbody?ConsidertheneckofTiktaalik.AllfishpriortoTiktaalikhaveasetofbonesthatattachtheskulltotheshoulder,sothateverytimetheanimalbentitsbody,italsobentitshead.Tiktaalikisdifferent.Theheadiscompletelyfreeoftheshoulder.Thiswholearrangementissharedwithamphibians,reptiles,birds,andmammals,includingus.TheentireshiftcanbetracedtothelossofafewsmallbonesinafishlikeTiktaalik.

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Tracingarmbonesfromfishtohumans.Icandoasimilaranalysisforthewrists,ribs,ears,and

otherpartsofourskeleton—allthesefeaturescanbetracedbacktoafishlikethis.ThisfossilisjustasmuchapartofourhistoryastheAfricanhominids,suchasAustralopithecusafarensis,thefamous“Lucy.”SeeingLucy,wecanunderstandourhistoryashighlyadvancedprimates.SeeingTiktaalikisseeingourhistoryasfish.Sowhathavewelearned?Ourworldissohighlyordered

thatwecanuseawalkthroughazootopredictthekindsoffossilsthatlieinthedifferentlayersofrocksaroundtheworld.Thosepredictionscanbringaboutfossildiscoveriesthattellusaboutancienteventsinthehistoryoflife.Therecordofthoseeventsremainsinsideus,aspartofour

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anatomicalorganization.WhatIhaven’tmentionedisthatwecanalsotraceour

historyinsideourgenes,throughDNA.Thisrecordofourpastdoesn’tlieintherocksoftheworld;itliesineverycellinsideus.We’llusebothfossilsandgenestotellourstory,thestoryofthemakingofourbodies.

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CHAPTERTWO

GETTINGAGRIP

Imagesofthemedicalschoolanatomylabareimpossibletoforget.Imaginewalkingintoaroomwhereyouwillspendseveralmonthstakingahumanbodyapartlayerbylayer,organbyorgan,allasawaytolearntensofthousandsofnewnamesandbodystructures.InthemonthsbeforeIdidmyfirsthumandissection,I

readiedmyselfbytryingtoenvisionwhatIwouldsee,howIwouldreact,andwhatIwouldfeel.Itturnedoutthatmyimaginedworldinnowaypreparedmefortheexperience.Themomentwhenweremovedthesheetandsawthebodyforthefirsttimewasn’tnearlyasstressfulasI’dexpected.Weweretodissectthechest,soweexposeditwhileleavingthehead,arms,andlegswrappedinpreservative-drenchedgauze.Thetissuesdidnotlookveryhuman.Havingbeentreatedwithanumberofpreservatives,thebodydidn’tbleedwhencut,andtheskinandinternalorganshadtheconsistencyofrubber.Ibegantothinkthatthecadaverlookedmorelikeadollthanahuman.Afewweekswentbyasweexposedtheorgansofthechestandabdomen.Icame

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tothinkthatIwasquitethepro;havingalreadyseenmostoftheinternalorgans,Ihaddevelopedacockyself-confidenceaboutthewholeexperience.Ididmyinitialdissections,mademycuts,andlearnedtheanatomyofmostofthemajororgans.Itwasallverymechanical,detached,andscientific.ThiscomfortableillusionwasrudelyshatteredwhenI

uncoveredthehand.AsIunwrappedthegauzefromthefingers—asIsawthejoints,fingertips,andfingernailsforthefirsttime—Iuncoveredemotionsthathadbeenconcealedduringthepreviousfewweeks.Thiswasnodollormannequin;thishadoncebeenalivingperson,whousedthathandtocarryandcaress.Suddenly,thismechanicalexercise,dissection,becamedeeplyandemotionallypersonal.Untilthatmoment,Iwasblindtomyconnectiontothecadaver.Ihadalreadyexposedthestomach,thegallbladder,andotherorgans;butwhatsanepersonformsahumanconnectionatthesightofagallbladder?Whatisitaboutahandthatseemsquintessentially

human?Theanswermust,atsomelevel,bethatthehandisavisibleconnectionbetweenus;itisasignatureforwhoweareandwhatwecanattain.Ourabilitytograsp,tobuild,andtomakeourthoughtsrealliesinsidethiscomplexofbones,nerves,andvessels.Theimmediatethingthatstrikesyouwhenyouseethe

insideofthehandisitscompactness.Theballofyourthumb,thethenareminence,containsfourdifferentmuscles.Twiddleyourthumbandtiltyourhand:ten

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differentmusclesandatleastsixdifferentbonesworkinunison.Insidethewristareatleasteightsmallbonesthatmoveagainstoneanother.Bendyourwrist,andyouareusinganumberofmusclesthatbegininyourforearm,extendingintotendonsastheytraveldownyourarmtoendatyourhand.Eventhesimplestmotioninvolvesacomplexinterplayamongmanypartspackedinasmallspace.Therelationshipbetweencomplexityandhumanity

withinourhandshaslongfascinatedscientists.In1822,theeminentScottishsurgeonSirCharlesBellwrotetheclassicbookontheanatomyofhands.Thetitlesaysitall:TheHand,ItsMechanismandVitalEndowmentsasEvincingDesign.ToBell,thestructureofthehandwas“perfect”becauseitwascomplexandideallyarrangedforthewaywelive.Inhiseye,thisdesignedperfectioncouldonlyhaveadivineorigin.ThegreatanatomistSirRichardOwenwasoneofthe

scientificleadersinthissearchfordivineorderwithinbodies.Hewasfortunatetobeananatomistinthemid-1800s,whentherewerestillentirelynewkindsofanimalstodiscoverlivinginthedistantreachesoftheearth.Asmoreandmorepartsoftheworldwereexploredbywesterners,allsortsofexoticcreaturesmadetheirwaybacktolaboratoriesandmuseums.Owendescribedthefirstgorilla,broughtbackfromexpeditionstocentralAfrica.Hecoinedthename“dinosaur”foranewkindoffossilcreaturediscoveredinrocksinEngland.Hisstudyofthesebizarrenewcreaturesgavehimspecialinsights:he

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begantoseeimportantpatternsintheseemingchaosoflife’sdiversity.Owendiscoveredthatourarmsandlegs,ourhandsand

feet,fitintoalargerscheme.Hesawwhatanatomistsbeforehimhadlongknown,thatthereisapatterntotheskeletonofahumanarm:oneboneintheupperarm,twobonesintheforearm,abunchofninelittlebonesatthewrists,thenaseriesoffiverodsthatmakethefingers.Thepatternofbonesinthehumanlegismuchthesame:onebone,twobones,lotsablobs,andfivetoes.Incomparingthispatternwiththediversityofskeletonsintheworld,Owenmadearemarkablediscovery.Owen’sgeniuswasnotthathefocusedonwhatmadethe

variousskeletonsdifferent.Whathefound,andlaterpromotedinaseriesoflecturesandvolumes,wereexceptionalsimilaritiesamongcreaturesasdifferentasfrogsandpeople.Allcreatureswithlimbs,whetherthoselimbsarewings,flippers,orhands,haveacommondesign.Onebone,thehumerusinthearmorthefemurintheleg,articulateswithtwobones,whichattachtoaseriesofsmallblobs,whichconnectwiththefingersortoes.Thispatternunderliesthearchitectureofalllimbs.Wanttomakeabatwing?Makethefingersreallylong.Makeahorse?Elongatethemiddlefingersandtoesandreduceandlosetheouterones.Howaboutafrogleg?Elongatethebonesofthelegandfuseseveralofthemtogether.Thedifferencesbetweencreatureslieindifferencesintheshapesandsizesofthebonesandthenumbersofblobs,fingers,andtoes.Despite

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radicalchangesinwhatlimbsdoandwhattheylooklike,thisunderlyingblueprintisalwayspresent.

Thecommonplanforalllimbs:onebone,followedbytwobones,thenlittleblobs,thenfingersortoes.

ForOwen,seeingadesigninthelimbswasonlythe

beginning:whenhelookedatskullsandbackbones,indeedwhenheconsideredtheentirearchitectureofthebody,he

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foundthesamething.Thereisafundamentaldesignintheskeletonofallanimals.Frogs,bats,humans,andlizardsarealljustvariationsonatheme.Thattheme,toOwen,wastheplanoftheCreator.ShortlyafterOwenannouncedthisobservationinhis

classicmonographOntheNatureofLimbs,CharlesDarwinsuppliedanelegantexplanationforit.Thereasonthewingofabatandthearmofahumanshareacommonskeletalpatternisbecausetheysharedacommonancestor.Thesamereasoningappliestohumanarmsandbirdwings,humanlegsandfroglegs—everythingthathaslimbs.ThereisamajordifferencebetweenOwen’stheoryandthatofDarwin:Darwin’stheoryallowsustomakeveryprecisepredictions.FollowingDarwin,wewouldexpecttofindthatOwen’sblueprinthasahistorythatwillberevealedincreatureswithnolimbsatall.Where,then,dowelookforthehistoryofthelimbpattern?Welooktofishandtheirfinskeletons.

SEEINGTHEFISH

InOwenandDarwin’sday,thegulfbetweenfinsandlimbsseemedimpossiblywide.Fishfinsdon’thaveanyobvioussimilaritiestolimbs.Ontheoutside,mostfishfinsarelargelymadeupoffinwebbing.Ourlimbshavenothinglikethis,nordothelimbsofanyothercreaturealivetoday.Thecomparisonsdonotgetanyeasierwhenyouremovethefin

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webbingtoseetheskeletoninside.Inmostfish,thereisnothingthatcanbecomparedtoOwen’sonebone–twobones–lotsablobs–digitspattern.Alllimbshaveasinglelongboneattheirbase:thehumerusintheupperarmandthefemurintheupperleg.Infish,thewholeskeletonlooksutterlydifferent.Thebaseofatypicalfinhasfourormorebonesinside.Inthemid-1800s,anatomistsbegantolearnof

mysteriouslivingfishfromthesoutherncontinents.OneofthefirstwasdiscoveredbyGermananatomistsworkinginSouthAmerica.Itlookedlikeanormalfish,withfinsandscales,butbehinditsthroatwerelargevascularsacs:lungs.Yetthecreaturehadscalesandfins.SoconfusedwerethediscoverersthattheynamedthecreatureLepidosirenparadoxa,“paradoxicallyscaledamphibian.”Otherfishwithlungs,aptlynamedlungfish,weresoonfoundinAfricaandAustralia.AfricanexplorersbroughtonetoOwen.ScientistssuchasThomasHuxleyandtheanatomistCarlGegenbaurfoundlungfishtobeessentiallyacrossbetweenanamphibianandafish.Localsfoundthemdelicious.Aseeminglytrivialpatterninthefinsofthesefishhada

profoundimpactonscience.Thefinsoflungfishhaveattheirbaseasinglebonethatattachestotheshoulder.Toanatomists,thecomparisonwasobvious.Ourupperarmhasasinglebone,andthatsinglebone,thehumerus,attachestoourshoulder.Inthelungfish,wehaveafishwithahumerus.And,curiously,itisnotjustanyfish;itisafishthatalsohaslungs.Coincidence?

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Asahandfuloftheselivingspeciesbecameknowninthe1800s,cluesstartedtocomefromanothersource.Asyoumightguess,theseinsightscamefromancientfish.Oneofthefirstofthesefossilscamefromtheshoresof

theGaspéPeninsulainQuebec,inrocksabout380millionyearsold.Thefishwasgivenatongue-twistername,Eusthenopteron.Eusthenopteronhadasurprisingmixoffeaturesseeninamphibiansandfish.OfOwen’sonebone–twobones–lotsablobs–digitsplanoflimbs,Eusthenopteronhadtheonebone–twobonespart,butinafin.Somefish,then,hadstructureslikethoseinalimb.Owen’sarchetypewasnotadivineandeternalpartofalllife.Ithadahistory,andthathistorywastobefoundinDevonianagerocks,rocksthatarebetween390millionand360millionyearsold.Thisprofoundinsightdefinedawholenewresearchprogramwithawholenewresearchagenda:somewhereintheDevonianrocksweshouldfindtheoriginoffingersandtoes.Inthe1920s,therocksprovidedmoresurprises.Ayoung

Swedishpaleontologist,GunnarSave-Soderbergh,wasgiventheextraordinaryopportunitytoexploretheeastcoastofGreenlandforfossils.Theregionwasterraincognita,butSave-SoderberghrecognizedthatitfeaturedenormousdepositsofDevonianrocks.Hewasoneoftheexceptionalfieldpaleontologistsofalltime,whothroughouthisshortcareeruncoveredremarkablefossilswithbothaboldexploringspiritandapreciseattentiontodetail.(Unfortunately,hewastodietragicallyof

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tuberculosisatayoungage,soonafterthestunningsuccessofhisfieldexpeditions.)Inexpeditionsbetween1929and1934,Save-Soderbergh’steamdiscoveredwhat,atthetime,waslabeledamajormissinglink.Newspapersaroundtheworldtrumpetedhisdiscovery;editorialsanalyzeditsimportance;cartoonslampoonedit.Thefossilsinquestionweretruemosaics:theyhadfish-likeheadsandtails,yettheyalsohadfullyformedlimbs(withfingersandtoes),andvertebraethatwereextraordinarilyamphibian-like.AfterSave-Soderberghdied,thefossilsweredescribedbyhiscolleagueErikJarvik,whonamedoneofthenewspeciesIchthyostegasoderberghiinhonorofhisfriend.

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Thefinsofmostfish—forexample,azebrafish(top)—havelargeamountsoffinwebbingandmanybonesatthebase.Lungfishcapturedpeople’sinterestbecauselikeustheyhaveasingleboneatthebaseoftheappendage.Eusthenopteron(middle)showedhowfossilsbegintofillthegap;ithasbonesthatcomparetoourupperarmandforearm.Acanthostega(bottom)sharesEusthenopteron’spatternofarmboneswiththeadditionoffullyformeddigits.

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Forourstory,Ichthyostegaisabitofaletdown.True,itisaremarkableintermediateinmostaspectsofitsheadandback,butitsaysverylittleabouttheoriginoflimbsbecause,likeanyamphibian,italreadyhasfingersandtoes.Anothercreature,onethatreceivedlittlenoticewhenSave-Soderberghannouncedit,wastoproviderealinsightsdecadeslater.Thissecondlimbedanimalwastoremainanenigmauntil1988,whenapaleontologicalcolleagueofmine,JennyClack,whoweintroducedinthefirstchapter,returnedtoSave-Soderbergh’ssitesandfoundmoreofitsfossils.Thecreature,calledAcanthostegagunnaribackinthe1920sonthebasisofSave-Soderbergh’sfragments,nowrevealedfulllimbs,withfingersandtoes.Butitalsocarriedarealsurprise:Jennyfoundthatthelimbwasshapedlikeaflipper,almostlikethatofaseal.Thissuggestedtoherthattheearliestlimbsarosetohelpanimalsswim,notwalk.Thatinsightwasasignificantadvance,butaproblemremained:Acanthostegahadfullyformeddigits,witharealwristandnofinwebbing.Acanthostegahadalimb,albeitaveryprimitiveone.Thesearchfortheoriginsofhandsandfeet,wristsandankleshadtogostilldeeperintime.Thisiswheremattersstooduntil1995.

FINDINGFISHFINGERSANDWRISTS

In1995,TedDaeschlerandIhadjustreturnedtohishouse

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inPhiladelphiaafterdrivingallthroughcentralPennsylvaniainanefforttofindnewroadcuts.WehadfoundalovelycutonRoute15northofWilliamsport,wherePennDOThadcreatedagiantcliffinsandstonesabout365millionyearsold.Theagencyhaddynamitedthecliffandleftpilesofbouldersalongsidethehighway.Thiswasperfectfossil-huntinggroundforus,andwestoppedtocrawlovertheboulders,manyofthemroughlythesizeofasmallmicrowaveoven.Somehadfishscalesscatteredthroughout,sowedecidedtobringafewbackhometoPhiladelphia.UponourreturntoTed’shouse,hisfour-year-olddaughter,Daisy,camerunningouttoseeherdadandaskedwhatwehadfound.InshowingDaisyoneoftheboulders,wesuddenly

realizedthatstickingoutofitwasasliveroffinbelongingtoalargefish.Wehadcompletelymisseditinthefield.And,asweweretolearn,thiswasnoordinaryfishfin:itclearlyhadlotsofbonesinside.Peopleinthelabspentaboutamonthremovingthefinfromtheboulder—andthere,exposedforthefirsttime,wasafishwithOwen’spattern.Closesttothebodywasonebone.Thisoneboneattachedtotwobones.Extendingawayfromthefinwereabouteightrods.Thislookedforalltheworldlikeafishwithfingers.Ourfinhadafullsetofwebbing,scales,andevenafish-

likeshoulder,butdeepinsidewerebonesthatcorrespondedtomuchofthe“standard”limb.Unfortunately,wehadonlyanisolatedfin.Whatweneededwastofindaplacewherewholebodiesofcreaturescould

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berecoveredintact.Asingleisolatedfincouldneverhelpusanswertherealquestions:Whatdidthecreatureuseitsfinsfor,anddidthefishfinshavebonesandjointsthatworkedlikeours?Theanswerwouldcomeonlyfromwholeskeletons.

Ourtantalizingfin.Sadly,wefoundonlythisisolatedspecimen.StipplediagramusedwiththepermissionofScottRawlins,ArcadiaUniversity.Photobytheauthor.

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Forthatfind,wehadtosearchalmosttenyears.AndIwasn’tthefirsttorecognizewhatwewerelookingat.Thefirstweretwoprofessionalfossilpreparators,FredMullisonandBobMasek.Preparatorsusedentaltoolstoscratchattherockswefindinthefieldandtherebyexposethefossilsinside.Itcantakemonths,ifnotyears,forapreparatortoturnabigfossil-filledboulderlikeoursintoabeautiful,research-qualityspecimen.Duringthe2004expedition,wehadcollectedthree

chunksofrock,eachaboutthesizeofapieceofcarry-onluggage,fromtheDevonianofEllesmereIsland.Eachcontainedaflat-headedanimal:theoneIfoundiniceatthebottomofthequarry,Steve’sspecimen,andathirdspecimenwediscoveredinthefinalweekoftheexpedition.Inthefieldwehadremovedeachhead,leavingenoughrockintactaroundittoexploreinthelabfortherestofthebody.Thentherockswerewrappedinplasterforthetriphome.Openingthesekindsofplastercoveringsinthelabismuchlikeencounteringatimecapsule.BitsandpiecesofourlifeontheArctictundraarethere,asarethefieldnotesandscribbleswemakeonthespecimen.Eventhesmellofthetundracomeswaftingoutofthesepackagesaswecracktheplasteropen.FredinPhiladelphiaandBobinChicagowerescratching

ondifferentbouldersatthesamegeneraltime.FromoneoftheseArcticblocks,BobhadpulledoutaparticularsmallboneinabigfinoftheFish(wehadn’tnameditTiktaalikyet).Whatmadethiscube-shapedblobofbonedifferent

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fromanyotherfinbonewasajointattheendthathadspacesforfourotherbones.Thatis,thebloblookedscarilylikeawristbone—butthefinsintheblockthatBobwaspreparingweretoojumbledtotellforsure.ThenextpieceofevidencecamefromPhiladelphiaaweeklater.Fred,amagicianwithhisdentaltools,uncoveredawholefininhisblock.Attherightplace,justattheendoftheforearmbones,thefinhadthatbone.Andthatboneattachedtofourmorebeyond.Wewerestaringattheoriginofapieceofourownbodiesinsidethis375-million-year-oldfish.Wehadafishwithawrist.

ThebonesofthefrontfinofTiktaalik—afishwithawrist.

Overthenextmonths,wewereabletoseemuchofthe

restoftheappendage.Itwaspartfin,partlimb.Ourfishhadfinwebbing,butinsidewasaprimitiveversionofOwen’sonebone–twobones–lotsablobs–digitsarrangement.JustasDarwin’stheorypredicted:attherighttime,attherightplace,wehadfoundintermediatesbetweentwoapparently

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differentkindsofanimals.Findingthefinwasonlythebeginningofthediscovery.

TherealfunforTed,Farish,andmecamefromunderstandingwhatthefindidandhowitworked,andinguessingwhyawristjointaroseinthefirstplace.Solutionstothesepuzzlesarefoundinthestructureofthebonesandjointsthemselves.WhenwetookthefinofTiktaalikapart,wefound

somethingtrulyremarkable:allthejointsurfaceswereextremelywellpreserved.Tiktaalikhasashoulder,elbow,andwristcomposedofthesamebonesasanupperarm,forearm,andwristinahuman.Whenwestudythestructureofthesejointstoassesshowonebonemovesagainstanother,weseethatTiktaalikwasspecializedforaratherextraordinaryfunction:itwascapableofdoingpush-ups.Whenwedopush-ups,ourhandslieflushagainstthe

ground,ourelbowsarebent,andweuseourchestmusclestomoveupanddown.Tiktaalik’sbodywascapableofallofthis.Theelbowwascapableofbendinglikeours,andthewristwasabletobendtomakethefish’s“palm”lieflatagainsttheground.Asforchestmuscles,Tiktaaliklikelyhadtheminabundance.Whenwelookattheshoulderandtheundersideofthearmboneatthepointwheretheywouldhaveconnected,wefindmassivecrestsandscarswherethelargepectoralmuscleswouldhaveattached.Tiktaalikwasableto“dropandgiveustwenty.”

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Afull-scalemodelofTiktaalik’sbody(top)andadrawingofitsfin(bottom).Thisisafininwhichtheshoulder,elbow,andproto-wristwerecapableofperformingatypeofpush-up.

Whywouldafisheverwanttodoapush-up?Ithelpsto

considertherestoftheanimal.Withaflathead,eyesontop,andribs,Tiktaalikwaslikelybuilttonavigatethebottomandshallowsofstreamsorponds,andeventofloparoundonthemudflatsalongthebanks.Finscapableofsupportingthebodywouldhavebeenveryhelpfulindeedforafishthatneededtomaneuverinalltheseenvironments.ThisinterpretationalsofitswiththegeologyofthesitewherewefoundthefossilsofTiktaalik.Thestructureoftherocklayersandthepatternofthegrainsin

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therocksthemselveshavethecharacteristicsignatureofadepositthatwasoriginallyformedbyashallowstreamsurroundedbylargeseasonalmudflats.Butwhyliveintheseenvironmentsatall?What

possessedfishtogetoutofthewaterorliveinthemargins?Thinkofthis:virtuallyeveryfishswimminginthese375-million-year-oldstreamswasapredatorofsomekind.Somewereuptosixteenfeetlong,almosttwicethesizeofthelargestTiktaalik.ThemostcommonfishspecieswefindalongsideTiktaalikissevenfeetlongandhasaheadaswideasabasketball.Theteetharebarbsthesizeofrailroadspikes.Wouldyouwanttoswimintheseancientstreams?Itisnoexaggerationtosaythatthiswasafish-eat-fish

world.Thestrategiestosucceedinthissettingwereprettyobvious:getbig,getarmor,orgetoutofthewater.Itlooksasifourdistantancestorsavoidedthefight.Butthisconflictavoidancemeantsomethingmuch

deepertous.Wecantracemanyofthestructuresofourownlimbstothefinsofthesefish.Bendyourwristbackandforth.Openandcloseyourhand.Whenyoudothis,youareusingjointsthatfirstappearedinthefinsoffishlikeTiktaalik.Earlier,thesejointsdidnotexist.Later,wefindtheminlimbs.ProceedfromTiktaaliktoamphibiansallthewayto

mammals,andonethingbecomesabundantlyclear:theearliestcreaturetohavethebonesofourupperarm,ourforearm,evenourwristandpalm,alsohadscalesandfinwebbing.Thatcreaturewasafish.

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Whatdowemakeoftheonebone–twobones–lotsablobs–digitsplanthatOwenattributedtoaCreator?Somefish,forexamplethelungfish,havetheoneboneatthebase.Otherfish,forexampleEusthenopteron,havetheonebone–twobonesarrangement.ThentherearecreatureslikeTiktaalik,withonebone–twobones–lotsablobs.Thereisn’tjustasinglefishinsideofourlimbs;thereisawholeaquarium.Owen’sblueprintwasassembledinfish.Tiktaalikmightbeabletodoapush-up,butitcouldnever

throwabaseball,playthepiano,orwalkontwolegs.ItisalongwayfromTiktaaliktohumanity.Theimportant,andoftensurprising,factisthatmostofthemajorboneshumansusetowalk,throw,orgraspfirstappearinanimalstenstohundredsofmillionsofyearsbefore.Thefirstbitsofourupperarmandlegarein380-million-year-oldfishlikeEusthenopteron.Tiktaalikrevealstheearlystagesintheevolutionofourwrist,palm,andfingerarea.Thefirsttruefingersandtoesareseenin365-million-year-oldamphibianslikeAcanthostega.Finally,thefullcomplementofwristandanklebonesfoundinahumanhandorfootisseeninreptilesmorethan250millionyearsold.Thebasicskeletonofourhandsandfeetemergedoverhundredsofmillionsofyears,firstinfishandlaterinamphibiansandreptiles.Butwhatarethemajorchangesthatenableustouseour

handsorwalkontwolegs?Howdotheseshiftscomeabout?Let’slookattwosimpleexamplesfromlimbsforsomeanswers.

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Wehumans,likemanyothermammals,canrotateourthumbrelativetoourelbow.Thissimplefunctionisveryimportantfortheuseofourhandsineverydaylife.Imaginetryingtoeat,write,orthrowaballwithoutbeingabletorotateyourhandrelativetoyourelbow.Wecandothisbecauseoneforearmbone,theradius,rotatesalongapivotpointattheelbowjoint.Thestructureofthejointattheelbowiswonderfullydesignedforthisfunction.Attheendofourupper-armbone,thehumerus,liesaball.Thetipoftheradius,whichattacheshere,formsabeautifullittlesocketthatfitsontheball.Thisball-and-socketjointallowstherotationofourhand,calledpronationandsupination.Wheredoweseethebeginningsofthisability?IncreatureslikeTiktaalik.InTiktaalik,theendofthehumerusformsanelongatedbumpontowhichacup-shapedjointontheradiusfits.WhenTiktaalikbentitselbow,theendofitsradiuswouldrotate,orpronate,relativetotheelbow.Refinementsofthisabilityareseeninamphibiansandreptiles,wheretheendofthehumerusbecomesatrueball,muchlikeourown.Lookingnowatthehindlimb,wefindakeyfeaturethat

givesusthecapacitytowalk,onewesharewithothermammals.Unlikefishandamphibians,ourkneesandelbowsfaceinoppositedirections.Thisfeatureiscritical:thinkoftryingtowalkwithyourkneecapfacingbackward.AverydifferentsituationexistsinfishlikeEusthenopteron,wheretheequivalentsofthekneeandelbowfacelargelyinthesamedirection.Westartdevelopmentwithlittlelimbs

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orientedmuchlikethoseinEusthenopteron,withelbowsandkneesfacinginthesamedirection.Aswegrowinthewomb,ourkneesandelbowsrotatetogiveusthestateofaffairsweseeinhumanstoday.Ourbipedalpatternofwalkingusesthemovementsof

ourhips,knees,ankles,andfootbonestopropelusforwardinanuprightstanceunlikethesprawledpostureofcreatureslikeTiktaalik.Onebigdifferenceisthepositionofourhips.Ourlegsdonotprojectsidewayslikethoseofacrocodile,amphibian,orfish;rather,theyprojectunderneathourbodies.Thischangeinposturecameaboutbychangestothehipjoint,pelvis,andupperleg:ourpelvisbecamebowlshaped,ourhipsocketbecamedeep,ourfemurgaineditsdistinctiveneck,thefeaturethatenablesittoprojectunderthebodyratherthantotheside.Dothefactsofourancienthistorymeanthathumansare

notspecialoruniqueamonglivingcreatures?Ofcoursenot.Infact,knowingsomethingaboutthedeeporiginsofhumanityonlyaddstotheremarkablefactofourexistence:allofourextraordinarycapabilitiesarosefrombasiccomponentsthatevolvedinancientfishandothercreatures.Fromcommonpartscameaveryuniqueconstruction.Wearenotseparatefromtherestofthelivingworld;wearepartofitdowntoourbonesand,aswewillseeshortly,evenourgenes.Inretrospect,themomentwhenIfirstsawthewristofa

fishwasasmeaningfulasthefirsttimeIunwrappedthefingersofthecadaverbackinthehumananatomylab.Both

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timesIwasuncoveringadeepconnectionbetweenmyhumanityandanotherbeing.

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CHAPTERTHREE

HANDYGENES

WhilemycolleaguesandIwerediggingupthefirstTiktaalikintheArcticinJuly2004,RandyDahn,aresearcherinmylaboratory,wassweatingitoutontheSouthSideofChicagodoinggeneticexperimentsontheembryosofsharksandskates,cousinsofstingrays.You’veprobablyseensmallblackeggcases,knownasmermaid’spurses,onthebeach.Insidethepurseoncelayaneggwithyolk,whichdevelopedintoanembryonicskateorray.Overtheyears,Randyhasspenthundredsofhoursexperimentingwiththeembryosinsidetheseeggcases,oftenworkingwellpastmidnight.Duringthefatefulsummerof2004,RandywastakingthesecasesandinjectingamolecularversionofvitaminAintotheeggs.Afterthathewouldlettheeggsdevelopforseveralmonthsuntiltheyhatched.Hisexperimentsmayseemtobeabizarrewaytospend

thebetterpartofayear,letaloneforayoungscientisttolaunchapromisingscientificcareer.Whysharks?WhyaformofvitaminA?

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Tomakesenseoftheseexperiments,weneedtostepbackandlookatwhatwehopetheymightexplain.Whatwearereallygettingatinthischapteristherecipe,writteninourDNA,thatbuildsourbodiesfromasingleegg.Whenspermfertilizesanegg,thatfertilizedeggdoesnotcontainatinyhand,forinstance.Thehandisbuiltfromtheinformationcontainedinthatsinglecell.Thistakesustoaveryprofoundproblem.Itisonethingtocomparethebonesofourhandswiththebonesinfishfins.Whathappensifyoucomparethegeneticrecipethatbuildsourhandswiththerecipethatbuildsafish’sfin?Tofindanswerstothisquestion,justlikeRandy,wewillfollowatrailofdiscoverythattakesusfromourhandstothefinsofsharksandeventothewingsofflies.Aswe’veseen,whenwediscovercreaturesthatreveal

differentandoftensimplerversionsofourbodiesinsidetheirown,awonderfullydirectwindowopensintothedistantpast.Butthereisabiglimitationtoworkingwithfossils.Wecannotdoexperimentsonlong-deadanimals.Experimentsaregreatbecausewecanactuallymanipulatesomethingtoseetheresults.Forthisreason,mylaboratoryissplitdirectlyintwo:halfisdevotedtofossils,theotherhalftoembryosandDNA.Lifeinmylabcanbeschizophrenic.ThelockedcabinetthatholdsTiktaalikspecimensisadjacenttothefreezercontainingourpreciousDNAsamples.ExperimentswithDNAhaveenormouspotentialto

revealinnerfish.Whatifyoucoulddoanexperimentin

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whichyoutreatedtheembryoofafishwithvariouschemicalsandactuallychangeditsbody,makingpartofitsfinlooklikeahand?Whatifyoucouldshowthatthegenesthatbuildafish’sfinarevirtuallythesameasthosethatbuildourhands?Webeginwithanapparentpuzzle.Ourbodyismadeup

ofhundredsofdifferentkindsofcells.Thiscellulardiversitygivesourtissuesandorganstheirdistinctshapesandfunctions.Thecellsthatmakeourbones,nerves,guts,andsoonlookandbehaveentirelydifferently.Despitethesedifferences,thereisadeepsimilarityamongeverycellinsideourbodies:allofthemcontainexactlythesameDNA.IfDNAcontainstheinformationtobuildourbodies,tissues,andorgans,howisitthatcellsasdifferentasthosefoundinmuscle,nerve,andbonecontainthesameDNA?TheanswerliesinunderstandingwhatpiecesofDNA

(thegenes)areactuallyturnedonineverycell.Askincellisdifferentfromaneuronbecausedifferentgenesareactiveineachcell.Whenageneisturnedon,itmakesaproteinthatcanaffectwhatthecelllookslikeandhowitbehaves.Therefore,tounderstandwhatmakesacellintheeyedifferentfromacellinthebonesofthehand,weneedtoknowaboutthegeneticswitchesthatcontroltheactivityofgenesineachcellandtissue.Here’stheimportantfact:thesegeneticswitcheshelpto

assembleus.Atconception,westartasasinglecellthatcontainsalltheDNAneededtobuildourbody.Theplanforthatentirebodyunfoldsviatheinstructionscontainedin

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thissinglemicroscopiccell.Togofromthisgeneralizedeggcelltoacompletehuman,withtrillionsofspecializedcellsorganizedinjusttherightway,wholebatteriesofgenesneedtobeturnedonandoffatjusttherightstagesofdevelopment.Likeaconcertocomposedofindividualnotesplayedbymanyinstruments,ourbodiesareacompositionofindividualgenesturningonandoffinsideeachcellduringourdevelopment.

GenesarestretchesofDNAcontainedineverycellofourbodies.

Thisinformationisaboontothosewhoworkto

understandbodies,becausewecannowcomparetheactivityofdifferentgenestoassesswhatkindsofchangesareinvolvedintheoriginofneworgans.Takelimbs,forexample.Whenwecomparetheensembleofgenesactiveinthedevelopmentofafishfintothoseactiveinthedevelopmentofahumanhand,wecancataloguethegeneticdifferencesbetweenfinsandlimbs.Thiskindofcomparisongivesussomelikelyculprits—thegenetic

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switchesthatmayhavechangedduringtheoriginoflimbs.Wecanthenstudywhatthesegenesaredoingintheembryoandhowtheymighthavechanged.Wecanevendoexperimentsinwhichwemanipulatethegenestoseehowbodiesactuallychangeinresponsetodifferentconditionsorstimuli.Toseethegenesthatbuildourhandsandfeet,weneed

totakeapagefromascriptfortheTVshowCSI:CrimeSceneInvestigation—startatthebodyandworkourwayin.Wewillbeginbylookingatthestructureofourlimbs,andzoomallthewaydowntothetissues,cells,andgenesthatmakeit.

MAKINGHANDS

Ourlimbsexistinthreedimensions:theyhaveatopandabottom,apinkysideandathumbside,abaseandatip.Thebonesatthetips,inourfingers,aredifferentfromthebonesattheshoulder.Likewise,ourhandsaredifferentfromonesidetotheother.Ourpinkiesareshapeddifferentlyfromourthumbs.TheHolyGrailofourdevelopmentalresearchistounderstandwhatgenesdifferentiatethevariousbonesofourlimb,andwhatcontrolsdevelopmentinthesethreedimensions.WhatDNAactuallymakesapinkydifferentfromathumb?Whatmakesourfingersdistinctfromourarmbones?Ifwecanunderstandthegenesthatcontrolsuchpatterns,wewillbe

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privytotherecipethatbuildsus.Allthegeneticswitchesthatmakefingers,armbones,

andtoesdotheirthingduringthethirdtoeighthweekafterconception.Limbsbegintheirdevelopmentastinybudsthatextendfromourembryonicbodies.Thebudsgrowovertwoweeks,untilthetipformsalittlepaddle.Insidethispaddlearemillionsofcellswhichwillultimatelygiverisetotheskeleton,nerves,andmusclesthatwe’llhavefortherestofourlives.

Thedevelopmentofalimb,inthiscaseachickenwing.Allofthekeystagesinthedevelopmentofawingskeletonhappeninsidetheegg.

Tostudyhowthispatternemerges,weneedtolookat

embryosandsometimesinterferewiththeirdevelopmenttoassesswhathappenswhenthingsgowrong.Moreover,weneedtolookatmutantsandattheirinternalstructuresandgenes,oftenbymakingwholemutantpopulations

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throughcarefulbreeding.Obviously,wecannotstudyhumansintheseways.Thechallengeforthepioneersinthisfieldwastofindtheanimalsthatcouldbeusefulwindowsintoourowndevelopment.Thefirstexperimentalembryologistsinterestedinlimbsinthe1930sand1940sfacedseveralproblems.Theyneededanorganisminwhichthelimbswereaccessibleforobservationandexperiment.Theembryohadtoberelativelylarge,sothattheycouldperformsurgicalproceduresonit.Importantly,theembryohadtogrowinaprotectedplace,inacontainerthatsheltereditfromjostlingandotherenvironmentaldisturbances.Also,andcritically,theembryoshadtobeabundantandavailableyear-round.Theobvioussolutiontothisscientificneedisatyourlocalgrocerystore:chickeneggs.Inthe1950sand1960sanumberofbiologists,including

EdgarZwillingandJohnSaunders,didextraordinarilycreativeexperimentsonchickeneggstounderstandhowthepatternoftheskeletonforms.Thiswasaneraofsliceanddice.Embryoswerecutupandvarioustissuesmovedabouttoseewhateffectthishadondevelopment.Theapproachinvolvedverycarefulmicrosurgery,manipulatingpatchesoftissuenomorethanamillimeterthick.Inthatway,bymovingtissuesaboutinthedevelopinglimb,SaundersandZwillinguncoveredsomeofthekeymechanismsthatbuildlimbsasdifferentasbirdwings,whaleflippers,andhumanhands.Theydiscoveredthattwolittlepatchesoftissue

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essentiallycontrolthedevelopmentofthepatternofbonesinsidelimbs.Astripoftissueattheextremeendofthelimbbudisessentialforalllimbdevelopment.Removeit,anddevelopmentstops.Removeitearly,andweareleftwithonlyanupperarm,orapieceofanarm.Removeitslightlylater,andweendupwithanupperarmandaforearm.Removeitevenlater,andthearmisalmostcomplete,exceptthatthedigitsareshortanddeformed.Anotherexperiment,initiallydonebyMaryGasselingin

JohnSaunders’slaboratory,ledtoapowerfulnewlineofresearch.Takealittlepatchoftissuefromwhatwillbecomethepinkysideofalimbbud,earlyindevelopment,andtransplantitontheoppositeside,justunderwherethefirstfingerwillform.Letthechickdevelopandformawing.Theresultsurprisednearlyeverybody.Thewingdevelopednormallyexceptthatitalsohadafullduplicatesetofdigits.Evenmoreremarkablewasthepatternofthedigits:thenewfingersweremirrorimagesofthenormalset.Obviously,somethinginsidethatpatchoftissue,somemoleculeorgene,wasabletodirectthedevelopmentofthepatternofthefingers.Thisresultspawnedablizzardofnewexperiments,andwelearnedthatthiseffectcanbemimickedbyavarietyofothermeans.Forexample,takeachickenembryoanddabalittlevitaminAonitslimbbud,orsimplyinjectvitaminAintotheegg,andlettheembryodevelop.IfyousupplythevitaminAattherightconcentrationandattherightstage,you’llgetthesamemirror-imageduplicationthatGasseling,Saunders,and

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Zwillinggotfromthegraftingexperiments.Thispatchoftissuewasnamedthezoneofpolarizingactivity(ZPA).Essentially,theZPAisapatchoftissuethatcausesthepinkysidetobedifferentfromthethumbside.Obviouslychicksdonothaveapinkyandathumb.Theterminologyweuseistonumberthedigits,withourpinkycorrespondingtodigitfiveofotheranimalsandourthumbcorrespondingtodigitone.

MovingalittlepatchoftissuecalledtheZPAcausesthefingerstobeduplicated.

TheZPAdrewinterestbecauseitappeared,insomeway,

tocontroltheformationoffingersandtoes.Buthow?SomepeoplebelievedthatthecellsintheZPAmadeamoleculethatthenspreadacrossthelimbtoinstructcellstomakedifferentfingers.Thekeyproposalwasthatitwastheconcentrationofthisunnamedmoleculethatwasthe

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importantfactor.InareasclosetotheZPA,wherethereisahighconcentrationofthismolecule,cellswouldrespondbymakingapinky.Intheoppositesideofthedevelopinghand,fartherfromtheZPAsothatthemoleculewasmorediffused,thecellswouldrespondbymakingathumb.Cellsinthemiddlewouldeachrespondaccordingtotheconcentrationofthismoleculetomakethesecond,third,andfourthfingers.Thisconcentration-dependentideacouldbetested.In

1979,DenisSummerbellplacedanextremelysmallpieceoffoilbetweentheZPApatchandtherestofthelimb.TheideawastousethisbarriertopreventanykindofmoleculefromdiffusingfromtheZPAtotheotherside.Summerbellstudiedwhathappenedtothecellsoneachsideofthebarrier.CellsontheZPAsideformeddigits.Cellsontheoppositesideoftendidnotformdigits;iftheydid,thedigitswerebadlymalformed.Theconclusionwasobvious.SomethingwasemanatingfromtheZPAthatcontrolledhowthedigitsformedandwhattheylookedlike.Toidentifythatsomething,researchersneededtolookatDNA.

THEDNARECIPE

Thatprojectwaslefttoanewgenerationofscientists.Notuntilthe1990s,whennewmoleculartechniquesbecameavailable,wasthegeneticcontrolfortheZPA’soperationunraveled.

