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Questions of which human popula-tions first arrived in the Americas,and when, where and how this hap-

pened, have been debated by scientists fordecades1. It has long been presumed thatthe first people entering the New Worldwere the direct ancestors of present-dayNative Americans and that they arrived inAmerica from northeast Asia about 12,000years ago2. But this theory has been chal-lenged by new archaeological discoveriesand by findings of early human remains

bearing anatomical similarities to thepeople of south Asia and the southernPacific Rim3,4 (Fig. 1). Writing on page 62of this issue,Gonzlez-Joset al.5 add morefuel to this heated debate. They present acomparative study of early historic humanskulls from Baja California, Mexico, andtheir findings lend weight to the view thatnot all early American populations weredirectly related to present-day Native Amer-icans.

Human skeletal remains have long beenused by palaeoanthropologists to model earlyhuman migration. The conventional view

is that different skeletal populations withsimilar craniofacial features (skull form)shared a common ancestry and were geneti-cally related,whereas different features reflect

different ancestry.Migration histories andevolutionary forces explain the simi-lari ties or differences.

Piecing together the ancestr yof the Americas has been diffi-cult ,as early human remainsdating from about 10,000years ago (the end of thelast ice age) are fragmentaryand scarce. Scientists havetypically reconstructed themissing pieces of the most

ancient skulls by extrapo-lating backwards fromlater, more complete skel-etons. Ancient Americanskulls reconstructed inthis way were anatomically indistinguishablefrom early northeast Asians and also frompresent-day Native Americans2. So a theoryarose, supported by dental and otherarchaeological data6, that the first humansentering the Americas were northeast Asianswho arrived in three successive migrationsbeginning around 12,000 years ago. Thesefounding colonizers were thought to be big-

game hunters, equipped with so-calledClovis spears7, who rapidly populated theWestern Hemisphere and gave rise to present-day Native Americans (Fig.2a).

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But more recent archaeological discover-ies suggest that there were several differentfounding populations, arr iving from differ-ent places, each with different l ifestyles andtechnologies8. Some populations not onlyhunted big game but also exploited a widerange of plant and animal li fe.To complicatematters further, it is no longer certain thatthe first colonizers arrived about 12,000

years ago some archaeological sites inSouth America date from 12,500 years ago,which suggests that the first humans arrivedat least 15,000 years ago.

A simi lar pattern of diversity is emergingfrom statistical analyses of cranial and facialmeasurements of some of the oldest skel-etons found in the Americas. The archaeo-logical and skeletal data have led to a newmodel, in which the Palaeoamericans theproposed fi rst arrivals in the New World were not northeast Asians. They cameinstead from south Asia and the southernPacific Rim, and they probably shared

ancestr y with ancient Australians and othersouthern populations3,9.A second group ofhumans then arr ived from nor theast Asia orMongolia, and it was this second popula-tion that adapted to the warming climateafter the Ice Age and gave rise to the modernAmerindians (an ancient population ofAmeri cans whose skeletal remains make upmost of the human material found in theNew World) and the present-day NativeAmericans (Fig. 2b). So according to thistheory, the Palaeoamericans are unrelatedto most modern Amerindians and to theNative Americans.

Gonzlez-Jos et al.5 now propose amore complex view of American ancestr y.

Tracking the f irst AmericansTom D. Dilleha y

A study of 33 anc ient skulls excavated from M exico invites us to rec onsider ourview of the anc estry of the early Americans. Unlike m ost o ther early Am erican

rem ains, the skulls resem ble those from south Asian pop ulations.

Figure 1The skull ofan

early American.The

Kennewick Man,

excavated from

Washington

state,lived

about 9,300

years ago.His

craniofacial

features,which

do not resemble

living Native

Americans,are typicalofthe skulls that are

increasingly being found

in ancient American sites.

