The Mysterious Origin of Maize - Transtutors...Economic Botany 55(4) pp. 492–514. 2001 2001 by The...

BIOL 3431: Plants and People

Unit 3

The Mysterious Origin of Maize

By

Mary W. Eubanks

Eubanks, M. (2001). The Mysterious Origin of Maize, Economic Botany 55(4) pp. 492-513. Bronx, NY: The New York Botanical Garden Press. “Copied under Permission from Access Copyright. Further reproduction, distribution, or transmission is prohibited except as otherwise permitted by Law.”

Economic Botany 55(4) pp. 492–514. 2001� 2001 by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.

FEATURE ARTICLE

THE MYSTERIOUS ORIGIN OF MAIZE

MARY W. EUBANKS

Eubanks, Mary W. (Department of Biology, Duke University, Durham, NC 27708-0338 USA).THE MYSTERIOUS ORIGIN OF MAIZE. Economic Botany 55(4):492–514, 2001. Domesticated maizeemerged from human selection, exploitation, and cultivation of natural recombinants betweentwo wild grasses that had novel characteristics desired by humans for food. Crossing experi-ments reconstructing prototypes of ancient archaeological specimens demonstrate how the sim-ple flowering spike of the wild relatives of maize was transformed into the prolific grain-bearingear within a few generations of intergenomic recombination between teosinte and Tripsacum.The high degree of morphological similarities of segregating intercross progeny to archaeo-logical specimens from Tehuacan, Oaxaca, and Tamaulipas provides strong support for thisevolutionary scenario. Comparative genomic analysis of maize, teosinte, and Tripsacum con-firms that maize has inherited unique polymorphisms from a Tripsacum ancestor and otherunique polymorphisms from a teosinte progenitor. This supports the hypothesis that Tripsacumintrogression provided the mutagenic action for the transformation of the teosinte spike intothe maize ear. This model for the origin of maize explains its sudden appearance, rapid evo-lutionary trajectory, and genesis of its spectacular biodiversity.Key Words: evolution of maize; Tripsacum; teosinte; archaeological maize.

‘‘In the future, as population pressure and our understanding of maize both increase, this crop’s role as oursymbiotic partner in survival will also increase. As the most intelligent species on the only planet known tocontain life, it behooves us to use our intelligence not only to harmonize our symbiotic relationships with plantsbut with each other. . .’’

(Galinat 1992:11)

Maize (Zea mays L.), or corn as it is calledin the United States, is one of three cerealgrains upon which world civilizations werefounded. It has been described as ‘‘having apassport without a birth certificate’’ (Wilkesand Goodman 1996) because, although it is oneof the most widely grown food crops aroundthe globe today, its precise parentage has beencontroversial. With over 200 land races indig-enous to Latin America (Brieger et al. 1958;Brown 1960; Grant et al. 1963; Grobman, Sal-huana, and Sevilla 1961; Hatheway 1957; Ra-mırez et al. 1960; Roberts et al. 1957; Timothyet al. 1961; Timothy et al. 1963; Wellhausen,Fuentes, and Hernandez C. 1957; Wellhausenet al. 1952), maize is phenomenally rich in ge-netic biodiversity. A study of botanical facsim-iles depicted on pre-Columbian pottery (Eu-banks 1999a) demonstrated that six of the 14categories into which the land races have been

grouped (Bird and Goodman 1977) have deeproots in antiquity.It has been estimated that over 90% of maize

found archaeologically is cobs or cob fragmentswithout kernels in place or in direct associationwith cobs (Lenz 1948). Therefore, most of whatwe know about prehistoric maize has been in-ferred from observable characteristics of cobmacrofossils. Classification of maize is based onfeatures of the ear with kernels in situ. Althoughsome of the ear characters can be inferred fromwell-preserved cobs, there is still considerableambiguity in identifications based on cob mor-phology alone (Bird 1970; Mangelsdorf,MacNeish, and Galinat 1967a). This limitationis overcome by ceramic models of maize thatprovide a unique window into maize evolution(Eubanks 1999a). Ancient potters made moldsby pressing real maize ears into clay, then firingthe negative impression (Fig. 1). They then fash-ioned positive casts by pressing damp clay into

2001] 493EUBANKS: MYSTERIOUS ORIGIN OF MAIZE

Fig. 1. Ceramic mold formed from an actual earof maize. Muse Amano, Lima, Peru.

Fig. 2. Map showing the homelands of the Zapo-tec (Mexico) and Moche (Peru) cultures in prehistory.These groups displayed naturalistic mold made maizeon their ceremonial vessels.

a mold and adorned ceremonial wares with themaize replicas. These botanical facsimiles of thenatural ear provide the morphometric data thatis essential for identifying the race of maize de-picted. This particular technique of using moldsto produce maize replicas true to nature was em-ployed by two contemporaneous cultures, theZapotec of the Valley of Oaxaca in Mexico andthe Moche on the north coast of Peru (Fig. 2)during the Classic period (�200–900 A.D.).Both cultures regarded maize as the sacredsource and sustenance of life.The Zapotec displayed maize on ceremonial

urns representing revered ancestral figures bear-ing attributes of different deities in the Zapotecpantheon associated with growing maize. Thefigure was usually depicted in front of a cylin-drical container. These urns, often referred to asfunerary urns, were placed in tombs or in nicheson the outside of tombs. Figure 3 illustrates aZapotec urn depicting an anthropomorphic fig-ure with attributes of the maize god, the God ofGlyph ‘‘L,’’ with maize ears in the headdress.The Moche depicted maize in association withtheir anthropomorphic creator god Ai-apaec(Fig. 4). Other Moche jars depict maize alone or

associated with a variety of animals includingbirds, frogs, fish, and rodents.Aided by numerical taxonomy in a study of