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Amajorbreakthroughhappenedin1993,whenCliffTabin’slaboratoryatHarvardstartedhuntingforthegenesthatcontroltheZPA.TheirpreywasthemolecularmechanismsthatgavetheZPAitsabilitytomakeourpinkydifferentfromourthumb.Bythetimehisgroupstartedtoworkintheearly1990s,anumberofexperimentsliketheonesI’vedescribedhadledustobelievethatsomesortofmoleculecausedthewholething.Thiswasagrandtheory,butnobodyknewwhatthismoleculewas.Peoplewouldproposeonemoleculeafteranother,onlytofindthatnonewasuptothejob.Finally,theTabinlabcameupwithanovelnotion,andoneveryrelevanttothethemeofthisbook.Looktofliesfortheanswer.Geneticexperimentsinthe1980shadrevealedthe

wonderfulpatternofgeneactivitythatsculptsthebodyofaflyfromasingle-celledegg.Thebodyofafruitflyisorganizedfromfronttoback,withtheheadatthefrontandthewingsattheback.Wholebatteriesofgenesareturnedonandoffduringflydevelopment,andthispatternofgeneactivityservestodemarcatethedifferentregionsofthefly.Tabindidn’tknowitatthetime,buttwoother

laboratories—thoseofAndyMacMahonandPhilIngham—hadalreadycomeupwiththesamegeneralideaindependently.Whatemergedwasaremarkablysuccessfulcollaborationamongthreedifferentlabgroups.OneoftheflygenescaughttheattentionofTabin,McMahon,andIngham.Theynotedthatthisgenemadeoneendofabodysegmentlookdifferentfromtheother.Flygeneticists

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namedithedgehog.Doesn’tthefunctionofhedgehogintheflybody—tomakeoneregiondifferentfromanother—soundlikewhattheZPAdoesinmakingthepinkydifferentfromthethumb?Thatparallelwasnotlostonthethreelabs.Soofftheywent,lookingforahedgehoggeneincreatureslikechickens,mice,andfish.Becausethelabgroupsknewthestructureofthefly’s

hedgehoggene,theyhadasearchimagetohelpthemsingleoutthegeneinchickens.Eachgenehasadistinctivesequence;usinganumberofmoleculartools,theresearcherscouldscanthechicken’sDNAforthehedgehogsequence.Afteralotoftrialanderror,theyfoundachickenhedgehoggene.Justaspaleontologistsgettonamenewspecies,

geneticistsgettonamenewgenes.Theflygeneticistswhodiscoveredhedgehoghadnameditthatbecausetheflieswithamutationinthegenehadbristlesthatremindedthemofalittlehedgehog.Tabin,McMahon,andInghamnamedthechickenversionofthegeneSonichedgehog,aftertheSegaGenesisvideogame.Nowcamethefunquestion:WhatdoesSonichedgehog

actuallydointhelimb?TheTabingroupattachedadyetoamoleculethatwouldsticktothegene,enablingthemtovisualizewherethegeneisactiveinthelimb.Totheirgreatsurprise,theyfoundthatonlycellsinatinypatchofthelimbhadgeneactivity:theZPA.Sothenextstepsbecameobvious.Thepatternsof

activityintheSonichedgehoggeneshouldmimicthoseof

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theZPAtissueitself.Recallthatwhenyoutreatthelimbwithretinoicacid,aformofvitaminA,yougetaZPAactiveontheoppositeside.Guesswhathappenswhenyoutreatalimbwithretinoicacid,thenmapwhereSonichedgehogisactive?Sonichedgehogbecomesactiveonbothsides—pinkyandthumb—justastheZPAdoeswhenitistreatedwithretinoicacid.KnowingthestructureofthechickenSonichedgehog

gaveotherresearchersthetoolstolookforitineverythingelsethathasfingers,fromfrogstohumans.EverylimbedanimalhastheSonichedgehoggene.Andineverysingleanimalthatwehavestudied,SonichedgehogisactiveintheZPAtissue.IfSonichedgehoghadn’tturnedonproperlyduringtheeighthweekofyourowndevelopment,thenyoueitherwouldhaveextrafingersoryourpinkyandthumbwouldlookalike.Occasionally,whenthingsgowrongwithSonichedgehog,thehandendsuplookinglikeabroadpaddlewithasmanyastwelvefingersthatalllookalike.WenowknowthatSonichedgehogisoneofdozensof

genesthatacttosculptourlimbsfromshouldertofingertipbyturningonandoffattherighttime.Remarkably,workinchickens,frogs,andmicewastellingusthesamething.TheDNArecipetobuildupperarms,forearms,wrists,anddigitsisvirtuallyidenticalineverycreaturethathaslimbs.HowfarbackcanwetraceSonichedgehogandtheother

bitsofDNAthatbuildlimbs?Isthisstuffactiveinbuildingtheskeletonoffishfins?Orarehandsgeneticallycompletelydifferentfromfishfins?Wesawaninnerfishin

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theanatomyofourarmsandhands.WhatabouttheDNAthatbuildsit?EnterRandyDahnwithhismermaid’spurses.

GIVINGSHARKSAHAND

RandyDahnenteredmylaboratorywithasimplebutveryelegantidea:treatskateembryosjustthewayCliffTabintreatedchickeneggs.Randy’sgoalwastoperformalltheexperimentsonskatesthatchickenbiologistshadperformedonchickeneggs,fromSaundersandZwilling’stissuesurgeriesallthewaytoCliffTabin’sgeneexperiments.Skatesdevelopinaneggwithakindofshellandayolk.Skatesevenhavebigembryos,justaschickensdo.Becauseoftheseconvenientfacts,wecouldapplytoskatesmanyofthegeneticandexperimentaltoolspeoplehaddevelopedtounderstandchickens.Whatcouldwelearnbycomparingthedevelopmentofa

sharkfintothatofachickenleg?Evenmorerelevant,whatcouldwelearnaboutourselvesfromallthis?Chickens,asSaunders,Zwilling,andTabinshowed,area

surprisinglygoodproxyforourownlimbs.EverythingthatwasdiscoveredbySaundersandZwilling’scuttingandgraftingexperimentsandbyTabin’sDNAworkappliestoourownlimbsaswell:wehaveaZPA,wehaveSonichedgehog,andbothhaveagreatbearingonourwell-being.Aswesaw,amalfunctioningZPAoramutationinSonic

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hedgehogcancausemajormalformationsinhumanhands.Randywantedtodeterminehowdifferenttheapparatus

isthatbuildsourhands.Howdeepisourconnectiontotherestoflife?Istherecipethatbuildsourhandsnew,ordoesit,too,havedeeprootsinothercreatures?Ifso,howdeep?Sharksandtheirrelativesaretheearliestcreaturesthat

havefinswithaskeletoninside.Ideally,toanswerRandy’squestion,youwouldwanttobringa400-million-year-oldsharkfossilintothelaboratory,grinditup,andlookatitsgeneticstructure.Thenyou’dtrytomanipulateitsfossilembryostolearnwhetherSonichedgehogisactiveinthesamegeneralplaceasinourlimbstoday.Thiswouldbeawonderfulexperiment,butitisimpossible.WecannotextractDNAfromfossilssoold,and,evenifwecould,wecouldneverfindembryosofthosefossilanimalsonwhichtodoexperiments.Livingsharksandtheirrelativesarethenextbestthing.

Nobodywouldeverconfuseasharkfinwithahumanhand:youcouldn’taskfortwomoredifferentkindsofappendages.Notonlyaresharksandhumansverydistantlyrelated,butalsotheskeletalstructuresoftheirappendageslooknothingalike.NothingevenremotelysimilartoOwen’sonebone–twobones–lotsablobs–digitspatternisinsideashark’sfin.Instead,thebonesinsideareshapedlikerods,longandshort,thinandwide.Wecallthemboneseventhoughtheyaremadeofcartilage(sharksandskatesareknownascartilaginousfish,becausetheirskeletonsneverturnintohardbone).IfyouwanttoassesswhetherSonic

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hedgehog’sroleinlimbsisuniquetolimbedanimals,whynotchooseaspeciesutterlydifferentinalmosteveryway?Inaddition,whynotchoosethespeciesthatisthemostprimitivelivingfishwithanykindofpairedappendage,whetherfinorlimb?Sharksfitbothbillsperfectly.Ourfirstproblemwasasimpleone.Weneededareliable

sourcefortheembryosofsharksandskates.Sharksproveddifficulttoobtainwithanydegreeofregularity,butskates,theircloserelatives,wereanothermatter.Sowestartedwithsharksandusedskatesasoursupplyofsharksdwindled.Wefoundasupplierwhowouldshipuseverymonthortwoabatchoftwentyorthirtyeggcaseswithembryosinside.Webecameavirtualcargocultaswewaitedeachmonthforourshipmentofpreciouseggcases.WorkbyTabin’sgroupandothersgaveRandyimportant

cluestobeginhissearch.SinceTabin’sworkin1993,peoplehadfoundSonichedgehoginanumberofdifferentspecies,everythingfromfishtohumans.Withtheknowledgeofthestructureofthegene,RandywasabletosearchalltheDNAoftheskateandsharkforSonichedgehog.Inaveryshorttimehefoundit:asharkSonichedgehoggene.ThekeyquestionstoanswerwereWhereisSonic

hedgehogactive?,and,evenmoreimportant,Whatisitdoing?TheeggcaseswereputtouseasRandyvisualizedwhere

andwhenSonichedgehogisactiveinthedevelopmentofskates.HefirststudiedwhetherSonichedgehogturnsonat

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thesametimeinskatefindevelopmentasitdoesinchickenlimbs.Yes,itdoes.Thenhestudiedwhetheritisturnedoninthepatchoftissueatthebackendofthefin,theequivalentofourpinky.Yesagain.NowhedidhisvitaminAexperiment.Thiswasthemillion-dollarmoment.Ifyoutreatthelimbofachickenormammalwiththiscompound,yougetapatchoftissuethathasSonichedgehogactivityontheoppositeside,andthisresultiscoupledwithaduplicationofthebones.Randyinjectedtheegg,waitedadayorso,andthencheckedwhether,asinchickens,thevitaminAcausedSonichedgehogtoturnonintheoppositesideofthelimb.Itdid.Nowcamethelongwait.WeknewthatSonichedgehogwasbehavingthesamewayinourhandsandinskates’andsharks’fins.Butwhatwouldtheeffectofallthisbeontheskeleton?Wewouldhavetowaittwomonthsfortheanswer.Theembryosweredevelopinginsideanopaqueeggcase.

Allwecouldtellwaswhetherthecreaturewasalive;theinsideofthefinwasinvisibletous.Theendresultwasastunningexampleofsimilarity

amongus,sharks,andskates:amirror-imagefin.Thedorsalfinsduplicatedtheirstructuresinawonderfulfront-to-backpattern,thesamekindwesawwithexperimentsinlimbs.Limbsduplicatealimbstructure.Sharkfinsduplicateasharkfinstructureasdoskates.Sonichedgehoghasasimilareffectineventhemostdifferentkindsofappendageskeletonsfoundonearthtoday.OneeffectofSonichedgehog,youmayrecall,istomake

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thefingersdistinctfromoneanother.AswesawwithrespecttotheZPA,whatkindofdigitdevelopsdependsonhowclosethedigitistothesourceofSonichedgehog.Anormaladultskatefincontainsmanyskeletalrods,whichalllookalike.Couldwemaketheserodsdifferentfromoneanother,likeourdigits?RandytookasmallbeadimpregnatedwiththeproteinmadebySonichedgehogandputitinbetweentheseidenticalskeletalrods.ThekeytohisexperimentisthatheusedmouseSonichedgehog.Sonowwehavearealcontraption:askateembryowithabeadinsidethatisgraduallyleakingmouseSonichedgehogprotein.Wouldthatmouseproteinhaveanyeffectonasharkoraskate?Therearetwoextremeoutcomestoanexperimentlike

this.Oneisthatnothinghappens.ThiswouldmeanthatskatesaresodifferentfrommicethatSonichedgehogproteinhasnoeffect.Theotherextremeoutcomewouldpresentastunningexampleofourinnerfish.Thisoutcomewouldbethattherodsdevelopdifferentlyfromoneanother,demonstratingthatSonichedgehogdoessomethingsimilarinskatesandinus.Andlet’snotforgetthatsinceRandyisusingtheproteinfromamammal,itmeansthatthegeneticrecipewouldbereally,reallysimilar.Notonlydidtherodsenduplookingdifferentfromone

another,theyrespondedtoSonichedgehog,muchasfingersdo,onthebasisofhowclosetheyweretotheSonichedgehogbead:thecloserrodsdevelopedadifferentshapefromtheonesfartheraway.Totopmattersoff,itwasthe

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mouseproteinthatdidthejobsoeffectivelyintheskates.

Normalfins(left)andRandy’streatedfins.Thetreatedfinsshowedamirror-imageduplicationjustaschickenwingsdid.PhotographscourtesyofRandallDahn,UniversityofChicago.

The“innerfish”thatRandyfoundwasnotasinglebone,

orevenasectionoftheskeleton.Randy’sinnerfishlayinthebiologicaltoolsthatactuallybuildfins.Experimentafterexperimentoncreaturesasdifferentasmice,sharks,andfliesshowsusthatthelessonsofSonichedgehogareverygeneral.Allappendages,whethertheyarefinsorlimbs,arebuiltbysimilarkindsofgenes.Whatdoesthismeanfortheproblemwelookedatinthefirsttwochapters—the

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transitionoffishfinsintolimbs?ItmeansthatthisgreatevolutionarytransformationdidnotinvolvetheoriginofnewDNA:muchoftheshiftlikelyinvolvedusingancientgenes,suchasthoseinvolvedinsharkfindevelopment,innewwaystomakelimbswithfingersandtoes.Butthereisadeeperbeautytotheseexperimentson

limbsandfins.Tabin’slabusedworkinfliestofindageneinchickensthattellsusabouthumanbirthdefects.RandyusedtheTabinlabdiscoverytotellussomethingaboutourconnectionstoskates.An“innerfly”helpedfindan“innerchicken,”whichultimatelyhelpedRandyfindan“innerskate.”Theconnectionsamonglivingcreaturesrundeep.

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CHAPTERFOUR

TEETHEVERYWHERE

Thetoothgetsshortshriftinanatomyclass:wespendalloffiveminutesonit.Inthepantheonoffavoriteorgans—I’llleaveittoeachofyoutomakeyourlist—teethrarelyreachthetopfive.Yetthelittletoothcontainssomuchofourconnectiontotherestoflifethatitisvirtuallyimpossibletounderstandourbodieswithoutknowingteeth.Teethalsohavespecialsignificanceforme,becauseitwasinsearchingforthemthatIfirstlearnedhowtofindfossilsandhowtorunafossilexpedition.Thejobofteethistomakebiggercreaturesintosmaller

pieces.Whenattachedtoamovingjaw,teethslice,dice,andmacerate.Mouthsareonlysobig,andteethenablecreaturestoeatthingsthatarebiggerthantheirmouths.Thisisparticularlytrueofcreaturesthatdonothavehandsorclawsthatcanshredorcutthingsbeforetheygettothemouth.True,bigfishtendtoeatlittlerfish.Butteethcanbethegreatequalizer:smallerfishcanmunchonbiggerfishiftheyhavegoodteeth.Smallerfishcanusetheirteethtoscrapescales,feedonparticles,ortakeoutwholechunksof

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fleshfrombiggerfish.Wecanlearnalotaboutananimalbylookingatitsteeth.

Thebumps,pits,andridgesonteethoftenreflectthediet.Carnivores,suchascats,haveblade-likemolarstocutmeat,whileplanteatershaveamouthfullofflatterteeththatcanmacerateleavesandnuts.Theinformationalvalueofteethwasnotlostontheanatomistsofhistory.TheFrenchanatomistGeorgesCuvieroncefamouslyboastedthathecouldreconstructananimal’sentireskeletonfromasingletooth.Thisisalittleoverthetop,butthegeneralpointisvalid;teethareapowerfulwindowintoananimal’slifestyle.Humanmouthsrevealthatweareall-purposeeaters,for

wehaveseveralkindsofteeth.Ourfrontteeth,theincisors,areflatbladesspecializedforcutting.Therearmostteeth,themolars,areflatter,withadistinctivepatternthatcanmacerateplantoranimaltissue.Thepremolars,inbetween,areintermediateinfunctionbetweenincisorsandmolars.Themostremarkablethingaboutourmouthsisthe

precisionwithwhichwechew.Openandcloseyourmouth:yourteethalwayscometogetherinthesameposition,withupperandlowerteethfittingtogetherprecisely.Becausetheupperandlowercusps,basins,andridgesmatchclosely,weareabletobreakupfoodwithmaximalefficiency.Infact,amismatchbetweenupperandlowerteethcanshatterourteeth,andenrichourdentists.Paleontologistsfindteethwonderfullyinformative.Teeth

arethehardestpartsofourbodies,becausetheenamelincludesahighproportionofthemineralhydroxyapatite—

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highereventhanisfoundinbones.Thankstotheirhardness,teethareoftenthebest-preservedanimalpartwefindinthefossilrecordformanytimeperiods.Thisislucky;sinceteetharesuchagreatcluetoananimal’sdiet,thefossilrecordcangiveusagoodwindowonhowdifferentwaysoffeedingcameabout.Thisisparticularlytrueofmammalhistory:whereasmanyreptileshavesimilarteeth,thoseofmammalsaredistinctive.ThemammalsectionofatypicalpaleontologycoursefeelsalmostlikeDentistry101.Livingreptiles—crocodiles,lizards,snakes—lackmuch

ofwhatmakesmammalianmouthsunique.Acrocodile’steeth,forexample,allhaveasimilarblade-likeshape;theonlydifferencebetweenthemisthatsomearebigandotherssmall.Reptilesalsolackthepreciseocclusion—thefitbetweenupperandlowerteeth—thathumansandothermammalshave.Also,whereaswemammalsreplaceourteethonlyonce,reptilestypicallyreceivevisitsfromthetoothfairyfortheirentirelives,replacingtheirteethcontinuallyastheywearandbreakdown.Averybasicpieceofus—ourmammalianwayofprecise

chewing—emergesinthefossilrecordfromaroundtheworldthatrangesfrom225millionto195millionyearsago.Atthebase,intheolderrocks,wefindanumberofreptilesthatlooksuperficiallydog-like.Walkingonfourlegs,theyhavebigskulls,andmanyofthemhavesharpteeth.Theretheresemblancestops.Unlikedogs,thesereptileshaveajawmadeupofmanybones,andtheirteethdon’treallyfitwelltogether.Also,theirteetharereplacedin

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adecidedlyreptilianway:newteethpopinandoutthroughouttheanimals’lives.Gohigherintherocksandweseesomethingutterly

different:theappearanceofmammalness.Thebonesofthejawgetsmallerandmovetotheear.Wecanseethefirstevidenceofupperandlowerteethcomingtogetherinpreciseways.Thejaw’sshapechanges,too:whatwasasimplerodinreptileslooksmorelikeaboomeranginmammals.Atthistime,too,teetharereplacedonlyonceperlifetime,asinus.Wecantraceallthesechangesinthefossilrecord,especiallyfromcertainsitesinEurope,SouthAfrica,andChina.Therocksofabout200millionyearsagocontainrodent-

likecreatures,suchasMorganucodonandEozostrodon,thathavebeguntolooklikemammals.Theseanimals,nobiggerthanamouse,holdimportantpiecesofusinside.Picturescannotconveyjusthowwonderfultheseearlymammalsare.Forme,itwasarealthrilltoseecreatureslikethemforthefirsttime.WhenIenteredgraduateschool,Iwantedtostudyearly

mammals.IchoseHarvardbecauseFarishA.Jenkins,Jr.,whomwemetinthefirstchapter,wasleadingexpeditionstotheAmericanWestthatsystematicallyscouredtherocksforsignsofhowmammalsdevelopedtheirdistinctabilitiestochew.Theworkwasrealexploration;Farishandhisteamwerelookingfornewlocalitiesandsites,notreturningtoplacesotherpeoplehaddiscovered.Farishhadassembledatalentedgroupoffossilfinderscomprising

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stafffromHarvard’sMuseumofComparativeZoologyandafewfree-lancemercenaries.ChiefamongthemwereBillAmaral,ChuckSchaff,andthelateWillDowns.Thesepeopleweremyintroductiontotheworldofpaleontology.Farishandtheteamhadstudiedgeologicalmapsand

aerialphotostochoosepromisingareaswheretheymightfindearlymammals.Then,eachsummer,theygotintheirtrucksandheadedoffintothedesertsofWyoming,Arizona,andUtah.BythetimeIjoinedthem,in1983,theyhadalreadyfoundanumberofimportantnewmammalsandfossilsites.Iwasstruckbythepowerofpredictions:simplybyreadingscientificarticlesandbooks,Farish’steamcouldidentifylikelyandunlikelyplacestofindearlymammals.Mybaptisminfieldpaleontologycamefromwalkingout

intheArizonadesertwithChuckandBill.Atfirst,thewholeenterpriseseemedutterlyrandom.Iexpectedsomethingakintoamilitarycampaign,anorganizedandcoordinatedreconnaissanceofthearea.WhatIsawlookedliketheextremeopposite.Theteamwouldplunkdownonaparticularpatchofrock,andpeoplewouldscatterineveryconceivabledirectiontolookforfragmentsofboneonthesurface.Forthefirstfewweeksoftheexpedition,theyleftmealone.I’dsetofflookingforfossils,systematicallyinspectingeveryrockIsawforascrapofboneatthesurface.Attheendofeachdaywewouldcomehometoshowoffthegoodieswefound.Chuckwouldhaveseveralbagsofbones.Billwouldhavehiscomplement,usuallywithsomesortoflittleskullorotherprize.AndIhadnothing,my

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emptybagasadreminderofhowmuchIhadtolearn.Afterafewweeksofthis,Idecideditwouldbeagood

ideatowalkwithChuck.Heseemedtohavethefullestbagseachday,sowhynottakesomecuesfromtheexpert?Chuckwashappytowalkwithmeandexpoundonhislongcareerinfieldpaleontology.ChuckisallWestTexaswithaBrooklynflourish:cowboybootsandwesternvalueswithaNewYorkaccent.Whileheregaledmewithtalesofhispastexpeditions,Ifoundthewholeexperienceutterlyhumbling.First,Chuckdidnotlookateveryrock,andwhenhechoseonetolookat,forthelifeofmeIcouldn’tfigureoutwhy.Thentherewasthereallyembarrassingaspectofallthis:ChuckandIwouldlookatthesamepatchofground.Isawnothingbutrock—barrendesertfloor.Chucksawfossilteeth,jaws,andevenchunksofskull.Anaerialviewwouldhaveshowntwopeoplewalking

aloneinthemiddleofaseeminglylimitlessplain,wherethevistaofdustyredandgreensandstonemesas,buttes,andbadlandsextendedformiles.ButChuckandIwerestaringonlyattheground,attherubbleandtalusofthedesertfloor.Thefossilswesoughtweretiny,nomorethanafewincheslong,andourswasaverysmallworld.Thisintimateenvironmentstoodinextremecontrasttothevastnessofthedesertpanoramathatsurroundedus.Ifeltasifmywalkingpartnerwastheonlypersonontheentireplanet,andmywholeexistencewasfocusedonpiecesofrubble.ChuckwasextraordinarilypatientwithmeasIpestered

himwithquestionsforthebetterpartofeachday’swalk.I

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wantedhimtodescribeexactlyhowtofindbones.Overandover,hetoldmetolookfor“somethingdifferent,”somethingthathadthetextureofbonenotrock,somethingthatglistenedliketeeth,somethingthatlookedlikeanarmbone,notapieceofsandstone.Itsoundedeasy,butIcouldn’tgraspwhathewastellingme.TryasImight,Istillreturnedhomeeachdayempty-handed.Nowitwasevenmoreembarrassing,asChuck,whowaslookingatthesamerocks,camehomewithbagafterbag.Finally,oneday,Isawmyfirstpieceoftoothglisteningin

thedesertsun.Itwassittinginsomesandstonerubble,butthereitwas,asplainasday.Theenamelhadasheenthatnootherrockhad;itwaslikenothingIhadseenbefore.Well,notexactly—Iwaslookingatthingslikeiteveryday.ThedifferencewasthistimeIfinallysawit,sawthedistinctionbetweenrockandbone.Thetoothglistened,andwhenIsawitglistenIspotteditscusps.Thewholeisolatedtoothwasaboutthesizeofadime,notincludingtherootsthatprojectedfromitsbase.Tome,itwasasgloriousasthebiggestdinosaurinthehallsofanymuseum.Allofasudden,thedesertfloorexplodedwithbone;

whereonceIhadseenonlyrock,nowIwasseeinglittlebitsandpiecesoffossileverywhere,asifIwerewearingaspecialnewpairofglassesandaspotlightwasshiningonallthedifferentpiecesofbone.Nexttothetoothweresmallfragmentsofotherbones,thenmoreteeth.Iwaslookingatajawthathadweatheredoutonthesurfaceandfragmented.Istartedtoreturnhomewithmyownlittle

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bagseachnight.NowthatIcouldfinallyseebonesformyself,whatonce

seemedahaphazardgroupeffortstartedtolookdecidedlyordered.Peopleweren’tjustscatteringrandomlyacrossthedesert;therewererealthoughunspokenrules.Rulenumberone:gotothemostproductive-lookingrocks,judgingbywhateversearchimageorvisualcuesyou’vegainedfrompreviousexperience.Rulenumbertwo:don’tfollowinanybody’sfootsteps;covernewground(Chuckhadgraciouslyletmebreakthisone).Rulethree:ifyourplumareaalreadyhassomebodyonit,findanewplum,orsearchalesspromisingsite.Firstcome,firstserved.Overtime,Ibegantolearnthevisualcuesforotherkinds

ofbones:longbones,jawbones,andskullparts.Onceyouseethesethingsyouneverlosetheabilitytofindthem.Justasagreatfishermancanreadthewaterandseethefishwithin,soafossilfinderusesacatalogueofsearchimagesthatmakefossilsseemtojumpoutfromtherocks.Iwasbeginningtogainmyownvisualimpressionsofwhatfossilboneslooklikeindifferentrocksandindifferentlightingconditions.Findingfossilsinthemorningsunisverydifferentfromfindingthemintheafternoon,becauseofthewaythelightplaysalongtheground.Twentyyearslater,IknowthatImustgothrougha

similarexperienceeverytimeIlookforfossilssomeplacenew,fromtheTriassicofMoroccototheDevonianofEllesmereIsland.I’llstruggleforthefirstfewdays,almostasIdidthosedayswithChuckinArizonatwentyyearsago.

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ThedifferenceisthatnowIhavesomeconfidencethatasearchimagewillkickineventually.ThewholegoaloftheprospectingIdidwithChuckwasto

findasitewithenoughbonestomarkafossil-richlayerthatwecouldexpose.BythetimeIjoinedthecrew,Farish’steamhadalreadydiscoveredsuchazone,apatchofrockaboutahundredfeetlongthatcontainedskeletonafterskeletonofsmallanimals.Farish’sfossilquarrywasinsomeveryfine-grained

mudstone.Thetricktoworkingonitwastorealizethatthefossilswerecomingfromonethinlayer,nomorethanamillimeterthick.Onceyouexposedthatsurface,youhadaverygoodchanceofseeingbones.Theyweretiny,nomorethananinchortwolong,andblack,sotheylookedalmostlikeblacksmudgesagainstthebrownishrock.Thelittleanimalswefoundincludedfrogs(someoftheearliest),leglessamphibians,lizardsandotherreptiles,and,importantly,someoftheearliestmammals.Thekeypointisthattheearlymammalsweresmall.Very

small.Theirteethwerenotmuchmorethan2millimeterslong.Tospotthem,youhadtobeverycarefuland,moreoften,verylucky.Ifthetoothwascoveredbyacrumbofrockorevenbyafewgrainsofsand,youmightneverseeit.Itwasthesightoftheseearlymammalsthatreally

hookedme.I’dexposethefossillayer,thenscantheentiresurfacethroughmy10-powerhandlens.I’dscrutinizethewholethingonmyhandsandknees,withmyeyeandhandlensonlyabouttwoinchesfromthesurfaceoftheground.

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Thusengrossed,I’doftenforgetwhereIwasandaccidentallytrespassonmyneighbor’sspotonlytohaveabagofdirtdumpedonmyheadasasharpremindertokeeptomyspace.Occasionally,though,I’dhitthejackpotandseeadeepconnectionforthefirsttime.Theteethwouldlooklikelittleblades,withcuspsandroots.Thecuspsonthoselittleteethrevealedsomethingveryspecial.Eachtoothhadacharacteristicpatternofwearatthefacewhereupperandlowerteethfittogether.Iwasseeingsomeofthefirstevidenceofourpatternofprecisechewing,onlyinatinymammal190millionyearsold.ThepowerofthosemomentswassomethingI’llnever

forget.Here,crackingrocksinthedirt,Iwasdiscoveringobjectsthatcouldchangethewaypeoplethink.Thatjuxtapositionbetweenthemostchild-like,evenhumbling,activitiesandoneofthegreathumanintellectualaspirationshasneverbeenlostonme.ItrytoremindmyselfofiteachtimeIdigsomewherenew.Returningtoschoolthatfall,Idevelopedtheexpedition

bugbig-time.Iwantedtoleadmyownexpeditionbutlackedtheresourcestodoanythingbig,soIsetofftoexplorerocksinConnecticutthatwereabout200millionyearsold.Wellstudiedduringthenineteenthcentury,theyhadbeenthesettingforanumberofimportantfossildiscoveries.IfiguredthatifIhitthosesamerockswithmyhandlensandmywonderfullysuccessfulearlymammalsearchimage,I’dfindlotsofgoodies.Irentedaminivan,grabbedacaseofcollectingbags,andsetoff.

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Yetanotherlessonlearned:Ifoundnothing.Backtothedrawingboard,ormoreprecisely,thegeologylibraryatschool.Ineededaplacewhere200-million-year-oldrockswere

wellexposed:inConnecticuttherewereonlyroadcuts.Theidealplacewouldbealongthecoast,wherewaveactionwouldprovidelotsoffreshlybrokenrocksurfacetolookat.Lookingatamapmademychoiceclear:upinNovaScotia,TriassicandJurassicrocks(roughly200millionyearsold)layalongthesurface.Totopitoff,thetouristliteratureabouttheareaadvertisedtheworld’shighesttides,occasionallyoverfiftyfeet.Icouldn’tbelievemyluck.Icalledtheexpertontheserocks,PaulOlsen,whohad

juststartedteachingatColumbiaUniversity.IfIwasexcitedaboutfossil-findingprospectsbeforeItalkedtoPaul,Iwasfrothingafterward.Hedescribedtheperfectgeologyforfindingsmallmammalsorreptiles:ancientstreamsanddunesthathadjusttherightpropertiestopreservetinybones.Evenbetter,hehadalreadyfoundsomedinosaurbonesandfootprintsalongastretchofbeachnearthetownofParrsboro,NovaScotia.PaulandIhatchedaplantovisitParrsborotogetherandscanthebeachforlittlefossils.ThiswaswonderfullygenerousonPaul’spartbecausehehaddibsontheareaandwasundernoresponsibilitytohelpmeout,letalonecollaborate.IconsultedwithFarishonmyemergingplans,andhenot

onlyofferedmoneybutsuggestedthatItakethefossil-findingexperts,BillandChuck.Money,Bill,Chuck,Paul

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Olsen,excellentrocks,anddecentexposures—whatmorecouldyouwant?Thefollowingsummer,Iledmyveryfirstfossilexpedition.OffIwentinarentedstationwagontothebeachesof

NovaScotiawithmyfieldcrew,BillandChuck.Thejoke,ofcourse,wasonme.WithBillandChuckalong,whobetweenthemhadmoreyearsoffieldexperiencethanIhadbirthdays,Iwastheleaderinnameonly.Theycalledthefossil-findingshots,whileIpaidthedinnerbills.TherocksinNovaScotiawereexposedinabsolutely

gorgeousorangesandstonecliffsalongtheBayofFundy.Thetideswouldgoinandoutabouthalfamileeachday,exposingenormousflatsoforangebedrock.Itwasn’tlongbeforewestartedtofindbonesinmanydifferentareas.Smallwhiteflecksofbonewerecomingoutalongthecliffs.Paulwasfindingfootprintseverywhere,evenintheflatsopenedbythemovingtideseachday.

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PaulOlsenfindingfootprintsinthetidalflatsofNovaScotia.Athightide,thewaterwouldcomeallthewaytothecliffsatleft.Thearrowheadpointstoaspotwhere,ifwetimedourtripwrong,wewouldbestuckonthecliffsforhoursatatime.Photographbytheauthor.

Chuck,Bill,Paul,andIspenttwoweeksdigginginNova

Scotia,findingbits,flakes,andfragmentsofbonesstickingoutoftherocks.Bill,beingthefossilpreparatorofthegroup,continuallywarnedmenottoexposemuchofthebonesinthefieldbutrathertowrapthemupstillcoveredinsandstonesothathecouldtracethebonesinthelaboratoryunderamicroscopeinmorecontrolledconditions.Wedidthis,butI’lladmittobeingdisappointedwithwhatwebroughthome:justafewshoeboxesofrocks,withsmallchipsandflakesofbonesshowing.Aswedrovehome,Irecallthinkingthateventhoughwehadn’tfoundmuch,ithadbeenagreatexperience.ThenItookaweek’svacation;ChuckandBillreturnedtothelab.WhenIreturnedtoBoston,ChuckandBillwereoutto

lunch.Somecolleagueswerevisitingthemuseumand,havingcaughtsightofme,cameuptoshakemyhand,offercongratulations,andslapmeontheback.Iwasbeingtreatedlikeaconqueringhero,butIhadnoideawhy;itseemedlikeabizarrejoke,asiftheyweresettingmeupforsomebigcon.TheytoldmetogotoBill’slabtoseemytrophy.Notknowingwhattothink,Iran.UnderBill’smicroscopewasatinyjaw,notmorethan

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halfaninchlong.Initwereafewminuteteeth.Thejaw’sownerwasclearlyareptile,becausetheteethhadonlyasinglerootatthebase,whereasmammalteethhavemany.ButontheteethweretinybumpsandridgesthatIcouldseeevenwiththenakedeye.Lookingattheteethunderthemicroscopegavemethebiggestsurprise:thecuspshadlittlepatchesofwear.Thiswasareptilewithtooth-to-toothocclusion.Myfossilwaspartmammal,partreptile.Unbeknownsttome,Billhadunwrappedoneofour

blocksofrock,seenafleckofbone,andprepareditwithaneedleunderthemicroscope.Noneofushadknownitinthefield,butourexpeditionwasahugesuccess.AllbecauseofBill.WhatdidIlearnthatsummer?First,Ilearnedtolistento

ChuckandBill.Second,Ilearnedthatmanyofthebiggestdiscoverieshappeninthehandsoffossilpreparators,notinthefield.Asitturnedout,mybiggestlessonsaboutfieldworkwereyettocome.ThereptileBillhadfoundwasatritheledont,acreature

knownfromSouthAfricaaswellasnowfromNovaScotia.Thesewereveryrare,sowewantedtoreturntoNovaScotiathenextsummertofindmore.Ispentthewholewintertensewithanticipation.IfIcouldhavechippedthroughthewintericetofindfossils,Iwouldhavedoneit.Inthesummerof1985,wereturnedtothesitewherewe

hadfoundthetritheledont.Thefossilbedwasjustatbeachlevel,wherealittlepieceofthecliffhadfallenoffseveralyearsbefore.Wehadtotimeourdailyvisitjustso:thesite

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wasinaccessibleathightidebecausethewatercameuptoohigharoundapointwehadtonavigate.I’llneverforgetthatfirstdayofexcitementwhenweroundedthepointtofindourlittlepatchofbrightorangerock.Theexperiencewasmemorableforwhatwasmissing:mostoftheareawehadworkedtheyearbefore.Ithadweatheredawaythepreviouswinter.Ourlovelyfossilsite,containingbeautifultritheledonts,wasgonewiththetides.Thegoodnews,ifyoucouldcallitthat,wasthatthere

wasalittlemoreorangesandstonetoscanalongthebeach.Mostofthebeach,inparticularthepointwehadtogoaroundeachmorning,wasmadeupofbasaltfroma200-million-year-oldlavaflow.Wewerepositivenofossilscouldbefoundthere,foritisvirtuallyaxiomaticthattheserocks,whichwereoncesuperhot,wouldneverpreservefossilbone.Wespentfiveormoredaystimingourvisitstothesitesbythetides,pawingawayattheorangesandstonesbeyondit,andfindingabsolutelynothing.Ourbreakthroughcamewhenthepresidentofthelocal

LionsClubcamebyourcabinonenightlookingforjudgesforthelocalbeautycontest,tocrownParrsboro’sMissOldHomeWeek.Thetownalwaysreliedonvisitorsforthisoneroustask,becauseinternecinepassionstypicallyrunhighduringtheevent.Theusualjudges,anelderlycouplefromQuebec,werenotvisitingthisyear,andthecrewandIwereinvitedtosubstitute.Butinjudgingthebeautycontestandarguingoverits

conclusion,westayedupwaytoolate,forgotaboutthenext

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morning’stides,andendeduptrappedaroundabendinthebasaltcliffs.Forabouttwohours,wewerestuckonalittlepromontoryaboutfiftyfeetwide.Therockwasvolcanicandnotthetypeonewouldeverchoosetosearchforfossils.Weskippedstonesuntilwegotbored,thenwelookedattherocks:maybewe’dfindinterestingcrystalsorminerals.Billdisappearedaroundacorner,andIlookedatsomeofthebasaltbehindus.AfteraboutfifteenminutesIheardmyname.I’llneverforgetBill’sunderstatedtone:“Uh,Neil,youmightwanttocomeoverhere.”AsIroundedthecorner,IsawtheexcitementinBill’seyes.ThenIsawtherocksathisfeet.Stickingoutoftherocksweresmallwhitefragments.Fossilbones,thousandsofthem.Thiswasexactlywhatwewerelookingfor,asitewith

smallbones.Itturnedoutthatthevolcanicrockswerenotentirelyvolcanic:sliversofsandstonecutthroughthecliff.Therockshadbeenproducedbyanancientmudflowassociatedwithavolcaniceruption.Thefossilswerestuckintheancientmuds.Webroughttonsoftheserockshome.Insideweremore

tritheledonts,someprimitivecrocodiles,andotherlizard-likereptiles.Thetritheledontswerethegems,ofcourse,becausetheyshowedthatsomekindsofreptilesalreadydisplayedourmammaliankindofchewing.Earlymammals,suchasthoseFarish’steamuncovered

inArizona,hadveryprecisepatternsofbiting.Scrapesonthecuspsofanuppertoothfitagainstmirrorimagesofthesescrapesonalowertooth.Thesepatternsofwearare

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sofinethatdifferentspeciesofearlymammalscanbedistinguishedbytheirpatternsoftoothwearandocclusion.Farish’sArizonamammalshaveadifferentpatternofcuspsandchewingthanthoseofthesameagefromSouthAmerica,Europe,orChina.Ifallwehadtocomparethesefossilstowerelivingreptiles,thentheoriginofmammalianfeedingwouldappeartobeabigmystery.AsI’vementioned,crocodilesandlizardsdonothaveanykindofmatchingpatternofocclusion.Hereiswherecreaturesliketritheledontscomein.Whenwegobackintime,torocksabout10millionyearsolder,suchasthoseinNovaScotia,wefindtritheledontswithanincipientversionofthiswayofchewing.Intritheledonts,individualcuspsdonotinterlockinapreciseway,astheydoinmammals;instead,theentireinnersurfaceoftheuppertoothshearsagainsttheoutersurfaceofthelowertooth,almostlikeascissors.Ofcourse,thesechangesinocclusiondidnothappeninavacuum.Itshouldcomeasnosurprisethattheearliestcreaturestoshowamammaliankindofchewingalsodisplaymammalianfeaturesofthelowerjaw,skull,andskeleton.

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AtritheledontandapieceofitsupperjawdiscoveredinNovaScotia.JawfragmentillustratedbyLazloMeszoley.

Becauseteethpreservesowellinthefossilrecord,we

haveverydetailedinformationabouthowmajorpatternsofchewing—andtheabilitytousenewdiets—aroseovertime.Muchofthestoryofmammalsisthestoryofnewwaysofprocessingfood.Soonafterweencountertritheledontsinthefossilrecord,westartseeingallsortsofnewmammalspecieswithnewkindsofteeth,aswellasnewwaysofoccludingandusingthem.Byabout150millionyearsago,inrocksfromaroundtheworld,wefindsmallrodent-sizemammalswithanewkindoftoothrow,onethatpavedthewayforourownexistence.Whatmadethesecreaturesspecialwasthecomplexityoftheirmouths:thejawhaddifferentkindsofteethsetinit.Themouthdevelopedakindofdivisionoflabor.Incisorsinthefrontbecamespecializedtocutfood,caninesfurtherbacktopunctureit,andmolarsintheextremebacktoshearormashit.Theselittlemammals,whichresemblemice,haveafundamentalpieceofourhistoryinsideofthem.Ifyoudoubtthis,imagineeatinganapplelackingyourincisorteethor,betteryet,alargecarrotwithnomolars.Ourdiversediet,rangingfromfruittomeattoTwinkie,ispossibleonlybecauseourdistantmammalianancestorsdevelopedamouthwithdifferentkindsofteeththatcanoccludeprecisely.Andyes,initialstagesofthisareseenintritheledontsandotherancientrelatives:theteethinthe

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fronthaveadifferentpatternofbladesandcuspsthanthoseintheback.