Figure 2Tracing American ancestry.Analysis ofskeletal remains has led to at least two models to

explain the origin ofearly human populations in the Americas.a, It was originally thought that

the first colonizers were the direct ancestors ofpresent-day Native Americans,who arrived from

northeast Asia and possibly central Asia in three successive migrations about 12,000 years ago.b,

More recent analyses ofthe craniofacial features ofskulls dating from the end ofthe Ice Age suggest

that the first arrivals were from south Asia or the Pacific Rim.These Palaeoamericanswere thought

to be unrelated to the majority ofmodern Amerindian remains a later group ofcolonizers from

northeast Asia were thought to have given rise to these late-prehistoric populations.Now,Gonzlez-

Joset al.5have found that a group ofearly historic Amerindian skulls from the Baja peninsula in

Mexico bear a strong resemblance to the early Palaeoamericans,suggesting that the colonization

ofthe Americas was more complex than had previously been suspected.

Arriva l from

northeast

Asia /Mong olia

Arriva l from

northeast

Asia/Mong olia

Modern

Amerindians

Arriva l from

northeastAsia/Mong olia

NativeAmericans

NativeAmericans

BajaAmerindians

ModernAmerindians

Palaeoamericans

Arrival from south Asia/

southern P ac ific Rim

Arriva l from

northeast

Asia/Mong olia

Present day

10,000

to

15,000

years ago

Three s ucces sive migrations

from northeast Asia

Two m igra tions

a b

E

T H O M P S O N / A P P H O T O

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8/12/2019 Tracking the First Americans

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These authors analysed the skeletalremains of 33 modern Amerindians fromearly histor ic times, excavated from the tipof the Baja peninsula in Mexico. Surpr is-ingly,the craniofacial features of these BajaAmerindians show closer affinity to thePalaeoameri can skul ls than to other mod-ern Amerindian remains. The Baja Amer-indian and Palaeoamerican skulls have

similar long and narrow braincases andrelatively short , narrow faces, implying acommon ancestr y with the ancient inhabi-tants of south Asia and the Pacific Rim.Gonzlez-Jos et al . confirm that modernAmerindian skulls from other areas aresimilar to ancient northeast Asian remains.Their new data add to accumulatingevidence of morphological differencesbetween early humans from different areasof the Americas8,9.

The authors consider several potentialexplanations to account for the presence ofPalaeoamerican traits in the Baja Amer-

indian skulls, but they suggest that the bestexplanation is that the Palaeoamericanswere the direct ancestors of the BajaAmerindians.After the Ice Age,the increasedaridity could have geographically isolatedthe founding Palaeoamerican population inthe Baja area,and limited its gene flow withother modern Amerindian groups.

Do the new findings tell us anythingmore about when the first humans arrivedin the Americas? The authors do not fullydiscuss the chronological implications oftheir work, but their interpretation ofshared ancestry between the Palaeoameri-

cans and the Baja Amerindians might bestfi t a model of Palaeoamerican arrival about11,00012,000 years ago.There is no directevidence to support this view, but if thePalaeoamericans had arrived 15,000 yearsago or earl ier, the Baja population wouldhave remained isolated for much longer.This seems unlikely, given the rate ofpopulation growth and movement thatprobably occurred after initial coloniza-tion and then after the Ice Age when theclimate warmed.

But could the similarities between theancient Palaeoamericans and the later Baja

Amerindians instead reflect the influence ofother evolutionary forces,such as gene flowor natural selection and convergent adapta-tion of different populations to simi lar localenvironments?Answering this question wil ldepend upon finding more isolated prehis-toric populations showing ancient Palaeo-american traits, and then establishingwhether parallel evolutionary forces wereacting on them and whether they werederived from a single ancestry. But this wil lbe a difficult task.Human remains from theend of the Ice Age are scarce and often frag-mentary, so we have only a vague notionof the skeletal characteristics of the ancientPalaeoamericans. And we have a poor

understanding of the migration history ofdifferent American populations and whatkind of evolutionary forces might haveinfluenced them10.