129 vessels with depictions of moldmade maizein museum collections in Mexico, Peru, theUnited States, and Europe, Eubanks (1999a)identified twenty land races in ten countries fromthe maize replicas (see cover illustration). In ac-cordance with the classification scheme of Birdand Goodman (1977), seven races identified onthe pre-Columbian jars belong to two categoriesof pop corn; six types of flint/popcorns; one isin the Cuzco group; one is a Caribbean dent; oneis a lowland flour maize, and four were not clas-sified by Bird and Goodman. This broad spec-trum of maize biodiversity that had developedas early as the Classic period can be traced asfar back as preceramic times. Evidence for thiswas preserved iconographically by ancient Za-potec potters who frequently added stylizedmale flowers (staminate tips) to the naturalisti-cally depicted maize ears (Fig. 5). The signifi-cance of this as a clue for solving the mysteryof the origin of maize becomes apparent as thisstory unfolds.The oldest maize cobs are over 6000 years old

494 [VOL. 55ECONOMIC BOTANY

Fig. 3. Zapotec urn of anthropomorphic figure as-sociated with attributes of the maize god. Note mold-made maize adorning the headdress. Yale UniversityMuseum of Art, New Haven.

Fig. 4. Moche jar depicting Ai-apaec the creatorgod associated with maize. Field Museum of NaturalHistory, Chicago.

and are from rockshelters in the Valleys of Te-huacan and Oaxaca in southern Mexico (Benz2001; Flannery 1986; MacNeish 2001; Mac-Neish and Eubanks 2000; Mangelsdorf, Mac-Neish, and Galinat 1964, 1967a; Piperno andFlannery 2001). These tiny cobs (Fig. 6) havethe basic characteristics that distinguish domes-ticated maize from its wild relatives. The hardfruitcases have converted to open cupules (cup-like structures) that expose the kernels; theglumes (basal bract of a grass spikelet) have be-come herbaceous like the male glumes as op-posed to indurated by lignification for protectionof the grain; each cupule bears a pair of kernelsinstead of one, and the cupulate segments of thefemale flowering spike have fused to form a rig-id rachis, the cob.Most scientists agree that maize evolved

from teosinte, a closely related wild grass alsoin the genus Zea that is endemic to Mexicowhere maize is believed to have originated. Thegreat botanical mystery is how was the simplespike of teosinte with five to seven hard seeds(Fig. 7) so radically transformed into a struc-ture that is unparalleled anywhere else in thebotanical kingdom, the highly prolific ear thatproduces hundreds of kernels (Fig. 8). Possible

scenarios include: (1) a progressive evolutionduring which mutations accumulated over along period of time (Beadle 1939; Doebley1990); (2) a sudden catastrophic sexual trans-mutation that converted the male flowers intofemale flowers (Iltis 1983); (3) a rapid majorshift due to human intervention by premeditat-ed, deliberate selection in the F2 of two teosintemutants that recombined a mutation for pairedfemale spikelets with one for four-ranked singlespikelets (Galinat 1977, 1992); or (4) establish-ment by humans selecting naturally occurringmutant recombinants formed by hybridizationbetween wild grasses (Eubanks 1995, 1997;MacNeish and Eubanks 2000; Mangelsdorf andReeves 1939; Mangelsdorf 1983, 1986; Wilkes1979). Although the hypothesis that teosintewas transformed into maize by gradual accu-mulation of mutations over thousands of yearsis currently the most widely accepted paradigm(Bennetzen et al. 2001; Smith 2001), a growingbody of evidence indicates that the staple food


Fig. 5. Drawing of a maize ear with staminate tipas seen on Zapotec urns. This unusual feature that waspreserved iconographically by ancient potters was animportant clue to solving the mystery of the origin ofmaize.

Fig. 6. Primitive maize cob from Coxcatlan Cavein the Valley of Tehuacan.

and sacred grain of prehistoric America is anintrogressive polyploid that combines the germ-plasm of teosinte (Zea sp.) and gamagrass(Tripsacum sp.) (Eubanks 1995, 1997, 2001a,b; MacNeish and Eubanks 2000). If the latterinterpretation is accurate, maize could haveevolved by punctuated equilibrium (Gould1984) within a few generations of human ex-ploitation and artificial selection. This paper isan interdisciplinary synthesis of the latest evi-dence from biosystematics, experimental cross-es, archaeology, and comparative genomics thatpoints to a subtle, but dramatic, genetic leapforward that set the course for a rapid, com-plex, reticulate evolutionary trajectory inti-mately linked to human intervention, manipu-lation, cultivation, and dispersal.