TEETHANDBONES—THEHARDSTUFF

Italmostgoeswithoutsayingthatwhatmakesteethspecialamongorgansistheirhardness.Teethhavetobeharderthanthebitsoffoodtheybreakdown;imaginetryingtocutasteakwithasponge.Inmanyways,teethareashardasrocks,andthereasonisthattheycontainacrystalmoleculeontheinside.Thatmolecule,knownashydroxyapatite,impregnatesthemolecularandcellularinfrastructureofbothteethandbones,makingthemresistanttobending,compression,andotherstresses.Teethareextrahardbecausetheirouterlayer,enamel,isfarricherinhydroxyapatitethananyotherstructureinthebody,includingbone.Enamelgivesteeththeirwhitesheen.Ofcourse,enamelisonlyoneofthelayersthatmakeupourteeth.Theinnerlayers,suchasthepulpanddentine,arealsofilledwithhydroxyapatite.Therearelotsofcreatureswithhardtissues—clamsand

lobsters,forexample.Buttheydonotusehydroxyapatite;lobstersandclamsuseothermaterials,suchascalciumcarbonateorchitin.Also,unlikeus,theseanimalshaveanexoskeletoncoveringthebody.Ourhardnesslieswithin.Ourparticularbrandofhardness,withteethinsideour

mouthsandbonesinsideourbodies,isanessentialpartof

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whoweare.Wecaneat,moveabout,breathe,evenmetabolizecertainmineralsbecauseofourhydroxyapatite-containingtissues.Forthesecapabilities,wecanthankthecommonancestorwesharewithallfish.Everyfish,amphibian,reptile,bird,andmammalontheplanetislikeus.Allofthemhavehydroxyapatite-containingstructures.Butwheredidthisallcomefrom?Thereisanimportantintellectualissueatstakehere.By

knowingwhere,when,andhowhardbonesandteethcameabout,wewillbeinapositiontounderstandwhy.Whydidourkindofhardtissuesarise?Didtheycomeabouttoprotectanimalsfromtheirenvironment?Didtheycomeabouttohelpthemmove?Answerstothesequestionslieinthefossilrecord,inrocksapproximately500millionyearsold.Someofthemostcommonfossilsinancientoceans,500

millionto250millionyearsold,areconodonts.Conodontswerediscoveredinthe1830sbytheRussianbiologistChristianPander,whowillreappearinafewchapters.Theyaresmallshellyorganismswithaseriesofspikesprojectingoutofthem.SincePander’stime,conodontshavebeendiscoveredoneverycontinent;thereareplaceswhereyoucannotcrackarockwithoutfindingvastnumbersofthem.Hundredsofkindsofconodontsareknown.Foralongtime,conodontswereenigmas:scientists

disagreedoverwhethertheywereanimal,vegetable,ormineral.Everybodyseemedtohaveapettheory.Conodontswereclaimedtobepiecesofclams,sponges,

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vertebrates,evenworms.Thespeculationendedwhenwholeanimalsstartedtoshowupinthefossilrecord.Thefirstspecimenthatmadesenseofeverythingwas

foundbyaprofessorofpaleontologyrummagingthroughthebasementattheUniversityofEdinburgh:therewasaslabofrockwithwhatlookedlikealampreyinit.Youmightrecalllampreysfrombiologyclass—theseareveryprimitivefishthathavenojaws.Theymaketheirlivingbyattachingtootherfishandfeedingontheirbodilyfluids.Embeddedinthefrontofthelampreyimpressionweresmallfossilsthatlookedstrangelyfamiliar.Conodonts.Otherlamprey-likefossilsstartedtocomeoutofrocksinSouthAfricaandlaterthewesternUnitedStates.Thesecreaturesallhadanexceptionaltrait:theyhadwholeassemblagesofconodontsintheirmouths.Theconclusionbecameabundantlyclear:conodontswereteeth.Andnotjustanyteeth.Conodontsweretheteethofanancientjawlessfish.Wehadtheearliestteethinthefossilrecordforover150

yearsbeforewerealizedwhattheywere.Thereasoncomesdowntohowfossilsarepreserved.Thehardbits,forexampleteeth,tendtogetpreservedeasily.Softparts,suchasmuscle,skin,andguts,usuallydecaywithoutfossilizing.Wehavemuseumcabinetsfulloffossilskeletons,shells,andteeth,butpreciousfewgutsandbrains.Ontherareoccasionswhenwefindevidenceofsofttissues,theyaretypicallypreservedonlyasimpressionsorcasts.Ourfossilrecordisloadedwithconodontteeth,butittookus150

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yearstofindthebodies.Thereissomethingelseremarkableaboutthebodiestowhichconodontsbelonged.Theyhavenohardbones.Theseweresoft-bodiedanimalswithhardteeth.Foryears,paleontologistshavearguedaboutwhyhard

skeletons,thosecontaininghydroxyapatite,aroseinthefirstplace.Forthosewhobelievedthatskeletonsbeganwithjaws,backbones,orbodyarmor,conodontsprovidean“inconvenienttooth,”ifyouwill.Thefirsthardhydroxyapatite-containingbodypartswereteeth.Hardbonesarosenottoprotectanimals,buttoeatthem.Withthis,thefish-eat-fishworldreallybeganinearnest.First,bigfishatelittlefish;then,anarmsracebegan.Littlefishdevelopedarmor,bigfishobtainedbiggerjawstocrackthearmor,andsoon.Teethandbonesreallychangedthecompetitivelandscape.Thingsgetmoreinterestingstillaswelookatsomeof

thefirstanimalswithbonyheads.Aswemoveupintimefromtheearliestconodontanimals,weseewhatthefirstbony-headskeletonslookedlike.Theybelongedtofishcalledostracoderms,areabout500millionyearsold,andarefoundinrocksallovertheworld,fromtheArctictoBolivia.Thesefishlooklikehamburgerswithfleshytails.Theheadregionofanostracodermisabigdiskcovered

byashieldofbone,lookingalmostlikearmor.IfIweretoopenamuseumdrawerandshowyouone,youwouldimmediatelynoticesomethingodd:theheadskeletonisreallyshiny,muchlikeourteethorthescalesofafish.

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Aconodont(left)andanostracoderm(right).Conodontswereoriginallyfoundisolated.Then,aswholeanimalsbecameknown,welearnedthatmanyofthemfunctionedtogetherasatoothrowinthemouthsofthesesoft-bodiedjawlessfish.Ostracodermshaveheadscoveredwithabonyshield.Themicroscopiclayersofthatshieldlookliketheyarecomposedoflittletooth-likestructures.ConodonttoothrowreconstructioncourtesyofDr.MarkPurnell,UniversityofLeicester,andDr.PhilipDonoghue,UniversityofBristol.

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Oneofthejoysofbeingascientististhatthenaturalworldhasthepowertoamazeandsurprise.Here,inostracoderms,anobscuregroupofancientjawlessfish,liesaprimeexample.Ostracodermsareamongtheearliestcreatureswithbonyheads.Cuttheboneoftheskullopen,embeditinplastic,popitunderthemicroscope,andyoudonotfindjustanyoldtissuestructure;rather,youfindvirtuallythesamestructureasinourteeth.Thereisalayerofenamelandevenalayerofpulp.Thewholeshieldismadeupofthousandsofsmallteethfusedtogether.Thisbonyskull—oneoftheearliestinthefossilrecord—ismadeentirelyoflittleteeth.Teethoriginallyarosetobitecreatures;later,aversionofteethwasusedinanewwaytoprotectthem.

TEETH,GLANDS,ANDFEATHERS

Teethnotonlyheraldawholenewwayofliving,theyrevealtheoriginofawholenewwayofmakingorgans.Teethdevelopbyaninteractionoftwolayersoftissueinourdevelopingskin.Basically,twolayersapproacheachother,cellsdivide,andthelayerschangeshapeandmakeproteins.Theouterlayerspitsoutthemolecularprecursorsofenamel,theinnerlayerthedentineandpulpoftheinsideofthetooth.Overtime,thestructureofthetoothislaiddown,thentweakedtomakethepatternsofcuspsandtroughsthatdistinguisheachspecies.

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Thekeytotoothdevelopmentisthataninteractionbetweenthesetwolayersoftissue,anoutersheetofcellsandaninnerlooselayerofcells,causesthetissuetofoldandmakesbothlayerssecretethemoleculesthatbuildtheorgan.Itturnsoutthatexactlythesameprocessunderliesthedevelopmentofallthestructuresthatdevelopwithinskin:scales,hair,feathers,sweatglands,evenmammaryglands.Ineachcase,twolayerscometogether,fold,andsecreteproteins.Indeed,thebatteriesofthemajorgeneticswitchesthatareactiveinthisprocessineachkindoftissuearelargelysimilar.

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Teeth,breasts,feathers,andhairalldevelopfromtheinteractionsbetweenlayersofskin.

Thisexampleisakintomakinganewfactoryorassembly

process.Onceplasticinjectionwasinvented,itwasusedinmakingeverythingfromcarpartstoyo-yos.Teetharenodifferent.Oncetheprocessthatmakesteethcameintobeing,itwasmodifiedtomakethediversekindsoforgans

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thatliewithinskin.Wesawthistakentoaverygreatextremeintheostracoderms.Birds,reptiles,andhumansarejustasextremeinmanyways.Wewouldneverhavescales,feathers,orbreastsifwedidn’thaveteethinthefirstplace.Thedevelopmentaltoolsthatmaketeethhavebeenrepurposedtomakeotherimportantskinstructures.Inaveryrealsenseorgansasdifferentasteeth,feathers,andbreastsareinextricablylinkedbyhistory.Athemeofthesefirstfourchaptersishowwecantrace

thesameorganindifferentcreatures.InChapter1wesawthatwecanmakepredictionsandfindversionsofourorgansinancientrocks.InChapter2wesawhowwecantracesimilarbonesallthewayfromfishtohumans.Chapter3showshowtherealheritablepartofourbodies—theDNAandgeneticrecipethatbuildsorgans—canbefollowedinverydifferentcreatures.Here,inteeth,mammaryglands,andfeathers,wefindasimilartheme.Thebiologicalprocessesthatmakethesedifferentorgansareversionsofthesamething.Whenyouseethesedeepsimilaritiesamongdifferentorgansandbodies,youbegintorecognizethatthediverseinhabitantsofourworldarejustvariationsonatheme.

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CHAPTERFIVE

GETTINGAHEAD

ItwastwonightsbeforemyanatomyfinalandIwasinthelabataroundtwointhemorning,memorizingthecranialnerves.Therearetwelvecranialnerves,eachbranchingtotakebizarretwistsandturnsthroughtheinsideoftheskull.Tostudythem,webisectedtheskullfromforeheadtochinandsawedopensomeofthebonesofthecheek.SothereIwas,holdinghalfoftheheadineachhand,tracingthetwistedpathsthatthenervestakefromourbrainstothedifferentmusclesandsenseorgansinside.Iwasenrapturedbytwoofthecranialnerves,the

trigeminalandthefacial.Theircomplicatedpatternboileddowntosomethingsosimple,sooutrageouslyeasythatIsawthehumanheadinanewway.Thatinsightcamefromunderstandingthefarsimplerstateofaffairsinsharks.Theeleganceofmyrealization—thoughnotitsnovelty;comparativeanatomistshadhaditacenturyormoreago—andthepressureoftheupcomingexamledmetoforgetwhereIwas.Atsomepoint,Ilookedaround.ItwasthemiddleofthenightandIwasaloneinthelab.Ialso

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happenedtobesurroundedbythebodiesoftwenty-fivehumanbeingsundersheets.Forthefirstandlasttime,Igotthewillies.Iworkedmyselfintosuchalatherthatthehairsonthebackofmyneckrose,myfeetdidtheirjob,andwithinananosecondIfoundmyselfatthebusstop,outofbreath.ItgoeswithoutsayingthatIfeltridiculous.Iremembertellingmyself:Shubin,you’vebecomehard-core.Thatthoughtdidnotlastlong;IsoondiscoveredIhadlockedmyhousekeysinthelab.Whatmademesohard-coreisthatheadanatomyis

deeplymesmerizing,infact,beautiful.Oneofthejoysofscienceisthat,onoccasion,weseeapatternthatrevealstheorderinwhatinitiallyseemschaotic.Ajumblebecomespartofasimpleplan,andyoufeelyouareseeingrightthroughsomethingtofinditsessence.Thischapterisaboutseeingthatessenceinsideourownheads.And,ofcourse,theheadsoffish.

THEINNERCHAOSOFTHEHEAD

Headanatomyisnotonlycomplicatedbuthardtosee,since,unlikeotherpartsofthebody,thetissuesoftheheadareencapsulatedinabonybox.Weliterallyhavetosawthroughthecheek,forehead,andcraniumtoseethevesselsandorgans.Havingthusopenedahumanhead,wefindaclumpofwhatlooksliketangledfishinglines.Vesselsandnervesmakecuriousloopsandturnsastheytravelthrough

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theskull.Thousandsofnervebranches,muscles,andbonessitwithinthissmallbox.Atfirstglance,thewholearrayisabewilderingmess.Ourskullsaremadeupofthreefundamentalparts:think

plates,blocks,androds.Theplatescoverourbrain.Patthetopofyourhead,andyouarefeelingthem.Theselargeplatesfittogetherlikejigsaw-puzzlepiecesandformmuchofourcranium.Whenwewereborn,theplateswereseparate;theopenspacesbetweenthem,thefontanelles,arevisibleininfants,occasionallythrobbingwiththebraintissueunderneath.Aswegrow,thebonesenlarge,andbythetimewereachtheageoftwotheyhavefused.Anotherpartofourskullliesunderneaththebrain,

formingaplatformthatholdsitup.Unliketheplate-likebonesatthetop,theseboneslooklikecomplicatedblocksandhavemanyarteriesandnervesrunningthroughthem.Thethirdkindofbonemakesupourjaws,somebonesinourears,andotherbonesinourthroats;thesebonesstartdevelopmentlookinglikerods,whichultimatelybreakupandchangeshapetohelpuschew,swallow,andhear.Insidetheskullareanumberofcompartmentsand

spacesthathousedifferentorgans.Obviously,thebrainoccupiesthelargestofthese.Otherspacescontainoureyes,partsofourears,andournasalstructures.Muchofthechallengeinunderstandingheadanatomycomesfromseeingthesedifferentspacesandorgansinthreedimensions.Attachedtothebonesandorgansintheheadarethe

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musclesweusetobite,totalk,andtomoveoureyesandwholehead.Twelvenervessupplythesemuscles,eachexitingthebraintotraveltoadifferentregioninsideourhead.Thesearethedreadedcranialnerves.

Plates,blocks,androds:thethemeforskulls.Everyboneinourheadcanbetracedtooneofthesethings.

Thekeytounlockingthebasicsoftheheadistoseethe

cranialnervesasmorethanajumble.Indeed,mostofthemreallyaresimple.Thesimplestcranialnerveshaveonlyonefunction,andtheyattachtoonemuscleororgan.Thecranialnervethatgoestoournasalstructures,theolfactory,hasonejob:totakeinformationfromournasaltissuestoourbrain.Someofthenervesthatgotooureyesandearsarealsosimpleinthisway:theopticnerveisinvolvedwithvision;theacousticnerveworksinhearing.Aboutfourothercranialnervesonlyservemuscles—

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workingtomovetheeyesinsidetheorbits,forexample,ortomovetheheadaroundontheneck.Butfourofthecranialnerveshavegivenmedical

studentsfitsfordecades.Forgoodreason:thefourhaveverycomplexfunctionsandtaketortuouspathsthroughtheheadtodotheirjobs.Thetrigeminalnerveandthefacialnervedeservespecialmention.Bothexitthebrainandbreakupintoabewilderingnetworkofbranches.Muchlikeacablethatcancarrytelevision,Internet,andvoiceinformation,asinglebranchofthetrigeminalorfacialnervecancarryinformationaboutbothsensationandaction.Individualfibersforsensationandactionemanatefromdifferentpartsofthebrain,areconsolidatedincables(whatweendupcallingthetrigeminalandfacialnerves),thenbreakupagain,sendingbranchesalloverthehead.Thetrigeminal’sbranchesdotwomajorthings:they

controlmuscles,andtheycarrysensoryinformationfrommuchofourfacebacktoourbrain.Themusclescontrolledbythetrigeminalnerveincludethoseweusetochewaswellastinymusclesdeepinsidetheear.Thetrigeminalisalsothemajornerveforsensationintheface.Thereasonaslaptothefacehurtssomuch,beyondtheemotionalpain,isbecausethetrigeminalcarriessensoryinformationfromtheskinofourfacebacktoourbrain.Yourdentistalsoknowsthebranchesofyourtrigeminalnervewell.Differentbranchesgototherootsofourteeth;asinglejabofanestheticalongoneofthesebranchescandeadenthesensationofdifferentpartsofourtoothrow.

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Thefacialnervealsocontrolsmusclesandrelayssensoryinformation.Asitsnameimplies,itisthemainnervethatcontrolsthemusclesoffacialexpression.Weusethesetinymusclestosmile,tofrown,toraiseandloweroureyebrows,toflareournostrils,andsoon.Theyhavewonderfullyevocativenames.Oneofthemajormusclesthatweuseinfrowning—itmovesthecornersofourmouthdown—iscalledthedepressorangulioris.Anothergreatnamebelongstothemuscleweusetofurrowourbrowinconcern:thecorrugatorsupercilli.Flareyournostrilsandyouareusingyournasalis.Eachofthesemuscles,likeeveryothermuscleoffacialexpression,iscontrolledbybranchesofthefacialnerve.Thingslikeanunevensmileorasymmetricallydroopingeyelidsareasignthatsomethingmightbewrongwiththefacialnerveononesideofaperson’sface.YouareprobablybeginningtoseewhyIwasstayingup

solatetostudythesenerves.Nothingaboutthemseemstomakeanysense.Forexample,boththetrigeminalandthefacialnervessendtinybranchestomusclesinsideourears.Whydotwodifferentnerves,whichinnervateentirelydifferentpartsofthefaceandjaw,sendbranchestoearmusclesthatlieadjacenttooneanother?Evenmoreconfusing,thetrigeminalandfacialalmostcrisscrossastheysendbranchestoourfaceandjaw.Why?Withsuchoddlyredundantfunctionsandtortuouspaths,thereseemstobenorhymeorreasontotheirstructure,muchlesstohowthesenervesmatchupwiththeplates,blocks,androds

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thatmakeupourskull.Inthinkingaboutthesenerves,Iamremindedofmyfirst

dayshereinChicagoin2001.Ihadbeengivenspaceforaresearchlaboratoryinahundred-year-oldbuildingandthelabneedednewutilitycables,plumbing,andairhandling.Irememberthedaywhenthecontractorsfirstopenedthewallstogetaccesstotheinnardsofthebuilding.TheirreactiontotheplumbingandwiringinsidemywallwasalmostexactlylikeminewhenIopenedthehumanheadandsawthetrigeminalandfacialnervesforthefirsttime.Thewires,cables,andpipesinsidethewallswereajumble.Nobodyinhisrightmindwouldhavedesignedabuildingfromscratchthisway,withcablesandpipestakingbizarreloopsandturnsthroughoutthebuilding.Andthat’sexactlythepoint.Mybuildingwasconstructed

in1896,andtheutilitiesreflectanolddesignthathasbeenjerry-riggedfurtherwitheachrenovation.Ifyouwanttounderstandthewiringandplumbinginmybuilding,youhavetounderstanditshistory,howitwasrenovatedforeachnewgenerationofscientists.Myheadhasalonghistoryalso,andthathistoryexplainscomplicatednerveslikethetrigeminalandthefacial.Forus,thathistorybeginswithafertilizedegg.

THEESSENCEINEMBRYOS

Nobodystartslifewithahead:spermandeggcome

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togethertomakeasinglecell.Betweenthemomentofconceptionandthethirdweekthereafter,wegofromthatsinglecelltoaballofcells,thentoaFrisbee-shapedcollectionofcells,thentosomethingthatlooksvaguelylikeatubeandincludesdifferentkindsoftissues.Betweenthetwenty-thirdandtwenty-eighthdaysafterconception,thefrontendofthetubethickensandfoldsoverthebody,sotheembryolooksasifit’salreadycurledupinthefetalposition.Theheadatthisstagelookslikeabigglob.Thebaseofthisglobholdsthekeytomuchofthebasicorganizationofourheads.Fourlittleswellingsdeveloparoundtheareathatwill

becomethethroat.Ataboutthreeweeksweseethefirsttwo;theothertwoemergeaboutfourdayslater.Eachswellinglooksquitehumbleontheoutside:asimpleblob,separatedfromthenextbyalittlecrease.Whenyoufollowwhathappenstotheblobsandcreases,youbegintoseetheorderandbeautyofthehead,includingthetrigeminalandfacialnerves.Ofthecellsinsideeachblob,knownasarches,somewill

formbonetissueandothersmuscleandbloodvessels.Thereisacomplexmixofcellsinsideeacharch;somecellsdividedrighttherewhileothersmigratedalongwaytoenterthearchitself.Whenweidentifythecellsineacharchaccordingtowheretheyendupintheadult,thingsstarttomakealotofsense.Ultimately,thefirstarchtissuesformtheupperand

lowerjaws,twotinyearbones(themalleusandincus),and

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allthevesselsandmusclesthatsupplythem.Thesecondarchformsthethirdsmallearbone(thestapes),atinythroatbone,andmostofthemusclesthatcontrolfacialexpression.Thethirdarchformsbones,muscles,andnervesdeeperinthethroat;weusethesetoswallow.Finally,thefourtharchformsthedeepestpartsofourthroat,includingpartsofourlarynxandthemusclesandvesselsthatsurrounditandhelpitfunction.Ifyouweretoshrinkyourselftothesizeofapinheadand

travelinsidethemouthofthedevelopingembryo,youwouldseeindentationsthatcorrespondtoeachswelling.Therearefouroftheseindentations.And,likethearchesontheoutside,cellsontheindentationsformimportantstructures.ThefirstelongatestoformourEustachiantubeandsomestructuresintheear.Thesecondformsthecavitythatholdsourtonsils.Thethirdandfourthformimportantglands,includingtheparathyroid,thymus,andthyroid.WhatI’vejustgivenyouisoneofthebigtricksfor

understandingthemostcomplicatedcranialnervesandlargeportionsofthehead.Whenyouthinktrigeminalnerve,thinkfirstarch.Facialnerve,secondarch.Thereasonthetrigeminalnervegoestoboththejawsandtheearisthatallthestructuresitsuppliesoriginallydevelopedinthefirstarch.Thesamethingistrueforthefacialnerveandthesecondarch.Whatdothemusclesoffacialexpressionhaveincommonwiththemusclesintheearthatthefacialnervesupplies?Theyareallsecondarchderivatives.Asforthenervesofthethirdandfourtharches,theircomplexpaths

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allrelatetothefactthattheyinnervatestructuresthatarosefromtheirrespectivearches.Thosethirdandfourtharchnerves,amongthemtheglossopharyngealandvagus,followthesamepatternastheonesinfront,eachgoingtostructuresthatdevelopedfromthearchtheyareassociatedwith.

Ifwefollowthegillarchesfromanembryotoanadult,wecantracetheoriginsofjaws,ears,larynx,andthroat.Bones,muscles,nerves,andarteriesalldevelopinsidethesegillarches.

Thisfundamentalblueprintofheadshelpsusmakesense

ofoneoftheapocryphaltalesinanatomy.In1820,sothestorygoes,JohannesGoethewaswalkingthroughtheJewishcemeteryinViennawhenhespottedthedecomposingskeletonofaram.Thevertebraewereexposedandabovethemlayadamagedskull.Goethe,inamomentofepiphany,sawthatthebreaksintheskullmade

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itlooklikeagnarledmessofvertebrae.ToGoethe,thisrevealedtheessentialpatternwithin:theheadismadeupofvertebraethatfusedandgrewavaulttoholdourbrainsandsenseorgans.Thiswasarevolutionaryideabecauseitlinkedheadsandbodiesastwoversionsofthesamefundamentalplan.Thenotionmusthavebeeninthedrinkingwaterintheearly1800sbecauseotherpeople,amongthemLorenzOken,allegedlycameupwithvirtuallythesameideainasimilarsetting.GoetheandOkenwerebothpickingupsomethingvery

profound,althoughtheycouldnothaveknownitatthetime.Ourbodyissegmented,andthispatternismostclearlyseeninourvertebrae.Eachvertebraisablockthatrepresentsasegmentofourbody.Theorganizationofournervesisalsosegmental,correlatingcloselywiththepatternofthevertebrae.Nervesexitthespinalcordtosupplythebody.Thesegmentalconfigurationisobviouswhenyoulookatthelevelsofthespinalcordthatareassociatedwitheachpartofourbody.Forexample,themusclesinourlegsaresuppliedbynervesthatexitfromlowerpartsofthespinalcordthanthosethatsupplyourarms.Headsmaynotlookit,buttheyalsocontainaverydeepsegmentalpattern.Ourarchesdefinesegmentsofbones,muscles,arteries,andnerves.Lookintheadult,andyouwon’tseethispattern.Weseeitonlyintheembryo.Ourskullsloseallovertevidenceoftheirsegmental

originsaswegofromembryotoadult.Theplate-likebonesofourskullsformoverourgillarches,andthemuscles,

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nerves,andarteries,whichallhadaverysimplesegmentalpatternearlyon,arerewiredtomakeouradultheads.Knowingsomethingaboutdevelopmentcanhelpus

predictwheretolookforwhatismissinginchildrenwhohavecertainbirthdefects.Forexample,childrenbornwithfirstarchsyndromehaveatinyjawandnonfunctioningearswithnomalleusorincusbone.Missingarestructuresthatnormallywouldhaveformedfromthefirstarch.Thearchesaretheroadmapformajorchunksofthe

skull,fromthemostcomplicatedcranialnervestothemuscles,arteries,bones,andglandsinside.Thearchesarealsoaguidetosomethingelse:ourverydeepconnectionwithsharks.

OURINNERSHARK

Thetake-homemessageofmanyalawyerjokeisthatlawyersareanespeciallyvoraciouskindofshark.Teachingembryologyduringoneoftherecurringvoguesforthesejokes,Irememberthinkingthatthejokeisonallofus.We’reallmodifiedsharks—or,worse,thereisalawyerinsideeachofus.Aswe’veseen,muchofthesecretofheadsliesinthe

arches,theswellingsthatgaveustheroadmapforthecomplicatedcranialnervesandkeystructuresinsidethehead.Thoseinsignificant-lookingswellingsandindentationshavecapturedtheimaginationofanatomists

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for150years,becausetheylooklikethegillslitsinthethroatregionsoffishandsharks.Fishembryoshavethesebulgesandindentations,too.In

fish,theindentationsultimatelyopenuptoformthespacesbetweenthegillswherewaterflows.Inus,theindentationsnormallysealover.Inabnormalcases,gillslitsfailtocloseandremainopenaspouchesorcysts.Abranchialcyst,forexample,isoftenabenignfluid-filledcystthatformsinanopenpouchinsidetheneck;thepouchiscreatedbythefailureofthethirdorfourtharchtoclose.Rarely,childrenarebornwithanactualvestigeofanancientgillarchcartilage,alittlerodthatrepresentsagillbarfromthethirdarch.Intheseinstances,mysurgicalcolleaguesareoperatingonaninnerfishthatunfortunatelyhascomebacktobiteus.Everyheadoneveryanimalfromasharktoahuman

sharesthosefourarchesindevelopment.Therichnessofthestoryliesinwhathappensinsideeacharch.Here,wecanmakeapoint-by-pointcomparisonbetweenourheadsandthoseofsharks.

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Thegillregionofadevelopinghumanandadevelopingsharklookthesameearlyon.

Lookatthefirstarchinahumanandashark,andyou

findaverysimilarstateofaffairs:jaws.Themajordifferenceisthathumans’firstarchalsoformssomeearbones,whichwedonotseeinsharks.Unsurprisingly,thecranialnervethatsuppliesthejawsofhumansandsharksisthefirstarchnerve,thetrigeminalnerve.Thecellsinsidethesecondgillarchdivide,change,and

giverisetoabarofcartilageandmuscle.Inus,thebarofcartilagebreaksuptoformoneofthethreebonesofourmiddleear(thestapes)andsomeothersmallstructuresatthebaseoftheheadandthroat.Oneofthesebones,calledthehyoid,assistsusinswallowing.Takeagulp,listentomusic,andthankthestructuresthatformfromyoursecondarch.Inashark,thesecondarchrodbreaksuptoformtwo

bonesthatsupportthejaws:aloweronethatcompares

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withourhyoidandanupperonethatsupportstheupperjaw.Ifyouhaveeverwatchedagreatwhitesharktrytochompsomething—adiverinacage,forexample—youhaveprobablynoticedthattheupperjawcanextendandretractasthesharkbites.Theupperboneofthissecondarchispartoftheleversystemofbonesthatrotatetomakethatpossible.Thatupperboneisremarkableinanotherway,too.Itcompareswithoneofthebonesinourmiddleear:thestapes.Bonesthatsupporttheupperandlowerjawsinsharksareusedinustoswallowandhear.Asforthethirdandfourtharches,wefindthatmanyof

thestructuresweusetotalkandswalloware,insharks,partsoftissuesthatsupportthegills.Themusclesandcranialnervesweusetoswallowandtalkmovethegillsinsharksandfish.

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Atfirstglance,ourcranialnerves(bottomright)appeardifferentfromthoseofashark(bottomleft).Butlookcloselyandyouwillfindprofoundsimilarities.Virtuallyallofournervesarepresentinsharks.Theparallelsgodeeperstill:equivalentnervesinsharksandhumanssupplysimilarstructures,andtheyevenexitthebraininthesameorder(topleftandright).

Ourheadmaylookincrediblycomplicated,butitisbuilt

fromasimpleandelegantblueprint.Thereisapatterncommontoeveryskullonearth,whetheritbelongstoashark,abonyfish,asalamander,orahuman.Thediscovery

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ofthispatternwasamajoraccomplishmentofnineteenth-centuryanatomy,atimewhenanatomistswereputtingembryosofallkindsofspeciesunderthemicroscope.In1872,theOxfordanatomistFrancisMaitlandBalfourfirstsawthebasicplanofheadswhenhelookedatsharksandsawthebulges,thegillarches,andthestructuresinside.Unfortunately,hediedsoonafterinamountaineeringaccidentintheSwissAlps.Hewasonlyinhisthirties.

GILLARCHGENES

Duringthefirstthreeweeksafterconception,wholebatteriesofgenesareturnedonandoffinourgillarchesandthroughoutthetissuesthatwillbecomeourfuturebrain.Thesegenesinstructcellstomakethedifferentportionsofourhead.Thinkofeachregionofourheadasgainingageneticaddressthatmakesitdistinctive.Modifythisgeneticaddressandwecanmodifythekindsofstructuresthatdevelopthere.Forexample,ageneknownasOtxisactiveinthefront

region,wherethefirstgillarchforms.Behindit,towardthebackofthehead,anumberofso-calledHoxgenesareactive.EachgillarchhasadifferentcomplementofHoxgenesactiveinit.Withthisinformation,wecanmakeamapofourgillarchesandtheconstellationofgenesactiveinmakingeach.Nowwecandoexperiments:changethegeneticaddress

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ofonegillarchintothatofanother.Takeafrogembryo,turnoffsomegenes,makethegeneticsignalssimilarinthefirstandsecondarches,andyouendupwithafrogthathastwojaws:amandibledevelopswhereahyoidbonewouldnormallybe.Thisshowsthepowerofthegeneticaddressesinmakingourgillarches.Changetheaddress,andyouchangethestructuresinthearch.Thepowerofthisapproachisthatwecannowexperimentwiththebasicdesignofheads:wecanmanipulatetheidentityofthegillarchesalmostatwill,bychangingtheactivityofthegenesinside.

TRACINGHEADS:FROMHEADLESSWONDERSTOOURHEADEDANCESTORS

Whystopatfrogsandsharks?Whynotextendourcomparisontoothercreatures,likeinsectsorworms?Butwhywouldwedothiswhennoneofthesecreatureshasaskull,muchlesscranialnerves?Noneofthemevenhasbones.Whenweleavefishforworms,wegettoaverysoftandheadlessworld.Bitsofourselvesarethere,though,ifyoulookclosely.Thoseofuswhoteachcomparativeanatomyto

undergraduatesusuallybeginthecoursewithaslideofAmphioxus.EverySeptember,hundredsofAmphioxusslidesappearonscreensincollegelecturehallsfromMainetoCalifornia.Why?Rememberthesimpledichotomybetweeninvertebratesandvertebrates?Amphioxusisaworm,an

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invertebrate,thatsharesmanyfeatureswithbackbonedanimalssuchasfish,amphibians,andmammals.Amphioxuslacksabackbone,butlikeallcreatureswithbackbones,ithasanervecordthatrunsalongitsback.Inaddition,arodrunsthelengthofitsbody,paralleltothenervecord.Thisrod,knownasthenotochord,isfilledwithajelly-likesubstanceandprovidessupportforthebody.Asembryos,wehaveanotochord,too,butunlikeAmphioxus’s,oursbreaksupandultimatelybecomespartofthedisksthatliebetweenourvertebrae.Ruptureadiskandthejelly-likesubstanceofwhatwasonceanotochordcanwreakhavocwhenitpinchesnervesorinterfereswiththeabilityofonevertebratomovealongthenext.Whenweinjureadisk,averyancientpartofourbodyplanisrupturing.Thanksalot,Amphioxus.

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Theclosestrelativestoanimalswithheadsarewormswithgillslits.ShownareAmphioxusandareconstructionofafossilworm(Haikouella)over530millionyearsold.Bothwormshaveanotochord,anervecord,andgillslits.ThefossilwormisknownfromoverthreehundredindividualspecimensfromsouthernChina.

Amphioxusisnotuniqueamongworms.Someofthebest

examplesarenotintheoceansoftodaybutinancientrocksofChinaandCanada.Buriedinsedimentsover500millionyearsoldaresmallwormsthatlackheads,complexbrains,orcranialnerves.Theymaynotlooklikemuch,beingsmallsmudgesintherock,butthepreservationofthesefossilsis

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incredible.Whenyoulookunderamicroscope,youfindbeautifullypreservedimpressionsthatdisplaytheirsoftanatomyinfinedetail,occasionallyevenwithimpressionsofskin.Theyshowsomethingelsewonderful,too.Theyaretheearliestcreatureswithnotochordsandnervecords.Thesewormsaretellingussomethingabouttheoriginofpartsofourbodies.Butthereissomethingelsewesharewiththeselittle

worms:gillarches.Amphioxus,forexample,hastheminabundance,andassociatedwitheacharchisalittlebarofcartilage.Likethecartilagesthatformourjaws,ourearbones,andpartsofourvoicebox,theserodssupportthegillslit.Theessenceofourheadgoesbacktoworms,organismsthatdonotevenhaveahead.WhatdoesAmphioxusdowiththegillarches?Itpumpswaterthroughthemtofilteroutlittleparticlesoffood.Fromsohumbleabeginningcomesthebasicstructuresofourownhead.Justasteeth,genes,andlimbshavebeenmodifiedandtheirfunctionsrepurposedovertheages,so,too,hasthebasicstructureofourhead.

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CHAPTERSIX

THEBEST-LAID(BODY)PLANS

Weareapackageofabouttwotrillioncellsassembledinaverypreciseway.Ourbodiesexistinthreedimensions,withourcellsandorgansintheirproperplaces.Theheadisontop.Thespinalcordistowardourback.Ourgutsareonthebellyside.Ourarmsandlegsaretothesides.Thisbasicarchitecturedistinguishesusfromprimitivecreaturesorganizedasclumpsordisksofcells.Thesamedesignisalsoanimportantpartofthebodies

ofothercreatures.Likeus,fish,lizards,andcowshavebodiesthataresymmetricalwithafront/back,top/bottom,andleft/right.Theirfrontends(correspondingtothetopofanuprighthuman)allhaveheads,withsenseorgansandbrainsinside.Theyhaveaspinalcordthatrunsthelengthofthebodyalongtheback.Alsolikeus,theyhaveananus,whichisattheoppositeendoftheirbodiesfromthemouth.Theheadisontheforwardend,inthedirectiontheytypicallyswimorwalk.Asyoucanimagine,“anus-forward”wouldn’tworkverywellinmostsettings,particularlyaquaticones.Socialsituationswouldbeaproblem,too.

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Itismoredifficulttofindourbasicdesigninreallyprimitiveanimals—jellyfish,forexample.Jellyfishhaveadifferentkindofbodyplan:theircellsareorganizedintodisksthathaveatopandbottom.Lackingafrontandback,aheadandtail,andaleftandright,jellyfishbodyorganizationappearsverydifferentfromourown.Don’tevenbothertryingtocompareyourbodyplanwithasponge.Youcouldtry,butthemerefactthatyouweretryingwouldrevealsomethingmorepsychiatricthananatomical.Toproperlycompareourselveswiththeseprimitive

animals,weneedsometools.Justaswithheadsandlimbs,ourhistoryiswrittenwithinourdevelopmentfromeggtoadult.Embryosholdthecluestosomeoftheprofoundmysteriesoflife.Theyalsohavetheabilitytoderailmyplans.

THECOMMONPLAN:COMPARINGEMBRYOS

Ienteredgraduateschooltostudyfossilmammalsandendedupthreeyearslaterstudyingfishandamphibiansformydissertation.Myfallfromgrace,ifyouwanttocallitthat,happenedwhenIstartedtolookatembryos.Wehadalotofembryosinthelab:salamanderlarvae,fishembryos,evenfertilizedchickeneggs.I’droutinelypopthemunderthemicroscopetoseewhatwasgoingon.Theembryosofallthespecieslookedlikelittlewhitishbatchesofcells,no

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morethananeighthofaninchlong.Itwasexcitingwatchingdevelopmentprogress;astheembryogotbigger,theyolk,itsfoodsupply,gotsmallerandsmaller.Bythetimetheyolkwasgone,theembryowasusuallybigenoughtohatch.Watchingtheprocessofdevelopmentbroughtabouta

hugeintellectualtransformationinme.Fromsuchsimpleembryonicbeginnings—smallblobsofcells—camewonderfullycomplexbirds,frogs,andtroutcomprisingtrillionsofcellsarrangedinjusttherightway.Buttherewasmore.Thefish,amphibian,andchickenembryoswerelikenothingIhadeverseenbeforeinbiology.Theyalllookedgenerallyalike.Allofthemhadaheadwithgillarches.Allofthemhadalittlebrainthatbeganitsdevelopmentwiththreeswellings.Allofthemhadlittlelimbbuds.Infact,thelimbsweretobecomemythesis,thefocusofmynextthreeyears’work.Here,incomparinghowtheskeletondevelopedinbirds,salamanders,frogs,andturtles,Iwasfindingthatlimbsasdifferentasbirdwingsandfroglegslookedverysimilarduringtheirdevelopment.Inseeingtheseembryos,Iwasseeingacommonarchitecture.Thespeciesendeduplookingdifferent,buttheystartedfromagenerallysimilarplace.Lookingatembryos,italmostseemsthatthedifferencesamongmammals,birds,amphibians,andfishsimplypaleincomparisonwiththeirfundamentalsimilarities.ThenIlearnedoftheworkofKarlErnstvonBaer.Inthe1800s,somenaturalphilosopherslookedto

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embryostotrytofindthecommonplanforlifeonearth.ParamountamongtheseobserverswasKarlErnstvonBaer.Borntoanoblefamily,heinitiallytrainedtobeaphysician.Hisacademicmentorsuggestedthathestudychickendevelopmentandtrytounderstandhowchickenorgansdeveloped.Unfortunately,vonBaercouldnotaffordincubatorsto

workonchickens,norcouldheaffordmanyeggs.Thiswasnotverypromising.Luckyforhim,hehadanaffluentfriend,ChristianPander,whocouldaffordtodotheexperiments.Astheylookedatembryos,theyfoundsomethingfundamental:allorgansinthechickencanbetracedtooneofthreelayersoftissueinthedevelopingembryo.Thesethreelayersbecameknownasthegermlayers.Theyachievedalmostlegendarystatus,whichtheyretaineventothisday.Pander’sthreelayersgavevonBaerthemeanstoask

importantquestions.Doallanimalssharethispattern?Arethehearts,lungs,andmusclesofallanimalsderivedfromtheselayers?And,importantly,dothesamelayersdevelopintothesameorgansindifferentspecies?VonBaercomparedthethreelayersofPander’schicken

embryoswitheverythingelsehecouldgethishandson:fish,reptiles,andmammals.Yes,everyanimalorganoriginatedinoneofthesethreelayers.Significantly,thethreelayersformedthesamestructuresineveryspecies.Everyheartofeveryspeciesformedfromthesamelayer.Anotherlayergaverisetoeverybrainofeveryanimal.Andsoon.Nomatterhowdifferentthespecieslookasadults,as

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tinyembryostheyallgothroughthesamestagesofdevelopment.Tofullyappreciatetheimportanceofthis,weneedto

lookagainatourfirstthreeweeksafterconception.Atthemomentoffertilization,majorchangeshappeninsidetheegg—thegeneticmaterialofthespermandeggfusesandtheeggbeginstodivide.Ultimately,thecellsformaball.Inhumans,overaboutfivedays,thesingle-cellbodydividesfourtimes,toproduceaballofsixteencells.Thisballofcells,knownasablastocyst,resemblesafluid-filledballoon.Athinsphericalwallofcellssurroundssomefluidinthecenter.Atthis“blastocyststage”therestilldoesnotappeartobeanybodyplan—thereisnofrontandback,andcertainlytherearenotyetanydifferentorgansortissues.Onaboutthesixthdayafterconception,theballofcellsattachestoitsmother’suterusandbeginstheprocessofconnectingtoitsothatmotherandembryocanjoinbloodstreams.Thereisstillnoevidenceofthebodyplan.Itisafarcryfromthisballofcellstoanythingthatyou’drecognizeasanymammal,reptile,orfish,muchlessahuman.Ifwearelucky,ourballofcellshasimplantedinour

mother’suterus.Whenablastocystimplantsinthewrongplace—whenthereisan“ectopicimplantation”—theresultscanbedangerous.About96percentofectopicimplantationshappenintheuterine(orfallopian)tubes,nearwhereconceptionhappens.Sometimesmucusblockstheeasypassageoftheblastocysttotheuterus,causingit

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toimplantimproperlyinthetubes.Ectopicpregnancycancausevarioustissuerupturesifnotcaughtintime.Inreallyrarecases,theblastocystisexpelledintothemother’sbodycavity,thespacebetweenhergutsandbodywall.Inevenrarercases,theseblastocystswillimplantontheoutsideliningofthemother’srectumoruterusandthefetusdevelopstofullterm!Althoughthesefetusescansometimesbedeliveredbyanabdominalincision,suchimplantationisgenerallyverydangerousbecauseitincreasestheriskofmaternaldeathbybleedingbyafactorof90,ascomparedwithanormalimplantationinsidetheuterus.Inanyevent,atthisstageofdevelopmentweare

extremelyhumble-lookingcreatures.Aroundthebeginningofoursecondweekafterconception,theblastocysthasimplanted,withonepartoftheballembeddedinthewalloftheuterus,andtheotherfree.Thinkofaballoonpushedintoawall:thisflatteneddiskbecomesthehumanembryo.Ourentirebodyformsfromonlythetoppartofthisball,thepartthatismushedintothewall.Thepartoftheblastocystbelowthediskcoverstheyolk.Atthisstageofdevelopment,welooklikeaFrisbee,asimpletwo-layereddisk.HowdoesthisovalFrisbeeendupwithvonBaer’sthree

germlayersandgoontolookanythinglikeahuman?First,cellsdivideandmove,causingtissuestofoldinonthemselves.Eventually,astissuesmoveandfold,webecomeatubewithafoldedswellingattheheadendand

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anotheratthetail.Ifweweretocutourselvesinhalfrightaboutnow,wewouldfindatubewithinatube.Theoutertubewouldbeourbodywall,theinnertubeoureventualdigestivetract.Aspace,thefuturebodycavity,separatesthetwotubes.Thistube-within-a-tubestructurestayswithusourentirelives.Theguttubegetsmorecomplicated,withabigsacforastomachandlongintestinaltwistsandturns.Theoutertubeiscomplicatedbyhair,skin,ribs,andlimbsthatpushout.Butthebasicplanpersists.Wemaybemorecomplicatedthanwewereattwenty-onedaysafterconception,butwearestillatubewithinatube,andallofourorgansderivefromoneofthethreelayersoftissuethatappearedinoursecondweekafterconception.Thenamesofthesethreeall-importantlayersare

derivedfromtheirposition:theouterlayeriscalledectoderm,theinnerlayerendoderm,andthemiddlelayermesoderm.Ectodermformsmuchoftheouterpartofthebody(theskin)andthenervoussystem.Endoderm,theinsidelayer,formsmanyoftheinnerstructuresofthebody,includingourdigestivetractandnumerousglandsassociatedwithit.Themiddlelayer,themesoderm,formstissueinbetweenthegutsandskin,includingmuchofourskeletonandourmuscles.Whetherthebodybelongstoasalmon,achicken,afrog,oramouse,allofitsorgansareformedbyendoderm,ectoderm,andmesoderm.