Given these limitations, the findings ofGonzlez-Joset al. do not allow us to drawfirm conclusions about the relationshipbetween the ancient Palaeoamericans andthe later Baja Amerindians. But the impor-

tance of this and other studies7,11 is that theysuggest a different view of the origins andinteractions of early human populations inthe Americas.What we really want to know iswhat took place within and between thesepopulations, how they changed over time,and how quickly they changed. These issuescan be resolved only by obtaining moreskeletal data11 and by combining themwith regional archaeological records,whichshould provide information on the socialand cultural histories of the different

populations. Slowly, we are realizing thatthe ancestr y of the Americas is as complexand as dif ficult to trace as that of otherhuman lineages around the world. Tom D. Di llehay is in the Department of

Anthropology, Universit y of Kentucky, Lexington,

Kentucky 40506, USA.

e-mai l: dill eha@uky.edu

1. Dixon,E.J.Quat. Sci. Rev.20,277299 (2001).

2. Dalton,R.Nature422,1012 (2002).3. Neves, W.A. & Pucciarelli, H. M. J. Hum. Evol. 21,

261273 (1991).

4. Dillehay, T. D.The Settl ement of the Americas: A New Prehistory

(Basic Books,New York, 2001).

5. Gonzlez-Jos, R.et al. Nature425,6265 (2003).

6. Turner,C.G.Mem.Calif. Acad.Sci. 27,123158 (2002).

7. Meltzer,D. J.Mem.Calif. Acad.Sci 27,2758 (2002).

8. Lahr,M. M.The Evolution of Modern Human Di versity

(Cambri dge Univ.Press,1998).

9. Neves, W.A. et al.Homo50,258263 (1999).

10.Steele,D. G.& Powell, J. F.Mem.Calif. Acad.Sci. 27,

93122 (2002).

11.Jantz,R. E.& Owsley,D. W.Nevada Hist. Soc. Q. 40,

6284 (1997).

gases and other incompatible elements.The primitive layer serves as a reservoir forthese elements (which were depleted fromthe upper mantle when Earths crust was

formed) and it is occasionally sampled bydeep-mantle plumes (Fig.1a).Over the past several years,however, seis-

mic tomography has given us increasinglydetailed images of apparently cold slabs(characterized by fast seismic velocities)descending into the deep mantle rightthrough the 660-km boundary, effectivelycutting to shreds the simple picture of themantle convecting in two nearly isolatedlayers. If cold slabs descend into the deepmantle, there must be a correspondingupward flow of deep-mantle material toshallow levels (Fig. 1b). No matter what

specifi c form the exchange across the 660-kmboundary takes, in this whole-mantlemodel of mantle convection,it would withina few hundred million years destroy anycompositional layering that had possiblybeen inheri ted from early in Earths history.

Meanwhile, the geochemical argumentsfor a separate deep reservoir have not disap-peared. Primordial noble gases are sti llpreferentially associated with hot spots2, atleast some ofwhich seem to come from deep-mantle plumes3. And much of the uppermantle remains highly depleted of incom-patible trace elements, including the heat-producing thorium,uranium and potassium also suggesting the presence of a less

Sandwiched between Earths thin crustand its metallic core lies a layer ofpressurized rock at high temperature

the mantle. Convection in this layer

drives plate tectonics and sea-floor spread-ing,but we know litt le about the pattern ofcirculation. Indeed, current thinking aboutmantle dynamics is in a state of turmoil. Aswe cannot observe convection directly, wemust piece together indirect evidence fromseismology, geochemistry, mineral physics,fluid dynamics and numerical simulationsof convection. But the evidence is contradic-tory and has led to at least two conflictingviews about mantle movement. On page39of this issue, Bercovici and Karato1 proposea new model that might resolve this confl ict.They suggest that water dissolved in the

mantle might create a thin layer of melt at adepth of 400 km, causing an unexpectedpattern of circulation in the mantle.

The two conflicting models for mantleconvection (Fig. 1a, b, overleaf) are usuallydescribed as layered convection (suppor-ted by geochemists) and whole-mantleconvection (supported by seismologists).Geochemists have long insisted on the two-layered model, in which the mantle consistsof a relatively primitive layer below a depth of660 km containing primordial noblegases, trapped 4.5 bil lion years ago when theEarth formed and an upper layer that ishighly depleted of heat-producing elements(uranium, thorium and potassium), noble

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Earth science

Just add w aterAlbrec ht W. Ho fma nn

A new model could explain why Earths upper mantle is depleted ofmany trace elements. At a certain depth, minerals might release water,creating a molten filter that traps trace elements in the mantle beneath.

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