TAXONOMY AND CROSSABILITYMaize belongs to the Tripsacinae (syn. May-

deae), a sub-tribe of the Andropogoneae (Steb-bins and Crampton 1961), a large tribe of trop-ical grasses that includes the other importanteconomic plants sorghum and sugar cane. Likemost grasses, the Tripsacinae are wind-pollinat-ed. A distinguishing characteristic for the Amer-ican members and Coix in the Orient is they aremonoecious, i.e., the male and female flowers(spikelets) are borne separately on the sameplant. In teosinte (Zea sp.) and gamagrass (Trip-

sacum sp.), the grain is borne inside a boneyfruitcase composed of two indurated outerglumes. Generally, there are five to nine seedson a spike that shatters upon maturity for naturaldispersal. A distinct difference between Zea andTripsacum is that in Zea the male (staminate)flowers are produced in their own inflorescence(the tassel) that appears at the tips of the stalksand sometimes at the tips of ears, and the female(pistillate) flowers usually develop separatelyterminating the lateral branches. On the otherhand, in Tripsacum the male flowers are usuallyonly borne directly above the female flowers onthe same spike (Fig. 9). The flowering spikeswith unisex divisions are produced on numeroustall canes all originating at ground level from thebasal rosette of leaves. In contrast to the teosinteplant habit of multiple branches that producetiny ears in the leaf axils, Tripsacum’s floweringshoot provides a genetic preview of the singlestalk of domesticated maize.Teosinte.—According to Iltis and Doebley

(1980), the teosintes include three subspecies ofZea mays: Z. m. ssp. mexicana, Z. m. ssp. par-viglumis, and Z. m. huehuetenangensis; andthree separate species: Z. luxurians, Z. diploper-ennis, and Z. perennis. Iltis and Benz (2000) re-cently reported teosinte from the Pacific coast ofNicaragua, and they have assigned it a new spe-cies name, Z. nicaraguensis. The teosintes areendemic to Mexico and Central America. Exceptfor two rhizome forming perennials: the tetra-ploid Z. perennis and Z. diploperennis, the teo-sintes are annuals that must reproduce everyyear from seed. With the exception of the tet-


Fig. 7. A flowering spike and mature seeds of te-osinte (Zea diploperennis).

Fig. 8. An example of the highly productive maizeear with hundreds of kernels.

raploid Z. perennis, all have the same diploidchromosome number (n � 10) and readily re-combine with maize.Gamagrass.—Gamagrass, Tripsacum spp.,

is a rhizomatous perennial that ranges fromeastern North America to South America. Cut-ler and Anderson (1941) recognized seven spe-cies, and two more were added a few years later(Randolph 1950; Randolph and Hernandez-X.1950). Subsequent surveys of Tripsacumthroughout Latin America expanded the genusto include as many as 16 species (Berthaud etal. 1997; Brink and de Wet 1983; de Wet et al.1976, 1983; Gray 1974; Randolph 1976), eachwith a gametic chromosome number of n � 18.Most species have 36 or 72 chromosomes, butplants with up to 108 chromosomes have beenreported. There are two sections of the genus:Tripsacum and Fasciculata. Section Tripsacumis distinguished by erect flowers and both mem-bers of the pair of male flowers are sessile (at-tached directly to the rachis). In section Fasci-culata, the inflorescences are pendulous, andlike Zea, one member of the pair of male flow-ers is sessile, but the other one is pedicellate

(attached to a small stem called a pedicel). Thefirst successful crosses between Tripsacum andZea were made by applying Tripsacum pollento maize silks. Special pollination and embryorescue techniques were required to recover hy-brid progeny (Mangelsdorf and Reeves 1939).(Maize � Tripsacum) hybrids have a high de-gree of sterility. Although the chromosomenumber most often reported in the literature for(maize � Tripsacum) progeny is 28 or 46, hy-brids with 20 chromosomes are also produced(James 1969). Numerous attempts were madeto cross Tripsacum and teosinte (Mangelsdorf1974; Tantravahi 1968), but none yielded via-ble progeny until the diploid perennial teosintewas discovered on the verge of extinction in themountains of Jalisco, Mexico in the late 1970s(Iltis et al. 1979). Wilkes (1982) had the fore-sight to recognize that this new teosinte couldpossibly break the crossability barrier betweenTripsacum and teosinte.


Fig. 9. Inflorescence of gamagrass (Tripsacumdactyloides) showing the male flowers above the fe-male flowers on same spike.

Fig. 10. Female spike of F1 (teosinte � gamagrass)showing steps in the transformation of teosinte intomaize that are missing from the fossil record, i.e. two-ranked with paired kernels in fused cupulate fruitcases.

EXPERIMENTAL CROSSESAlthough the chromosome number of this

new perennial teosinte (Zea diploperennis Iltis,Doebley and Guzman) is the same as maize andthe annual teosintes (2n � 20), its chromosomesresemble Tripsacum chromosomes in size andstructure more closely than any of the Zeas. Eu-banks hypothesized that this similarity in chro-mosome architecture would enhance crossabilitybetween the two species, and conducted cross-ability tests between this unusual teosinte and itsEastern gamagrass cousin (Tripsacum dactyloi-des L.). Fully fertile recombinant progeny wererecovered from replicated experimental crosses(Eubanks 1989, 1992, 1994, 1995, 1996). Thesuccess of these crosses is significant for fourreasons: (1) these are the first viable recombi-nants recovered from crossing teosinte and gam-agrass; (2) recombinant derivatives are fully fer-tile; (3) they are cross-fertile with maize and

thus provide a genetic bridge for employing ben-eficial genes from Tripsacum in maize improve-ment using conventional breeding practices (Eu-banks 1994, 1998, 2002a, 2002b); (4) some ofthe flowering spikes produced by recombinantsegregants have paired kernels in fused yokesalternating in a decussate (arranged in oppositepairs at right angles to those above and below)manner to each other (Fig. 10), a key link in thetransformation of teosinte into maize that ismissing from the fossil record (Galinat 1985,1992, 2001). Plants in a population of segregat-ing intercross progeny whose maternal lineagetraced through Tripsacum produced ears (Fig.11) with all the features of ancient maize (Benz2001; Eubanks 2001a, b; MacNeish and Eu-banks 2000; Mangelsdorf, MacNeish, and Gali-nat 1964, 1967a). They are small with four toeight rows of paired kernels in shallow cupulesborne on a slender cob, and the kernels are en-closed individually by long, soft glumes. Theseplants are perennial, have 20 chromosomes, and