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Ourearlydays,thefirstthreeweeksafterconception.Wegofrombeingasinglecelltoaballofcellsandendupasatube.

VonBaersawhowembryosrevealfundamentalpatterns

oflife.Hecontrastedtwokindsoffeaturesindevelopment:featuressharedbyeveryspecies,andfeaturesthatvaryfromspeciestospecies.Featuressuchasthetube-within-a-tubearrangementaresharedbyallanimalswithabackbone:fish,amphibians,reptiles,birds,andmammals.Thesecommonfeaturesappearrelativelyearlyindevelopment.Thefeaturesthatdistinguishus—biggerbrainsinhumans,shellsonturtles,feathersonbirds—arise

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relativelylater.VonBaer’sapproachisverydifferentfromthe“ontogeny

recapitulatesphylogeny”ideayoumighthavelearnedinschool.VonBaersimplycomparedembryosandnotedthattheembryosofdifferentspecieslookedmoresimilartoeachotherthandotheadultsofthosespecies.The“ontogenyrecapitulatesphylogeny”approachchampioneddecadeslaterbyErnstHaeckelmadetheclaimthateachspeciestrackeditsevolutionaryhistoryasitproceededthroughdevelopment.Accordingly,theembryoofahumanwentthroughafish,areptile,andamammalstage.Haeckelwouldcompareahumanembryotoanadultfishoralizard.ThedifferencesbetweentheideasofvonBaerandHaeckelmightseemsubtle,buttheyarenot.Inthepastonehundredyears,timeandnewevidencehavetreatedvonBaermuchmorekindly.Incomparingembryosofonespeciestoadultsofanother,Haeckelwascomparingapplestooranges.Amoremeaningfulcomparisonisonewherewecanultimatelyuncoverthemechanismsthatdriveevolution.Forthat,wecompareembryosofonespeciestoembryosofanother.Theembryosofdifferentspeciesarenotcompletelyidentical,buttheirsimilaritiesareprofound.Allhavegillarches,notochords,andlooklikeatubewithinatubeatsomestageoftheirdevelopment.And,importantly,embryosasdistinctasfishandpeoplehavePanderandvonBaer’sthreegermlayers.

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Atfourweeksafterconception,weareatubewithinatubeandhavethethreegermlayersthatgiverisetoallourorgans.

Allofthesecomparisonsleadustotherealissueatstake.

Howdoestheembryo“know”todevelopaheadatthefrontendandananusattheback?Whatmechanismsdrivedevelopmentandmakecellsandtissuesabletoformbodies?Toanswerthesequestionsrequiredawholenew

approach.RatherthansimplycomparingembryosasinvonBaer’sday,wehadtofindanewwayofanalyzingthem.Thelatterpartofthenineteenthcenturyusheredintheera,

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whichwefirstdiscussedinChapter3,whenembryoswerechopped,grafted,split,andtreatedwithvirtuallyeverykindofchemicalimaginable.Allinthenameofscience.

EXPERIMENTINGWITHEMBRYOS

Biologistsattheturnofthetwentiethcenturyweregrapplingwithfundamentalquestionsaboutbodies.Whereintheembryodoestheinformationtobuildthemlie?Isthisinformationcontainedineverycellorinpatchesofcells?Andwhatformdoesthisinformationtake—isitaspecialkindofchemical?Beginningin1903,theGermanembryologistHans

Spemannbegantoinvestigatehowcellslearnedtobuildbodiesduringdevelopment.Hisgoalwastofindwherethebody-buildinginformationresides.ThebigquestionforSpemannwaswhetherallthecellsintheembryohaveenoughinformationtobuildwholebodies,orwhetherthatinformationisconfinedtocertainpartsofthedevelopingembryo.Workingwithnewteggs,whichwereeasytoobtainand

relativelyeasytofiddlewithinthelab,Spemanndevisedacleverexperiment.Hecutoffastrandofhisinfantdaughter’shairandmadeaminiaturelassooutofit.Babyhairisremarkablestuff;soft,thin,andpliant,itmadetheidealmaterialfortyingupatinyspheresuchasanewtegg.Spemanndidexactlythattoadevelopingnewtegg,

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pinchingonesideofffromtheother.Manipulatingthenucleiofthecellsabit,helettheresultingcontraptiondevelopandwatchedwhathappened.Theembryoformedtwins:twocompletesalamandersemerged,eachwithanormalbodyplanandeachentirelyviable.Theconclusionwasobvious:fromoneeggcancomemorethanoneindividual.Thisiswhatidenticaltwinsare.Biologically,Spemannhaddemonstratedthatintheearlyembryosomecellshavethecapacitytoformawholenewindividualontheirown.Thisexperimentwasonlythebeginningofawholenew

phaseofdiscovery.Inthe1920sHildeMangold,agraduatestudentin

Spemann’slaboratory,startedtoworkwithsmallembryos.Thefinecontrolshehadofherfingersmadeherabletodosomeincrediblydemandingexperiments.AtthestageofdevelopmentwithwhichMangoldworked,thesalamanderembryoisasphereaboutasixteenthofaninchindiameter.Sheloppedoffatinypieceoftissue,smallerthanapinhead,fromonepartoftheembryoandgrafteditontotheembryoofanotherspecies.WhatMangoldtransplantedwasn’tjustanypatch,butanareawherecellsthatweretoformmuchofthethreegermlayersweremovingandfolding.Mangoldwassoskilledthatthegraftedembryosactuallycontinuedtodevelop,givingherapleasantsurprise.Thegraftedpatchledtotheformationofawholenewbody,includingaspinalcord,back,belly,evenahead.

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Justbymovingasmallpatchoftissueintheembryo,Mangoldproducedtwins.

Whyisallthisimportant?Mangoldhaddiscovereda

smallpatchoftissuethatwasabletodirectothercellstoformanentirebodyplan.Thetiny,incrediblyimportantpatchoftissuecontainingallthisinformationwastobeknownastheOrganizer.Mangold’sdissertationworkwasultimatelytowinthe

NobelPrize,butnotforher.HildeMangolddiedtragically(thegasolinestoveinherkitchencaughtfire)beforeherthesiscouldevenbepublished.SpemannwontheNobelPrizeinMedicinein1935,andtheawardcites“hisdiscoveryoftheOrganizeranditseffectinembryonicdevelopment.”Today,manyscientistsconsiderMangold’sworktobe

thesinglemostimportantexperimentinthehistoryofembryology.AtroughlythesametimethatMangoldwasdoing

experimentsinSpemann’slab,W.Vogt(alsoinGermany)wasdesigningclevertechniquestolabelcells,orbatchesofthem,andthusallowtheexperimentertowatchwhathappensastheeggdevelops.Vogtwasabletoproducea

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mapoftheembryothatshowswhereeveryorganoriginatesintheegg.Weseetheantecedentsofthebodyplaninthecellfatesoftheearlyembryo.Fromtheearlyembryologists,peoplelikevonBaer,

Pander,Mangold,andSpemann,wehavelearnedthatallthepartsofouradultbodiescanbemappedtoindividualbatchesofcellsinthesimplethree-layeredFrisbee,andthegeneralstructureofthebodyisinitiatedbytheOrganizerregiondiscoveredbyMangoldandSpemann.Cut,slice,anddice,andyou’llfindthatallmammals,

birds,amphibians,andfishhaveOrganizers.Youcanevensometimesswaponespecies’Organizerforanother.TaketheOrganizerregionfromachickenandgraftittoasalamanderembryo:yougetatwinnedsalamander.ButjustwhatisanOrganizer?Whatinsideittellscells

howtobuildbodies?DNA,ofcourse.AnditisinthisDNAthatwewillfindtheinnerrecipethatwesharewiththerestofanimallife.

OFFLIESANDMEN

VonBaerwatchedembryosdevelop,comparedonespeciestoanother,andsawfundamentalpatternsinbodies.MangoldandSpemannphysicallydistortedembryostolearnhowtheirtissuesbuildbodies.IntheDNAage,wecanaskquestionsaboutourowngeneticmakeup.Howdoourgenescontrolthedevelopmentofourtissuesandour

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bodies?Ifyoueverthoughtthatfliesareunimportant,considerthis:mutationsinfliesgaveusimportantcluestothemajorbodyplangenesactiveinhumanembryos.Weputthiskindofthinkingtouseinthediscoveryofgenesthatbuildfingersandtoes.Nowwe’llseehowittellsusaboutthewaysentirebodiesarebuilt.Flieshaveabodyplan.Theyhaveafrontandaback,atop

andabottom,andsoon.Theirantennae,wings,andotherappendagespopoutofthebodyintherightplace.Exceptwhentheydon’t.Somemutantflieshavelimbsgrowingoutoftheirheads.Othershaveduplicatewingsandextrabodysegments.Theseareamongtheflymutantsthattelluswhyourvertebraechangeshapefromtheheadendtotheanalendofthebody.Peoplehavebeenstudyingabnormalfliesforovera

hundredyears.Mutantswithoneparticularkindofabnormalitygotspecialattention.Theseflieshadorgansinthewrongplaces—alegwhereanantennashouldhavebeen;anextrasetofwings—orweremissingbodysegments.Somethingwasmessingwiththeirfundamentalbodyplan.Ultimately,thesemutantsarisefromsomesortoferrorintheDNA.RememberthatgenesarestretchesofDNAthatlieonthechromosome.Usingavarietyoftechniquesthatallowustovisualizethechromosome,wecanidentifythepatchofthechromosomeresponsibleforthemutanteffect.Essentially,webreedmutantstomakeawholepopulationwhereeveryindividualhasthegeneticerror.Then,usingavarietyofmolecularmarkers,we

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comparethegenesofindividualswiththemutationtothosewithout.Thisallowsustopinpointtheregionandthelikelystretchofchromosomeresponsibleforthemutanteffect.Itturnsoutthataflyhaseightgenesthatmakesuchmutants.ThesegeneslienexttooneanotherononeofthelongDNAstrandsofthefly.Thegenesthataffecttheheadsegmentslienexttothosethataffectthesegmentsinthemiddleofthefly,thepartofthebodythatcontainsthewings.ThesebitsofDNA,inturn,lieadjacenttotheonesthatcontrolthedevelopmentoftherearpartofthefly.Thereisawonderfulordertothewaythegenesareorganized:theirpositionalongtheDNAstrandparallelsthestructureofthebodyfromfronttoback.Nowthechallengewastoidentifythestructureofthe

DNAactuallyresponsibleforthemutation.MikeLevineandBillMcGinnis,inWalterGehring’slabinSwitzerland,andMattScott,inTomKauffman’slabinIndiana,noticedthatinthemiddleofeachgenewasashortDNAsequencethatwasvirtuallyidenticalineachspeciestheylookedat.Thislittlesequenceiscalledahomeobox.TheeightgenesthatcontainthehomeoboxarecalledHoxgenes.Whenthescientistsfishedaroundforthisgenesequenceinotherspecies,theyfoundsomethingsouniformthatitcameasatruesurprise:versionsoftheHoxgenesappearineveryanimalwithabody.

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Hoxgenesinfliesandpeople.Thehead-to-tailorganizationofthebodyisunderthecontrolofdifferentHoxgenes.Flieshaveonesetofeighthoxgenes,eachrepresentedasalittleboxinthediagram.Humanshavefoursetsofthesegenes.Infliesand

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people,theactivityofagenematchesitspositionontheDNA:genesactiveintheheadlieatoneend,thoseinthetailatanother,withgenesaffectingthemiddleofthebodylyinginbetween.

Versionsofthesamegenessculptthefront-to-back

organizationofthebodiesofcreaturesasdifferentasfliesandmice.MesswiththeHoxgenesandyoumesswiththebodyplaninpredictableways.Ifyoumakeaflythatlacksageneactiveinamiddlesegment,themidsectionoftheflyismissingoraltered.Makeamousethatlacksoneofthegenesthatspecifiesthoracicsegments,andyoutransformpartsoftheback.Hoxgenesalsoestablishtheproportionsofourbodies—

thesizesofthedifferentregionsofourhead,chest,andlowerback.Theyareinvolvedinthedevelopmentofindividualorgans,limbs,genitalia,andguts.Changesinthembringaboutchangesinthewaysourbodiesareputtogether.Differentkindsofcreatureshavedifferentnumbersof

Hoxgenes.Fliesandotherinsectshaveeight,miceandothermammalsthirty-nine.Thethirty-nineHoxgenesinmiceareallversionsoftheonesthatarefoundinflies.ThissimilarityhasledtotheideathatthelargenumberofmammalianHoxgenesarosefromaduplicationofthesmallercomplementofgenesinthefly.Despitethesedifferencesinnumber,themousegenesareactivefromfronttobackinaverypreciseorderjustastheflygenesare.Canwegoevendeeperinourfamilytree,findingsimilar

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stretchesofDNAinvolvedinmakingevenmorefundamentalpartsofourbodies?Theanswer,surprisingly,isyes.Anditlinksustoanimalsevensimplerthanflies.

DNAANDTHEORGANIZER

AtthetimewhenSpemannwontheNobelPrize,theOrganizerwasalltherage.Scientistssoughtthemysteriouschemicalthatcouldinducetheentirebodyplan.Butjustaspopularculturehasyo-yosandTickleMeElmodolls,sosciencehasfadsthatwaxandwane.Bythe1970s,theOrganizerwasviewedaslittlemorethanacuriosity,acleveranecdoteinthehistoryofembryology.Thereasonforthisfallfromgracewasthatnoonecoulddecipherthemechanismsthatmadeitwork.ThediscoveryofHoxgenesinthe1980schanged

everything.Intheearly1990s,whentheOrganizerconceptwasstilldecidedlyunfashionable,EddieDeRobertis’slaboratoryatUCLAwaslookingforHoxgenesinfrogs,usingtechniqueslikeLevineandMcGinnis’s.Thesearchwasbroadanditnettedmanydifferentkindsofgenes.Oneofthesehadaveryspecialpatternofactivity.ItwasactiveattheexactsiteintheembryothatcontainstheOrganizer,anditwasactiveatexactlytherighttimeofdevelopment.IcanonlyimaginewhatDeRobertisfeltwhenhefoundthatgene.HewaslookingattheOrganizer,andthereintheOrganizerwasagenethatseemedspecificallytocontrolit

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orbelinkedtoitsactivityintheembryo.TheOrganizerwasback.Organizergenesstartedpoppingupinlaboratories

everywhere.Whiledoingadifferentkindofexperiment,RichardHarlandatBerkeleyfoundanothergene,whichhecalledNoggin.NoggindoesexactlywhatanOrganizergeneshould.WhenHarlandtooksomeNogginandinjecteditintotherightplaceinanembryo,itfunctionedexactlyliketheOrganizer.Theembryodevelopedtwobodyaxes,includingtwoheads.AreDeRobertis’sgeneandNoggintheactualbitsofDNA

thatmakeuptheOrganizer?Theanswerisyesandno.Manygenes,includingthesetwo,interacttoorganizethebodyplan.Suchsystemsarecomplex,becausegenescanplaymanydifferentrolesduringdevelopment.Noggin,forexample,playsaroleinthedevelopmentofthebodyaxisbutisalsoinvolvedwithahostofotherorgans.Furthermore,genesdonotactalonetospecifycomplicatedcellbehaviorslikethoseweseeinheaddevelopment.Genesinteractwithothergenesatallstagesofdevelopment.Onegenemayinhibittheactivityofanotherorpromoteit.Sometimesmanygenesinteracttoturnanothergeneonoroff.Fortunately,newtoolsallowustostudytheactivityofthousandsofgenesinacellatonce.Couplethistechnologywithnewcomputer-basedwaysofinterpretinggenefunctionandwehaveenormouspotentialtounderstandhowgenesbuildcells,tissues,andbodies.Understandingthesecomplexinteractionsbetween

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batteriesofgenesshedslightontheactualmechanismsthatbuildbodies.Nogginservesasagreatexample.Nogginalonedoesnotinstructanycellintheembryoaboutitspositiononthetop–bottomaxis;rather,itactsinconcertwithseveralothergenestodothis.Anothergene,BMP-4,isabottomgene;itisturnedonincellsthatwillmakethebottom,orbellyside,ofanembryo.ThereisanimportantinteractionbetweenBMP-4andNoggin.WhereverNogginisactive,BMP-4cannotdoitsjob.TheupshotisthatNoggindoesnottellcellstodevelopas“cellsonthetopofthebody”instead,itturnsoffthesignalthatwouldmakethembottomcells.Theseoff-oninteractionsunderlievirtuallyalldevelopmentalprocesses.

ANINNERSEAANEMONE

Itisonethingtocompareourbodieswiththoseoffrogsandfish.Inarealsenseweandtheyaremuchalike:weallhaveabackbone,twolegs,twoarms,ahead,andsoon.Whatifwecompareourselveswithsomethingutterlydifferent,forexamplejellyfishandtheirrelatives?Mostanimalshavebodyaxesdefinedbytheirdirection

ofmovementorbywheretheirmouthandanuslierelativetoeachother.Thinkaboutit:ourmouthisontheoppositeendofthebodyfromouranusand,asinfishandinsects,itisusuallyinthedirection“forward.”Howcanwetrytoseeourselvesinanimalsthathaveno

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nervecordatall?Howaboutnoanusandnomouth?Creatureslikejellyfish,corals,andseaanemoneshaveamouth,butnoanus.Theopeningthatservesasamouthalsoservestoexpelwaste.Whilethatoddarrangementmaybeconvenientforjellyfishandtheirrelatives,itgivesbiologistsvertigowhentheytrytocomparethesecreaturestoanythingelse.Anumberofcolleagues,MarkMartindaleandJohn

Finnertyamongthem,havedivedintothisproblembystudyingthedevelopmentofthisgroupofanimals.Seaanemoneshavebeenremarkablyinformative,becausetheyarecloserelativesofjellyfishandtheyhaveaveryprimitivebodypattern.Also,seaanemoneshaveaveryunusualshape,onethatatfirstglancewouldseemtomakethemworthlessasaformtocomparetous.Aseaanemoneisshapedlikeatreetrunkwithalongcentralstumpandabunchoftentaclesattheend.Thisoddshapemakesitparticularlyappealing,sinceitmighthaveafrontandaback,atopandabottom.Drawalinefromthemouthtothebaseoftheanimal.Biologistshavegiventhatlineaname:theoral–aboralaxis.Butnamingitdoesn’tmakeitmorethananarbitraryline.Ifitisreal,thenitsdevelopmentshouldresemblethedevelopmentofoneofourownbodydimensions.Martindaleandhiscolleaguesdiscoveredthatprimitive

versionsofsomeofourmajorbodyplangenes—thosethatdetermineourhead-to-anusaxis—areindeedpresentintheseaanemone.And,moreimportant,thesegenesare

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activealongtheoral–aboralaxis.Thisinturnmeansthattheoral–aboralaxisoftheseprimitivecreaturesisgeneticallyequivalenttoourhead-to-anusaxis.Oneaxisdown,anothertogo.Doseaanemoneshave

anythinganalogoustoourbelly-to-backaxis?Seaanemonesdon’tseemtohaveanythingcomparable.Despitethis,Martindaleandhiscolleaguestooktheboldstepofsearchingintheseaanemoneforthegenesthatspecifyourbelly-to-backaxis.Theyknewwhatourgeneslookedlike,andthisgavethemasearchimage.Theyuncoverednotone,butmanydifferentbelly-to-backgenesintheseaanemone.Butalthoughthesegeneswereactivealonganaxisintheseaanemone,thataxisdidn’tseemtocorrelatewithanypatterninhowtheadultanimal’sorgansareputtogether.

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Jellyfishrelatives,suchasseaanemones,haveafrontandabackaswedo,abodyplansetupbyversionsofthesamegenes.

Justwhatthishiddenaxiscouldbeisnotapparentfrom

theoutsideoftheanimal.Ifwecutoneinhalf,however,wefindanimportantclue,anotheraxisofsymmetry.Calledthedirectiveaxis,itseemstodefinetwodistinctsidesofthecreature,almostaleftandaright.Thisobscureaxiswasknowntoanatomistsbackinthe1920sbutremainedacuriosityinthescientificliterature.Martindale,Finnerty,andtheirteamchangedthat.Allanimalsarethesamebutdifferent.Likeacakerecipe

passeddownfromgenerationtogeneration—withenhancementstothecakeineach—therecipethatbuildsourbodieshasbeenpasseddown,andmodified,foreons.Wemaynotlookmuchlikeseaanemonesandjellyfish,buttherecipethatbuildsusisamoreintricateversionoftheonethatbuildsthem.Powerfulevidenceforacommongeneticrecipefor

animalbodiesisfoundwhenweswapgenesbetweenspecies.Whathappenswhenyouswapabody-buildinggenefromananimalthathasacomplexbodyplanlikeourswithonefromaseaanemone?RecallthegeneNoggin,whichinfrogs,mice,andhumansisturnedoninplacesthatwilldevelopintobackstructures.InjectextraamountsoffrogNogginintoafrogegg,andthefrogwillgrowextrabackstructures,sometimesevenasecondhead.Inseaanemoneembryos,aversionofNogginisalsoturnedonat

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oneendofthedirectiveaxis.Now,themillion-dollarexperiment:taketheproductofNogginfromaseaanemoneandinjectitintoafrogembryo.Theresult:afrogwithextrabackstructures,almostthesameresultasifthefrogwereinjectedwithitsownNoggin.Now,though,aswegobackintime,weareleftwithwhat

lookslikeahugegap.Everythinginthischapterhadabody.Howdowecompareourselveswiththingsthathavenobodiesatall—withsingle-celledmicrobes?

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CHAPTERSEVEN

ADVENTURESINBODYBUILDING

WhenIwasn’toutinthefieldcollectingfossils,muchofmygraduatecareerwasspentstaringintoamicroscope,lookingathowcellscometogethertomakebones.Iwouldtakethedevelopinglimbofasalamanderora

frog,andstainthecellswithdyesthatturndevelopingcartilageblueandbonesred.Icouldthenmaketherestofthetissuesclearbytreatingthelimbwithglycerin.Thesewerebeautifulpreparations:theembryoentirelyclearandallthebonesradiatingthecolorsofthedyes.Itwaslikelookingatcreaturesmadeofglass.Duringtheselonghoursatthemicroscope,Iwasliterally

watchingananimalbeingbuilt.Theearliestembryoswouldhavetinylittlelimbbudsandthecellsinsidewouldbeevenlyspaced.Then,atlaterstages,thecellswouldclumpinsidethelimbbud.Insuccessivelyolderembryos,thecellswouldtakedifferentshapesandtheboneswouldform.EachofthoseclumpsIsawduringtheearlystagesbecameabone.Itishardnottofeelawestruckwatchingananimal

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assembleitself.Justlikeabrickhouse,alimbisbuiltbysmallerpiecesjoiningtomakealargerstructure.Butthereisahugedifference.Houseshaveabuilder,somebodywhoactuallyknowswhereallthebricksneedtogo;limbsandbodiesdonot.Theinformationthatbuildslimbsisnotinsomearchitecturalplanbutiscontainedwithineachcell.Imagineahousecomingtogetherspontaneouslyfromalltheinformationcontainedinthebricks:thatishowanimalbodiesaremade.Muchofwhatmakesabodyislockedinsidethecell;in

fact,muchofwhatmakesusuniqueisthere,too.Ourbodylooksdifferentfromthatofajellyfishbecauseofthewaysourcellsattachtooneanother,thewaystheycommunicate,andthedifferentmaterialstheymake.Beforewecouldevenhavea“bodyplan”—letalonea

head,brain,orarm—therehadtobeawaytomakeabodyinthefirstplace.Whatdoesthismean?Tomakeallofabody’stissuesandstructures,cellshadtoknowhowtocooperate—tocometogethertomakeanentirelynewkindofindividual.Tounderstandthemeaningofthis,let’sfirstconsider

whatabodyis.Then,let’saddressthethreegreatquestionsaboutbodies:When?How?AndWhy?Whendidbodiesarise,howdidtheycomeabout,and,mostimportant,whyaretherebodiesatall?

HABEASCORPUS:SHOWMETHEBODY

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Noteveryclumpofcellscanbeawardedthehonorofbeingcalledabody.Amatofbacteriaoragroupofskincellsisaverydifferentthingfromanarrayofcellsthatwewouldcallanindividual.Thisisanessentialdistinction;athoughtexperimentwillhelpusseethedifference.Whathappensifyoutakeawaysomebacteriafromamat

ofbacteria?Youendupwithasmallermatofbacteria.Whathappenswhenyouremovesomecellsofahumanorfish,sayfromtheheartorbrain?Youcouldendupwithadeadhumanorfish,dependingonwhichcellsyouremove.Sothethoughtexperimentrevealsoneofthedefining

featuresofbodies:ourcomponentpartsworktogethertomakeagreaterwhole.Butnotallpartsofbodiesareequal;somepartsareabsolutelyrequiredforlife.Moreover,inbodies,thereisadivisionoflaborbetweenparts;brains,hearts,andstomachshavedistinctfunctions.Thisdivisionoflaborextendstothesmallestlevelsofstructure,includingthecells,genes,andproteinsthatmakebodies.Thebodyofawormorapersonhasanidentitythatthe

constituentparts—organs,tissues,andcells—lack.Ourskincells,forexample,arecontinuallydividing,dying,andbeingsloughedoff.Yetyouarethesameindividualyouweresevenyearsago,eventhoughvirtuallyeveryoneofyourskincellsisnowdifferent:theonesyouhadbackthenaredeadandgone,replacedbynewones.Thesameistrueofvirtuallyeverycellinourbodies.Likeariverthatremainsthesamedespitechangesinitscourse,watercontent,evensize,weremainthesameindividualsdespitethecontinual

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turnoverofourparts.Anddespitethiscontinualchange,eachofourorgans

“knows”itssizeandplaceinthebody.Wegrowinthecorrectproportionsbecausethegrowthofthebonesinourarmsiscoordinatedwiththegrowthofthebonesinourfingersandourskulls.Ourskinissmoothbecausecellscancommunicatetomaintainitsintegrityandtheregularityofitssurface.Untilsomethingoutoftheordinaryhappens,like,forinstance,wegetawart.Thecellsinsidethewartaren’tfollowingtherules:theydonotknowwhentostopgrowing.Whenthefinelytunedbalanceamongthedifferentparts

ofbodiesbreaksdown,theindividualcreaturecandie.Acanceroustumor,forexample,isbornwhenonebatchofcellsnolongercooperateswithothers.Bydividingendlessly,orbyfailingtodieproperly,thesecellscandestroythenecessarybalancethatmakesalivingindividualperson.Cancersbreaktherulesthatallowcellstocooperatewithoneanother.Likebullieswhobreakdownhighlycooperativesocieties,cancersbehaveintheirownbestinterestuntiltheykilltheirlargercommunity,thehumanbody.Whatmadeallthiscomplexitypossible?Forourdistant

ancestorstogofromsingle-celledcreaturestobodiedones,astheydidoverabillionyearsago,theircellshadtoutilizenewmechanismstoworktogether.Theyneededtobeabletocommunicatewithoneanother.Theyneededtobeabletosticktogetherinnewways.Andtheyneededtobeableto

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makenewthings,suchasthemoleculesthatmakeourorgansdistinct.Thesefeatures—thegluebetweencells,thewayscellscan“talk”toeachother,andthemoleculesthatcellsmake—constitutethetoolkitneededtobuildallthedifferentbodiesweseeonearth.Theinventionofthesetoolsamountedtoarevolution.

Theshiftfromsingle-celledanimalstoanimalswithbodiesrevealsawholenewworld.Newcreatureswithwholenewcapabilitiescameabout:theygotbig,theymovedaround,andtheydevelopedneworgansthathelpedthemsense,eat,anddigesttheirworld.

DIGGINGUPBODIES

Here’sahumblingthoughtforallofusworms,fish,andhumans:mostoflife’shistoryisthestoryofsingle-celledcreatures.Virtuallyeverythingwehavetalkedaboutthusfar—animalswithhands,heads,senseorgans,evenbodyplans—hasbeenaroundforonlyasmallfractionoftheearth’shistory.Thoseofuswhoteachpaleontologyoftenusetheanalogyofthe“earthyear”toillustratehowtinythatfractionis.Taketheentire4.5-billion-yearhistoryoftheearthandscaleitdowntoasingleyear,withJanuary1beingtheoriginoftheearthandmidnightonDecember31beingthepresent.UntilJune,theonlyorganismsweresingle-celledmicrobes,suchasalgae,bacteria,andamoebae.Thefirstanimalwithaheaddidnotappearuntil

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October.ThefirsthumanappearsonDecember31.We,likealltheanimalsandplantsthathaveeverlived,arerecentcrashersatthepartyoflifeonearth.Thevastnessofthistimescalebecomesabundantlyclear

whenwelookattherocksintheworld.Rocksolderthan600millionyearsaregenerallydevoidofanimalsorplants.Inthemwefindonlysingle-celledcreaturesorcoloniesofalgae.Thesecoloniesformmatsorstrands;somecoloniesaredoorknob-shaped.Innowayarethesetobeconfusedwithbodies.Thefirstpeopletoseetheearliestbodiesinthefossil

recordhadnoideawhattheywerelookingat.Between1920and1960reallyoddfossilsstartedpoppingupfromallaroundtheworld.Inthe1920sand1930s,MartinGurich,aGermanpaleontologistworkinginwhatistodayNamibia,discoveredavarietyofimpressionsofwhatlookedlikeanimalbodies.Shapedlikedisksandplates,thesethingsseemedunremarkable:theycouldhavebeenprimitivealgaeorjellyfishlivinginancientseas.In1947,anAustralianmininggeologistnamedReginald

Sprigghappeneduponalocalitywheretheundersidesoftherockscontainedimpressionsofdisks,ribbons,andfronds.WorkingaroundanabandonedmineintheEdiacaraHillsofSouthAustralia,Sprigguncoveredacollectionofthesefossilsanddescribedthemdutifully.Overtime,similarimpressionsbecameknownfromeverycontinentoftheworldexceptAntarctica.Sprigg’screaturesseemedstrange,butfewpeoplereallycaredaboutthem.

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ThereasonforthecollectivepaleontologicalyawnwasthatthesefossilswerethoughttocomefromtherelativelyyoungrocksoftheCambrianera,whenmanyanimalfossilswithprimitivebodieswerealreadyknown.Sprigg’sandGurich’sfossilssatrelativelyunnoticed,anassemblageofnotterriblyexciting,ifweird,impressionsfromaperiodalreadywellrepresentedinthemuseumcollectionsoftheworld.Inthemid-1960s,MartinGlaessner,acharismatic

Austrianex-patlivinginAustralia,changedallthat.Aftercomparingtheserockstothoseinotherpartsoftheworld,Glaessnershowedthatwithoutadoubtthesefossilswere15millionto20millionyearsolderthanoriginallythought.Theywerenodullcollectionofimpressions—rather,Gurich,Sprigg,andotherswereseeingtheearliestbodies.

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Atimescaleforeventsinthehistoryoflife.Noticetheextremelylongperiodoftimeduringwhichtherewerenobodiesonearth,onlysingle-celledorganismslivingaloneorincolonies.

Thesefossilscamefromtheperiodknownasthe

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Precambrian,whosenameliterallymeans“BeforeLife.”Ourunderstandingoftheantiquityoflifehadjustexploded.Paleontologicalcuriositiesbecamescientificjewels.ThePrecambriandisks,ribbons,andfrondsareclearly

theoldestcreatureswithbodies.Aswe’dexpectfromotherearlyanimalfossils,theyincluderepresentativesofsomeofthemostprimitiveanimalsontheplanettoday:spongesandjellyfish.OtherPrecambrianfossilslooklikenothingknown.Wecantellthattheyareimpressionsofsomethingwithabody,buttheirpatternsofblobs,stripes,andshapesmatchnolivingcreature.Onemessagefromthisisveryclear:creatureswithmany

cellsbegantopopulatetheseasoftheplanetby600millionyearsago.Thesecreatureshadwell-definedbodiesandweren’tjustcoloniesofcells.Theyhavepatternsofsymmetrythat,insomecases,resemblethoseoflivingforms.Asforthosethatcannotbecompareddirectlywithlivingforms,differentpartsoftheirbodiesneverthelesshavespecializedstructures.ThisimpliesthatthePrecambrianorganismshadalevelofbiologicalorganizationthatatthetimewasutterlynewontheplanet.Evidenceofthesechangesisseennotonlyinthefossil

bodiesbutalsointherocksthemselves.Withthefirstbodiescomethefirsttrackways.Etchedintherocksarethefirstsignsthatcreatureswereactuallycrawlingandsquirmingthroughtheooze.Theearliesttrackways,smallribbon-shapedscrapesintheancientmud,showthatsomeofthesecreatureswithbodieswerecapableofrelatively

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complicatedmotions.Notonlydidtheyhavebodieswithidentifiableparts,buttheywereactuallyusingthemtomoveinnewways.Allofthismakestotalsense.Weseethefirstbodies

beforeweseethefirstbodyplans.Weseethefirstprimitivebodyplansbeforeweseethefirstbodyplanswithheads,andsoon.Liketheimaginaryzoowewalkedthroughinthefirstchapter,therocksoftheworldarehighlyordered.Aswesaidatthebeginningofthissection,weareafter

thewhen,how,andwhyofbodies.ThePrecambriandiscoveriestellusthewhen.Toseethehow,andultimatelythewhy,weneedtotakeaslightlydifferenttack.

OUROWNBODYOFEVIDENCE

AphotocouldnevercapturejusthowmuchofourbodiesistobefoundwithinthosePrecambriandisks,fronds,andribbons.Whatcouldwehumans,withallourcomplexity,eversharewithimpressionsinrocks,particularlyonesthatlooklikecrinkledjellyfishandsquashedrollsoffilm?Theanswerisprofoundand,whenweseetheevidence,

inescapable:the“stuff”thatholdsustogether—thatmakesourbodiespossible—isnodifferentfromwhatformedthebodiesofGurich’sandSprigg’sancientimpressions.Infact,thescaffoldingofourentirebodyoriginatedinasurprisinglyancientplace:single-celledanimals.

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Whatholdsaclumpofcellstogether,whethertheyformajellyfishoraneyeball?Increatureslikeus,thatbiologicalglueisastoundinglycomplicated;itnotonlyholdsourcellstogether,butalsoallowscellstocommunicateandformsmuchofourstructure.Theglueisnotonething;itisavarietyofdifferentmoleculesthatconnectandliebetweenourcells.Atthemicroscopiclevel,itgiveseachofourtissuesandorgansitsdistinctiveappearanceandfunction.Aneyeballlooksdifferentfromalegbonewhetherwelookatitwiththenakedeyeorunderamicroscope.Infact,muchofthedifferencebetweenalegboneandaneyerestsinthewaysthecellsandmaterialsarearrangeddeepinside.Everyfallforthepastseveralyears,Ihavedrivenmedical

studentscrazywithjusttheseconcepts.Nervousfirst-yearstudentsmustlearntoidentifyorgansbylookingatrandomslidesoftissueunderamicroscope.Howdotheydothis?Thetaskisalittlelikefiguringoutwhatcountryyouare

inbylookingatastreetmapofasmallvillage.Thetaskisdoable,butweneedtherightclues.Inorgans,someofthebestclueslieintheshapeofcellsandhowtheyattachtooneanother;itisalsoimportanttobeabletoidentifythestuffthatliesbetweenthem.Tissueshaveallkindsofdifferentcells,whichattachtooneanotherindifferentways:someregionshavestripsorcolumnsofcells;inothers,cellsarerandomlyscatteredandlooselyattachedtooneanother.Theseareas,wherecellsarelooselypacked,

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areoftenfilledwithmaterialsthatgiveeachtissueitscharacteristicphysicalproperties.Forinstance,themineralsthatliebetweenbonecellsdeterminethehardnessofbone,whereasthelooserproteinsinthewhitesofoureyesmakethewalloftheeyeballmorepliant.Ourstudents’abilitytoidentifyorgansfrommicroscope

slides,then,comesfromknowinghowcellsarearrangedandwhatliesbetweenthecells.Forus,thereisadeepermeaning.Themoleculesthatmakethesecellulararrangementspossiblearethemoleculesthatmakebodiespossible.Iftherewerenowaytoattachcellstooneanother,oriftherewerenomaterialsbetweencells,therewouldbenobodiesontheearth—justbatchesofcells.Thismeansthatthestartingpointforunderstandinghowandwhybodiesaroseistoseethesemolecules:themoleculesthathelpcellssticktogether,themoleculesthatallowthemtocommunicatewithoneanother,andthesubstancesthatliebetweencells.Tounderstandtherelevanceofthismolecularstructure

toourbodies,let’sfocusindetailononepart:ourskeleton.Ourskeletonisapowerfulexampleofhowtinymoleculescanhaveabigimpactonthestructureofourbodyandexemplifiesgeneralprinciplesthatapplytoallthebody’sparts.Withoutskeletons,wewouldbeformlessmassesofgoo.Livingonlandwouldnotbeeasyorevenpossible.Somuchofourbasicbiologyandbehaviorismadepossiblebyourskeletonthatweoftentakeitforgranted.Everytimewewalk,playpiano,inhale,orchewfoodwehaveour

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skeletontothank.Agreatanalogyfortheworkingsofourskeletonisa

bridge.Thestrengthofabridgedependsonthesizes,shapes,andproportionsofitsgirdersandcables.Butalso,importantly,thestrengthofthebridgedependsonthemicroscopicpropertiesofthematerialsfromwhichitismade.Themolecularstructureofsteeldetermineshowstrongitisandhowfaritwillbendbeforebreaking.Inthesameway,ourskeleton’sstrengthisbasedonthesizesandshapesofourbones,butalsoonthemolecularpropertiesofourbonesthemselves.Let’sgoforaruntoseehow.Aswejogalongapath,our

musclescontract,ourback,arms,andlegsmove,andourfeetpushagainstthegroundtomoveusforward.Ourbonesandjointsfunctionlikeagiantcomplexofleversandpulleysthatmakeallthatmovementpossible.Ourbody’smovementsaregovernedbybasicphysics:ourabilitytorunisinlargepartbasedonthesize,shape,andproportionsofourskeletonandtheconfigurationofourjoints.Atthislevel,welooklikeabigmachine.Andlikeamachine,ourdesignmatchesourfunctions.Aworld-classhighjumperhasdifferentboneproportionsfromachampionsumowrestler.Theproportionsofthelegsofarabbitorafrog,specializedtohopandjump,aredifferentfromthoseofahorse.Now,let’stakeamoremicroscopicview.Popasliceofa

femurunderthemicroscope,andyouwillimmediatelyseewhatgivesboneitsdistinctivemechanicalproperties.The

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cellsarehighlyorganizedinplaces,particularlyontheouterrimofthebone.Somecellssticktogether,whereasothersareseparated.Betweentheseparatedcellsarethematerialsthatdefinethestrengthofbone.Oneofthemistherock,orcrystal,knownashydroxyapatite,whichwediscussedinChapter4.Hydroxyapatiteishardthewayconcreteis:strongwhencompressed,lessstrongiftwistedorbent.So,likeabuildingmadeofbricksorconcrete,bonesareshapedsoastomaximizetheircompressivefunctionsandminimizetwistingandbending,somethingGalileorecognizedintheseventeenthcentury.Theothermoleculefoundbetweenourbonecellsisthe

mostcommonproteinintheentirehumanbody.Ifwemagnifyit10,000timeswithanelectronmicroscope,weseesomethingthatlookslikearopeconsistingofbundlesoflittlemolecularfibers.Thismolecule,collagen,alsohasthemechanicalpropertiesofarope.Ropeisrelativelystrongwhenpulled,butitcollapseswhencompressed;thinkofthetwoteamsinatug-of-warrunningtowardthemiddle.Collagen,likerope,isstrongwhenpulledbutweakwhentheendsarepushedtogether.Boneiscomposedofcellsthatsitinaseaof

hydroxyapatite,collagen,andsomeother,lesscommonmolecules.Somecellssticktogether;othercellsfloatinsidethesematerials.Thestrengthofboneisbasedoncollagen’sstrengthwhenpulled,andonhydroxyapatite’sstrengthwhencompressed.Cartilage,theothertissueinourskeleton,behaves

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somewhatdifferently.Duringourjog,itwasthecartilageinourjointsthatprovidedthesmoothsurfaceswhereourbonesglidedagainstoneanother.Cartilageisamuchmoreplianttissuethanbone;itcanbendandsmushasforcesareappliedtoit.Thesmoothoperationofthekneejoint,aswellasmostoftheotherjointsweusedduringourjog,dependsonhavingrelativelysoftcartilage.Whenhealthycartilageiscompresseditalwaysreturnstoitsnativeshape,likeakitchensponge.Duringeachstepofourrun,ourentirebodymassslamsagainstthegroundatsomespeed.Withouttheseprotectivecapsatourjointsourboneswouldgrindagainstoneanother:averyunpleasantanddebilitatingoutcomeofarthritis.Thepliabilityofcartilageisapropertyofitsmicroscopic

structure.Thecartilageatourjointshasrelativelyfewcells,andthesecellsareseparatedbyalotoffillingbetweenthem.Aswithbone,itisthepropertiesofthisinterstitialfillingthatlargelydeterminethemechanicalpropertiesofthecartilage.Collagenfillsmuchofthespacebetweencartilagecells

(aswellasthecellsofourothertissues).Whatreallygivescartilageitspliancyisanotherkindofmolecule,oneofthemostextraordinaryinthewholebody.Thiskindofmolecule,calledaproteoglycancomplex,givescartilagestrengthwhensqueezedorcompressed.Shapedlikeagiantthree-dimensionalbrush,withalongstemandlotsoflittlebranches,theproteoglycancomplexisactuallyvisibleunderamicroscope.Ithasanamazingpropertyrelevantto

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ourabilitiestowalkandmove,thankstothefactthatthetiniestbrancheslovetoattachtowater.Aproteoglycan,then,isamoleculethatactuallyswellsupwithwater,fillingupuntilit’slikeagiantpieceofJell-O.Takethispieceofgelatin,wrapcollagenropesinandaroundit,andyouendupwithasubstancethatisbothpliantandsomewhatresistanttotension.This,essentially,iscartilage.Aperfectpadforourjoints.Theroleofthecartilagecellsistosecretethesemoleculeswhentheanimalisgrowingandmaintainthemwhentheanimalisnot.Theratiosamongthevariousmaterialsdefinemuchof

themechanicaldifferencesamongbone,cartilage,andteeth.Teethareveryhardand,predictably,thereislotsofhydroxyapatiteandrelativelylittlecollagenbetweenthecellsintheenamel.Bonehasrelativelymorecollagen,lesshydroxyapatite,andnoenamel.Consequently,itisnotashardasteeth.Cartilagehaslotsofcollagenandnohydroxyapatite,andisloadedwithproteoglycans.Itisthesoftestofthetissuesinourskeleton.Oneofthemainreasonsourskeletonslookandworkastheydoisthatthesemoleculesaredeployedintherightplacesintherightproportions.Whatdoesallthishavetodowiththeoriginofbodies?

Onepropertyiscommontoanimals,whethertheyhaveskeletonsornot:allofthem,includingclumpsofcells,havemoleculesthatliebetweentheircells,specificallydifferentkindsofcollagensandproteoglycans.Collagenseemsparticularlyimportant:themostcommonproteinin

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animals,itmakesupover90percentofthebody’sproteinbyweight.Bodybuildinginthedistantpastmeantthatmoleculeslikethesehadtobeinvented.Somethingelseisessentialforbodies:thecellsinour

boneshavetobeabletosticktogetherandtalktooneanother.Howdobonecellsattachtooneanother,andhowdodifferentpartsofboneknowtobehavedifferently?Hereiswheremuchofourbodybuildingkitlies.Bonecells,likeeverycellinourbodies,sticktoone

anotherbymeansoftinymolecularrivets,ofwhichthereisavastdiversity.Somebindcellsthewaycontactcementholdsthesolesofshoestogether:onemoleculeisfirmlyattachedtotheoutermembraneofonecell,anothertotheoutermembraneofaneighboringcell.Thusattachedtobothcellmembranes,theglueformsastablebondbetweenthecells.Othermolecularrivetsaresoprecisethattheybind

selectively,onlytothesamekindofrivet.Thisisahugelysignificantfeaturebecauseithelpsorganizeourbodiesinafundamentalway.Theseselectiverivetsenablecellstoorganizethemselvesandensurethatbonecellssticktobonecells,skintoskin,andsoon.Theycanorganizeourbodiesintheabsenceofotherinformation.Ifweputanumberofcells,eachwithadifferentkindofthistypeofrivet,onadishandletthecellsgrow,thecellswillorganizethemselves.Somemightformballs,otherssheets,asthecellssortoutbythenumbersandkindsofrivetstheyhave.Butarguablythemostimportantconnectionbetween

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cellsliesinthewaysthattheyexchangeinformationwithoneanother.Theprecisepatternofourskeleton,infactofourwholebody,ispossibleonlybecausecellsknowhowtobehave.Cellsneedtoknowwhentodivide,whentomakemolecules,andwhentodie.If,forexample,boneorskincellsbehavedrandomly—iftheydividedtoomuchordiedtoolittle—thenwewouldbeveryuglyor,worse,verydead.Cellscommunicatewithoneanotherusing“words”

writtenasmoleculesthatmovefromcelltocell.Onecellcan“talk”tothenextbysendingmoleculesbackandforth.Forinstance,inarelativelysimpleformofcell-to-cellcommunication,onecellwillemitasignal,inthiscaseamolecule.Thismoleculewillattachtotheoutercovering,ormembrane,ofthecellreceivingthesignal.Onceattachedtotheoutermembrane,themoleculewillsetoffachainreactionofmoleculareventsthattravelsfromtheoutermembranealltheway,inmanycases,tothenucleusofthecell.Rememberthatthegeneticinformationsitsinsidethenucleus.Consequently,thismolecularsignalcancausegenestobeturnedonandoff.Theendresultofallthisisthatthecellreceivingtheinformationnowchangesitsbehavior:itmaydie,divide,ormakenewmoleculesinresponsetothecuefromtheothercell.Atthemostbasiclevel,thesearethethingsthatmake

bodiespossible.Allanimalswithbodieshavestructuralmoleculeslikecollagensandproteoglycans,allofthemhavethearrayofmolecularrivetsthatholdcellstogether,andallofthemhavethemoleculartoolsthatallowcellsto

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communicatewithoneanother.Wenowhaveasearchimagetounderstandthehowof

bodyorigins.Toseehowbodiesarose,weneedtolookforthesemoleculesinthemostprimitivebodiesontheplanet,andthen,ultimately,increaturesthathavenobodyatall.

BODYBUILDINGFORBLOBS

Whatdoesthebodyofaprofessorsharewithablob?Let’slookatsomeofthemostprimitivebodiesalivetodaytofindtheanswer.Oneofthesecreatureshasthedubiousdistinctionof

almostneverbeingseeninthewild.Inthelate1880s,astrangelysimplecreaturewasdiscoveredlivingontheglasswallsofanaquarium.Unlikeanythingelsealive,itlookedlikeamassofgoo.TheonlythingwecancompareitwithisthealiencreatureintheSteveMcQueenmovieTheBlob.RecallthattheBlobwasanamorphousglopthat,afterdroppinginfromouterspace,engulfeditsprey:dogs,people,andeventuallysmalldinersinlittletownsinPennsylvania.TheBlob’sdigestiveendwasonitsunderside:weneversawit;weonlyheardtheshrieksofcreaturescaughtthere.ShrinktheBlobdowntobetween200and1,000cells,abouttwomillimetersindiameter,andwehavetheenigmaticlivingcreatureknownasaplacozoan.Placozoanshaveonlyfourtypesofcells,whichmakeaverysimplebodyshapedlikeasmallplate.Itisa

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realbody,though.Someofthecellsontheundersurfacearespecializedfordigestion;othershaveflagella,whichbeattomovethecreaturearound.Wehavelittleideaofwhattheyeatinthewild,wheretheylive,orwhattheirnaturalhabitatis.Yetthesesimpleblobsrevealsomethingterrificallyimportant:withasmallnumberofspecializedcells,theseprimitivecreaturesalreadyhaveadivisionoflaboramongtheirparts.Muchofwhatisinterestingaboutbodiesalreadyexists

inplacozoans.Theyhavetruebodies,albeitprimitivelyorganizedones.InsearchingthroughtheirDNAandexaminingthemoleculesonthesurfaceoftheircells,wefindthatmuchofourbodybuildingapparatusisalreadythere.Placozoanshaveversionsofthemolecularrivetsandcellcommunicationtoolsweseeinourownbodies.Ourbodybuildingapparatusisfoundinblobssimpler

thansomeofReginaldSprigg’sancientimpressions.Canwegofurther,toevenmoreprimitivekindsofbodies?Partoftheanswerliesinapieceofclassickitchenware:thesponge.Atfirstglance,spongesareunremarkable.Thebodyofaspongeconsistsofthespongematrixitself;notalivingmaterial,itisaformofsilica(glassymaterial)orcalciumcarbonate(ahardshell-likematerial)withsomecollageninterspersed.Rightoffthebat,thatmakesspongesinteresting.Recallthatcollagenisamajorpartofourintercellularspaces,holdingcellsandmanytissuestogether.Spongesmaynotlookit,buttheyalreadyhaveoneoftheearmarksofbodies.

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Intheearly1900s,H.V.P.Wilsonshowedjusthowamazingspongesreallyare.WilsoncametotheUniversityofNorthCarolinaasitsfirstprofessorofbiologyin1894.TherehewentontotrainacadreofAmericanbiologistswhoweretodefinethefieldofgeneticsandcellbiologyinNorthAmericaforthenextcentury.Asayoungman,Wilsondecidedtofocushislife’sresearchon,ofallthings,sponges.Oneofhisexperimentsrevealedatrulyremarkablecapabilityoftheseapparentlysimplecreatures.Heranthemthroughakindofsieve,whichbrokethemdowntoasetofdisaggregatedcells.Wilsonputthenowcompletelydisaggregated,amoeba-likecellsinadishandwatchedthem.Atfirst,theycrawledaroundonthesurfaceofthedish.Then,somethingsurprisinghappened:thecellscametogether.First,theyformedredcloudyballsofcells.Next,theygainedmoreorganization,withcellsbecomingpackedindefinitepatterns.Finally,theclumpofcellswouldformanentirenewspongebody,withthevarioustypesofcellsassumingtheappropriatepositions.Wilsonwaswatchingabodycometogetheralmostfromscratch.Ifwewerelikesponges,thentheSteveBuscemicharacterwhogetsmincedinthewoodchipperintheCoenbrothers’movieFargowouldhavebeenjustfine.Infact,hemighthavebeeninvigoratedbytheexperience,ashiscellsmighthaveaggregatedtoformmanydifferentversionsofhim.Itisthecellswithinspongesthatmakethemusefulin

understandingtheoriginofbodies.Theinsideofthespongeisusuallyahollowspacethatcanbedividedinto

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compartments,dependingonthespecies.Waterflowsthroughthespace,directedbyaveryspecialkindofcell.Thesecellsareshapedlikegobletswiththecuppartfacingtheinsideofthesponge.Tinyciliaextendingfromtherimofthegobletbeatandcapturefoodparticlesinthewater.Alsoextendingfromthegobletpartofeachofthesecellsisalargeflagellum.Theconcertedactionoftheflagellaoftheselittlebeatercellsmoveswaterandfoodthroughtheporesofthesponge.Othercellsontheinsideofthespongeprocesstheparticlesoffood.Stillotherslinetheoutsideandcancontractwhenthespongeneedstochangeitsshapeaswatercurrentschange.Aspongeseemsafarcryfromabody,yetithasmanyof

themostimportantpropertiesofbodies:itscellshaveadivisionoflabor;thecellscancommunicatewithoneanother;andthearrayofcellsfunctionsasasingleindividual.Aspongeisorganized,withdifferentkindsofcellsindifferentplacesdoingdifferentthings.Itisafarcryfromahumanbodywithtrillionsofpreciselypackagedcells,butitsharessomeofthehumanbody’sfeatures.Mostsignificantly,thespongehasmuchofthecelladhesion,communication,andscaffoldingapparatusthatwehave.Spongesarebodies,albeitveryprimitiveandrelativelydisorganizedones.Likeplacozoansandsponges,wehavemanycells.Like

them,ourbodiesshowadivisionoflaboramongparts.Thewholemolecularapparatusthatholdsbodiestogetherisalsopresent:therivetsthatholdcellstogether;thevarious

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devicesthathelpcellssignaltooneanother;andmanyofthemoleculesthatliebetweencells.Likeusandallotheranimals,placozoansandspongesevenhavecollagen.Unlikeus,theyhaveveryprimitiveversionsofallthesefeatures:insteadoftwenty-onecollagens,spongeshavetwo;whereaswehavehundredsofdifferenttypesofmolecularrivets,spongeshaveasmallfractionofthatnumber.Spongesaresimplerthanweandhavefewerkindsofcells,butthebasicbodybuildingapparatusisthere.Placozoansandspongesareaboutassimpleasbodies

getnowadays.Togoanyfurther,wehavetosearchforthethingsthatbuildourbodiesincreaturesthathavenobodiesatall:single-celledmicrobes.Howdoyoucompareamicrobetoananimalwitha

body?Arethetoolsthatbuildbodiesinanimalspresentinsingle-celledcreatures?Ifso,andiftheyarenotbuildingbodies,whataretheydoing?Themoststraightforwardwaytobegintoanswerthese

questionsinvolveslookinginsidethegenesofmicrobestosearchforanysimilaritiestoanimals.Theearliestcomparisonsbetweenanimalandmicrobialgenomesrevealedastrikingfact:inmanysingle-celledanimals,muchofthemolecularmachineryforcelladhesion,interaction,andsoonisjustnotthere.Someanalysesevensuggestedthatmorethaneighthundredofthesekindsofmoleculesarefoundonlyinanimalswithbodieswhiletheyareabsentinsingle-celledcreatures.Thiswouldseemtosupportthenotionthatthegenesthathelpcellsuniteto

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makebodiesarosetogetherwiththeoriginofbodies.Andatfirstglance,itseemstomakesensethatthetoolstobuildbodiesshouldariseinlockstepwithbodiesthemselves.ThestoryturnedupsidedownwhenNicoleKing,ofthe

UniversityofCaliforniaatBerkeley,studiedtheorganismscalledchoanoflagellates.King’schoiceofsubjectwasnoaccident.FromworkonDNA,sheknewthatchoanoflagellatesarelikelytheclosestmicroberelativesofanimalswithbodies,placozoans,andsponges.ShealsosuspectedthathiddeninthegenesofchoanoflagellatesareversionsoftheDNAthatmakeourbodies.NicolewasaidedinhersearchbytheHumanGenome

Project,anenterprisethathassucceededinmappingallthegenesinourbodies.WiththesuccessoftheHumanGenomeProjectcamemanyothermappingstudies:we’vehadtheRatGenomeProject,theFlyGenomeProject,theBumblebeeGenomeProject—thereareevenongoingprojectstosequencethegenomesofsponges,placozoans,andmicrobes.Thesemapsareagoldmineofinformationbecausetheyenableustocomparethebodybuildinggenesinmanydifferentspecies.TheyalsogaveNicolethegenetictoolstostudyherchoanoflagellates.Choanoflagellateslookremarkablylikethegoblet-shaped

cellsinsideasponge.Infact,foralongtime,manypeoplethoughtthattheywerejustdegeneratesponges—spongeswithoutalltheothercells.Ifthiswerethecase,thentheDNAofchoanoflagellatesshouldresemblethatofabizarresponge.Itdoesn’t.WhenpartsoftheDNAof

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choanoflagellateswerecomparedwithmicrobeandspongeDNA,thesimilaritytomicrobeDNAturnedouttobeextraordinary.Choanoflagellatesaresingle-celledmicrobes.Thegeneticdistinctionbetween“single-celledmicrobe”

and“animalwithbody”completelybrokedownthankstoNicole’sworkonchoanoflagellates.Mostofthegenesthatareactiveinchoanoflagellatesarealsoactiveinanimals.Infact,manyofthosegenesarepartofthemachinerythatbuildsbodies.Afewexamplesrevealthepowerofthiscomparison.Functionsofcelladhesionandcellcommunication,evenpartsofthemoleculesthatformthematrixbetweencellsandthemolecularcascadesthatferryasignalfromoutsidethecelltotheinside—allarepresentinchoanoflagellates.Collagensarepresentinchoanoflagellates.Thevariouskindsofmolecularrivetsthatholdcellstogetherarealsopresentinchoanoflagellates,althoughtheyaredoingslightlydifferentjobs.

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Choanoflagellates(left)andsponges(right).ChoanoflagellatesevengiveNicolearoadmapfor

comparingourbodybuildingapparatustothatofothermicrobes.Thefundamentalmolecularstructurethatmakescollagensandproteoglycanaggregatesisknownfromanumberofdifferentkindofmicrobes.Streptococcusbacteria—commoninourmouths(and,onehopes,rareinotherplaces)—haveontheircellsurfaceamoleculethatisverysimilartocollagen.Ithasthesamemolecular

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signature,butdoesnotaggregatetoformropesorsheetsascollagensdoinanimals.Likewise,someofthesugarsthatmakeupproteoglycancomplexesinsideourcartilageareseeninthewallsofdifferentkindsofbacteria.Theirfunctionsinbothvirusesandbacteriaarenotparticularlypleasant.Theyareassociatedwiththewaysthattheseagentsinvadeandinfectcellsand,inmanycases,becomemorevirulent.Manyofthemoleculesthatmicrobesusetocauseusmiseryareprimitiveversionsofthemoleculesthatmakeourownbodiespossible.Thissetsupapuzzle.Inthefossilrecord,weseenothing

butmicrobesforthefirst3.5billionyearsofearthhistory.Then,suddenly,overaspanofperhaps40millionyears,allkindsofbodiesappear:plantbodies,fungalbodies,animalbodies;bodieseverywhere.Bodieswerearealfad.But,ifyoutakeNicole’sworkatfacevalue,thepotentialtobuildbodieswasinplacewellbeforebodieseverhitthescene.Whytherushforbodiesaftersuchaverylongtimewithnobodiesatall?

APERFECTSTORMINTHEORIGINOFBODIES

Timingiseverything.Thebestideas,inventions,andconceptsdon’talwayswin.Howmanymusicians,inventors,andartistsweresofaraheadoftheirtimethattheyfloppedandwereforgotten,onlytoberediscoveredlater?WeneedlooknofurtherthanpoorHeronofAlexandria,who,

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perhapsinthefirstcenturya.d.,inventedthesteamturbine.Unfortunately,itwasregardedasatoy.Theworldwasn’treadyforit.Thehistoryoflifeworksthesameway.Thereisa

momentforeverything,perhapsevenforbodies.Toseethis,weneedtounderstandwhybodiesmighthavecomeaboutinthefirstplace.Onetheoryaboutthisisextremelysimple:Perhaps

bodiesarosewhenmicrobesdevelopednewwaystoeateachotheroravoidbeingeaten?Havingabodywithmanycellsallowscreaturestogetbig.Gettingbigisoftenaverygoodwaytoavoidbeingeaten.Bodiesmayhavearisenasjustthatkindofdefense.Whenpredatorsdevelopnewwaysofeating,prey

developnewwaysofavoidingthatfate.Thisinterplaymayhaveledtotheoriginofmanyofourbodybuildingmolecules.Manymicrobesfeedbyattachingandengulfingothermicrobes.Themoleculesthatallowmicrobestocatchtheirpreyandholdontothemarelikelycandidatesforthemoleculesthatformtherivetattachmentsbetweencellsinourbodies.Somemicrobescanactuallycommunicatewitheachotherbymakingcompoundsthatinfluencethebehaviorofothermicrobes.Predator-preyinteractionsbetweenmicrobesofteninvolvemolecularcues,eithertowardoffpotentialpredatorsortoserveasluresenticingpreytocomeclose.Perhapssignalslikethesewereprecursorstothekindsofsignalsthatourowncellsusetoexchangeinformationtokeepourbodiesintact.

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Wecouldspeculateonthisadinfinitum,butmoreexcitingwouldbesometangibleexperimentalevidencethatshowshowpredationcouldbringaboutbodies.ThatisessentiallywhatMartinBoraasandhiscolleaguesprovided.Theytookanalgathatisnormallysingle-celledandletitliveinthelabforoverathousandgenerations.Thentheyintroducedapredator:asingle-celledcreaturewithaflagellumthatengulfsothermicrobestoingestthem.Inlessthantwohundredgenerations,thealgarespondedbybecomingaclumpofhundredsofcells;overtime,thenumberofcellsdroppeduntiltherewereonlyeightineachclump.Eightturnedouttobetheoptimumbecauseitmadeclumpslargeenoughtoavoidbeingeatenbutsmallenoughsothateachcellcouldpickuplighttosurvive.Themostsurprisingthinghappenedwhenthepredatorwasremoved:thealgaecontinuedtoreproduceandformindividualswitheightcells.Inshort,asimpleversionofamulticellularformhadarisenfromano-body.Ifanexperimentcanproduceasimplebody-like

organizationfromano-bodyinseveralyears,imaginewhatcouldhappeninbillionsofyears.Thequestionthenbecomesnothowcouldbodiesarise,butwhydidn’ttheyarisesooner?Answerstothispuzzlemightlieintheancient

environmentinwhichbodiesarose:theworldmaynothavebeenreadyforbodies.Abodyisaveryexpensivethingtohave.Thereare

obviousadvantagesofbecomingacreaturewithalarge

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body:besidesavoidingpredators,animalswithbodiescaneatother,smallercreaturesandactivelymovelongdistances.Bothoftheseabilitiesallowtheanimalstohavemorecontrolovertheirenvironment.Butbothconsumealotofenergy.Bodiesrequireevenmoreenergyastheygetlarger,particularlyiftheyincorporatecollagen.Collagenrequiresarelativelylargeamountofoxygenforitssynthesisandwouldhavegreatlyincreasedourancestors’needforthisimportantmetabolicelement.Buttheproblemwasthis:levelsofoxygenontheancient

earthwereverylow.Forbillionsofyearsoxygenlevelsintheatmospheredidnotcomeclosetowhatwehavetoday.Then,roughlyabillionyearsago,theamountofoxygenincreaseddramaticallyandhasstayedrelativelyhigheversince.Howdoweknowthis?Fromthechemistryofrocks.Rocksfromaboutabillionyearsagoshowthetelltalesignatureofhavingbeenformedwithincreasingamountsofoxygen.Couldtheriseinoxygenintheatmospherebelinkedtotheoriginofbodies?Itmayhavetakenthepaleontologicalequivalentofa

perfectstormtobringaboutbodies.Forbillionsofyears,microbesdevelopednewwaysofinteractingwiththeirenvironmentandwithoneanother.Indoingso,theyhitonanumberofthemolecularpartsandtoolstobuildbodies,thoughtheyusedthemforotherpurposes.Acausefortheoriginofbodieswasalsoinplace:byabillionyearsago,microbeshadlearnedtoeateachother.Therewasareasontobuildbodies,andthetoolstodosowerealreadythere.

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Somethingwasmissing.Thatsomethingwasenoughoxygenontheearthtosupportbodies.Whentheearth’soxygenincreased,bodiesappearedeverywhere.Lifewouldneverbethesame.

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CHAPTEREIGHT

MAKINGSCENTS

Intheearly1980s,therewastensionbetweenmolecularbiologistsandpeoplewhoworkedonwholeorganisms—ecologists,anatomists,andpaleontologists.Anatomists,forexample,wereseenasquaintlyout-of-date,hopelesslyentrancedbyanantiquatedkindofscience.Molecularbiologywasrevolutionizingourapproachtoanatomyanddevelopmentalbiology,somuchsothattheclassicaldisciplines,suchaspaleontology,seemedtobedeadendsinthehistoryofbiology.Iwasmadetofeelthat,becauseofmyloveoffossils,IwasgoingtobereplacedbyoneofthosenewautomatedDNAsequencers.Twentyyearslater,I’mstilldigginginthedirtand

crackingrocks.I’malsocollectingDNAandlookingatitsroleindevelopment.Debatesusuallybeginaseither-orscenarios.Overtime,all-or-nothingpositionsgivewaytoamorerealisticapproach.Fossilsandthegeologicalrecordremainaverypowerfulsourceofevidenceaboutthepast;nothingelserevealstheactualenvironmentsandtransitionalstructuresthatexistedduringthehistoryof

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life.Aswe’veseen,DNAisanextraordinarilypowerfulwindowintolife’shistoryandtheformationofbodiesandorgans.Itsroleisparticularlyimportantwherethefossilrecordissilent.Largepartsofbodies—softtissues,forexample—simplydonotfossilizereadily.Inthesecases,theDNArecordisvirtuallyallwehave.ExtractingDNAfrombodiesisincrediblyeasy,soeasy

youcandoitinyourkitchen.Takeahandfuloftissuefromsomeplantoranimal—peas,orsteak,orchickenliver.Addsomesaltandwaterandpopeverythinginablendertomushupthetissue.Thenaddsomedishsoap.Soapbreaksupthemembranesthatsurroundallthecellsinthetissuethatweretoosmallfortheblendertohandle.Afterthat,addsomemeattenderizer.ThemeattenderizerbreaksupsomeoftheproteinsthatattachtoDNA.Nowyouhaveasoapy,meat-tenderizedsoup,withDNAinside.Finally,addsomerubbingalcoholtothemix.You’llhavetwolayersofliquid:soapymushonthebottom,clearalcoholontop.DNAhasarealattractiontoalcoholandwillmoveintoit.Ifagoopywhiteballappearsinthealcohol,you’vedoneeverythingright.ThatgoopistheDNA.Youarenowinapositiontousethatwhiteglopto

understandmanyofthebasicconnectionswehavewiththerestoflife.Thetrick,onwhichwespendcountlesshoursanddollars,comesdowntocomparingDNA’sstructureandfunctionindifferentspecies.Hereisthecounterintuitivebit.ByextractingDNAfromanytissue,saytheliver,ofdifferentspecies,youcanactuallydecipherthehistoryof

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virtuallyanypartofourbody,includingoursenseofsmell.LockedinsidethatDNA,whetheritcomesfromliver,blood,ormuscle,ismuchoftheapparatusweusetodetectodorsinourenvironment.RecallthatallourcellscontainthesameDNA;whatdiffersiswhichbitsofDNAareactive.Thegenesinvolvedinthesenseofsmellarepresentinallofourcells,althoughtheyareactiveonlyinthenasalarea.Asweallknow,odorselicitimpulsesinourbrainsthat

canhaveaprofoundimpactonthewayweperceiveourworld.Awhiffmightleadustorecalltheschoolroomsofourchildhoodorthemustycozinessofourgrandparents’attic,eachoccasionbringinglong-buriedfeelingstothesurface.Moreessentially,smellscanhelpustosurvive.Thesmelloftastyfoodgetsushungry;thesmellofsewagemakesusfeelill.Wearehardwiredtoavoidrotteneggs.Wanttosellyourhome?Itwouldbefarbettertohavebreadbakingintheoventhancabbageboilingonthestovetopwhenprospectivebuyerscomeby.Wecollectivelyinvestvastsumsinoursenseofsmell:in2005theperfumeindustrygenerated$24billionofbusinessintheUnitedStatesalone.Allofthisatteststohowdeeplyembeddedoursenseofsmellisinsideofus.Itisalsoveryancient.Oursenseofsmellallowsustodiscriminateamongfive

thousandtotenthousandodors.Somepeoplecandetecttheodormoleculesinagreenbellpepperataconcentrationoflessthanonepartpertrillion.Thatislikepickingoutonegrainofsandfromamile-longbeach.Howdowedothat?Whatweperceiveasasmellisourbrain’sresponsetoa

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cocktailofmoleculesfloatingintheair.Themoleculesthatweultimatelyregisterasanodoraretiny,lightenoughtobesuspendedintheair.Aswebreatheorsniff,wesucktheseodormoleculesintoournostrils.Fromthere,theodormoleculesgotoanareabehindournosewheretheyaretrappedbythemucousliningofournasalpassages.Insidethisliningisapatchoftissuecontainingmillionsofnervecells,eachwithlittleprojectionsintothemucousmembrane.Whenthemoleculesintheairbindtothenervecells,signalsaresenttoourbrain.Ourbrainrecordsthesesignalsasasmell.Themolecularpartofsmellingworkslikealock-and-key

mechanism.Thelockistheodormolecule;thekeyisthereceptoronthenervecell.Amoleculecapturedbythemucousmembranesinournoseinteractswithareceptoronthenervecell.Onlywhenthemoleculeattachestothereceptorisasignalsenttoourbrain.Eachreceptoristunedtoadifferentkindofmolecule,soaparticularodormightinvolvelotsofmoleculesand,accordingly,lotsofreceptorssendingsignalstoourbrains.Thebestanalogyforsmellcomesfrommusic:achord.A

chordismadeupofseveralnotesactingtogetherasone.Inthesameway,anodoristheproductofsignalsfromlotsofreceptorskeyedtodifferentodormolecules.Ourbrainperceivesthesedifferentimpulsesasonesmell.

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Molecules(enlargedmany,manytimes)fromaflowerwaftthroughtheair.Thesemoleculesattachtoreceptorsinsidetheliningofournasalcavities.Oncethemoleculesattach,asignalissenttoourbrain.Eachsmelliscomposedofmanydifferentmoleculesattachingtodifferentreceptors.Ourbrainintegratesthesesignalsasweperceiveasmell.

Asinfish,amphibians,reptiles,mammals,andbirds,

muchofoursenseofsmellishousedinsideourskull.Liketheotheranimals,wehaveoneormoreholesthroughwhichwebringairinside,andthenasetofspecialized

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tissueswherethechemicalsintheaircaninteractwithneurons.Wecantracethepatternsoftheseholes,spaces,andmembranesfromfishtomanandfindageneralpattern.Themostprimitivelivinganimalswithskulls,jawlessfishsuchaslampreysandhagfish,haveasinglenostrilthatleadstoasacinsidetheskull.Watergoesintothisblindsac,anditistherethatsmellingtakesplace;themaindifferencefromusbeingthatlampreysandhagfishextractodorsfromwaterinsteadofair.Ourclosestfishrelativeshaveanarrangementsomewhatlikeours:thewaterentersanostrilandultimatelygoestoacavitylinkedwiththemouth.FishlikelungfishorTiktaalikhavetwokindsofnostrils:anexternaloneandaninternalone.Inthis,theyarealotlikeus.Sitwithyourmouthclosedandbreathe.Airentersanexternalnostrilandtravelsthroughyournasalcavitiestoenterthebackofyourthroatviainternalpassageways.Ourfishancestorshadinternalandexternalnostrils,too,andtonobody’ssurprisethesearethesamefishthathavearmbonesandotherfeaturesincommonwithus.Oursenseofsmellcontainsadeeprecordofourhistory

asfish,amphibians,andmammals.Amajorbreakthroughinunderstandingthisoccurredin1991whenLindaBuckandRichardAxeldiscoveredthelargefamilyofgenesthatgiveusoursenseofsmell.

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Nasalopeningsandtheflowofodormoleculesfromjawlessfishtoman.

BuckandAxelusedthreemajorassumptionstodesign

theirexperiments.First,theycameupwithareasonedhypothesis,basedonworkdoneinotherlaboratories,aboutwhatthegenesthatmakeodorreceptorsmighthavelookedlike.Experimentsshowedthatodorreceptorshaveacharacteristicstructurewithanumberofmolecularloopsthathelpthemconveyinformationacrossacell.Thiswasabigclue,becauseBuckandAxelcouldthensearchthegenomeofamouseforeverygenethatmakesthisstructure.Second,theyassumedthatthegenesforthesereceptorshadtohaveaveryspecificactivity—theyshouldbeactiveonlyinthetissuesinvolvedwithsmell.This

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makessense:ifsomethingisinvolvedinsmelling,thenitshouldberestrictedtothetissuesspecializedforthatpurpose.Third—andthislastwasabigassumption—AxelandBuckreasonedthattherewasn’tonlyoneorevenasmallnumberofthesegenes,therehadtobelotsofthem.Thishypothesiswasbasedonthefactthatmanydifferentkindsofchemicalscanstimulatedifferentsmells.Iftherewasaone-to-onematchbetweeneachchemicaltypeandareceptor/genespecializedforit,thentherehadtobemany,manygenes.But,giventhedatatheyhadatthetime,thisneedn’thavebeentrue.BuckandAxel’sthreeassumptionswereborneout

perfectly.Theyfoundgenesthathadthecharacteristicstructureofthereceptortheywerelookingfor.Theyfoundthatallofthesegeneswereactiveonlyinthetissuesinvolvedinsmelling,theolfactoryepithelium.Andfinally,theyfoundahugenumberofthesegenes.Itwasahomerun.Then,BuckandAxeldiscoveredsomethingtrulyastounding:fully3percentofourentiregenomeisdevotedtogenesfordetectingdifferentodors.Eachofthesegenesmakesareceptorforanodormolecule.Forthiswork,BuckandAxelsharedtheNobelPrizein2004.FollowingBuckandAxel’ssuccess,peoplestartedfishing

aroundforolfactoryreceptorgenesinotherspecies.Itturnsoutthatsuchgenesarealivingrecordofsomemajortransitionsinthehistoryoflife.Takethewater-to-landtransition,over365millionyearsago.Therearetwokindsofsmellinggenes:oneisspecializedforpickingupchemical

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scentsinthewater,theotherspecializedforair.Thechemicalreactionbetweenodormoleculeandreceptorisdifferentinwaterandair,hencetheneedforslightlydifferentreceptors.Aswe’dexpect,fishhavewater-basedreceptorsintheirnasalneurons,mammalsandreptileshaveair-basedones.Thisdiscoveryhelpsusmakesenseofthestateofaffairs

inthemostprimitivefishaliveontheplanettoday—thejawlessfishsuchaslampreysandhagfish.Itturnsoutthatthesecreatureshave,unlikemoreadvancedfishandmammals,neither“air”nor“water”genes;rather,theirreceptorscombinebothtypes.Theimplicationisclear:theseprimitivefisharosebeforethesmellinggenessplitintotwotypes.Jawlessfishrevealanotherveryimportantpoint:they

haveaverysmallnumberofodorgenes.Bonyfishhavemore,andstillmoreareseeninamphibiansandreptiles.Thenumberofodorgeneshasincreasedovertime,fromrelativelyfewinprimitivecreaturessuchasjawlessfish,totheenormousnumberseeninmammals.Wemammals,withoverathousandofthesegenes,devoteahugepartofourentiregeneticapparatusjusttosmelling.Presumably,themoreofthesegenesananimalhas,themoreacuteitsabilitytodiscerndifferentkindsofsmells.Inthislight,ourlargenumberofodorgenesmakessense—mammalsarehighlyspecializedsmellinganimals.Justthinkofwhateffectivetrackersdogscanbe.Butwheredoallourextraodorgenescomefrom?Did

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theyjustpopoutoftheblue?Howthisexpansionhappenedseemsobviouswhenwelookatthestructureofthegenes.Ifyoucomparetheodorgenesofamammalwiththehandfulofodorgenesinajawlessfish,the“extra”genesinmammalsareallvariationsonatheme:theylooklikecopies,albeitmodifiedones,ofthegenesinjawlessfish.Thismeansthatourlargenumberofodorgenesarosebymanyroundsofduplicationofthesmallnumberofgenespresentinprimitivespecies.Thisleadsustoaparadox.Humansdevoteabout3

percentofourgenometoodorgenes,justlikeeveryothermammal.Whengeneticistslookedatthestructureofthehumangenesinmoredetail,theyfoundabigsurprise:fullythreehundredofthesethousandgenesarerenderedcompletelyfunctionlessbymutationsthathavealteredtheirstructurebeyondrepair.(Othermammalsdousethesegenes.)Whyhavesomanyodorgenesifsomanyofthemareentirelyuseless?Dolphinsandwhales,ofallcreatures,offeraninsightto

helpusanswerthisquestion.Likeallmammals,dolphinsandwhaleshavehair,breasts,andathree-bonedmiddleear.Theirmammalianhistoryisalsorecordedintheirsmellinggenes:lackingfish-likewater-specializedgenes,cetaceanshavemammalianair-specializedgenes.ThemammalianhistoryofwhalesanddolphinsisevenwrittenintheDNAoftheirodorperceptionapparatus.Butthereisaninterestingpuzzle:dolphinsandwhalesnolongerusetheirnasalpassagestosmell.Whatarethesegenesdoing?

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Theformernasalpassagehasbeenmodifiedintoablowhole,whichisusedinbreathing,notinsmelling.Thishashadaremarkableeffectonthesmellinggenes:allofacetacean’sodorgenesarepresent,butnotoneisfunctional.Whathashappenedtothesmellgenesofdolphinsand

whalesalsohappensinmanyotherspecies’genes.Mutationscropupingenomesfromgenerationtogeneration.Ifamutationknocksoutthefunctionofagene,theresultcanbedangerous,orevenlethal.Butwhathappensifamutationknocksoutthefunctionofagenethatdoesnothing?Thereisalotofmathematicaltheorythatsaystheobvious:suchmutationswilljustsilentlygetpassedonfromgenerationtogeneration.Thisisexactlywhatappearstohavehappenedindolphins.Theirsmellgenesarenolongerneeded,giventheblowhole,sothemutationsthatknockedouttheirfunctionjustaccumulateovertime.Thegeneshavenouse,buttheyremainpresentintheDNAassilentrecordsofevolution.Buthumansdohaveasenseofsmell,sowhyhaveso

manyofourodorgenesbeenknockedout?YoavGiladandhiscolleaguesansweredthisquestionbycomparinggenesamongdifferentprimates.Hefoundthatprimatesthatdevelopcolorvisiontendtohavelargenumbersofknocked-outsmellgenes.Theconclusionisclear.Wehumansarepartofalineagethathastradedsmellforsight.Wenowrelyonvisionmorethanonsmell,andthisisreflectedinourgenome.Inthistrade-off,oursenseofsmellwasdeemphasized,andmanyofourolfactorygenes

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becamefunctionless.Wecarryalotofbaggageinournoses—or,more

precisely,intheDNAthatcontrolsoursenseofsmell.Ourhundredsofuselessolfactorygenesareleftoverfrommammalancestorswhoreliedmoreheavilyonthesenseofsmelltosurvive.Infact,wecantakethesecomparisonsdeeperstill.Likephotocopiesthatlosetheirfidelityastheyarerepeatedlycopied,ourolfactorygenesgetmoredissimilaraswecompareourselvestosuccessivelymoreprimitivecreatures.Ourgenesaresimilartoprimates’,lesssimilartoothermammals’,lesssimilarstilltoreptiles’,amphibians’,fishes’,andsoon.Thatbaggageisasilentwitnesstoourpast;insideournosesisaveritabletreeoflife.

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CHAPTERNINE

VISION

OnlyonceinmyentirecareerhaveIfoundtheeyeofafossilcreature.Iwasn’tinthefieldonanexpedition,IwasinthebackroomofamineralshopinasmalltowninnortheastChina.MycolleagueGaoKeqinandIwerestudyingtheearliestknownsalamanders,beautifulfossilscollectedfromChineserocksabout160millionyearsold.WehadjustfinishedacollectingtriptosomesitesGaoknewabout.Thelocationsweresecret,becausethesesalamanderfossilshaveseriousmonetaryvalueforthefarmerswhotypicallyfindthem.Whatmakesthemspecialisthatimpressionsofthesofttissue,suchasgills,guts,andthenotochord,areoftenpreserved.Privatecollectorslovethembecausefossilsofthisqualityareexceedinglyrare.Bythetimeweendedupatthemineralshop,GaoandIhadalreadycollectedanumberofreallybeautifulancientsalamandersofourownfromhissites.Thisparticularmineraldealerhadgottenhishandson

oneofthebestsalamanderfossilsofalltime.Gaowantedustoseeitandspentthebetterpartofadaytryingtowork

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thedeal.Thewholevisithadaterrificallyillicitfeel.Gaospentseveralhourssmokingcigaretteswiththegentleman,speakingandgesturinginChinese.Clearlytherewassomebarteringgoingon,butnotknowingChineseIhadnoideawhatofferswerebeingputonthetable.Afterlotsofheadshakingandultimatelyabighandshake,Iwaspermittedtogotothebackroomandlookatafossilonthedealer’sdesk.Itwasastunningsight:thebodyofalarvalsalamander,nomorethanthreeincheslong.Init,Icouldseeimpressionsofthewholeanimal,allthewaydowntothelittleshellsitateasitslastmeal.And,forthefirstandonlytimeinmycareer,Iwasstaringattheeyeofanancientfossilanimal.Eyesrarelymakeitintothefossilrecord.Aswe’veseen,

thebestcandidatesforpreservationasfossilsarethehardpartsoftheanimals—bones,teeth,andscales.Ifwewanttounderstandthehistoryofeyes,thenwecanuseanimportantfacttoouradvantage.Thereisaremarkablediversityoforgansandtissuesthatanimalsusetocapturelight,fromsimplephotoreceptororgansininvertebrateanimalstothecompoundeyesofvariousinsectsandourowncamera-typeeye.Howdoweputthisvariationtouseinunderstandinghowourabilitytoseedevelopedovertime?Thehistoryofoureyesisalotlikethatofacar.Takea

ChevyCorvette,forexample.Wecantracethehistoryofthemodelasawhole—theCorvette—andthehistoryofeachofitsparts.The’Vettehasahistory,beginningwithitsorigins

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in1953andcontinuingthroughthedifferentmodeldesignseachyear.Thetiresusedonthe’Vettealsohaveahistory,asdoestherubberusedinmakingthem.Thissuppliesagreatanalogyforbodiesandorgans.Oureyeshaveahistoryasorgans,butsodoeyes’constituentparts,thecellsandtissues,andsodothegenesthatmakethoseparts.Onceweidentifythesemultiplelayersofhistoryinourorgans,weunderstandthatwearesimplyamosaicofbitsandpiecesfoundinvirtuallyeverythingelseontheplanet.Muchoftheprocessingoftheimagesweseeactually

happensinsideourbrains:theroleoftheeyeistocapturelightinawaythatitcanbecarriedtothebrainforprocessingasanimage.Oureyes,likethoseofeverycreaturewithaskullandbackbone,arelikelittlecameras.Afterlightfromtheoutsideenterstheeye,itisfocusedonascreenatthebackoftheeyeball.Lighttravelsthroughseverallayersasittraversesthispath.Firstitpassesthroughthecornea,athinlayerofcleartissuethatcoversthelens.Theamountoflightthatenterstheeyeiscontrolledbyadiaphragm,calledtheiris,whichdilatesandcontractsbytheactionofinvoluntarymuscles.Thelightthenpassesthroughthelens,which,asacameradoes,focusestheimage.Tinymusclessurroundthelens;asthesemusclescontract,theychangethelens’sshape,thusfocusingimagesfromnearandfar.Ahealthylensisclearandmadeupofspecialproteinsthatgiveititsdistinctiveshapeandlight-gatheringproperties.Theseproteins,knownaslenscrystallins,areexceptionallylong-lived,

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allowingthelenstocontinuefunctioningasweage.Thescreenonwhichallofthelightisprojected,theretina,isloadedwithbloodvesselsandlightreceptors.Theselightreceptorssendsignalstoourbrainthatwetheninterpretasimages.Theretinaabsorbsthelightviasensitivelight-gatheringcells.Therearetwotypesofsuchcells:oneisverysensitivetolight,theotherlessso.Themoresensitivecellsrecordonlyinblackandwhite;thelesssensitivecellsrecordincolor.Ifwelookaroundtheanimalworld,wecanassesswhetheranimalsarespecializedfordaylightornightbylookingatthepercentagesofeachtypeoflight-sensingcellintheireyes.Inhumansthesecellsmakeupabout70percentofallthesensorycellsinourbody.Thatisaclearstatementabouthowimportantvisionistous.Ourcamera-likeeyeiscommontoeverycreaturewitha

skull,fromfishtomammals.Inothergroupsofanimalswefinddifferenteyes,rangingfromsimplepatchesofcellsspecializedtodetectlight,toeyeswithcompoundlensessuchasthosefoundinflies,toprimordialversionsofourowneye.Thekeytounderstandingthehistoryofoureyesistounderstandtherelationshipbetweenthestructuresthatmakeourcamera-eyeandthosethatmakealltheotherkindsofeyes.Todothis,wewillstudythemoleculesthatgatherlight,thetissuesweusetosee,andthegenesthatmakethewholething.

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Eyescomeintofocus:fromprimitivelight-capturingdevicesininvertebratestoourcamera-typeeyewithalens.Aseyesevolve,visualacuityincreases.

LIGHT-GATHERINGMOLECULES

Thereallyimportantworkinthelight-gatheringcellshappensinsidethemoleculethatactuallycollectslight.