Fig. 11. F2 (teosinte � gamagrass) derivative thatreconstructs a prototype of ancient maize from Tehua-can illustrated in the insert. Drawing courtesy of Wal-ton C. Galinat.

can be propagated by seed or vegetatively. Be-cause prototypes that closely match ancientspecimens can be reconstructed experimentallyby crossing diploperennis teosinte and gama-grass, it is hypothesized that domesticated maizeemerged from human selection and cultivationof natural hybrids between teosinte and Tripsa-cum.

ANCIENT MAIZE

The question of the origin of maize involvesbiology and archaeology. In order to be mean-ingful, the findings of experimental researchmust be consistent with the archaeological rec-ord documenting maize evolution. Therefore,viewing the biological picture through the ar-chaeological lens is critical to bringing themechanism of how teosinte was transformedinto maize into sharper focus and alignment withthe concomitant cultural developments intimate-

ly connected with the biological evolutionarytrajectory of maize.

THE ARCHAEOLOGICAL RECORDThere is no evidence in the archaeological rec-

ord for a long, gradual evolution during whichthe mutations that transformed teosinte intomaize might have accumulated (Benz 1999).Segregating experimental (teosinte � gama-grass) crosses reveal that the transition from te-osinte to maize could have happened rapidly andmay have required only a few generations of in-tercrossing. Therefore, the archaeological recordis re-examined in light of the new biological ev-idence from experimental crosses. Ears pro-duced by some segregating F1 (teosinte � gam-agrass) progeny (Fig. 10) simulate a missing linkin the evolutionary history of maize (Eubanks1995, 1997, 2001a, b; MacNeish and Eubanks2000). They have a pair of kernels in a singlecupule like maize, and the cupulate segments donot break apart easily as they do in teosinte andTripsacum. On the other hand, these first gen-eration recombinants lack other distinguishingfeatures of maize: (1) the kernels are not fullyexposed; (2) the glumes are not soft; and (3)they are not multi-ranked. Although the kernelsare slightly exposed, they are still partially en-cased by hard outer glumes. Like maize, thereis a pair of kernels in each cupule, but the rachisis two-ranked like teosinte and gamagrass.When I grew an F2 population of intercross re-combinants in which gamagrass was the mater-nal parent (Fig. 11), however, I recovered prog-eny with the features of early maize: paired ker-nels in open cupules; long, soft glumes; a multi-rowed cob; staminate tips on many of the ears.Different segregants from that population pro-duced ears that match macrofossils from Tehua-can, Oaxaca, and Tamaulipas.Tehuacan.—Cobs excavated from dry caves

in the Valley of Tehuacan, originally radiocar-bon dated to �5000 B.C. (Johnson and MacNeish1972), along with cobs from Guila Naquıtz cavein the Valley of Oaxaca in southern Mexico thatdate to around 4200 B.C. (Piperno and Flannery2001), are the earliest macrofossils of maize yetuncovered. The original Tehuacan 14C dates havebeen challenged by subsequent accelerator massspectrometry (AMS) dating of cobs in the col-lection at the Instituto Nacional de Antropologıay Historia Laboratorio Paleobotanico Laboratoryin Mexico City (Long et al. 1989). The AMS


Fig. 12. Archaeological cobs from the Valley of Tehuacan illustrate the complete evolutionary sequence ofmaize domestication from around 5000 B.C. (small cob on the top) until 1500 A.D. (largest cob on the bottom).Photo courtesy of the Robert S. Peabody Museum, Phillips Academy, Andover, MA.

dates indicated the early cobs might be 1500years younger than first reported, but concernswere raised about the AMS dates for a numberof reasons (Flannery and MacNeish 1997;MacNeish 2001; MacNeish and Eubanks 2000).Since subsequent AMS dates on maize macro-fossils from Oaxaca (Piperno and Flannery2001) and Zea pollen from Tabasco (Pope et al.2001) securely place early maize in the range of4000–5000 calendar years B.C., which alignswell with the original 14C dates, it is probablethat the original date for the first appearance ofmaize in the Tehuacan Valley of around 7000years ago is closer than more recently believed(Fritz 1994; Long et al. 1989; Long and Fritz2001). The Tehuacan Valley macrofossils rep-

resent the full spectrum of maize evolution (Fig.12). They chart the transformation of the firstcobs beginning around 5000 B.C. through the in-termediate stages of early cultivated maize, fol-lowed by early tripsacoid maize, and the devel-opment of the Nal Tel-Chapalote complex, latetripsacoid and slender popcorns that were pre-sent by 1500 A.D. when the Spanish arrived inthe New World (Mangelsdorf, MacNeish, andGalinat 1964, 1967a).The earliest cobs from the Tehuacan caves

were remarkably uniform in size, ranged from19 mm to 25 mm long, had four to eight rowsof kernels surrounded by long, soft glumes onthin cobs with shallow cupules (Mangelsdorf,MacNeish, and Galinat 1964, 1967a). Note how


Fig. 13. (Teosinte � gamagrass) F2 derivative earand cob that match ancient four-rowed cobs from Te-huacan and Oaxaca.