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Whenthismoleculeabsorbslight,itchangesshapeandbreaksupintotwoparts.OnepartisderivedfromvitaminA,theotherfromaproteinknownasanopsin.Whentheopsinbreaksoff,itinitiatesachainreactionthatleadstoaneuronsendinganimpulsetoourbrain.Weusedifferentopsinstoseeinblackandwhiteandincolor.Justasaninkjetprinterneedsthreeorfourinkstoprintincolor,weneedthreelight-gatheringmoleculestoseeincolor.Forblack-and-whitevision,weuseonlyone.Theselight-gatheringmoleculeschangeshapeinthe

light,thenrechargeinthedarkandgobacktotheirnormalstate.Theprocesstakesafewminutes.Weallknowthisfrompersonalexperience:gofromabrightplaceintoadarkroomanditisvirtuallyimpossibletoseefaintobjects.Thereasonisthatthelight-gatheringmoleculesneedtimetorecharge.Afterafewminutes,visioninthedarkreturns.Despitethestunningvarietyofphotoreceptororgans,

everyanimalusesthesamekindoflight-capturingmoleculetodothisjob.Insects,humans,clams,andscallopsalluseopsins.Notonlycanwetracethehistoryofeyesthroughdifferencesinthestructureoftheiropsins,butwehavegoodevidencethatwecanthankbacteriaforthesemoleculesinthefirstplace.Essentially,anopsinisakindofmoleculethatconveys

informationfromtheoutsideofacelltotheinside.Topulloffthisfeat,itneedstocarryachemicalacrossthemembranethatencirclesacell.Opsinsuseaspecializedkindofconductorthattakesaseriesofbendsandloopsas

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ittravelsfromtheoutsidetotheinsideofthecell.Butthistwistedpaththereceptortakesthroughthecellmembraneisnotrandom—ithasacharacteristicsignature.Whereelseisthistwistedpathseen?Itisidenticaltopartsofcertainmoleculesinbacteria.Theveryprecisemolecularsimilaritiesinthismoleculesuggestaveryancientpropertyofallanimalsextendingallthewaytooursharedhistorywithbacteria.Inasense,modifiedbitsofancientbacterialieinsideourretinas,helpingustosee.Wecaneventracesomemajoreventsinthehistoryof

oureyesbyexaminingopsinsindifferentanimals.Takeoneofthemajoreventsinourprimatepast,thedevelopmentofrichcolorvision.Recallthathumansandourclosestaperelatives,theOldWorldmonkeys,haveaverydetailedkindofcolorvisionthatreliesonthreedifferentkindsoflightreceptors.Eachofthesereceptorsistunedtoadifferentkindoflight.Mostothermammalshaveonlytwokindsofreceptorsandsocannotdiscriminateasmanycolorsaswecan.Itturnsoutthatwecantracetheoriginofourcolorvisionbylookingatthegenesthatmakethereceptors.Thetwokindsofreceptorsmostmammalshavearemadebytwokindsofgenes.Ofourthreereceptor-makinggenes,twoareremarkablylikeoneofthoseinothermammals.Thisseemstoimplythatourcolorvisionbeganwhenoneofthegenesinothermammalsduplicatedandthecopiesspecializedovertimefordifferentlightsources.Asyou’llremember,asimilarthinghappenedwithodorreceptorgenes.

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Thisshiftmayberelatedtochangesinthefloraoftheearthmillionsofyearsago.Ithelpstothinkwhatcolorvisionwaslikelygoodforwhenitfirstappeared.Monkeysthatliveintreeswouldbenefitbecausecolorvisionenabledthemtodiscriminatebetteramongmanykindsoffruitsandleavesandselectthemostnutritiousamongthem.Fromstudyingtheotherprimatesthathavecolorvision,wecanestimatethatourkindofcolorvisionaroseabout55millionyearsago.Atthistimewefindfossilevidenceofchangesinthecompositionofancientforests.Beforethistime,theforestswererichinfigsandpalms,whicharetastybutallofthesamegeneralcolor.Laterforestshadmoreofadiversityofplants,likelywithdifferentcolors.Itseemsagoodbetthattheswitchtocolorvisioncorrelateswithaswitchfromamonochromaticforesttoonewitharicherpaletteofcolorsinfood.

TISSUES

Animaleyescomeintwoflavors;oneisseenininvertebrates,theotherinvertebrates,suchasfishandhumans.Thecentralideaisthattherearetwodifferentwaysofincreasingthelight-gatheringsurfaceareaineyetissue.Invertebrates,suchasfliesandworms,accomplishthisbyhavingnumerousfoldsinthetissue,whileourlineageexpandsthesurfaceareabyhavinglotsoflittleprojectionsextendingfromthetissueliketinybristles.A

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hostofotherdifferencesalsorelatetothesedifferentkindsofdesigns.Lackingfossilsattherelevantphaseofhistory,itwouldseemthatwewouldneverbeabletobridgethedifferencesbetweenoureyesandthoseofinvertebrates.Thatis,until2001,whenDetlevArendtthoughttostudytheeyesofaveryprimitivelittleworm.Polychaetesareamongthemostprimitivelivingworms

known.Theyhaveaverysimplesegmentedbodyplan,andtheyalsohavetwokindsoflight-sensingorgans:aneyeand,buriedundertheirskin,apartoftheirnervoussystemthatisspecializedtopickuplight.Arendttookthesewormsapartbothphysicallyandgenetically.Knowledgeofthegenesequenceofouropsingenesandthestructureofourlight-gatheringneuronsgaveArendtthetoolstostudyhowpolychaetesaremade.Hefoundthattheyhadelementsofbothkindsofanimalphotoreceptors.Thenormal“eye”wasmadeupofneuronsandopsinsliketheeyeofanyinvertebrate.Thetinyphotoreceptorsundertheskinwereanothermatteraltogether.Theyhad“vertebrate”opsinsandcellularstructureevenwiththelittlebristle-likeprojections,butinprimitiveform.Arendthadfoundalivingbridge,ananimalwithbothkindsofeyes,oneofwhich—ourkind—existedinaveryprimitiveform.Whenwelooktoprimitiveinvertebrates,wefindthatthedifferentkindsofanimaleyessharecommonparts.

GENES

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Arendt’sdiscoveryleadstoyetanotherquestion.Itisonethingforeyestosharecommonparts,buthowcaneyesthatlooksodifferent—suchasthoseofworms,flies,andmice—becloselyrelated?Fortheanswer,letusconsiderthegeneticrecipethatbuildseyes.Attheturnofthetwentiethcentury,MildredHogewas

recordingmutationsinfruitflieswhenshefoundaflythathadnoeyeswhatsoever.Thismutantwasnotanisolatedcase,andHogediscoveredthatshecouldbreedawholelineofsuchflies,whichshenamedeyeless.Later,asimilarmutationwasdiscoveredinmice.Someindividualshadsmalleyes;otherslackedwholeportionsoftheheadandface,includingtheireyes.Asimilarconditioninhumansisknownasaniridia;affectedindividualsaremissinglargepiecesoftheireyes.Intheseverydifferentcreatures—flies,mice,andhumans—geneticistswerefindingsimilarkindsofmutants.Abreakthroughcameintheearly1990s,when

laboratoriesappliednewmoleculartechniquestounderstandhoweyelessmutantsaffectedeyedevelopment.Mappingthegenes,theywereabletolocalizethebitsofDNAresponsibleforthemutations.WhentheDNAwassequenced,itturnedoutthatthefly,mouse,andhumangenesresponsibleforeyelessnesshadsimilarDNAstructuresandsequences.Inaveryrealsense,theyarethesamegene.Whatdidwelearnfromthis?Scientistshadidentifieda

singlegenethat,whenmutated,producedcreatureswith

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smalleyesornoeyesatall.Thismeantthatthenormalversionofthegenewasamajortriggerfortheformationofeyes.Nowcamethechancetodoexperimentstoaskawholeotherkindofquestion.Whathappenswhenwemesswiththegene,turningitonandoffinthewrongplaces?Flieswereanidealsubjectforthiswork.Duringthe

1980s,anumberofverypowerfulgenetictoolsweredevelopedthroughworkonflies.Ifyouknewagene,oraDNAsequence,youcouldmakeaflylackingthegeneor,thereverse,aflywiththegeneactiveinthewrongplaces.Usingthesetools,WalterGehringstartedplayingaround

withtheeyelessgene.Gehring’steamwasabletomaketheeyelessDNAactiveprettymuchanywheretheywanted:intheantenna,onthelegs,onthewings.Whenhisteamdidthis,theyfoundsomethingstunning.Iftheyturnedontheeyelessgeneintheantenna,aneyegrewthere.Iftheyturnedontheeyelessgeneonabodysegment,aneyedevelopedthere.Everywheretheyturnedonthegene,theywouldgetaneweye.Totopitalloff,someofthemisplacedeyesshowedanascentabilitytorespondtolight.Gehringhaduncoveredamajortriggerintheformationofeyes.Gehringdidn’tstopthere;hebeganswappinggenes

betweenspecies.Theytookthemouseequivalentofeyeless,Pax6,andturneditoninafly.Themousegeneproducedaneweye.Andnotjustanyeye—aflyeye.Gehring’slabfoundtheycouldusethemousegenetotriggertheformationofanextraflyeyeanywhere:ontheback,onawing,nearthemouth.WhatGehringhadfoundwasamasterswitchfor

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eyedevelopmentthatwasvirtuallythesameinamouseandafly.Thisgene,Pax6,initiatedacomplexchainreactionofgeneactivitythatultimatelyledtoanewflyeye.Wenowknowthateyeless,orPax6,controls

developmentineverythingthathaseyes.Theeyesmaylookdifferent—somewithalens,somewithout;somecompound,somesimple—butthegeneticswitchesthatmakethemarethesame.Whenyoulookintoeyes,forgetaboutromance,creation,

andthewindowsintothesoul.Withtheirmolecules,genes,andtissuesderivedfrommicrobes,jellyfish,worms,andflies,youseeanentiremenagerie.

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CHAPTERTEN

EARS

Thefirsttimeyouseetheinsideoftheearisaletdown:therealmachineryishiddendeepinsidetheskull,encasedinawallofbone.Onceyouopentheskullandremovethebrain,youneedtochipwithachiseltoremovethatwall.Ifyouarereallygood,orverylucky,you’llmaketherightstrokeandseeit—theinnerear.Itresemblesthekindoftinycoiledsnailshellyoufindinthedirtinyourlawn.Theearmaynotlooklikemuch,butitisawonderful

RubeGoldbergcontraption.Whenwehear,soundwavesarefunneledintotheoutsideflap,theexternalear.Thesoundwavesentertheearandmaketheeardrumrattle.Theeardrumisattachedtothreelittlebones,whichshakealongwithit.Oneoftheseearbonesisattachedtothesnail-shellstructurebyakindofplunger.Theshakingoftheearbonecausestheplungertogoupanddown.Thiscausessomegelinsidethesnailshelltomovearound.Swishinggelbendsnerves,whichsendasignaltothebrain,whichinterpretsitassound.Nexttimeyouareataconcert,justimagineallthestuffflyingaroundinyourhead.

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Thisstructureallowsustodistinguishthreepartstotheear:external,middle,andinner.Theexternalearisthevisiblepart.Themiddlecontainsthelittleearbones.Finally,theinnerearconsistsofthenerves,thegel,andthetissuesthatsurroundthem.Thesethreecomponentsofearsenableustostructureourdiscussioninaveryconvenientway.

Ofthethreepartsofourear—theouter,middle,andinner—theinnerearisthemostancientandthepartthatcontrolsthenerveimpulsessenttothebrain.

Thepartoftheearwecansee,theflaponwhichwehang

ourglasses,isarelativelynewevolutionaryadditiontobodies.Confirmthisonyournexttriptotheaquariumorzoo.Howmanysharks,bonyfish,amphibians,andreptileshaveexternalears?Thepinna—theflapoftheexternalear—isfoundonlyinmammals.Someamphibiansandreptileshavevisibleexternalears,buttheyhavenopinna.Oftenthe

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externalearisonlyamembranethatlookslikethetopofadrum.Theeleganceofourconnectiontosharksandbonyfishis

revealedwhenwelookinsideourears.Earsmightseemanunlikelyplaceforahuman-sharkconnection,especiallysincesharksdon’thaveears.Buttheconnectionisthere.Let’sstartwiththeearbones.

THEMIDDLEEAR—THETHREEEARBONES

Mammalsareveryspecial.Withhairandmilk-producingglands,wecaneasilybedistinguishedfromothercreatures.Itsurprisesmostpeopletolearnthatsomeofthemostdistinctivetraitsofmammalslieinsidetheear.Thebonesofthemammalianmiddleeararelikethoseofnootheranimal:mammalshavethreebones,whereasreptilesandamphibianshaveonlyone.Fishhavenoneatall.Wheredidourmiddleearbonescomefrom?Someanatomy:recallthatourthreemiddleearbonesare

knownasthemalleus,incus,andstapes.Aswe’veseen,eachoftheseearbonesisderivedfromthegillarches:thestapesfromthefirstarch,andthemalleusandincusfromthesecondarch.Itisherethatourstorybegins.In1837,theGermananatomistKarlReichertwaslooking

atembryosofmammalsandreptilestounderstandhowtheskullforms.Hefollowedthegillarchesofdifferentspeciestounderstandwheretheyendedupinthevariousskulls.As

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hedidthisagainandagain,hefoundsomethingthatappearednottomakeanysense:twooftheearbonesinthemammalscorrespondedtopiecesofthejawinthereptiles.Reichertcouldnotbelievehiseyes,andhismonographrevealshisexcitement.Ashedescribestheear-jawcomparison,hisprosedepartsfromthenormallystaiddescriptionofnineteenth-centuryanatomytoexpressshock,evenwonderment,atthisdiscovery.Theconclusionwasinescapable:thesamegillarchthatformedpartofthejawofareptileformedearbonesinmammals.Reichertproposedanotionthatevenhecouldbarelybelieve—thatpartsoftheearsofmammalsarethesamethingasthejawsofreptiles.ThingsgetmoredifficultwhenwerealizethatReichertproposedthisseveraldecadesbeforeDarwinpropoundedhisnotionofafamilytreeforlife.Whatdoesitmeantocallstructuresintwodifferentspecies“thesame”withoutanotionofevolution?Muchlater,in1910and1912,theGermananatomist

ErnstGaupppickeduponReichert’sworkandpublishedanexhaustivestudyontheembryologyofmammalianears.Gauppprovidedmoredetailand,giventhetimes,interpretedReichert’sworkinanevolutionaryframework.Gaupp’sstorywentlikethis:thethreemiddleearbonesrevealthetiebetweenreptilesandmammals.Thesingleboneinthereptilianmiddleearisthesameasthestapesofmammals;botharesecond-archderivatives.Theexplosivebitofinformation,though,wasthatthetwoothermiddleearbonesofmammals—themalleusandtheincus—

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evolvedfrombonessetinthebackofthereptilianjaw.Ifthiswasindeedthecase,thenthefossilrecordshouldshowbonesshiftingfromthejawtotheearduringtheoriginofmammals.TheproblemwasthatGauppworkedonlyonlivingcreaturesanddidn’tfullyappreciatetherolethatfossilscouldplayinhistheory.Beginninginthe1840sanumberofnewkindsoffossil

creatureswerebecomingknownfromdiscoveriesinSouthAfricaandRussia.Oftenabundantlypreserved,wholeskeletonsofdog-sizeanimalswerebeingunearthed.Astheywerediscovered,manyofthemwerecratedandshippedtoRichardOweninLondonforidentificationandanalysis.Owenwasstruckthatthesecreatureshadamélangeoffeatures.Partsoftheirskeletonlookedreptile-like.Otherparts,notablytheirteeth,lookedlikemammals.Andthesewerenotisolatedfinds.Itturnsoutthatthese“mammal-likereptiles”werethemostcommonskeletonsbeinguncoveredatmanyfossilsites.Notonlyweretheyverycommon,thereweremanykinds.IntheyearsafterOwen,thesemammal-likereptilesbecameknownfromotherpartsoftheworldandfromseveraldifferenttimeperiodsinearthhistory.Theyformedabeautifultransitionalseriesinthefossilrecordbetweenreptileandmammal.Until1913,embryologistsandpaleontologistswere

workinginisolationfromoneanother.Atthistime,theAmericanpaleontologistW.K.Gregory,oftheAmericanMuseumofNaturalHistory,sawanimportantlinkbetween

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Gaupp’sembryosandtheAfricanfossils.Themostreptilianofthemammal-likereptileshadonlyasingleboneinitsmiddleear;likeotherreptiles,ithadajawcomposedofmanybones.SomethingremarkablewasrevealedasGregorylookedatthesuccessivelymoremammalianmammal-likereptiles,somethingthatwouldhaveflooredReicherthadhebeenalive:acontinuumofformsshowingbeyonddoubtthatovertimethebonesatthebackofthereptilianjawgotsmallerandsmaller,untiltheyultimatelylayinthemiddleearofmammals.Themalleusandincusdidindeedevolvefromjawbones.WhatReichertandGauppobservedinembryoswasburiedinthefossilrecordallalong,justwaitingtobediscovered.Whywouldmammalsneedathree-bonedmiddleear?

Thislittlelinkageformsaleversystemthatallowsmammalstohearhigher-frequencysoundsthananimalswithasinglemiddleearbone.Theoriginofmammalsinvolvednotonlynewpatternsofchewing,aswesawinChapter4,butnewwaysofhearing.Infact,thisshiftwasaccomplishednotbyevolvingnewbonesperse,butbyrepurposingexistingones.Bonesoriginallyusedbyreptilestochewevolvedinmammalstoassistinhearing.Somuchforthemalleusandincus.Where,though,does

thestapescomefrom?IfIsimplyshowedyouanadulthumanandashark,you

wouldneverguessthatthistinybonedeepinsideahuman’searisthesamethingasalargerodintheupperjawofafish.Yet,developmentally,thesebonesarethe

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samething.Thestapesisasecond-archbone,asisthecorrespondingboneinasharkandafish—thehyomandibula.Butthehyomandibulaisnotanearbone;recallthatfishandsharksdonothaveears.Inouraquaticcousins,thisboneisalargerodthatconnectstheupperjawtothebraincase.Despitetheapparentdifferencesinthefunctionandshapeofthesebones,thesimilaritiesbetweenthehyomandibulaandthestapesextendeventothenervesthatsupplythem.Thekeynerveforthefunctioningofbothbonesisthesecond-archnerve,thefacialnerve.Wethushaveasituationwheretwoverydifferentboneshavesimilardevelopmentaloriginsandpatternsofinnervation.Isthereanexplanationforthis?Again,welooktothefossils.Aswetracethe

hyomandibulafromsharkstocreatureslikeTiktaaliktoamphibians,wecanseehowitgetssmallerandsmaller,ultimatelyshiftingpositionfromtheupperjawtoplayaroleinhearing.Thenamechanges,too.Whenitisbigandsupportingthejaw,wecallitahyomandibula;whenitissmallandfunctionsinhearing,itisknownasastapes.Thisshifthappenedwhenthedescendantsoffishbegantowalkonland.Hearinginwaterisdifferentfromhearingonland,andthesmallsizeandpositionofthestapesmakesitidealforpickingupvibrationsinair.Thenewabilitycameaboutbymodifyingtheupperjawboneofafish.

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Wecantracebonesfromgillarchestoourears,firstduringthetransitionfromfishtoamphibian(right),andlaterduringtheshiftfromreptiletomammal(left).

Ourmiddleearcontainsarecordoftwoofthegreat

transformationsinthehistoryoflife.Theoriginofourstapes,anditstransformationfromajawsupportboneto

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anearbone,beganwhenfishstartedtowalkonland.Theotherbigeventtookplaceduringtheoriginofmammals,whenbonesatthebackofareptilejawbecameourmalleusandincus.Nowlet’sgofurtherinsidetheear—totheinnerear.

THEINNEREAR—GELSMOVINGANDHAIRSBENDING

Movethroughtheexternalear,godeeperinside,pasttheeardrumandthreemiddleearbones,andyouendupdeepinsidetheskull.Hereyouwillfindtheinnerear—tubesandsomegel-filledsacs.Inhumans,asinothermammals,thebonytubestakethesnail-shellshapethatissostrikinglyapparentintheanatomylab.Theinnerearhasdifferentpartsdedicatedtodifferent

functions.Onepartisusedinhearing,anotherintellinguswhichwayourheadistilted,andstillanotherinrecordinghowfastourheadisacceleratingorstopping.Incarryingouteachofthesefunctions,theinnerearworksinroughlythesameway.Theseveralpartsoftheinnereararefilledwithagelthat

canmove.Specializednervecellssendhairlikeprojectionsintothisgel.Whenthegelmoves,thehairsontheendsofthenervecellsbend.Whenthesehairsbend,thenervecellssendanelectricalimpulsetothebrain,whereitisrecordedassound,position,oracceleration.

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Eachtimeyoutiltyourhead,thetinyrocksonthefluid-filledsacsmove.Indoingso,theybendnerveendingsinsidethesacsandcauseanimpulsetobesenttoyourbrainsaying“Yourheadistilted.”

Toenvisionthestructurethattellsuswhereourheadis

inspace,imagineaStatueofLibertysnowglobe.Thesnowglobeismadeofplasticandfilledwithgel.Whenyoushakeit,thegelmovesandthe“snow”fallsonLadyLiberty.Nowimagineasnowglobemadeofaflexiblemembrane.Pickitupandtiltit,andthewholethingwillflopabout,causing

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thegelinsidetoswisharound.This,onamuchsmallerscale,iswhatwehaveinsideourears.Whenwebendourheads,thesecontraptionsfloparound,causingtheusualchainofevents:thegelinsideswishes,thehairprojectionsonthenervesbend,andanimpulseissentbacktoourbrains.Inus,thiswholesystemismadeevenmoresensitiveby

thepresenceoftinyrock-likestructuresontopofthemembrane.Aswebendourheads,therocksaccentuatethefloppingofthemembrane,causingthegeltomoveevenmore.Thisincreasesthesensitivityofthesystem,enablingustoperceivesmalldifferencesinposition.Tiltyourhead,andlittlerocksinsideyourskullmove.Youcanprobablyimaginehowtoughitwouldbetolive

inouterspace.Oursensorsaretunedtoworkintheearth’sgravity,notinagravity-freespacecapsule.Floatingaround,oureyesrecordingoneversionofupanddown,ourinnerearsensorstotallyconfused,itisalltooeasytogetsick.Spacesicknesshasbeenarealproblemfortheseveryreasons.Thewayweperceiveaccelerationisbasedonyetanother

partofourinnerear,connectedtotheprevioustwo.Therearethreegel-filledtubesinsidetheear;eachtimeweaccelerateorstop,thegelinsidethetubesmoves,causingthenervecellstobendandstimulateacurrent.Thewholesystemweusetoperceivepositionand

accelerationisconnectedtooureyemuscles.Themotionofoureyesiscontrolledbyeightsmallmusclesattachedto

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thesidewallsoftheeyeball.Themusclescontracttomovetheeyeup,down,left,andright.Wecanmoveoureyesvoluntarilybycontractingthesemuscleseachtimewedecidetolookinanewdirection;butsomeofthemostfascinatingpropertiesofthesemusclesrelatetotheirinvoluntaryaction.Theymoveoureyesallthetime,withoutoureventhinkingaboutit.Toappreciatethesensitivityofthiseye-musclelink,

moveyourheadbackandforthwhilelookingatthepage.Keepyoureyesfixedinoneplaceasyoumoveyourhead.Whathappenedduringthisexperiment?Youreyes

stayedfixedonasinglepointwhileyourheadmoved.Thismotionissocommonplacethatwetakeitforgranted,butitisincrediblycomplex.Eachoftheeightmusclesinbotheyesisrespondingtothemovementofthehead.Sensorsinyourhead,whichI’lldescribeinthenextsection,recordthedirectionandvelocityofyourhead’smovement.Thesesignalsarecarriedtothebrain,whichthensendsoutsignalstellingyoureyemusclestofire.Thinkaboutthatthenexttimeyoufixyourgazeasyourheadismoving.Thissystemcanmisfire,andmisfireshavemuchtotellusaboutourgeneralwell-being.

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Everytimeweaccelerate,fluidintheinnerearswishes.Theswishistransformedintoanerveimpulsethatissenttothebrain.

Aneasywaytounderstandtheinnerear–eyeconnection

istointerferewithit.Onewayhumansdothisistoimbibetoomuchalcohol.Drinkingtoomuchethanolleadsustodosillythingsbecauseourinhibitionsarelowered.Drinkingwaytoomuchgivesusthespins.Andthespinsoftenpredictalousymorningahead,hungover,withmorespins,

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nausea,andheadache.Whenwedrinktoomuch,weareputtinglotsofethanol

intoourbloodstream,butthefluidinsideoureartubesinitiallycontainsverylittle.Astimepasses,however,thealcoholdiffusesfromourbloodintothegeloftheinnerear.Alcoholislighterthanthegel,sotheresultofthediffusionisliketheresultofpouringalcoholintoaglassofoliveoil.Justastheoilmovesaroundintheglassasthealcoholenters,sothegelinsideourearswirls.Theconvectionwreakshavocontheintemperateamongus.Ourhaircellsarestimulatedandourbrainthinkswearemoving.Butwearenotmoving;weareslumpedinacornerorhunchedonabarstool.Ourbrainhasbeentricked.Theproblemextendstooureyes.Ourbrainthinksweare

spinning,anditpassesthisinformationtooureyemuscles.Theeyestwitchinonedirection(usuallytotheright)aswetrytotrackanobjectmovingfromsidetoside.Ifyoupropopentheeyesofsomeonewhoisstonedrunk,youmightseethisstereotypicaltwitch,callednystagmus.Policeknowthiswell,andoftenlookfornystagmusinpeoplewhomtheyhavestoppedfordrivingerratically.Massivehangoversinvolveaslightlydifferentresponse.

Thedayafterthebinge,yourliverhasdonearemarkablyefficientjobofremovingthealcoholfromyourbloodstream.Tooefficient,forwestillhavealcoholinthetubesinourears.Thatalcoholthendiffusesfromthegelbackintothebloodstream,andindoingsoitoncemoresetsthegelinmotion:thespinsagain.Takethesameheavy

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drinkerwhoseeyesyousawtwitchtotherightthenightbeforeandlookathimduringthehangover.Hiseyesmightstilltwitch,butintheoppositedirection.Wecanthankoursharedhistorywithsharksandfishfor

this.Ifyouhaveevertriedtocatchatrout,thenyouhavecomeupagainstanorganthatislikelyanantecedenttoourinnerear.Aseveryfishermanknows,troutholdonlyincertainpartsofastream,typicallyspotswheretheycangetthebestmealwhileavoidingpredators.Oftensuchplacesareintheshadeandintheeddiesofthestream’scurrent.Greatplacesforbigfishtoholdarebehindbigrocksorfallenlogs.Trout,likeallfish,haveamechanismthatallowsthemtosensethecurrentandthemotionofthewateraroundthem,almostlikeasenseoftouch.Withintheskinandbonesofthefish,arrangedinlines

thatrunthelengthofthebodyandhead,aresmallorganswithsensoryreceptors.Thesereceptorslieinsmallbundlesfromwhichtheysendsmallhair-likeprojectionsintoajelly-filledsaccalledaneuromastorgan.IthelpstothinkofthesnowglobeStatueofLibertyagain.Aneuromastorganislikeatinyoneofthese,withnervesprojectinginside.Whenthewaterflowsaroundthefish,itdeformsthissmallsac,therebybendingthehair-likeprojectionsofthenerve.Muchlikethewholesysteminourears,thisapparatusthensendsasignalbacktothebrainandgivesthefishasenseofwhatthewaterisdoingaroundthem.Sharksandfishcandiscernthedirectioninwhichthewaterisflowing,andsomesharkscanevendetect

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distortionsofthewater,suchasareproducedbyotherfishswimmingnearthem.Weusedaversionofthissystemwhenwemovedourheadwithafixedgaze,andwesawitgoawrywhenweproppedopentheeyesoftheinebriatedindividualatthestartofthissection.Iftheancestorwehaveincommonwithsharksandfishhadusedsomeotherkindofinnereargel,sayonethatdoesnotswirlwhenalcoholisadded,wewouldneverspinwhendrunk.Ifyouthinkofourinnerearsandneuromastorgansas

versionsofthesamething,youwouldnotbefaroff.Bothcomefromthesamesortoftissueduringdevelopment,andtheyshareasimilarstructure.Butwhichcamefirst:neuromastsorinnerears?Heretheevidencegetssketchy.Ifyoulookatsomeoftheearliestfossilswithheads,creaturesabout500millionyearsold,you’llfindlittlepitsintheirexternalarmorthatsuggesttheyhadneuromastorgans.Unfortunately,wedonotknowmuchabouttheinnerearsofthesecreaturesbecausethepreservationofthatareaoftheheadiswanting.Untilmoreevidencerollsin,weareleftwithoneoftwoalternatives:eitherourinnerearsarosefromneuromastorgansortheotherwayaround.Bothscenarios,attheircore,reflectaprinciplewe’veseenatworkinotherpartsofthebody.Organscancomeaboutforonefunction,onlytoberepurposedovertimeforanynumberofnewuses.Inourownears,thereoccurredanexpansionoftheinner

ear.Thepartofourinnereardevotedtohearingis,asinothermammals,hugeandcoiled.Moreprimitivecreatures,

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suchasamphibiansandreptiles,haveasimpleuncoiledinnerear.Clearly,ourmammalianforebearsobtainedanewandbettertypeofhearing.Thesameistrueforthestructuresthatperceiveacceleration.Wehavethreecanalstorecordaccelerationbecauseweperceivespaceinthreedimensions.Theearliestknownfishwiththesecanals,akindofjawlessfishlikeahagfish,hasonlyone.Then,inotherprimitivefish,weseetwo.Finally,mostmodernfish,andothervertebrates,havethree,likeus.Wehaveseenthatourinnerearhasahistorythatcanbe

tracedtotheearliestfish.Remarkably,theneuronsinsidethegelofourearshaveanevenmoreancienthistory.Theseneurons,calledhaircells,havespecialfeatures

thatareseeninnootherneuron.Withfinehair-likeprojections,consistingofonelong“hair”andaseriesofsmallerones,theseneuronsliewithafixedorientationinourinnerearandinafish’sneuromastorgan.Recently,peoplehavesearchedforthesecellsinothercreatures,andhavefoundthemnotonlyinanimalsthatdonothavesenseorganslikeoursatallbutalsoinanimalsthathavenoheads.TheyareseenincreatureslikeAmphioxus,whichwemetinChapter5,thathavenoears,eyes,heads,orskulls.Haircells,then,werearounddoingotherthingsbeforeoursenseorgansevenhitthescene.

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Aprimitiveversionofpartofourinnerearisembeddedintheskinoffish.Smallsacs—theneuromasts—aredistributedaroundthebody.Whentheybend,theygivethefishinformationabouthowtheflowofwaterischanging.

Allthisisrecordedinourgenes,ofcourse.Ifhumansor

micehaveamutationthatknocksoutagenecalledPax2,theinnerearfailstoformproperly.Pax2isactiveintheearregionandappearstostartachainreactionofgeneactivitythatleadstothedevelopmentoftheinnerear.GofishingforthisgeneinmoreprimitiveanimalsandwefindPax2activeintheheadand,loandbehold,intheneuromasts.

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Thespinningdrunkandthefish’swater-sensingorganshavecommongenes:evidenceofacommonhistory.

JELLYFISHANDTHEORIGINSOFEYESANDEARS

JustlikePax6,whichwediscussedearlierinconnectionwitheyes,Pax2inearsisamajorgene,essentialforproperdevelopment.Interestingly,alinkbetweenPax2andPax6suggeststhatearsandeyesmighthavehadaveryancientcommonhistory.Thisiswheretheboxjellyfishentersourstory.Well

knowntoswimmersinAustraliabecausetheyhaveparticularlypoisonousvenom,thesejellyfisharedifferentfrommostothersinthattheyhaveeyes,morethantwentyofthem.Mostoftheseeyesaresimplepitsspreadoverthejellyfish’sepidermis.Othereyesonthebodyarestrikinglysimilartoourown,withakindofcornea,alens,evenanervousstructurelikeours.JellyfishdonothaveeitherPax6orPax2:theyarose

beforethosegeneshitthescene.Butintheboxjellyfish’sgenesweseesomethingremarkable.ThegenethatformstheeyesisnotPax6,aswe’dexpect,butasortofmosaicthathasthestructureofbothPax6andPax2.Inotherwords,thisgenelookslikeaprimitiveversionofotheranimals’Pax6andPax2.Themajorgenesthatcontroloureyeandearcorrespond

toasinglegeneinmoreprimitivecreatures,suchas

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jellyfish.You’reprobablythinking,Sowhat?Theancientconnectionbetweenearandeyegeneshelpstomakesenseofthingsweseeinhospitalclinicstoday:anumberofhumanbirthdefectsaffectboththeeyesandtheinnerear.Allthisisareflectionofourdeepconnectionstoprimitivecreatureslikethestingingboxjellyfish.

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CHAPTERELEVEN

THEMEANINGOFITALL

THEZOOINYOU

Myprofessionalintroductiontoacademiahappenedintheearly1980s,duringmycollegeyears,whenIvolunteeredattheAmericanMuseumofNaturalHistoryinNewYorkCity.Asidefromtheexcitementofworkingbehindthescenesinthecollectionsofthemuseum,oneofthemostmemorableexperienceswasattendingtheirraucousweeklyseminars.Eachweekaspeakerwouldcometopresentsomeesotericstudyonnaturalhistory.Followingthepresentation,oftenafairlylow-keyaffair,thelistenerswouldpickthetalkapartpointbypoint.Itwasmerciless.Onoccasion,thewholethingfeltlikeahumanbarbecue,withtheinvitedspeakerasthespit-roastedmaincourse.Frequently,thesedebateswoulddevolveintoshoutingsessionswithallthehighdudgeonandoperaticpantomimeofanoldsilentmovie,completewithshakenfistsandstompedfeet.HereIwas,inthehallowedhallsofacademe,listeningto

seminarsontaxonomy.Youknow,taxonomy—thescience

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ofnamingspeciesandorganizingthemintotheclassificationschemethatweallmemorizedinintroductorybiology.Icouldnotimagineatopiclessrelevanttoeverydaylife,letaloneonelesslikelytoleademinentseniorscientistsintoapoplexyandthelossofmuchoftheirhumandignity.Theinjunction“Getalife”couldnothaveseemedmoreapt.TheironyisthatInowseewhytheygotsoworkedup.I

didn’tappreciateitatthetime,buttheyweredebatingoneofthemostimportantconceptsinallofbiology.Itmaynotseemearth-shattering,butthisconceptliesattherootofhowwecomparedifferentcreatures—ahumanwithafish,orafishwithaworm,oranythingwithanythingelse.Ithasledustodeveloptechniquesthatallowustotraceourfamilylineages,identifycriminalsbymeansofDNAevidence,understandhowtheAIDSvirusbecamedangerous,andeventrackthespreadoffluvirusesthroughouttheworld.TheconceptI’mabouttodiscusssuppliestheunderpinningformuchofthelogicofthisbook.Oncewegraspit,weseethemeaningofthefish,worms,andbacteriathatlieinsideofus.Thearticulationoftrulygreatideas,ofthelawsofnature,

beginswithsimplepremisesthatallofusseeeveryday.Fromsimplebeginnings,ideasliketheseextendtoexplainthereallybigstuff,likethemovementofthestarsortheworkingsoftime.Inthatspirit,Icansharewithyouonetruelawthatallofuscanagreeupon.Thislawissoprofoundthatmostofustakeitcompletelyforgranted.Yet

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itisthestartingpointforalmosteverythingwedoinpaleontology,developmentalbiology,andgenetics.Thisbiological“lawofeverything”isthateveryliving

thingontheplanethadparents.Everypersonyou’veeverknownhasbiologicalparents,

asdoeseverybird,salamander,orsharkyouhaveeverseen.Technologymaychangethis,thankstocloningorsomeyet-to-be-inventedmethod,butsofarthelawholds.Toputitinamorepreciseform:everylivingthingsprangfromsomeparentalgeneticinformation.Thisformulationdefinesparenthoodinawaythatgetstotheactualbiologicalmechanismofheredityandallowsustoapplyittocreatureslikebacteriathatdonotreproducethewaywedo.Theextensionofthislawiswhereitspowercomesin.

Hereitis,inallitsbeauty:allofusaremodifieddescendantsofourparentsorparentalgeneticinformation.I’mdescendedfrommymotherandfather,butI’mnotidenticaltothem.Myparentsaremodifieddescendantsoftheirparents.Andsoon.Thispatternofdescentwithmodificationdefinesourfamilylineage.Itdoesthissowellthatwecanreconstructyourfamilylineagejustbytakingbloodsamplesofindividuals.Imaginethatyouarestandinginaroomfullofpeople

whomyouhaveneverseenbefore.Youaregivenasimpletask:findouthowcloselyrelatedeachpersonintheroomistoyou.Howdoyoutellwhoareyourdistantcousins,yoursuper-distantcousins,yourgreat-grandunclesseventy-five

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timesremoved?Toanswerthisquestion,weneedabiologicalmechanism

toguideourthinkingandgiveusawaytotesttheaccuracyofourhypothesizedfamilytree.Thismechanismcomesfromthinkingaboutourlawofbiology.Knowinghowdescentwithmodificationworksiskeytounlockingbiologicalhistory,becausedescentwithmodificationcanleaveasignature,whichwecandetect.Let’stakeahypotheticalhumorless,quiteunclown-like

couplewhohavechildren.Oneoftheirsonswasbornwithageneticmutationthatgavehimaredrubbernosethatsqueaks.Thissongrowsupandmarriesaluckywoman.Hepasseshismutatednosegenetohischildren,andtheyallhavehisredrubbernosethatsqueaks.Now,supposeoneofhisoffspringgetsamutationthatcauseshimtohavehugefloppyfeet.Whenthismutationpassestothenextgeneration,allofhischildrenarelikehim:theyhavearedrubbernosethatsqueaksandhugefloppyfeet.Goonegenerationfurther.Imaginethatoneofthesekids,theoriginalcouple’sgreat-grandchild,hasanothermutation:orangecurlyhair.Whenthismutationpassestothenextgeneration,allofhischildrenwillhaveorangecurlyhair,arubbernosethatsqueaks,andgiantfloppyfeet.Whenyouask“Whoisthisbozo?”you’llbeinquiringabouteachofourpoorcouple’sgreat-great-grandchildren.Thisexampleillustratesaveryseriouspoint.Descent

withmodificationcanbuildafamilytree,orlineage,thatwecanidentifybycharacters.Ithasasignaturethatwe

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immediatelyrecognize.LikeanestedsetofRussiandolls,ourhypotheticallineageformedgroupswithingroups,whichwerecognizebytheiruniquefeatures.Thegroupof“fullbozo”great-great-grandchildrenisdescendedfromanindividualwhohadonlythesqueakynoseandthehugefloppyfeet.Thisindividualwasinagroupof“proto-bozos,”whoaredescendedfromanindividualwhohadonlytherubbernosethatsqueaks.This“pre-proto-bozo”wasdescendedfromtheoriginalcouple,whodidn’tlookovertlyclown-like.

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Thebozofamilytree.Thispatternofdescentwithmodificationmeansthatyou

couldeasilyhavehypothesizedthebozofamilytreewithoutmetellingyouanythingaboutit.Ifyouhadaroomfullofthevariousgenerationsofbozos,youwouldhaveseenthatallclownkinareinagroupthatpossessesasqueakynose.Asubsetofthesehaveorangehairandfloppy

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feet.Nestedwithinthissubsetisanothergroup,thefullbozos.Thekeyisthatthefeatures—orangehair,squeakynose,bigfloppyfeet—enableyoutorecognizethegroups.Thesefeaturesareyourevidenceforthedifferentgroups,orinthiscasegenerations,ofclowns.Replacethisfamilycircuswithrealfeatures—genetic

mutationsandthebodychangesthattheyencode—andyouhavealineagethatcanbeidentifiedbybiologicalfeatures.Ifdescentwithmodificationworksthisway,thenourfamilytreeshaveasignatureintheirbasicstructure.Sopowerfulisthistruththatitcanhelpusreconstructfamilytreesfromgeneticdataalone,asweseefromthenumberofgenealogicalprojectscurrentlyunderway.Obviously,therealworldismorecomplexthanoursimplehypotheticalexample.Reconstructingfamilytreescanbedifficultiftraitsarisemanydifferenttimesinafamily,iftherelationshipbetweenatraitandthegenesthatcauseitisnotdirect,oriftraitsdonothaveageneticbasisandariseastheresultofchangesindietorotherenvironmentalconditions.Thegoodnewsisthatthepatternofdescentwithmodificationcanoftenbeidentifiedinthefaceofthesecomplications,almostlikefilteringoutnoisefromaradiosignal.Butwheredoourlineagesstop?Didthebozosstopatthe

humorlesscouple?DoesmylineagestopatthefirstShubins?Thatseemsawfullyarbitrary.DoesitstopatUkranianJews,ornorthernItalians?Howaboutatthefirsthumans?Ordoesitcontinueto3.8-billion-year-oldpond

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scum,andbeyond?Everybodyagreesthattheirownlineagegoesbacktosomepointintime,butjusthowfarbackistheissue.Ifourlineagegoesallthewaybacktopondscum,and

doessowhilefollowingourlawofbiology,thenweshouldbeabletomarshalevidenceandmakespecificpredictions.Ratherthanbeingarandomassortmentofcreatures,alllifeonearthshouldshowthesamesignatureofdescentwithmodificationthatwesawamongthebozos.Infact,thestructureoftheentiregeologicalrecordshouldn’tberandom,either.Recentadditionsshouldappearinrelativelyyoungrocklayers.JustasIamamorerecentarrivalthanmygrandfatherinmyfamilytree,sothestructureofthefamilytreeoflifeshouldalsohaveitsparallelsintime.Toseehowbiologistsactuallyreconstructour

relatednesstoothercreatures,weneedtoleavethecircusandreturntothezoowevisitedinthefirstchapterofthebook.