Fig. 14. Compare the ancient cob from Guila Na-quitz, Oaxaca labeled ‘‘e’’ to a (teosinte � gamagrass)derivative that shares the unusual features of the ar-chaeological specimen, i.e., single spikelets in trian-gular fruitcases borne in yokes. The AMS date for theancient cob is 6200 calendar years before present. Pho-to of archaeological specimen courtesy of Kent V.Flannery.

well these same primitive characteristics areseen in the two-ranked, four-rowed (teosinte �gamagrass) derivative with long, soft glumespictured in Fig. 13 (compare with the archaeo-logical specimen from Tehuacan in Fig. 6). Likemany of the early Tehuacan specimens, it had astaminate tip (male flowers) on the same rachisabove the kernels, a characteristic feature ofgamagrass.Oaxaca.—Four primitive maize specimens

were found just above Zone B1 in Guila Naquitzrockshelter in the Valley of Oaxaca (Benz 2001;Flannery 1986). The Oaxaca specimens wereexamined by Richard I. Ford and George Bea-dle, both of whom agreed they were either (1)‘‘maize-teosinte hybrids,’’ or (2) ‘‘primitivemaize’’ showing strong teosinte influence in itsgenetic background (Flannery 1986:8). One ofthese (Fig. 14e) may be the only known archae-ological ear with four ranks of single spikeletsresulting from a pathway of decussate yokesproducing four rows of spikelets. This pathwayalso appeared in ears of an F2 Tripsacum-teosin-te derivative in which Tripsacum was the ma-

ternal parent of its progenitor F1 hybrid (Fig.14). Both specimens have single spikelets in tri-angular fruitcases that are borne in yokes. Yok-ing is a precursor feature in the transition fromthe two-ranked, shattering fruitcases of the wildrelatives to the many-ranked, fused rachis thatforms the maize cob (Galinat 1970). AnotherOaxaca specimen is of a two-rowed cob (Benz2001). Similar two-rowed cobs were producedby segregants of the (teosinte � gamagrass) de-rivatives. AMS dating has determined these an-cient maize remains from Oaxaca are 6200 yearsold (Piperno and Flannery 2001). Since pollenwith the morphological attributes of modern ex-amples of Tripsacum and Zea was found in thestratigraphic layers below these macrofossils(Schoenwetter and Smith 1986), it can be in-


Fig. 15. An archaeological specimen of Tripsacum from Tamaulipas (on right) compared to an extant inflo-rescence of Tripsacum dactyloides (on left). Archaeobotanical specimen, Harvard University Herbaria, Cam-bridge, MA.

ferred that these specimens represent a segre-gating population of Tripsacum-teosinte recom-binants which were either being gathered in thewild, or were in the initial stages of domestictransformation into maize. This correlates withBeadle and Ford’s interpretation of the speci-mens as ‘‘primitive maize that shows strong te-osinte influence in its ancestry.’’Tamaulipas.—The only site where Tripsa-

cum (Fig. 15) and teosinte (Fig. 16) macrofossilshave been found together is from dry caves inTamaulipas in northeastern Mexico (Mangels-dorf, MacNeish, and Galinat 1967b). This is sig-nificant because it is outside the biogeographicrange of extant teosinte which is only foundwest of the Sierra Madre Oriental mountainrange today. Along with the Tripsacum and te-osinte remains is a specimen that was originallydescribed as a ‘‘maize-teosinte’’ hybrid becauseit had thicker stalks than teosinte and the rachisdid not break apart like the shattering fruitcasesof teosinte. However, Mangelsdorf and Galinatwere not confident about this designation andthe label accompanying the specimen reads‘‘maize-teosinte hybrid?’’ These specimens donot resemble ears produced by crosses betweenmaize and teosinte. Confounding features of

these specimens are they have a single kernelper cupule like teosinte and gamagrass, but theyhave long, soft papery glumes like early maize,and they have non-shattering rachises. This waswhy Mangelsdorf and Galinat had doubts aboutwhether the putative ‘‘maize-teosinte’’ hybridswere F1 hybrids or segregates appearing in sub-sequent generations. The specimen they sug-gested might represent the ancestral form ofmaize is a perfect match for an inflorescencephenotype produced by a Tripsacum-teosintesegregant (Fig. 17). Based on stratigraphy, thishybrid evidently postdates maize and Tripsa-cum. Therefore, it apparently represents a seg-regant of the Tripsacum-teosinte ancestral maizeprototype.North America.—Other archaeobotanical ev-

idence for human use of gamagrass in prehistoryis provided by large quantities of Tripsacumseeds recovered from rockshelters in easternNorth America (Gilmore 1931). If they are con-temporaneous with associated AMS dated Che-nopodium seeds from Newt Kash Hollow, Ken-tucky (Smith 1992), gamagrass was being ex-ploited by North American hunter-gatherers byor before 1400 B.C. At the time of their discov-ery, paleoethnobotanist Melvin R. Gilmore won-


Fig. 16. An archaeological example of teosinte from Tamaulipas (on right) compared to an extant inflores-cence of perennial teosinte (on left). Archaeobotanical specimen, Harvard University Herbaria, Cambridge, MA.

dered why they were being gathered because hethought it would be too difficult to extract themfrom the hard seedcoat (1931). However, thereis ethnohistorical documentation of Tripsacumfor food. Indigenous inhabitants of the FloridaEverglades frequently carry ‘‘a pocket full offruit cases to chew when out on hunting trips’’(Galinat and Craighead 1964). It is true the Trip-sacum grain is contained inside a hard cupulatefruitcase, but the edge of the outer glume doesnot overlap the inner glume as it does in teosin-te. Consequently, it is easy to extract the Trip-sacum kernel from the fruitcase with a simpletool or long fingernail (Fig. 18). Since gama-grass has three times the protein content ofmaize and teosinte (Bargman et al. 1988; Jack-son 1980), is delicious, and the kernels are aslarge as popcorn kernels, it is plausible that pre-historic hunters and foragers were exploiting thegrain for food, as well as the plant for other uses.The leaves and fibers can be used to make mats,bags, baskets, sandals, cordage, nets, etc.Feasibility for introgression between teosinte,

Tripsacum, and hybrids between them contrib-uting to the explosive evolution and wide radi-

ation of maize biodiversity is underscored byhuman dispersals of plants beyond their naturalranges (Darwin 1845; Dunn 1983; Gilmore1930; Yarnell 1965; Zeiner 1946). If, as indi-cated by experimental crosses and the archaeo-logical record, Tripsacum introgression initiatedthe mutagenic action leading to the transforma-tion of teosinte into maize, there should be ev-idence for this in the molecular signature ofmaize. Unique fragments of DNA that representalleles (alternate forms of a gene, also referredto as polymorphisms) should be found in Trip-sacum and maize that are not present in teosinte.Now we explore the molecular archaeology ofmaize to search for new genetic evidence to shedlight on this mystery.

MOLECULAR ARCHAEOLOGYEARLY MOLECULAR STUDIES

When molecular tools began to be employedfor elucidating the evolutionary histories ofplants in the 1980s, the early evidence was fromisozyme (an enzyme with different forms butsimilar chemical properties) studies (Doebley et


Fig. 17. The archaeobotanical ‘‘hybrid’’ from Tamaulipas (on left) is a perfect match for a (teosinte �gamagrass) derivative (on right). The specimens have a single kernel per cupule like teosinte and gamagrass,but long, soft glumes and non-shattering rachises like maize. Archaeobotanical specimen, Harvard UniversityHerbaria, Cambridge, MA.


Fig. 18. Rachid segments of Tripsacum dactyloi-des and removed kernels. One seed has a pair of ker-nels in the cupulate fruitcase, a distinguishing char-acteristic of maize that has precedent in Tripsacum.

Fig. 19. Diagram of the ten linkage groups of Zea showing the map locations of the RFLP probes used forthe comparative genomic study of Zea and Tripsacum. Not shown are the mitochondrial probes and someunmapped TDA markers.

al. 1984, 1985, 1987; Goodman and Stuber1983; Mastenbroek et al. 1981; Smith et al.1984, 1985; Stuber and Goodman 1983). In iso-zyme analysis, bands are formed by migrating

proteins on an electrophoretic gel. The visual-ized bands resemble bar codes. The banding pat-terns of different species are compared and onesthat share the greatest number of bands are in-terpreted as being most closely related. Analysisof 13 isozymes in maize and teosinte indicatedthat maize was most closely related to an annualteosinte endemic to the Balsas River drainage ofGuerrero in southwestern Mexico (Z. m. ssp.parviglumis). Although a valid explanation ofthese data is they signal recent introgression be-tween maize and extant teosinte, Doebley (1990)interpreted this early molecular evidence asproof that maize descended directly from par-viglumis teosinte. Subsequent DNA studies haveconfirmed: (1) a close relationship between te-osinte and maize (Buckler and Holtsford 1996;Zimmer, Jupe, and Walbot 1988); (2) a close re-lationship between teosinte and Tripsacum (Lar-son and Doebley 1994); (3) a Tripsacum endem-ic to Central America is a hybrid between Trip-sacum and Zea (Talbert et al. 1990); (4) exten-sive introgression between maize and teosinte(Gaut and Clegg 1993); (5) the genetic diversityof prehistoric maize is comparable to modernmaize (Goloubinoff, Paabo, and Wilson 1993);and (6) specific molecular markers are linked tofour or five genes that distinguish maize fromteosinte (Doebley 1992; Doebley et al. 1990;Doebley and Stec 1991; Doebley, Stec, and Gus-tus 1995; Dorweiler et al. 1993).


TABLE 1. TAXA EMPLOYED IN COMPARATIVE GENOMIC ANALYSIS.

Scientific name Region Source Accession No.

Zea mays ssp. maysNal TelChapalotePiraPolloW64AB73

MexicoMexicoColombiaColombiaU.S.U.S.