A(LONGER)WALKTHROUGHTHEZOO

Aswe’veseen,ourbodiesarenotputtogetheratrandom.Here,Iusetheword“random”inaveryspecificsense;Imeanthatthestructureofourbodiesisdefinitelynotrandomwithrespecttotheotheranimalsthatwalk,fly,swim,orcrawlacrossthisearth.Someanimalssharepart

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ofourstructure;othersdonot.Thereisordertowhatwesharewiththerestoftheworld.Wehavetwoears,twoeyes,onehead,apairofarms,andapairoflegs.Wedonothavesevenlegsortwoheads.Nordowehavewheels.Awalkinthezooimmediatelyshowsourconnectionsto

therestoflife.Infact,itwillshowthatwecangroupmuchoflifeinthesamewaywedidwiththebozos.Let’sgotojustthreeexhibitsatfirst.Startwiththepolarbears.Youcanmakealonglistofthefeaturesthatyousharewithpolarbears:hair,mammaryglands,fourlimbs,aneck,andtwoeyes,amonglotsofotherthings.Next,considertheturtleacrosstheway.Therearedefinitelysimilarities,butthelistisabitshorter.Yousharefourlimbs,aneck,andtwoeyes(amongotherthings)withtheturtle.Butunlikepolarbearsandyou,turtlesdon’thavehairormammaryglands.Asfortheturtle’sshell,thatseemsuniquetotheturtle,justasthewhitefurwasuniquetothepolarbear.NowvisittheAfricanfishexhibit.Itsinhabitantsarestillsimilartoyou,butthelistofcommonalitiesisevenshorterthanthelistforturtles.Likeyou,fishhavetwoeyes.Likeyou,theyhavefourappendages,butthoseappendageslooklikefins,notarmsandlegs.Fishlack,amongmanyotherfeatures,thehairandmammaryglandsthatyousharewithpolarbears.ThisisbeginningtosoundliketheRussiandollsetof

groups,subgroups,andsub-subgroupsthatappearedinthebozoexample.Fish,turtles,polarbears,andhumansallsharesomefeatures—heads,twoeyes,twoears,andsoon.Turtles,polarbears,andhumanshaveallthesefeatures,

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andtheyalsohavenecksandlimbs,featuresnotseeninfish.Polarbearsandhumansformanevenmoreelitegroup,whosemembershaveallofthesefeaturesandalsohairandmammaryglands.Thebozoexamplegivesusthemeanstomakesenseof

ourwalkthroughthezoo.Inthebozos,thepatternofgroupsreflecteddescentwithmodification.Theimplicationisthatthefull-bozokidssharedamorerecentrelativethantheydowiththekidswhohaveonlyasqueakynose.Thatmakessense:theparentofthesqueaky-nosedkidsisthegreat-greatgrandparentofthefullbozos.Applyingthissameapproachtothegroupsweencounteredduringourzoowalkmeansthathumansandpolarbearsshouldshareamorerecentancestorthantheydowithturtles.Thispredictionistrue:theearliestmammalismuchmorerecentthantheearliestreptile.Thecentralissuehereisdecipheringthefamilytreeof

species.Or,inmoreprecisebiologicalterms,theirpatternofrelatedness.ThispatternevengivesusthemeanstointerpretafossilsuchasTiktaalikinlightofourwalkthroughthezoo.Tiktaalikisawonderfulintermediatebetweenfishandtheirland-livingdescendants,buttheoddsofitbeingourexactancestorareveryremote.Itismorelikeacousinofourancestor.Nosanepaleontologistwouldeverclaimthatheorshehaddiscovered“TheAncestor.”Thinkaboutitthisway:WhatisthechancethatwhilewalkingthroughanyrandomcemeteryonourplanetIwoulddiscoveranactualancestorofmine?Diminishingly

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small.WhatIwoulddiscoveristhatallofthepeopleburiedinthesecemeteries—nomatterwhetherthatcemeteryisinChina,Botswana,orItaly—arerelatedtometodifferentdegrees.IcanfindthisoutbylookingattheirDNAwithmanyoftheforensictechniquesinuseincrimelabstoday.I’dseethatsomeofthedenizensofthecemeteriesaredistantlyrelatedtome,othersarerelatedmoreclosely.Thistreewouldbeaverypowerfulwindowintomypastandmyfamilyhistory.ItwouldalsohaveapracticalapplicationbecauseIcouldusethistreetounderstandmypredilectiontogetcertaindiseasesandotherfactsofmybiology.Thesameistruewhenweinferrelationshipsamongspecies.Therealpowerofthisfamilytreeliesinthepredictionsit

allowsustomake.Chiefamongtheseisthatasweidentifymoresharedcharacteristics,theyshouldbeconsistentwiththeframework.Thatis,asIidentifyfeaturesfromcells,DNA,andalltheotherstructures,tissues,andmoleculesinthebodiesoftheseanimals,theyshouldsupportthegroupingsthatweidentifiedduringourwalk.Conversely,wecanfalsifyourgroupingsbyfindingfeaturesinconsistentwiththem.Thatis,ifthereexistmanytraitssharedbyfishandpeoplethataren’tseeninpolarbears,ourframeworkisflawedandneedstoberevisedorjettisoned.Incaseswheretheevidenceisambiguous,weapplyanumberofstatisticaltoolstoassessthequalityofthecharacteristicssupportingthearrangementsinthefamilytree.Ininstanceswherethereisambiguity,the

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genealogicalarrangementistreatedasaworkinghypothesisuntilwecanfindsomethingconclusivetoallowustoeitheracceptorrejectit.Somegroupingsaresostrongthat,forallintentsand

purposes,weconsiderthemfact.Thefish–turtle–polarbear–humangrouping,forexample,issupportedbycharacteristicsfromhundredsofgenesandvirtuallyallfeaturesoftheanatomy,physiology,andcellularbiologyoftheseanimals.Ourfish-to-humanframeworkissostronglysupportedthatwenolongertrytomarshalevidenceforit—doingsowouldbelikedroppingaballfiftytimestotestthetheoryofgravity.Thesameholdsforourbiologicalexample.Youwouldhavethesamechanceofseeingyourballgoupthefifty-firsttimeyoudroppeditasyouwouldoffindingstrongevidenceagainsttheserelationships.Wecannowreturntotheopeningchallengeofthebook.

Howcanweconfidentlyreconstructtherelationshipsamonglong-deadanimalsandthebodiesandgenesofrecentones?Welookforthesignatureofdescentwithmodification,weaddcharacteristics,weevaluatethequalityoftheevidence,andweassessthedegreetowhichourgroupsarerepresentedinthefossilrecord.Theamazingthingisthatwenowhavetoolstoprobethishierarchy,usingcomputersandlargeDNAsequencinglabstoperformthesameanalysesyouperformedduringyourwalkthroughthezoo.Wenowhaveaccesstonewfossilsitesaroundtheworld.Wecanseeourbodies’placeinthenaturalworldbetterthanweevercould.

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FromChapter1throughChapter10,wehaveshownthatdeepsimilaritiesexistbetweencreatureslivingtodayandthoselongdeceased—ancientworms,livingsponges,andvariouskindsoffish.Now,armedwithknowledgeofthepatternofdescentwithmodification,wecanbegintomakesenseofitall.Enoughfunatthecircusandzoo.It’stimetogetdowntobusiness.Wehaveseenthatinsideourbodiesareconnectionstoamenagerieofothercreatures.Somepartsresemblepartsofjellyfish,otherspartsofworms,stillotherspartsoffish.Thesearen’thaphazardsimilarities.Somepartsofusareseenineveryotheranimal;othersareveryuniquetous.Itisdeeplybeautifultoseethatthereisanorderinallthesefeatures.HundredsofcharactersfromDNA,innumerableanatomicalanddevelopmentalfeatures—allfollowthesamelogicasthebozoswesawearlier.Let’sconsidersomeofthefeatureswe’vealreadytalked

aboutinthebookandshowyouhowtheyareordered.Witheveryotheranimalontheplanet,weshareabody

composedofmanycells.Callthisgroupmulticellularlife.Wesharethetraitofmulticellularitywitheverythingfromspongestoplacozoanstojellyfishtochimpanzees.Asubsetofthesemulticellularanimalshaveabodyplan

likeours,withafrontandaback,atopandabottom,andaleftandaright.TaxonomistscallthisgroupBilateria(meaning“bilaterallysymmetricalanimals”).Itincludeseveryanimalfrominsectstohumans.

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Asubsetofmulticellularanimalsthathaveabodyplanlikeours,withafrontandaback,atopandabottom,andaleftandaright,alsohaveskullsandbackbones.Callthesecreaturesvertebrates.Asubsetofthemulticellularanimalsthathaveabody

planlikeours,withafrontandaback,atopandabottom,andaleftandaright,andthathaveskulls,alsohavehandsandfeet.Callthesevertebratestetrapods(animalswithfourlimbs).Asubsetofthemulticellularanimalsthathaveabody

planlikeours,withafrontandaback,atopandabottom,andaleftandaright,thathaveskulls,andthathavehandsandfeet,alsohaveathree-bonedmiddleear.Callthesetetrapodsmammals.Asubsetofthemulticellularanimalsthathaveabody

planlikeourswithafrontandaback,atopandabottom,andaleftandaright,thathaveskullsandbackbones,thathavehandsandfeet,andthathaveathree-bonedmiddleear,alsohaveabipedalgaitandenormousbrains.Callthesemammalspeople.

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Ahumanfamilytree,allthewaybacktojellyfish.Ithasthesamestructureastheoneforthebozos.

Thepowerofthesegroupingsisseenintheevidenceon

whichtheyarebased.Hundredsofgenetic,embryological,andanatomicalfeaturessupportthem.Thisarrangementallowsustolookinsideourselvesinanimportantway.Thisexerciseisalmostlikepeelinganonion,exposing

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layerafterlayerofhistory.Firstweseefeatureswesharewithallothermammals.Then,aswelookdeeper,wefindthefeatureswesharewithfish.Deeperstillarethosewesharewithworms.Andsoon.Recallingthelogicofthebozos,thismeansthatweseeapatternofdescentwithmodificationdeeplyetchedinsideourownbodies.Thatpatternisreflectedinthegeologicalrecord.Theoldestmany-celledfossilisover600millionyearsold.Theearliestfossilwithathree-bonedmiddleearislessthan200millionyearsold.Theoldestfossilwithabipedalgaitisaround4millionyearsold.Areallthesefactsjustcoincidence,ordotheyreflectalawofbiologywecanseeatworkarounduseveryday?CarlSaganoncefamouslysaidthatlookingatthestarsis

likelookingbackintime.Thestars’lightbeganthejourneytooureyeseonsago,longbeforeourworldwasformed.Iliketothinkthatlookingathumansismuchlikepeeringatthestars.Ifyouknowhowtolook,ourbodybecomesatimecapsulethat,whenopened,tellsofcriticalmomentsinthehistoryofourplanetandofadistantpastinancientoceans,streams,andforests.Changesintheancientatmospherearereflectedinthemoleculesthatallowourcellstocooperatetomakebodies.Theenvironmentofancientstreamsshapedthebasicanatomyofourlimbs.Ourcolorvisionandsenseofsmellhasbeenmoldedbylifeinancientforestsandplains.Andthelistgoeson.Thishistoryisourinheritance,onethataffectsourlivestodayandwilldosointhefuture.

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WHYHISTORYMAKESUSSICK

Mykneewasswollentothesizeagrapefruit,andoneofmycolleaguesfromthesurgerydepartmentwastwistingandbendingittodeterminewhetherIhadstrainedorrippedoneoftheligamentsorcartilagepadsinside.This,andtheMRIscanthatfollowed,revealedatornmeniscus,theprobableresultoftwenty-fiveyearsspentcarryingabackpackoverrocks,boulders,andscreeinthefield.Hurtyourkneeandyouwillalmostcertainlyinjureoneormoreofthreestructures:themedialmeniscus,themedialcollateralligament,ortheanteriorcruciateligament.Soregularareinjuriestothesethreepartsofyourkneethatthesethreestructuresareknownamongdoctorsasthe“UnhappyTriad.”Theyareclearevidenceofthepitfallsofhavinganinnerfish.Fishdonotwalkontwolegs.Ourhumanitycomesatacost.Fortheexceptional

combinationofthingswedo—talk,think,grasp,andwalkontwolegs—wepayaprice.Thisisaninevitableresultofthetreeoflifeinsideus.Imaginetryingtojerry-rigaVolkswagenBeetletotravel

atspeedsof150milesperhour.In1933,AdolfHitlercommissionedDr.FerdinandPorschetodevelopacheapcarthatcouldget40milespergallonofgasandprovideareliableformoftransportationfortheaverageGermanfamily.TheresultwastheVWBeetle.Thishistory,Hitler’splan,placesconstraintsonthewayswecanmodifytheBeetletoday;theengineeringcanbetweakedonlysofar

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beforemajorproblemsariseandthecarreachesitslimit.Inmanyways,wehumansarethefishequivalentofa

hot-rodBeetle.Takethebodyplanofafish,dressituptobeamammal,thentweakandtwistthatmammaluntilitwalksontwolegs,talks,thinks,andhassuperfinecontrolofitsfingers—andyouhavearecipeforproblems.Wecandressupafishonlysomuchwithoutpayingaprice.Inaperfectlydesignedworld—onewithnohistory—wewouldnothavetosuffereverythingfromhemorrhoidstocancer.Nowhereisthishistorymorevisiblethaninthedetours,

twists,andturnsofourarteries,nerves,andveins.Followsomenervesandyou’llfindthattheymakestrangeloopsaroundotherorgans,apparentlygoinginonedirectiononlytotwistandendupinanunexpectedplace.Thedetoursarefascinatingproductsofourpastthat,aswe’llsee,oftencreateproblemsforus—hiccupsandhernias,forexample.Andthisisonlyonewayourpastcomesbacktoplagueus.Ourdeephistorywasspent,atdifferenttimes,inancient

oceans,smallstreams,andsavannahs,notofficebuildings,skislopes,andtenniscourts.Wewerenotdesignedtolivepasttheageofeighty,sitonourkeistersfortenhoursaday,andeatHostessTwinkies,norwerewedesignedtoplayfootball.Thisdisconnectbetweenourpastandourhumanpresentmeansthatourbodiesfallapartincertainpredictableways.Virtuallyeveryillnesswesufferhassomehistorical

component.Theexamplesthatfollowreflecthowdifferentbranchesofthetreeoflifeinsideus—fromancienthumans,

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toamphibiansandfish,andfinallytomicrobes—comebacktopesterustoday.Eachoftheseexamplesshowthatwewerenotdesignedrationally,butareproductsofaconvolutedhistory.

OURHUNTER-GATHERERPAST:OBESITY,HEARTDISEASE,ANDHEMORRHOIDS

Duringourhistoryasfishwewereactivepredatorsinancientoceansandstreams.Duringourmorerecentpastasamphibians,reptiles,andmammals,wewereactivecreaturespreyingoneverythingfromreptilestoinsects.Evenmorerecently,asprimates,wewereactivetree-livinganimals,feedingonfruitsandleaves.Earlyhumanswereactivehunter-gatherersand,ultimately,agriculturalists.Didyounoticeathemehere?Thatcommonthreadistheword“active.”Thebadnewsisthatmostofusspendalargeportionof

ourdaybeinganythingbutactive.Iamsittingonmybehindatthisveryminutetypingthisbook,andanumberofyouaredoingthesamereadingit(exceptforthevirtuousamonguswhoarereadingitinthegym).Ourhistoryfromfishtoearlyhumaninnowaypreparedusforthisnewregimen.Thiscollisionbetweenpresentandpasthasitssignatureinmanyoftheailmentsofmodernlife.Whataretheleadingcausesofdeathinhumans?Fourof

thetoptencauses—heartdisease,diabetes,obesity,andstroke—havesomesortofgeneticbasisand,likely,a

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historicalone.Muchofthedifficultyisalmostcertainlyduetoourhavingabodybuiltforanactiveanimalbutthelifestyleofaspud.In1962,theanthropologistJamesNeeladdressedthis

notionfromtheperspectiveofourdiet.Formulatingwhatbecameknownasthe“thriftygenotype”hypothesis,Neelsuggestedthatourhumanancestorswereadaptedforaboom-bustexistence.Ashunter-gatherers,earlyhumanswouldhaveexperiencedperiodsofbounty,whenpreywascommonandhuntingsuccessful.Theseperiodsofplentywouldbepunctuatedbytimesofscarcity,whenourancestorshadconsiderablylesstoeat.Neelhypothesizedthatthiscycleoffeastandfaminehad

asignatureinourgenesandinourillnesses.Essentially,heproposedthatourancestors’bodiesallowedthemtosaveresourcesduringtimesofplentysoastousethemduringperiodsoffamine.Inthiscontext,fatstoragebecomesveryuseful.Theenergyinthefoodweeatisapportionedsothatsomesupportsouractivitiesgoingonnow,andsomeisstored,forexampleinfat,tobeusedlater.Thisapportionmentworkswellinaboom-bustworld,butitfailsmiserablyinanenvironmentwhererichfoodsareavailable24/7.Obesityanditsassociatedmaladies—age-relateddiabetes,highbloodpressure,andheartdisease—becomethenaturalstateofaffairs.Thethriftygenotypehypothesisalsomightexplainwhywelovefattyfoods.Theyarehigh-valueintermsofhowmuchenergytheycontain,somethingthatwouldhaveconferredadistinctadvantageinour

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distantpast.Oursedentarylifestyleaffectsusinotherways,because

ourcirculatorysystemoriginallyappearedinmoreactiveanimals.Ourheartpumpsblood,whichiscarriedtoourorgansvia

arteriesandreturnedtotheheartbywayofveins.Becausearteriesareclosertothepump,thebloodpressureinthemismuchhigherthaninveins.Thiscanbeaparticularproblemforthebloodthatneedstoreturntoourheartfromourfeet.Bloodfromthefeetneedstogouphill,sotospeak,uptheveinsofourlegstoourchest.Ifthebloodisunderlowpressure,itmaynotclimballtheway.Consequently,wehavetwofeaturesthathelpthebloodmoveup.Thefirstarelittlevalvesthatpermitthebloodtomoveupbutstopitfromgoingdown.Theotherfeatureisourlegmuscles.Whenwewalkwecontractthem,andthiscontractionservestopumpthebloodupourlegveins.Theone-wayvalvesandtheleg-musclepumpsenableourbloodtoclimbfromfeettochest.Thissystemworkssuperblyinanactiveanimal,which

usesitslegstowalk,run,andjump.Itdoesnotworkwellinamoresedentarycreature.Ifthelegsarenotusedmuch,themuscleswillnotpumptheblooduptheveins.Problemscandevelopifbloodpoolsintheveins,becausethatpoolingcancausethevalvestofail.Thisisexactlywhathappenswithvaricoseveins.Asthevalvesfail,bloodpoolsintheveins.Theveinsgetbiggerandbigger,swellingandtakingtortuouspathsinourlegs.

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Needlesstosay,thearrangementofveinscanalsobearealpaininthebehind.Truckdriversandotherswhositforlongstretchesoftimeareparticularlypronetohemorrhoids,anothercostofoursedentarylives.Duringtheirlonghoursofsitting,bloodpoolsintheveinsandspacesaroundtherectum.Asthebloodpools,hemorrhoidsform—anunpleasantreminderthatwewerenotbuilttositfortoolong,particularlynotonsoftsurfaces.

PRIMATEPAST:TALKISNOTCHEAP

Talkingcomesatasteepprice:chokingandsleepapneaarehighonthelistofproblemswehavetolivewithinordertobeabletotalk.Weproducespeechsoundsbycontrollingmotionsofthe

tongue,thelarynx,andthebackofthethroat.Allofthesearerelativelysimplemodificationstothebasicdesignofamammalorareptile.AswesawinChapter5,thehumanlarynxismadeupmostlyofgillarchcartilages,correspondingtothegillbarsofasharkorfish.Thebackofthethroat,extendingfromthelastmolartoothtojustabovethevoicebox,hasflexiblewallsthatcanopenandclose.Wemakespeechsoundsbymovingourtongue,bychangingtheshapeofourmouth,andbycontractinganumberofmusclesthatcontroltherigidityofthiswall.Sleepapneaisapotentiallydangeroustrade-offforthe

abilitytotalk.Duringsleep,themusclesofourthroatrelax.

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Inmostpeople,thisdoesnotpresentaproblem,butinsomethepassagecancollapsesothatrelativelylongstretchespasswithoutabreath.This,ofcourse,canbeverydangerous,particularlyinpeoplewhohaveheartconditions.Theflexibilityofourthroat,sousefulinourabilitytotalk,makesussusceptibletoaformofsleepapneathatresultsfromobstructionoftheairway.Anothertrade-offofthisdesignischoking.Ourmouth

leadsbothtothetrachea,throughwhichwebreathe,andtoouresophagus,soweusethesamepassagetoswallow,breathe,andtalk.Thesethreefunctionscanbeatodds,forexamplewhenapieceoffoodgetslodgedinthetrachea.

FISHANDTADPOLEPAST:HICCUPS

Thisannoyancehasitsrootsinthehistorywesharewithfishandtadpoles.Ifthereisanyconsolationforgettinghiccups,itisthat

ourmiseryissharedwithmanyothermammals.Catscanbestimulatedtohiccupbysendinganelectricalimpulsetoasmallpatchoftissueintheirbrainstem.Thisareaofthebrainstemisthoughttobethecenterthatcontrolsthecomplicatedreflexthatwecallahiccup.Thehiccupreflexisastereotypedtwitchinvolvinga

numberofmusclesinourbodywall,diaphragm,neck,andthroat.Aspasminoneortwoofthemajornervesthatcontrolbreathingcausesthesemusclestocontract.This

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resultsinaverysharpinspirationofair.Then,about35millisecondslater,aflapoftissueinthebackofourthroat(theglottis)closesthetopofourairway.Thefastinhalationfollowedbyabriefclosureofthetubeproducesthe“hic.”Theproblemisthatwerarelyexperienceonlyasingle

hic.Stopthehiccupsinthefirstfivetotenhics,andyouhaveadecentchanceofendingtheboutaltogether.Missthatwindow,andtheboutofhiccupscanpersistforanaverageofaboutsixtyhics.Inhalingcarbondioxide(bybreathingintotheclassicpaperbag)andstretchingthebodywall(takingabiginhalationandholdingit)canendhiccupsearlyinsomeofus.Butnotall.Somecasesofpathologicalhiccupscanbeextremelyprolonged.Thelongestuninterruptedhiccupsinapersonlastedfrom1922to1990.Ourtendencytodevelophiccupsisanotherinfluenceof

ourpast.Therearetwoissuestothinkabout.Thefirstiswhatcausesthespasmofnervesthatinitiatesthehiccup.Thesecondiswhatcontrolsthatdistinctivehic,theabruptinhalation–glottisclosure.Thenervespasmisaproductofourfishhistory,whilethehicisanoutcomeofthehistorywesharewithanimalssuchastadpoles.First,fish.Ourbraincancontrolourbreathingwithout

anyconsciouseffortonourpart.Mostoftheworktakesplaceinthebrainstem,attheboundarybetweenthebrainandthespinalcord.Thebrainstemsendsnerveimpulsestoourmainbreathingmuscles.Breathinghappensinapattern.Musclesofthechest,diaphragm,andthroat

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contractinawell-definedorder.Consequently,thispartofthebrainstemisknownasa“centralpatterngenerator.”Thisregioncanproducerhythmicpatternsofnerveand,consequently,muscleactivation.Anumberofsuchgeneratorsinourbrainandspinalcordcontrolotherrhythmicbehaviors,suchasswallowingandwalking.Theproblemisthatthebrainstemoriginallycontrolled

breathinginfish;ithasbeenjerry-riggedtoworkinmammals.Sharksandbonyfishallhaveaportionofthebrainstemthatcontrolstherhythmicfiringofmusclesinthethroatandaroundthegills.Thenervesthatcontroltheseareasalloriginateinawell-definedportionofthebrainstem.Wecanevenseethisnervearrangementinsomeofthemostprimitivefishinthefossilrecord.Ancientostracoderms,fromrocksover400millionyearsold,preservecastsofthebrainandcranialnerves.Justasinlivingfish,thenervesthatcontrolbreathingextendfromthebrainstem.Thisworkswellinfish,butitisalousyarrangementfor

mammals.Infish,thenervesthatcontrolbreathingdonothavetotravelveryfarfromthebrainstem.Thegillsandthroatgenerallysurroundthisareaofthebrain.Wemammalshaveadifferentproblem.Ourbreathingiscontrolledbymusclesinthewallofourchestandbythediaphragm,thesheetofmusclethatseparatesourchestfromourabdomen.Contractionofthediaphragmcontrolsinspiration.Thenervesthatcontrolthediaphragmexitourbrainjustastheydoinfish,andtheyleavefromthebrain

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stem,nearourneck.Thesenerves,thevagusandthephrenicnerve,extendfromthebaseoftheskullandtravelthroughthechestcavitytoreachthediaphragmandtheportionsofthechestthatcontrolbreathing.Thisconvolutedpathcreatesproblems;arationaldesignwouldhavethenervestravelingnotfromtheneckbutfromnearerthediaphragm.Unfortunately,anythingthatinterfereswithoneofthesenervescanblocktheirfunctionorcauseaspasm.Iftheoddcourseofournervesisaproductofourfishy

past,thehiccupitselfislikelytheproductofourhistoryasamphibians.Hiccupsareuniqueamongourbreathingbehaviorsinthatanabruptintakeofairisfollowedbyaclosureoftheglottis.Hiccupsseemtobecontrolledbyacentralpatterngeneratorinthebrainstem:stimulatethisregionwithanelectricalimpulse,andwestimulatehiccups.Itmakessensethathiccupsarecontrolledbyacentralpatterngenerator,since,asinotherrhythmicbehaviors,asetsequenceofeventshappensduringahic.Itturnsoutthatthepatterngeneratorresponsiblefor

hiccupsisvirtuallyidenticaltooneinamphibians.Andnotinjustanyamphibians—intadpoles,whichusebothlungsandgillstobreathe.Tadpolesusethispatterngeneratorwhentheybreathewithgills.Inthatcircumstance,theywanttopumpwaterintotheirmouthandthroatandacrossthegills,buttheydonotwantthewatertoentertheirlungs.Topreventitfromdoingso,theyclosetheglottis,theflapthatclosesoffthebreathingtube.Andtoclosetheglottis,

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tadpoleshaveacentralpatterngeneratorintheirbrainstemsothataninspirationisfollowedimmediatelybyaclosingglottis.Theycanbreathewiththeirgillsthankstoanextendedformofhiccup.Theparallelsbetweenourhiccupsandgillbreathingin

tadpolesaresoextensivethatmanyhaveproposedthatthetwophenomenaareoneandthesame.Gillbreathingintadpolescanbeblockedbycarbondioxide,justlikeourhiccups.Wecanalsoblockgillbreathingbystretchingthewallofthechest,justaswecanstophiccupsbyinhalingdeeplyandholdingourbreath.Perhapswecouldevenblockgillbreathingintadpolesbyhavingthemdrinkaglassofwaterupsidedown.

SHARKPAST:HERNIAS

Ourpropensityforhernias,atleastforthoseherniasnearthegroin,resultsfromtakingafishbodyandmorphingitintoamammal.Fishhavegonadsthatextendtowardtheirchest,

approachingtheirheart.Mammalsdon’t,andthereinliestheproblem.Itisaverygoodthingthatourgonadsarenotdeepinourchestandnearourheart(althoughitmightmakerecitingthePledgeofAllegianceadifferentexperience).Ifourgonadswereinourchest,wewouldn’tbeabletoreproduce.Slitthebellyofasharkfrommouthtotail.Thefirstthing

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you’llseeisliver,alotofit.Theliverofasharkisgigantic.Somezoologistsbelievethatalargelivercontributestothebuoyancyoftheshark.Movetheliverawayandyou’llfindthegonadsextendingupneartheheart,inthe“chest”area.Thisarrangementistypicalofmostfish:thegonadslietowardthefrontofthebody.Inus,asinmostmammals,thisarrangementwouldbea

disaster.Malescontinuouslyproducespermthroughoutourlives.Spermarefinickylittlecellsthatneedexactlytherightrangeoftemperaturestodevelopcorrectlyforthethreemonthstheylive.Toohot,andspermaremalformed;toocold,andtheydie.Malemammalshaveaneatlittledeviceforcontrollingthetemperatureofthesperm-makingapparatus:thescrotum.Asweallknow,themalegonadssitinasac.Insidetheskinofthesacaremusclesthatcanexpandandcontractasthetemperaturechanges.Musclesalsolieinourspermcords.Hence,thecoldshowereffect:thescrotumwilltuckclosetothebodywhenitiscold.Thewholepackagerisesandfallswithtemperature.Thisisallawaytooptimizetheproductionofhealthysperm.Thedanglingscrotumalsoservesasasexualsignalin

manymammals.Betweenthephysiologicaladvantagesofhavinggonadsoutsidethebodywall,andtheoccasionalbenefitsthisprovidesinsecuringmates,thereareampleadvantagesforourdistantmammalianancestorsinhavingascrotum.

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Openasharkandyoufindahugeliver(top).Pushtheliverasideandyouseegonads,whichextendrelativelyclosetotheheart,astheydoinotherprimitivecreatures.PhotoscourtesyofDr.StevenCampana,CanadianSharkResearchLaboratory.

Theproblemwiththisarrangementisthattheplumbing

thatcarriesspermtothepenisiscircuitous.Spermtravelfromthetestesinthescrotumthroughthespermcord.Thecordleavesthescrotum,travelsuptowardthewaist,loopsoverthepelvis,thengoesthroughthepelvistotravelthroughthepenisandout.Alongthiscomplexpath,thespermgainseminalfluidsfromanumberofglandsthat

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connecttothetube.Thereasonforthisabsurdrouteliesinour

developmentalandevolutionaryhistory.Ourgonadsbegintheirdevelopmentinmuchthesameplaceasashark’s:upnearourlivers.Astheygrowanddevelop,ourgonadsdescend.Infemales,theovariesdescendfromthemidsectiontolieneartheuterusandfallopiantubes.Thisensuresthattheeggdoesnothavefartotraveltobefertilized.Inmales,thedescentgoesfarther.Thedescentofthegonads,particularlyinmales,createsa

weakspotinthebodywall.Toenvisionwhathappenswhenthetestesandspermaticcorddescendtoformascrotum,imaginepushingyourfistagainstarubbersheet.Inthisexample,yourfistbecomesequivalenttothetestesandyourarmtothespermaticcord.Theproblemisthatyouhavecreatedaweakspacewhereyourarmsits.Whereoncetherubbersheetwasasimplewall,you’venowmadeanotherspace,betweenyourarmandtherubbersheet,wherethingscanslip.Thisisessentiallywhathappensinmanytypesofinguinalherniasinmen.Someoftheseinguinalherniasarecongenital—whenapieceoftheguttravelswiththetestesasitdescends.Anotherkindofinguinalherniaisacquired.Whenwecontractourabdominalmuscles,ourgutspushagainstthebodywall.Aweaknessinthebodywallmeansthatgutscanescapethebodycavityandbesqueezedtolienexttothespermaticcord.Femalesarefartougherthanmales,particularlyinthis

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partofthebody.Becausefemalesdonothaveagianttuberunningthroughit,theirabdominalwallismuchstrongerthanaman’s.Thisisagoodthingwhenyouthinkoftheenormousstressesthatfemalebodywallsgothroughduringpregnancyandchildbirth.Atubethroughthebodywalljustwouldn’tdo.Men’stendencytodevelopherniasisatrade-offbetweenourfishancestryandourmammalpresent.

Thedescentofthetestes.Duringgrowth,thetestes

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descendfromthegonads’primitivepositionhighupinthebody.Theyenduplyinginthescrotum,whichisanoutpocketofthebodywall.Allofthisleavesthebodywallofhumanmalesweakinthegroinarea.

MICROBIALPAST:MITOCHONDRIALDISEASES

Mitochondriaexistinsideeverycellofourbodies,doingaremarkablenumberofthings.Theirmostobviousjobistoturnoxygenandsugarsintoakindofenergywecanuseinsideourcells.Otherfunctionsincludemetabolizingtoxinsinourliversandregulatingdifferentpartsofthefunctionofourcells.Wenoticeourmitochondriaonlywhenthingsgowrong.Unfortunately,thelistofdiseasescausedbymalfunctioningmitochondriaisextraordinarilylongandcomplex.Ifthereisaprobleminthechemicalreactionsinwhichoxygenisconsumed,energyproductioncanbeimpaired.Themalfunctionmaybeconfinedtoindividualtissues,saytheeyes,ormayaffecteverysysteminthebody.Dependingonthelocationandseverityofthemalfunction,itcanleadtoanythingfromweaknesstodeath.Manyoftheprocessesweusetolivereflectour

mitochondria’shistory.Thechainreactionofchemicaleventsthatturnssugarsandoxygenintousableenergyandcarbondioxidearosebillionsofyearsago,andversionsofit

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arestillseenindiversemicrobes.Mitochondriacarrythisbacterialpastinsideofthem:withanentiregeneticstructureandcellularmicrostructuresimilartobacteria,itisgenerallyacceptedthattheyoriginallyarosefromfree-livingmicrobesoverabillionyearsago.Infact,theentireenergy-generatingmachineryofourmitochondriaaroseinoneofthesekindsofancientbacteria.Thebacterialpastcanbeusedtoouradvantagein

studyingthediseasesofmitochondria—infact,someofthebestexperimentalmodelsforthesediseasesarebacteria.Thisispowerfulbecausewecandoallkindsofexperimentswithbacteriathatarenotpossiblewithhumancells.OneofthemostprovocativestudieswasdonebyateamofscientistsfromItalyandGermany.Thediseasethattheystudiedinvariablykillstheinfantswhoarebornwithit.Calledcardioencephalomyopathy,itresultsfromageneticchangethatinterruptsthenormalmetabolicfunctionofmitochondria.Instudyingapatientwhohadthedisease,theEuropeanteamidentifiedaplaceintheDNAthathadasuspiciouschange.Knowingsomethingaboutthehistoryoflife,theythenturnedtothemicrobeknownasParacoccusdenitrificans,whichisoftencalledafree-livingmitochondrionbecauseitsgenesandchemicalpathwaysaresosimilartothoseofmitochondria.JusthowsimilarwasrevealedbytheEuropeanteam.Theyproducedthesamechangeinthebacteria’sgenesthattheysawintheirhumanpatient.Whattheyfoundmakestotalsense,onceweknowourhistory.Theywereabletosimulatepartsofa

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humanmitochondrialdiseaseinabacterium,withvirtuallythesamechangeinmetabolism.Thisisputtingamany-billion-yearpartofourhistorytoworkforus.Theexamplefrommicrobesisnotunique.Judgingbythe

NobelPrizesawardedinmedicineandphysiologyinthepastthirteenyears,IshouldhavecalledthisbookYourInnerFly,YourInnerWorm,orYourInnerYeast.Pioneeringresearchonflieswonthe1995NobelPrizeinmedicineforuncoveringasetofgenesthatbuildsbodiesinhumansandotheranimals.Nobelsinmedicinein2002and2006wenttopeoplewhomadesignificantadvancesinhumangeneticsandhealthbystudyinganinsignificant-lookinglittleworm(C.elegans).Similarly,in2001,elegantanalysesofyeast(includingbaker’syeast)andseaurchinswontheNobelinmedicineforincreasingourunderstandingofsomeofthebasicbiologyofallcells.Thesearenotesotericdiscoveriesmadeonobscureandunimportantcreatures.Thesediscoveriesonyeast,flies,worms,and,yes,fishtellusabouthowourownbodieswork,thecausesofmanyofthediseaseswesuffer,andwayswecandeveloptoolstomakeourliveslongerandhealthier.

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EPILOGUE

Asaparentoftwoyoungchildren,Ifindmyselfspendingalotoftimelatelyinzoos,museums,andaquaria.Beingavisitorisastrangeexperience,becauseI’vebeeninvolvedwiththeseplacesfordecades,workinginmuseumcollectionsandevenhelpingtoprepareexhibitsonoccasion.Duringfamilytrips,I’vecometorealizehowmuchmyvocationcanmakemenumbtothebeautyandsublimecomplexityofourworldandourbodies.Iteachandwriteaboutmillionsofyearsofhistoryandaboutbizarreancientworlds,andusuallymyinterestisdetachedandanalytic.NowI’mexperiencingsciencewithmychildren—inthekindsofplaceswhereIdiscoveredmyloveforitinthefirstplace.Onespecialmomenthappenedrecentlywithmysonat

theMuseumofScienceandIndustryinChicago.We’vegonethereregularlyoverthepastthreeyearsbecauseofhisloveoftrainsandthefactthatthereisahugemodelrailroadsmackinthecenteroftheplace.I’vespentcountlesshoursatthatoneexhibittracingmodellocomotivesontheirlittletrekfromChicagotoSeattle.Afteranumberofweeklyvisits

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tothisshrineforthetrain-obsessed,NathanielandIwalkedtocornersofthemuseumwehadfailedtovisitduringourtrain-watchingventuresoroccasionalforaystothefull-sizetractorsandplanes.Inthebackofthemuseum,intheHenryCrownSpaceCenter,modelplanetshangfromtheceilingandspacesuitslieincasestogetherwithothermemorabiliaofthespaceprogramofthe1960sand1970s.IwasunderthepresumptionthatinthebackofthemuseumIwouldseethetriviathatdidn’tmakeittothemajorexhibitsupfront.Onedisplayconsistedofabatteredspacecapsulethatyoucouldwalkaroundandlookinside.Itdidn’tlooksignificant;itseemedwaytoosmallandjerry-riggedtobeanythingreallyimportant.Theplacardwasstrangelyformal,andIhadtoreaditseveraltimesbeforeitdawnedonme:herewastheoriginalCommandModulefromApollo8,theactualvesselthatcarriedJamesLovell,FrankBorman,andWilliamAndersonhumanity’sfirsttriptothemoonandback.ThiswasthespacecraftwhosepathIfollowedduringChristmasbreakinthirdgrade,andhereIwasthirty-eightyearslaterwithmyownson,lookingattherealthing.Ofcourseitwasbattered.Icouldseethescarsofitsjourneyandsubsequentreturntoearth.Nathanielwascompletelydisinterested,soIgrabbedhimandtriedtoexplainwhatitwas.ButIcouldn’tspeak;myvoicebecamesochokedwithemotionthatIcouldbarelyutterasingleword.Afterafewminutes,Iregainedmycomposureandtoldhimthestoryofman’striptothemoon.ButthestoryIcan’ttellhimuntilheisolderiswhyI

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becamespeechlessandemotional.TherealstoryisthatApollo8isasymbolforthepowerofsciencetoexplainandmakeouruniverseknowable.Peoplecanquibbleovertheextenttowhichthespaceprogramwasaboutscienceorpolitics,butthecentralfactremainsascleartodayasitwasin1968:Apollo8wasaproductoftheessentialoptimismthatfuelsthebestscience.Itexemplifieshowtheunknownshouldnotbeasourceofsuspicion,fear,orretreattosuperstition,butmotivationtocontinueaskingquestionsandseekinganswers.Justasthespaceprogramchangedthewaywelookatthe

moon,paleontologyandgeneticsarechangingthewayweviewourselves.Aswelearnmore,whatonceseemeddistantandunattainablecomeswithinourcomprehensionandourgrasp.Weliveinanageofdiscovery,whenscienceisrevealingtheinnerworkingsofcreaturesasdifferentasjellyfish,worms,andmice.Wearenowseeingtheglimmerofasolutiontooneofthegreatestmysteriesofscience—thegeneticdifferencesthatmakehumansdistinctfromotherlivingcreatures.Couplethesepowerfulnewinsightswiththefactthatsomeofthemostimportantdiscoveriesinpaleontology—newfossilsandnewtoolstoanalyzethem—havecometolightinthepasttwentyyears,andweareseeingthetruthsofourhistorywithever-increasingprecision.Lookingbackthroughbillionsofyearsofchange,everythinginnovativeorapparentlyuniqueinthehistoryoflifeisreallyjustoldstuffthathasbeenrecycled,recombined,repurposed,orotherwisemodifiedfornew

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uses.Thisisthestoryofeverypartofus,fromoursenseorganstoourheads,indeedourentirebodyplan.Whatdobillionsofyearsofhistorymeanforourlives

today?Answerstofundamentalquestionsweface—abouttheinnerworkingsofourorgansandourplaceinnature—willcomefromunderstandinghowourbodiesandmindshaveemergedfrompartscommontootherlivingcreatures.Icanimaginefewthingsmorebeautifulorintellectuallyprofoundthanfindingthebasisforourhumanity,andremediesformanyoftheillswesuffer,nestledinsidesomeofthemosthumblecreaturesthathaveeverlivedonourplanet.

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NOTES,REFERENCES,ANDFURTHERREADING

CHAPTERONEFINDINGANINNERFISH

Ihaveincludedamixofprimaryandsecondarysourcesforthoseinterestedinpursuingthetopicsinthebookfurther.Foraccountsthatuseexploratorypaleontologicalexpeditionsasavehicletodiscussmajorquestionsinbiologyandgeology,seeMikeNovacek’sDinosaursoftheFlamingCliffs(NewYork:Anchor,1997),AndrewKnoll’sLifeonaYoungPlanet(Princeton:PrincetonUniversityPress,2002),andJohnLong’sSwimminginStone(Melbourne:FreemantlePress,2006).Allbalancescientificanalysiswithdescriptionsofdiscoveryinthefield.ThecomparativemethodsthatIdiscuss,includingthe

methodsusedinourwalkthroughthezoo,arethemethodsofcladistics.AsuperboverviewisHenryGee’sInSearchofDeepTime(NewYork:FreePress,1999).Basically,Ipresentaversionofthethree-taxonstatement,thestartingpointforcladisticcomparisons.AgoodtreatmentwithbackgroundsourcesisfoundinRichardForeyetal.,“TheLungfish,theCoelacanthandtheCowRevisited,”inH.-P.

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SchultzeandL.Trueb,eds.,OriginoftheHigherGroupsofTetrapods(Ithaca,N.Y.:CornellUniversityPress,1991).Thecorrelationbetweenthefossilrecordandour“walk

throughthezoo”isdiscussedinseveralpapers.Asampling:Benton,M.J.,andHitchin,R.(1997)Congruencebetweenphylogeneticandstratigraphicdatainthehistoryoflife,ProceedingsoftheRoyalSocietyofLondon,B264:885–890;Norell,M.A.,andNovacek,M.J.(1992)Congruencebetweensuperpositionalandphylogeneticpatterns:Comparingcladisticpatternswithfossilrecords,Cladistics8:319–337;Wagner,P.J.,andSidor,C.(2000)Agerank/claderankmetrics—sampling,taxonomy,andthemeaningof“stratigraphicconsistency,”SystematicBiology49:463–479.Thelayersoftherockcolumnandthefossilscontained

thereinarebeautifullyandcomprehensivelydiscussedinRichardFortey’sLife:ANaturalHistoryoftheFirstFourBillionYearsofLifeonEarth(NewYork:Knopf,1998).ResourcesforvertebratepaleontologyincludeR.Carroll,VertebratePaleontologyandEvolution(SanFrancisco:W.H.Freeman,1987),andM.J.Benton,VertebratePaleontology(London:Blackwell,2004).Fortheoriginoftetrapods:CarlZimmerreviewedthe

stateoftheartinthefieldinhishighlyreadableandwell-researchedAttheWater’sEdge(NewYork:FreePress,1998).JennyClackhaswrittenthedefinitivetextonthewholetransition,GainingGround(Bloomington:IndianaUniversityPress,2002).Thebibleofthistransition,Clack’sbookwillbringanovicetoexpertstatusquickly.