M. M. GoodmanM. M. GoodmanUSDAM. M. GoodmanJ. CoorsA. Hallauer

Yuc7, 72-73Sin2, 70-75P.I. 4451271-72

Z. m. ssp. parviglumisZ. m. ssp. mexicanaZ. huehuetenangensisZ. luxuriansZ. diploperennisZ. perennisTripsacum dactyloidesT. d. meridionaleT. andersoniiT. maizarT. lanceolatumT. peruvianumT. cunidnamarce

MexicoMexico D.F.GuatemalaGuatemalaMexicoMexicoKansasColombiaVenezuelaGuatemalaArizonaPeruColombia

USDAUSDAUSDAUSDAH. H. IltisUSDAUSDAUSDAUSDAUSDAUSDAUSDAUSDA

P.I. 384061P.I. 566683P.I. 441934P.I. 306615Iltis no. 1250Ames 21875MIA 34680MIA 34597MIA 34435MIA 34744MIA 34713MIA 34503MIA 34631

RFLP GENOTYPINGRestriction fragment length polymorphism

(RFLP) genotyping is the DNA fingerprintingmethod employed by the maize seed industry toverify parentage and pedigree purity (Hoising-ton, Listman, and Morris 1998). It can also beused to analyze phylogenetic relationships, ge-netic diversity, and evolution of genome struc-ture (Doebley and Wendel 1989; Jansen, Wee,and Millie 1998; Kellogg and Birchler 1993). Toaddress the question of Tripsacum introgressioninto Zea, RFLP genotyping was employed forcomparative genomic analysis of the teosintes,several Tripsacum species, as well as ancient in-digenous land races and modern maize inbreds.If maize derived from recombination betweentwo or more progenitors, the maize fingerprintshould contain fragments of DNA that are foundexclusively in the molecular signatures of eachancestral taxa as well as fragments that areshared among the taxa. If maize and Tripsacumshare a substantial number of unique polymor-phisms at many genetic loci dispersed through-out the genome (all of the genetic material car-ried by a cell or individual) that are not presentin teosinte, it will provide robust evidence insupport of the hypothesis that maize arose as aresult of intergenomic recombination betweenteosinte and Tripsacum. On the other hand, if

maize is descended directly from teosinte, mostof the polymorphisms found in maize should bepresent in teosinte and few if any will be sharedwith Tripsacum.The important distinction of RFLP genotyping

is the bands visualize patterns of DNA (deoxy-ribonucleic acid, the hereditary material) ratherthan proteins as in isozyme analysis. The pro-cedures in RFLP genotyping are as follows: (1)extraction of genomic DNA from plant tissue(usually leaves); (2) incubation of the genomicDNA with restriction enzymes (molecular scis-sors) that cut the nucleotides (the basic buildingblocks of DNA) only where there is a precisesequence in the DNA code; (3) transfer of di-gested DNA into wells in an agarose gel matrix;(4) separation of the DNA fragments by size(molecular weight) as they migrate through thegel when an electric current is applied (electro-phoresis); (5) transfer of the DNA fragments toa membrane called a Southern blot; (6) hybrid-ization of radioactively labeled RFLP probes tothe Southern blot; (7) visualization of the patternof bands that light up wherever a genomic DNAfragment carries the same DNA sequence as theRFLP marker on an x-ray film (autoradiograph).The bands, referred to as polymorphisms, revealan individual’s genotype at any given locus. Anadvantage of RFLP genotyping in maize and its


TABLE 2. MOLECULAR MARKER LOCI WHEREPOLYMORPHISMS ARE SHARED BETWEEN ZEA ANDTRIPSACUM.

Probe

No. of bands shared by combined taxa

Maize/Trip*

Maize/Teosinte*

Trip/Teosinte*

Maize/Trip/Teosinte

UMC157UMC76ASG45CSU164UMC107UMC161UMC53UMC6UMC135

1——1——2—1

2—111—111

121—31132

6332—1311

UMC55UMC49UMC50UMC10BNL5.37UMC39bUMC15UMC63CSU25

——1—————2

12—21——2—

232214412

125415325

BNL5.46NPI386UMC42TDA62UMC19UMC52NPI409UMC27TDA66

—1—11—113

2—111311—

1231—264—

4222———31

TDA37UMC40UMC108UMC68UMC85TDA50NPI373

25111—2

1—————1

—1—2121

2232131

NPI393UMC28UMC62ASG8BNL15.40UMC110BNL8.32UMC80BNL16.06

12——11—3—

1—1—————4

11—114143

14112—143

P20020BNL9.11UMC124UMC48UMC7P10005UMC192UMC95

1—111—21

———2122—

14116331

32532—22

TABLE 2. CONTINUED.

Probe


Maize/Trip*

Maize/Teosinte*

Trip/Teosinte*

Maize/Trip/Teosinte

CSU61BNL5.09CSU54NPI285UMC163UMC44BNL10.13TDA168TDA48TDA53

111—2111—1

113—11——1—

—121111113

23123222—2

TDA80TDA204PMT1PMT2PMT3PMT4PMT5PMT6TDA16

1121———11

121——42—1

—27411212

—2223—31—

BNL7.71BNL6.32NPI114BNL5.62CSU3CSU92BNL8.29

1—2——11

13—12—1

4—12211

1—13—1—

NPI97UMC129UMC140UMC11UMC34UMC36UMC4UMC61BNL6.06

——11——211

23——211——

122———1—1

4—2211133

UMC97UMC2ASG24UMC121UMC32UMC60UMC54ASG7UMC3

—————1—1—

32131—141

1——12—1——

—11—12—11

UMC103UMC120UMC89BNL12.30NPI414UMC96UMC102P20725UMC104

————11—1—

122——42——

2—1111——1

—1——11123


TABLE 2. CONTINUED.