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OuroriginalpapersdescribingTiktaalikappearedintheApril6,2006,issueofNature.Thereferencesare:Daeschleretal.(2006)ADevoniantetrapod-likefishandtheoriginofthetetrapodbodyplan,Nature757:757–763;Shubinetal.(2006)ThepectoralfinofTiktaalikroseaeandtheoriginofthetetrapodlimb,Nature757:764–771.JennyClackandPerAhlberghadaveryreadableandcomprehensivecommentarypieceinthesameissue(Nature757:747–749).Everythingaboutourpastisrelative,eventhestructure

ofthisbook.Icouldhavecalledthisbook“OurInnerHuman”andwrittenitfromafish’spointofview.Thestructureofthatbookwouldhavebeenstrangelysimilar:afocusonthehistoryhumansandfishshareinbodies,brains,andcells.Aswe’veseen,alllifesharesadeeppartofitshistorywithotherspecies,whileanotherpartofitshistoryisunique.

CHAPTERTWOGETTINGAGRIP

Owenwasbynomeansthefirstpersontoseethepatternofonebone–twobones–lotsablobs–digits.Vicq-d’Azyrinthe1600sandGeoffroySt.Hilaire(1812)alsomadethispatternpartoftheirworldviews.WhatdistinguishedOwenwashisconceptofthearchetype.Thiswasatranscendentalorganizationofthebody,reflectingthedesignoftheCreator.St.Hilairewassearchinglessforanarchetypicalpatternhiddeninallstructurethanfor“lawsofform”that

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governtheformationofbodies.AnicetreatmentoftheseissuesisinT.Appel,TheCuvier-GeoffroyDebate:FrenchBiologyintheDecadesBeforeDarwin(NewYork:OxfordUniversityPress,1987),andE.S.Russell,FormandFunction:AContributiontotheHistoryofMorphology(Chicago:UniversityofChicagoPress,1982).ArecentvolumeeditedbyBrianHallisone-stop

shoppingforinformationonlimbdiversityanddevelopmentandcontainsanumberofimportantpapersondifferentkindsoflimbs:BrianK.Hall,ed.,FinsintoLimbs:Evolution,Development,andTransformation(Chicago:UniversityofChicagoPress,2007).UsefulreferencesforexploringtheshiftfromfinsandlimbsinmoredetailincludeShubinetal.(2006)ThepectoralfinofTiktaalikroseaeandtheoriginofthetetrapodlimb,Nature757:764–771;Coates,M.I.,Jeffery,J.E.,andRuta,M.(2002)Finstolimbs:whatthefossilssay,EvolutionandDevelopment4:390–412.

CHAPTERTHREEHANDYGENES

Thedevelopmentalbiologyoflimbdiversityhasseenanumberofreviewsandprimarypapers.ForareviewoftheclassicliteratureseeShubin,N.,andAlberch,P.(1986)Amorphogeneticapproachtotheoriginandbasicorganizationofthetetrapodlimb,EvolutionaryBiology20:319–387;andHinchliffe,J.R.,andGriffiths,P.,“ThePre-chondrogenicPatternsinTetrapodLimbDevelomentand

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TheirPhylogeneticSignificance,”inB.Goodwin,N.Holder,andC.Wylie,eds.,DevelopmentandEvolution(Cambridge,Eng.:CambridgeUniversityPress,1983),pp.99–121.Saunders’sandZwilling’sexperimentsarenowclassic,sosomeofthebestaccountsarenowseeninthemajortextbooksindevelopmentalbiology.TheseincludeS.Gilbert,DevelopmentalBiology,8thed.(Sunderland,Mass.:SinauerAssociates,2006);L.Wolpert,J.Smith,T.Jessell,F.Lawrence,E.Robertson,andE.Meyerowitz,PrinciplesofDevelopment(Oxford,Eng.:OxfordUniversityPress,2006).ForthefirstpaperdescribingSonichedgehog’srolein

limbpatterning,gotoRiddle,R.,Johnson,R.L.,Laufer,E.,Tabin,C.(1993)SonichedgehogmediatesthepolarizingactivityoftheZPA,Cell75:1401–1416.Randy’sworkonSonicsignalinginsharkandskatefinsis

inDahn,R.,Davis,M.,Pappano,W.,Shubin,N.(2007)Sonichedgehogfunctioninchondrichthyanfinsandtheevolutionofappendagepatterning,Nature445:311–314.Subsequentworkfromthelabontheoriginoflimbs,atleastfromageneticperspective,iscontainedinDavis,M.,Dahn,R.,andShubin,N.(2007)Alimbautopodial-likepatternofHoxexpressioninabasalactinopterygianfish,Nature447:473–476.Thestunninggeneticsimilaritiesinthedevelopmentof

flies,chickens,andhumansisdiscussedinShubin,N.,Tabin,C.,Carroll,S.(1997)Fossils,genes,andtheevolutionofanimallimbs,Nature388:639–648;andErwin,D.andDavidson,E.H.(2003)Thelastcommonbilaterian

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ancestor,Development129:3021–3032.

CHAPTERFOURTEETHEVERYWHERE

Theimportanceofteethtoanunderstandingofmammalsisevidentinthemanytreatmentsinthefield.Dentalstructureplaysaparticularlyimportantroleinunderstandingtheearlyrecordofmammals.ExtensivereviewsarefoundinZ.Kielan-Jaworowska,R.L.Cifelli,andZ.Luo,MammalsfromtheAgeofDinosaurs(NewYork:ColumbiaUniversityPress,2004);andJ.A.Lillegraven,Z.Kielan-Jaworowska,andW.Clemens,eds.,MesozoicMammals:TheFirstTwo-ThirdsofMammalianHistory(Berkeley:UniversityofCaliforniaPress,1979),p.311.Farish’smammalfromArizonaisanalyzedinJenkins,F.

A.,Jr.,Crompton,A.W.,Downs,W.R.(1983)MesozoicmammalsfromArizona:Newevidenceonmammalianevolution,Science222:1233–1235.ThetritheledontswefoundinNovaScotiaaredescribed

inShubin,N.,Crompton,A.W.,Sues,H.-D.,andOlsen,P.(1991)Newfossilevidenceonthesister-groupofmammalsandearlyMesozoicfaunaldistributions,Science251:1063–1065.Arecentreviewontheoriginofteeth,bone,andskulls,in

particularthenewevolutiongleanedfromconodontanimals,isfoundinDonoghue,P.,andSansomI.(2002)Originandearlyevolutionofvertebrateskeletonization,MicroscopyResearchandTechnique59:352–372.A

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thoroughreviewoftheevolutionaryrelationshipsamongconodontsandtheirsignificanceisinDonoghue,P.,Forey,P.,andAldridge,R.(2000)Conodontaffinityandchordatephylogeny,BiologicalReviews75:191–251.

CHAPTERFIVEGETTINGAHEAD

Awonderfullycomprehensiveanddetailedtreatmentofthedetailsofskullstructure,development,andevolutionisfoundinathree-volumeset:TheSkull,JamesHankenandBrianHall,eds.(Chicago:UniversityofChicagoPress,1993).Thisisamulti-authorupdateofoneoftheclassicvolumesonheaddevelopmentandstructure:G.R.deBeer,TheDevelopmentoftheVertebrateSkull(Oxford,Eng.:OxfordUniversityPress,1937).Detailsofheaddevelopmentandstructureinhumanscan

befoundintextsonhumananatomyandembryology.Forembryology,seeK.MooreandT.V.N.Persaud,TheDevelopingHuman,7thed.(Philadelphia:Elsevier,2006).ThecompanionanatomytextisK.MooreandA.F.Dalley,ClinicallyOrientedAnatomy(Philadelphia:LippincottWilliams&Wilkins,2006).FrancisMaitlandBalfour’sseminalworkisencapsulated

inBalfour,F.M.(1874)Apreliminaryaccountofthedevelopmentoftheelasmobranchfishes,Q.J.Microsc.Sci.14:323–364;F.M.Balfour,AMonographontheDevelopmentofElasmobranchFishes,4vols.(London:Macmillan&Co.,1878);F.M.Balfour,ATreatiseonComparativeEmbryology,

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2vols.(London:Macmillan&Co.,1880–81);M.FosterandA.Sedgwick,TheWorksofFrancisMaitlandBalfour,withanintroductorybiographicalnoticebyMichaelFoster,4vols.(London:Macmillan&Co.,1885).AsuccessoratOxford,EdwinGoodrich,producedoneoftheclassicsofcomparativeanatomy,StudiesontheStructureandDevelopmentofVertebrates(London:Macmillan,1930).Balfour,Oken,Goethe,Huxley,andotherswere

addressingtheproblemknownasheadsegmentation.Justasthevertebraedifferinaregularprogressionfromfronttoback,sotheheadhasasegmentalpattern.Aselectionofclassicandrecentresources(allwithgoodbibliographies)topursuethisfieldfurther:Olsson,L.,Ericsson,R.,Cerny,R.(2005)Vertebrateheaddevelopment:Segmentation,novelties,andhomology,TheoryinBiosciences124:145–163;Jollie,M.(1977)Segmentationofthevertebratehead,AmericanZoologist17:323–333;Graham,A.(2001)Thedevelopmentandevolutionofthepharyngealarches,JournalofAnatomy199:133–141.Arecentoverviewofthegeneticbasisofgillarch

formationisfoundinKuratani,S.(2004)Evolutionofthevertebratejaw:comparativeembryologyandmoleculardevelopmentalbiologyrevealthefactorsbehindevolutionarynovelty,JournalofAnatomy205:335–347.Examplesoftheexperimentalmanipulationofonegillarchintoanother,usinggenetictechnologies,includeBaltzinger,M.,Ori,M.,Pasqualetti,M.,Nardi,I.,Riji,F.(2005)Hoxa2knockdowninXenopusresultsinhyoidtomandibular

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homeosis,DevelopmentalDynamics234:858–867;Depew,M.,Lufkin,T.,Rubenstein,J.(2002)SpecificationofjawsubdivisionsbyDlxgenes,Science298:381–385.Acomprehensive,well-illustrated,andinformative

resourceforearlyfossilrecordsofskulls,heads,andprimitivefishisreviewedinP.Janvier,EarlyVertebrates(Oxford,Eng.:OxfordUniversityPress,1996).ThepaperdescribingHaikouella,the530-million-year-oldwormwithgills,isChen,J.-Y.,Huang,D.Y.,andLi,C.W.(1999)AnearlyCambriancraniate-likechordate,Nature402:518–522.

CHAPTERSIXTHEBEST-LAID(BODY)PLANS

Theoriginofbodyplanshasbeenthesubjectofanumberofbook-lengthtreatments.Foronewithanexceptionalscopeandbibliography,gotoJ.Valentine,OntheOriginofPhyla(Chicago:UniversityofChicagoPress,2004).TherehavebeenseveralbiographiesofvonBaer.Ashort

oneisJaneOppenheimer,“Baer,KarlErnstvon,”inC.Gillespie,ed.,DictionaryofScientificBiography,vol.1(NewYork:Scribners,1970).Formoredetailedtreatments,seeAutobiographyofDr.KarlErnstvonBaer,ed.JaneOppenheimer(1986;originallypublishedinGerman,2nded.,1886).SeealsoB.E.Raikov,KarlErnstvonBaer,1792–1876,trans.fromRussian(1968),andLudwigStieda,KarlErnstvonBaer,2nded.(1886).Alltheseresourceshavelargebibliographies.SeealsoS.Gould,OntogenyandPhylogeny(Cambridge,Mass.:HarvardUniversityPress,

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1977),foradiscussionofvonBaer’slaws.SpemannandMangold’sexperimentsarediscussedin

embryologytextbooks:S.Gilbert,DevelopmentalBiology,8thed.(Sunderland,Mass.:SinauerAssociates,2006).AmoderngeneticperspectiveontheOrganizeriscontainedinDeRobertis,E.M.(2006)Spemann’sorganizerandselfregulationinamphibianembryos,NatureReviews7:296–302,andDeRobertis,E.M.,andArecheaga,J.TheSpemannOrganizer:75yearson,InternationalJournalofDevelopmentalBiology45(specialissue).ForaccesstothehugeliteratureonHoxgenesand

evolution,thebeststartingreferenceisSeanCarroll’srecentbookEndlessFormsMostBeautiful(NewYork:Norton,2004).ArecentreviewandinterpretationofthewaysthatgenesallowustounderstandthecommonancestorofbilaterallysymmetricalanimalsisinErwin,D.,andDavidson,E.H.(2002)Thelastcommonbilaterianancestor,Development129:3021–3032.Anumberofinvestigatorsarguethatagenetic“flip”

betweenthebodyplanofananthropodandthebodyplanofahumanhappenedsometimeinthedistantpast.ThisideaisdiscussedinDeRobertis,E.,andSasai,Y.(1996)AcommonplanfordorsoventralpatterninginBilateria,Nature380:37–40.HistoricalperspectiveonSt.Hilaire’sviews,aswellasothercontroversiesintheearlyyearsofcomparativeanatomy,arefoundinT.Appel,TheCuvier-GeoffroyDebate:FrenchBiologyintheDecadesBeforeDarwin(NewYork:OxfordUniversityPress,1987).Data

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fromacornwormsdoesnoteasilyfitthismodel,andsuggeststhatinsometaxathemapbetweengeneactivityandaxisspecificationmayhaveevolved.Forthiswork,seeLowe,C.J.,etal.(2006)Dorsoventralpatterninginhemichordates:insightsintoearlychordateevolution,PLoSBiologyonlineaccess:http://dx.doi.org/journal.0040291.Theevolutionofthegenesthatdeterminethebodyaxes

isreviewedinMartindale,M.Q.(2005)Theevolutionofmetazoanaxialproperties,NatureReviewsGenetics6:917–927.Bodyplangenesincnidarians(jellyfish,seaanemones,andtheirrelatives)arediscussedinaseriesofprimarypapers:Martindale,M.Q.,Finnerty,J.R.,Henry,J.(2002)TheRadiataandtheevolutionaryoriginsofthebilaterianbodyplan,MolecularPhylogeneticsandEvolution24:358–365;Matus,D.Q.,Pang,K.,Marlow,H.,Dunn,C.,Thomsen,G.,Martindale,M.(2006)Molecularevidencefordeepevolutionaryrootsofbilateralityinanimaldevelopment,ProceedingsoftheNationalAcademyofSciences103:11195–11200;Chourrout,D.,etal.(2006)MinimalprotohoxclusterinferredfrombilaterianandcnidarianHoxcomplements,Nature442:684–687;Martindale,M.,Pang,K.,Finnerty,J.(2004)Investigatingtheoriginsoftriploblasty:“mesodermal”geneexpressioninadiploblasticanimal,theseaanemoneNemostellavectensis(phylum,Cnidaria;class,Anthozoa),Development131:2463–2474;Finnerty,J.,Pang,K.,Burton,P.,Paulson,D.,Martindale,M.Q.(2004)Deeporiginsforbilateralsymmetry:HoxandDppexpressioninaseaanemone,Science304:1335–1337.

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CHAPTERSEVENADVENTURESINBODYBUILDING

Threekeyarticlesreviewtheoriginsandevolutionofbodiesandofferanintegrativeperspectiveongenetics,geology,andecology:King,N.(2004)Theunicellularancestryofanimaldevelopment,DevelopmentalCell7:313–325;Knoll,A.H.,andCarroll,S.B.(1999)Earlyanimalevolution:Emergingviewsfromcomparativebiologyandgeology,Science284:2129–2137;Brooke,N.M.,andHolland,P.(2003)Theevolutionofmulticellularityandearlyanimalgenomes,CurrentOpinioninGeneticsandDevelopment13:599–603.Allthreepapersarewellreferencedandofferagoodintroductiontothetopicsofthechapter.Forstimulatingtreatmentsoftheconsequencesofthe

originofbodiesandofothernewformsofbiologicalorganization,seeL.W.Buss,TheEvolutionofIndividuality(Princeton:PrincetonUniversityPress,2006),andJ.MaynardSmith,andE.Szathmary,TheMajorTransitionsinEvolution(NewYork:OxfordUniversityPress,1998).ThestorybehindtheEdiacariananimalsiscovered,with

references,inRichardFortey’sLife:ANaturalHistoryoftheFirstFourBillionYearsofLifeonEarth(NewYork:Knopf,1998),andAndrewKnoll’sLifeonaYoungPlanet(Princeton:PrincetonUniversityPress,2002).Theexperimentthatyielded“proto-bodies”from“no-

bodies”isdescribedinBoraas,M.E.,Seale,D.B.,Boxhorn,J.(1998)Phagotrophybyaflagellateselectsforcolonialprey:Apossibleoriginofmulticellularity,EvolutionaryEcology

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12:153–164.

CHAPTEREIGHTMAKINGSCENTS

TheUniversityofUtahhasaneffectivewebsite,Learn.Genetics,thatprovidesawonderfullysimplekitchenprotocolforextractingDNA.TheURLishttp://learn.genetics.utah.edu/units/activities/extraction/.Theevolutionoftheso-calledodorgenesor,more

precisely,olfactoryreceptorgeneshasalargeliterature.BuckandAxel’sseminalpaperisBuck,L.,andAxel,R.(1991)Anovelmultigenefamilymayencodeodorantreceptors:amolecularbasisforodorrecognition,Cell65:175–181.Comparativeaspectsofolfactorygeneevolutionare

treatedinYoung,B.,andTrask,B.J.(2002)Thesenseofsmell:genomicsofvertebrateodorantreceptors,HumanMolecularGenetics11:1153–1160;Mombaerts,P.(1999)Molecularbiologyofodorantreceptorsinvertebrates,AnnualReviewsofNeuroscience22:487–509.Olfactoryreceptorgenesinjawlessfisharediscussedin

Freitag,J.,Beck,A.,Ludwig,G.,vonBuchholtz,L.,Breer,H.(1999)Ontheoriginoftheolfactoryreceptorfamily:receptorgenesofthejawlessfish(Lampetrafluviatilis),Gene226:165–174.ThedistinctionbetweenaquaticandterrestrialolfactoryreceptorgenesisdescribedinFreitag,J.,Ludwig,G.,Andreini,I.,Rossler,P.,Breer,H.(1998)Olfactoryreceptorsinaquaticandterrestrialvertebrates,

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JournalofComparativePhysiologyA183:635–650.Humanolfactoryreceptorevolutionisdiscussedina

numberofpapers.Thisselectionreflectstheissuesdiscussedinthetext:Gilad,Y.,Man,O.,Lancet,D.(2003)Humanspecificlossofolfactoryreceptorgenes,ProceedingsoftheNationalAcademyofSciences100:3324–3327;Gilad,Y.,Man,O.,andGlusman,G.(2005)Acomparisonofthehumanandchimpanzeeolfactoryreceptorgenerepertoires,GenomeResearch15:224–230;Menashe,I.,Man,O.,Lancet,D.,Gilad,Y.(2003)Differentnosesfordifferentpeople,NatureGenetics34:143–144;Gilad,Y.,Wiebe,V.,Przeworski,M.,Lancet,D.,Paabo,S.(2003)Lossofolfactoryreceptorgenescoincideswiththeacquisitionoffulltrichromaticvisioninprimates,PLoSBiologyonlineaccess:http://dx.doi.org/journal.pbio.0020005.Thenotionofgeneduplicationasanimportantsourceof

newgeneticvariationtracestotheseminalworkofOhnoalmostfortyyearsago:S.Ohno,EvolutionbyGeneDuplication(NewYork:Springer-Verlag,1970).ArecentreviewoftheissuethatcontainsadiscussionofbothopsinsandolfactoryreceptorgenesisfoundinTaylor,J.,andRaes,J.(2004)Duplicationanddivergence:theevolutionofnewgenesandoldideas,AnnualReviewsofGenetics38:615–643.

CHAPTERNINEVISION

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Opsingenesintheevolutionofeyeshavebeendescribedinanumberofpapersinrecentyears.ReviewsofthebasicbiologyandtheconsequencesofopsingeneevolutionincludeNathans,J.(1999)Theevolutionandphysiologyofhumancolorvision:insightsfrommoleculargeneticstudiesofvisualpigments,Neuron24:299–312;Dominy,N.,Svenning,J.C.,Li,W.H.(2003)Historicalcontingencyintheevolutionofprimatecolorvision,JournalofHumanEvolution44:25–45;Tan,Y.,Yoder,A.,Yamashita,N.,Li,W.H.(2005)Evidencefromopsingenesrejectsnocturnalityinancestralprimates,ProceedingsoftheNationalAcademyofSciences102:14712–14716;Yokoyama,S.(1996)Molecularevolutionofretinalandnonretinalopsins,GenestoCells1:787–794;Dulai,K.,vonDornum,M.,Mollon,J.,Hunt,D.M.(1999)TheevolutionoftrichromaticcolorvisionbyopsingeneduplicationinNewWorldandOldWorldprimates,Genome9:629–638.DetlevArendtandJoachimWittbrodt’sworkon

photoreceptortissueswasoriginallydescribedinapaperfromtheprimaryliterature:Arendt,D.,Tessmar-Raible,K.,Synman,H.,Dorresteijn,A.,Wittbrodt,J.(2004)Ciliaryphotoreceptorswithavertebrate-typeopsininaninvertebratebrain,Science306:869–871.Anassociatedcommentaryappearedwiththepiece:Pennisi,E.(2004)Worm’slight-sensingproteinssuggesteye’ssingleorigin,Science306:796–797.AnearlierreviewbyArendtprovidesthelargerframeworkthatheusestointerpretthediscovery:Arendt,D.(2003)Theevolutionofeyesand

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photoreceptorcelltypes,InternationalJournalofDevelopmentalBiology47:563–571.FurthercommentarycanbefoundinPlachetzki,D.C.,Serb,J.M.,Oakley,T.H.(2005)Newinsightsintophotoreceptorevolution,TrendsinEcologyandEvolution20:465–467.StillmorecommentaryonArendtandWittbrodt’sworkbyBerndFritzschandJoramPiatigorskyappearedinalaterissueofScience,withacomment-replythatdiscussedthenotionthattheoriginofeyesmaybeextremelyancient,andtracedtoaverydeepbranchofourevolutionarytree.ThistextcanbefoundinScience(2005)308:1113–1114.AreviewofWalterGehring’sworkonPax6andits

consequencesforeyeevolutioniscontainedinapersonalaccount:Gehring,W.(2005)Newperspectivesoneyedevelopmentandtheevolutionofeyesandphotoreceptors,JournalofHeredity96:171–184.Papersthatlookatthedifferentpossiblerelationships

betweenconservedeyeformationgenesandtheevolutionofeyeorgansincludeOakley,T.(2003)Theeyeasareplicatinganddivergingmodulardevelopmentalunit,TrendsinEcologyandEvolution18:623–627,andNilssonD.-E.(2004)Eyeevolution:aquestionofgeneticpromiscuity,CurrentOpinioninNeurobiology14:407–414.Therelationshipbetweenthelensproteinsinoureyes

andthoseoflarvalseasquirtsisdiscussedinShimeld,S.,Purkiss,A.G.,Dirks,R.P.H.,Bateman,O.,Slingsby,C.,Lubsen,N.(2005)Urochordateby-crystallinandtheevolutionaryoriginofthevertebrateeyelens,CurrentBiology15:1684–

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1689.

CHAPTERTENEARS

ThegeneticsofinnerearevolutionisdiscussedinBeisel,K.W.,andFritzsch,B.(2004)Keepingsensorycellsandevolvingneuronstoconnectthemtothebrain:molecularconservationandnoveltiesinvertebrateeardevelopment,BrainBehaviorandEvolution64:182–197.EardevelopmentandthegenesbehinditarediscussedinRepresa,J.,Frenz,D.A.,VandeWater,T.(2000)Geneticpatterningofembryoniceardevelopment,ActaOtolaryngolica120:5–10.Thetransformationofthehyomandibulaintothestapes

isreviewedincomprehensivebook-lengthtreatmentsoftheevolutionofprimitivefishortheoriginofland-livinganimals:J.Clack,GainingGround(Bloomington:IndianaUniversityPress,2002);P.Janvier,EarlyVertebrates(Oxford,Eng.:OxfordUniversityPress,1996).Itisalsodiscussedinrecentresearchpapers,includingClack,J.A.(1989)Discoveryoftheearliestknowntetrapodstapes,Nature342:425–427;Brazeau,M.,andAhlberg,P.(2005)Tetrapod-likemiddleeararchitectureinaDevonianfish,Nature439:318–321.Theoriginofthemammalianmiddleearisdiscussed

fromtheperspectiveofascientifichistorianinP.Bowler,Life’sSpendidJourney(Chicago:UniversityofChicagoPress,1996).Keyprimarysourcesinclude:Reichert,C.(1837)UberdieVisceralbogenderWirbeltiereimallgemeinenund

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derenMetamorphosenbeidenVogelnundSaugetieren,Arch.Anat.Physiol.Wiss.Med.1837:120–222;Gaupp,E.(1911)BeiträgezurKenntnisdesUnterkiefersderWirbeltiereI.DerProcessusanterior(Folii)desHammersderSaugerunddasGonialederNichtsäuger,AnatomischerAnzeiger39:97–135;Gaupp,E.(1911)BeiträgezurKenntnisdesUnterkiefersderWirbeltiereII.DieZusammensetzungdesUnterkiefersderQuadrupeden,AnatomischerAnzeiger,39:433–473;Gaupp,E.(1911)BeiträgezurKenntnisdesUnterkiefersderWirbeltiereIII.DasProblemederEntstehungeines“sekundären”KiefergelenkesbeidenSäugern,AnatomischerAnzeiger,39:609–666;Gregory,W.K.(1913)Critiqueofrecentworkonthemorphologyofthevertebrateskull,especiallyinrelationtotheoriginofmammals,JournalofMorphology24:1–42.Majorliteratureontheoriginofthemammalianjaw,

chewing,andthethree-bonedmiddleearincludesCrompton,A.W.(1963)Theevolutionofthemammalianjaw,Evolution17:431–439;Crompton,A.W.,andParker,P.(1978)Evolutionofthemammalianmasticatoryapparatus,AmericanScientist66:192–201;Hopson,J.(1966)Theoriginofthemammalianmiddleear,AmericanZoologist6:437–450;Allin,E.(1975)Evolutionofthemammalianear,JournalofMorphology147:403–438.TheevolutionaryoriginofPax2andPax6andthe

evolutionarylinkofearsandeyestoboxjellyfishisdiscussedinPiatigorsky,J.,andKozmik,Z.(2004)Cubozoanjellyfish:anevo/devomodelforeyesandothersensory

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systems,InternationalJournalforDevelopmentalBiology48:719–729.Linksofsensoryreceptormoleculestodifferent

moleculesinbacteriaarediscussedinKung,C.(2005)Apossibleunifyingprincipleformechanosensation,Nature436:647–654.

CHAPTERELEVENTHEMEANINGOFITALL

Themethodsofphylogeneticsystematicsarediscussedinanumberofsources.KeyprimaryliteratureincludestheclassicworkofWilliHennig,publishedoriginallyinGerman(GrundzügeeinerTheoriederphylogenetischenSystematik[Berlin:DeutscherZentralverlag,1950])andtranslatedintoEnglishmorethanadecadelater(PhylogeneticSystematics,trans.D.D.DavisandR.Zangerl[Urbana:UniversityofIllinoisPress,1966]).Methodsofphylogeneticreconstruction,whichformthe

basisforthechapter,arediscussedindetailinP.Forey,ed.,Cladistics:APracticalCourseinSystematics(Oxford,Eng.:ClarendonPress,1992);D.Hillis,C.Moritz,andB.Mable,eds.,MolecularSystematics(Sunderland,Mass.:SinauerAssociates,1996);R.DeSalle,G.Girbet,andW.Wheeler,MolecularSystematicsandEvolution:TheoryandPractice(Basel:BirkhauserVerlag,2002).Acomprehensivetreatmentofthephenomenonof

independentevolutionofsimilarfeaturesisinM.SandersonandL.Hufford,Homoplasy:TheRecurrenceof

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SimilarityinEvolution(SanDiego:AcademicPress,1996).Toseethetreeoflifeandthedifferenthypothesesforthe

relationshipsbetweenlivingcreatures,visithttp://tolweb.org/tree/.Thenotionthatourevolutionaryhistoryhasmedical

implicationshasbeenthesubjectofseveralgoodrecentbooks.Forcomprehensiveandwell-referencedtreatments,seeN.Boaz,EvolvingHealth:TheOriginsofIllnessandHowtheModernWorldIsMakingUsSick(NewYork:Wiley,2002);D.Mindell,TheEvolvingWorld:EvolutioninEverydayLife(Cambridge,Mass.:HarvardUniversityPress,2006);R.M.NesseandG.C.Williams,WhyWeGetSick:TheNewScienceofDarwinianMedicine(NewYork:Vintage,1996);W.R.Trevathan,E.O.Smith,andJ.J.McKenna,EvolutionaryMedicine(NewYork:OxfordUniversityPress,1999).TheapneaexampleIderivedfromdiscussionswithNino

Ramirez,chairmanoftheDepartmentofAnatomyattheUniversityofChicago.ThehiccupexampleisderivedfromStraus,C.,etal.(2003)Aphylogenetichypothesisfortheoriginofhiccoughs,Bioessays25:182–188.Thehuman-bacterialgeneswitchusedinthestudyofmitochondrialcardioencephalomyopathywasoriginallydiscussedinLucioli,S.,etal.(2006)IntroducinganovelhumanmtDNAmutationintotheParacoccusdenitriticansCOX1geneexplainsfunctionaldeficitsinapatient,Neurogenetics7:51–57.

ONLINERESOURCES

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Anumberofwebsitesandblogscarryaccurateinformationandareupdatedfrequently.

http://www.ucmp.berkeley.edu/ProducedbytheMuseumofPaleontologyattheUniversityofCalifornia–Berkeley,thisisoneofthebestonlineresourcesonpaleontologyandevolution.Itiscontinuouslyupdatedandrevised.

http://www.scienceblogs.com/loom/ThisisCarlZimmer’sblog,awell-written,timely,andthoughtfulsourceofinformationanddiscussiononevolution.

http://www.scienceblogs.com/pharyngula/P.Z.Myers,aprofessorofdevelopmentalbiology,writesthisaccessible,informative,andcutting-edgeblog.Thisisarichsourceofinformation,wellworthfollowing.

BothZimmer’sandMyers’sblogsareathttp://www.scienceblogs.com,asitethatcontainsanumberofexcellentblogsalsoworthfollowingforinformationandcommentaryonrecentdiscoveries.BlogsrelevanttothethemeofthisbookatthatsiteincludeAfarensis,TetrapodZoology,EvolvingThoughts,andGeneExpression.

http://www.tolweb.org/tree/TheTreeofLifeProjectprovidesaregularlyupdatedandauthoritativetreatmentoftherelationshipsamongallgroupsoflife.LiketheUCMP

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pageatBerkeley,italsoincludesresourcesforlearningabouthowevolutionarytreesaremadeandinterpreted.

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ACKNOWLEDGMENTS

Alltheillustrations,exceptwherenoted,arebyMs.KalliopiMonoyios(www.kalliopimonoyios.com).Kapireaddraftsofthemanuscriptandnotonlyimprovedthetextbutdesignedartthatmatchedit.Ihavebeentrulyfortunatetoworkwithsomeonewithsomanytalents.ScottRawlins(ArcadiaUniversity)generouslygavepermissiontousehiselegantrenderingofSauripterusinChapter2.TedDaeschler(AcademyofNaturalSciencesofPhiladelphia)graciouslyprovidedhissuperbphotosofthegreatTiktaalik“C”specimen.ThanksareduetoPhillipDonoghue(UniversityofBristol)andMarkPurnell(UniversityofLeicester)forpermissiontousetheirrenderingoftheconodonttootharray,McGraw-HillforpermissiontousethetextbookfigurethatstartedthehuntforTiktaalik,andStevenCampanaoftheCanadianSharkResearchLaboratoryforthephotosofsharkorgans.Oneofthegreatestdebtsstudentsofanatomyhaveisto

thepeoplewhodonatetheirbodiessothatwecanlearn.Itisarareprivilegetolearnfromarealbody.Sittingthroughlonghoursinthelab,onefeelsaveryprofoundconnection

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tothedonorswhomaketheexperiencepossible.Ifeltthatconnectionagainwhilewritingthisbook.TheideasIpresentherearerootedinresearchI’vedone

andinclassesI’vetaught.Colleaguesandstudentstoonumeroustoname—undergraduates,medicalstudents,andgraduatestudents—haveplayedaroleinthethinkingthatwentintothesepages.IoweagreatdebttothecolleaguesIhaveworkedwith

overtheyears.TedDaeschler,FarishA.Jenkins,Jr.,FredMullison,PaulOlsen,WilliamAmaral,JasonDowns,andChuckSchaffhaveallbeenpartofthestoriesItellhere.WithoutthesepeopleIwouldhavehadnoreservoirofexperienceonwhichtodraw,norwouldIhavehadasmuchfunalongtheway.MembersofmylaboratoryattheUniversityofChicago—RandallDahn,MarcusDavis,AdamFranssen,AndrewGillis,ChristianKammerer,KalliopiMonoyios,andBeckyShearman—allinfluencedmythinkingandtoleratedmytimeawayfromthebenchasIwrote.Colleagueswhogavetheirtimetoprovideneeded

backgroundorcommentsonthemanuscriptincludeKamlaAhluwalia,SeanCarroll,MichaelCoates,RandallDahn,MarcusDavis,AnnaDiRienzo,AndrewGillis,LanceGrande,ElizabethGrove,NicholasHatsopoulos,RobertHo,BettyKatsaros,MichaelLaBarbera,ChrisLowe,DanielMargoliash,KalliopiMonoyios,JonathanPritchard,VickyPrince,CliffRagsdale,NinoRamirez,CallumRoss,AviStopper,CliffTabin,andJohnZeller.HaythamAbu-Zaydhelpedwithmanyadministrativematters.Myownteachers

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ofanatomyintheHarvard–MITHealthSciencesandTechnologyprogram,FarishA.Jenkins,Jr.,andLeeGehrke,stimulatedaninterestthathaslastedovertwentyyears.Keyadviceattheinceptionoftheproject,andinspiration

throughout,camefromSeanCarrollandCarlZimmer.TheWellfleetPublicLibrary(Wellfleet,Massachusetts)

providedacomfortablehome,andmuch-neededretreat,whereIwrotesignificantpartsofthebook.AbriefstintattheAmericanAcademyinBerlinputmeinanenvironmentthatprovedcriticalwhenIwascompletingthemanuscript.Mytwobosses,Dr.JamesMadara,M.D.(CEO,University

ofChicagoMedicalCenter,VicePresidentforMedicalAffairs,DeanandSaraandHaroldThompsonDistinguishedServiceProfessorintheBiologicalSciencesDivisionandthePritzkerSchoolofMedicine),andJohnMcCarter,Jr.(CEO,TheFieldMuseum),supportedthisprojectandtheresearchbehindit.Ithasbeenatruepleasuretoworkwithsuchinsightfulandcompassionateleaders.IhavebeenfortunatetoteachattheUniversityof

ChicagoandtohavehadtheopportunitytointeractwiththeleadershipofthePritzkerSchoolofMedicinethere.Thedeans,HollyHumphreyandHalinaBruckner,graciouslywelcomedapaleontologisttotheirteam.ThroughinteractingwiththemIcametoappreciatethechallengesandimportanceofbasicmedicaleducation.IthasbeenagreatpleasuretobeassociatedwithThe

FieldMuseuminChicago,whereIhavehadtheopportunitytoworkwithauniquegroupofpeoplededicatedto

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scientificdiscovery,application,andoutreach.ThesecolleaguesincludeElizabethBabcock,JosephBrennan,SheilaCawley,JimCroft,LanceGrande,MelissaHilton,EdHorner,DebraMoskovits,LauraSadler,SeanVanDerziel,andDianeWhite.Iamalsogratefulforthesupport,guidance,andencouragementIhavereceivedfromtheleadersoftheCommitteeonScienceoftheBoardofTrusteesatTheFieldMuseum,JamesL.AlexanderandAdeleS.Simmons.Iamindebtedtomyagent,KatinkaMatson,forhelping

meturnanideaintoaproposalandforadvicethroughouttheprocess.IfeelprivilegedtohaveworkedwithMartyAsher,myeditor.Likeapatientteacher,hegavemeanurturingcombinationofadvice,time,andencouragementtohelpmefindmyway.ZacharyWagmancontributedtothisprojectincountlesswaysbybeingfreewithhistime,keeneditorialeye,andgoodcounsel.DanFrankmadeinsightfulsuggestionsthatstimulatedmetothinkaboutthestoryinnewways.JolantaBenalcopyeditedthetextandimprovedthepresentationimmeasurably.IamverygratefultoEllenFeldman,KristenBearse,andtheproductionteamfortheirhardworkunderatightschedule.Myparents,GloriaandSeymourShubin,alwaysknew

thatIwouldwriteabook,evenbeforeIdid.Withouttheirfaithinme,IdoubtthatIeverwouldhaveputawordonpaper.Mywife,MicheleSeidl,andourchildren,Nathanieland

Hannah,havebeenlivingwithfish—bothTiktaalikandthis

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book—forthebetterpartoftwoyears.MichelereadandcommentedoneverydraftofthistextandsupportedlongweekendabsenceswhileIwrote.Herpatienceandlovemadeitallpossible.

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Copyright©2008byNeilShubin

Allrightsreserved.PublishedintheUnitedStatesby

PantheonBooks,adivisionofRandomHouse,Inc.,NewYork,andinCanadabyRandomHouseofCanadaLimited,

Toronto.PantheonBooksandcolophonareregisteredtrademarksof

RandomHouse,Inc.AllillustrationsbyKalliopiMonoyiosunlessotherwise

noted.

LibraryofCongressCataloging-in-PublicationData

Shubin,Neil.Yourinnerfish:ajourneyintothe3.5-billion-yearhistory

ofthehumanbody/ByNeilShubin.—1sted.p.cm.

Includesbibliographicalreferences.eISBN:978-0-307-37716-6

1.Humananatomy—Popularworks.2.Humanevolution—Popularworks.I.Title.

QM26.s582008611—dc222007024699

www.pantheonbooks.com

v1.0

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TableofContentsTitlePageDedicationPrefaceONEFindingYourInnerFishTWOGettingaGripTHREEHandyGenesFOURTeethEverywhereFIVEGettingAheadSIXTheBest-Laid(Body)PlansSEVENAdventuresinBodybuildingEIGHTMakingScentsNINEVisionTENEarsELEVENTheMeaningofItAllEpilogueNotes,References,andFurtherReadingAcknowledgmentsCopyright

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TableofContents

TitlePage 2Dedication 4Preface 5ONEFindingYourInnerFish 7TWOGettingaGrip 39THREEHandyGenes 61FOURTeethEverywhere 81FIVEGettingAhead 108SIXTheBest-Laid(Body)Plans 129SEVENAdventuresinBodybuilding 154EIGHTMakingScents 185NINEVision 197TENEars 209ELEVENTheMeaningofItAll 229Epilogue 263Notes,References,andFurtherReading 267Acknowledgments 289Copyright 294

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THE PARADOXICAL RBC

Paradoxical Re ection in Quantum Mechanics

Paradoxical Re ection in Quantum Mechanics

Culture, Conditions and Paradoxical Frames

Culture, Conditions and Paradoxical Frames

A hypothesis on paradoxical privileged portal vein ...website60s.com/upload/files/a-hypothesis-on-paradoxical-privileged... · A hypothesis on paradoxical privileged portal vein metastasis

A hypothesis on paradoxical privileged portal vein ...website60s.com/upload/files/a-hypothesis-on-paradoxical-privileged... · A hypothesis on paradoxical privileged portal vein metastasis

chances - Umdasch Group · chances How Ali Mahlodji has taken advantage of his chances Page 14 Issue 01/2016 Education! The magazine that creates perspectives. 2 chances chances 3

chances - Umdasch Group · chances How Ali Mahlodji has taken advantage of his chances Page 14 Issue 01/2016 Education! The magazine that creates perspectives. 2 chances chances 3

Paradoxical Prognostic Implications of Visceral Adiposity ...

Paradoxical Prognostic Implications of Visceral Adiposity ...

Paradoxical buckling behaviour of a thin cylindrical shell ...imechanica.org/files/Paradoxical buckling behaviour of a thin... · Paradoxical buckling behaviour of a thin ... upper

Paradoxical buckling behaviour of a thin cylindrical shell ...imechanica.org/files/Paradoxical buckling behaviour of a thin... · Paradoxical buckling behaviour of a thin ... upper

Succeeding Through Collaborative Conflict: The Paradoxical ...

Succeeding Through Collaborative Conflict: The Paradoxical ...

Os Mandamentos Paradoxais - Paradoxical Commandments

Os Mandamentos Paradoxais - Paradoxical Commandments

Paradoxical vocal fold motion - 9th Call9thcall.ru/.../03/Paradoxical-vocal-fold-motion... · Paradoxical vocal fold motion (PVFM) can be seen during inspiration, expiration, or both

Paradoxical vocal fold motion - 9th Call9thcall.ru/.../03/Paradoxical-vocal-fold-motion... · Paradoxical vocal fold motion (PVFM) can be seen during inspiration, expiration, or both

Logic, models, and paradoxical inferences · Paradoxical inferences 2. Abstract . People reject ‘paradoxical’ inferences, such as: Luisa didn´t play music; therefore, if Luisa

Logic, models, and paradoxical inferences · Paradoxical inferences 2. Abstract . People reject ‘paradoxical’ inferences, such as: Luisa didn´t play music; therefore, if Luisa

The Paradoxical Commandments

The Paradoxical Commandments

The Paradoxical Politics of Viral Containment

The Paradoxical Politics of Viral Containment

The paradoxical foundation of strategic management

The paradoxical foundation of strategic management

Measuring Predictability: Theory and Macroeconomic ...fdiebold/papers/paper36/dk.pdf · Measuring Predictability: Theory and Macroeconomic Applications ... "Measuring Predictability:

Measuring Predictability: Theory and Macroeconomic ...fdiebold/papers/paper36/dk.pdf · Measuring Predictability: Theory and Macroeconomic Applications ... "Measuring Predictability:

Chasing Predictability

Chasing Predictability

Paradoxical Crystal Baffles Physicists | WIRED

Paradoxical Crystal Baffles Physicists | WIRED

The Paradoxical Prime Minister

The Paradoxical Prime Minister