Probe


Maize/Trip*

Maize/Teosinte*

Trip/Teosinte*

Maize/Trip/Teosinte

UMC65UMC66UMC147UMC90P10017UMC134UMC43UMC46UMC21

———31——1—

1151—1223

21—3112—3

—11——122—

UMC132BNL14.07UMC113CSU147P20075P6005KSU5

—111———

4213122

2———1——

——111—1

UMC152UMC130NPI306TDA17TDA250TDA68BNL5.71UMC144BNL8.35

2——1——132

—12—11—32

—13—1—120

1————1—11

* Bands found exclusively in these taxa.

wild relatives is that hundreds of these molecularmarkers have been mapped (Neuffer, Coe, andWessler 1997), and 90–100 probes dispersedacross the ten linkage groups (chromosomes)provides complete coverage of the Zea genome.The 140 probes (Fig. 19) used in this study pro-vide a thorough screen of the Zea genome. Formore information visit the Maize Genome Da-tabase web site (http://www.agron.missouri.edu/probes.html). The high stringency of the hybrid-ization conditions restricts RFLP probes to pair-ing only with those fragments of genomic DNAthat contain a homologous DNA sequence. Be-cause of the precision and accuracy of RFLPgenotyping (Helentjaris, Weber, and Wright1988; Melchinger et al. 1991; Smith and Smith1991), it is routinely used by industry and gov-ernment for quality control, parentage verifica-tion, and to monitor pedigree purity. The advan-tage of RFLP genotyping over DNA sequencingin population and phylogenetic studies is that incontrast to comparison of DNA sequences at a

single genetic locus, it allows a large number ofsites scattered throughout the genome to bescreened (Jansen, Wee, and Millie 1998). In anexperiment investigating efficacy of RFLP anal-ysis compared to DNA sequences for four genes,the RFLP data produced the correct phylogeny;the DNA sequence data did not (Hillis, Huelsen-beck, and Cunningham 1994).

COMPARATIVE GENOMICSFive to 13 individuals of each taxon listed in

Table 1 were sampled for analysis. The teosin-tes, Tripsacum species, and races of maize weregenotyped using 140 RFLP probes mapped tothe ten linkage groups of maize (Fig. 19). Thegenomic DNA was isolated using a standardprotocol (Helentjaris, Weber, and Wright 1988),digested with one of four restriction enzymes(EcoRI, EcoRV, BamHI or HindIII), separatedby gel electrophoresis, and transferred to South-ern blots that were hybridized with 140 radio-actively labeled probes (Fig. 19). The autoradio-graphs were digitized and scored using theProRFLP software program and the digitizeddata were converted into an Excel file. The au-toradiographs were then checked visually to ver-ify accuracy of digitized results. Each locus pro-duced a number of bands of varying molecularweights. RFLP genotyping of the same acces-sions with the same probes was conducted bytwo different commercial laboratories (Bioge-netics Inc., Brookings, SD, and Linkage Genet-ics Inc., Salt Lake City, UT) to independentlycross check results.The restriction fragment data were tabulated

by molecular marker/restriction enzyme combi-nation for every taxa and examined to determinehow many polymorphisms are shared betweenTripsacum and Zea (see Table 2). Bands areshared between Tripsacum and Zea at 129 of the140 molecular marker loci. Thus, 92% of theloci tested yield evidence of Tripsacum intro-gression in Zea. Out of all 140 loci, maize hasa total of 456 polymorphisms that are present inteosinte and/or Tripsacum. Of those 456 poly-morphisms, 92 (20.2%) are unique to maize andTripsacum; 166 (36.4%) are unique to maize andteosinte, and 198 (43.4%) are present in bothTripsacum and teosinte. It can be inferred thatpolymorphisms uniquely shared between Trip-sacum and maize were likely derived from aTripsacum ancestor, and polymorphisms unique-ly shared between teosinte and maize were ap-


TABLE 3. GENETIC LOCI OF KEY TRAITS DISTINGUISHING MAIZE AND TEOSINTE.

Trait Designation Chromosome RFLP Markers

Teosinte branched 1Two-ranked earTerminal ear 1Teosinte glume architecture 1Multiple controlling effects

tb1tr1te1tga1

1L2S3L4S5S

UMC107, UMC140UMC6, UMC61UMC102, BNL6.06UMC42, BNL5.46TDA66, UMC27

parently inherited from a teosinte progenitor.Polymorphisms shared among all three taxacould have derived from either a teosinte or aTripsacum ancestor.

PRIMORDIAL GENES OF MAIZEGenes involved in the morphological trans-

formation of the teosinte spike into the maizeear are believed to map to five regions of themaize genome. It has been postulated that theseregions of major effects contain tightly linkedgenes that affect a single trait (Mangelsdorf1974), or they represent individual loci withpleiotropic (a single gene controlling the ex-pression of several, seemingly unrelated observ-able characteristics) effects on more than onetrait (Beadle 1939). Through genetic analysesusing molecular markers Doebley (1992) and hiscollaborators (Doebely et al. 1990; Doebley andStec 1991; Doebley, Stec, and Gustus 1995;Dorweiler et al. 1993) have identified specificRFLP probes linked to these putative loci (seeTable 3). If maize descended directly from teo-sinte, maize and teosinte should have a high fre-quency of the same polymorphisms for theseparticular loci.A segment of the long arm of chromosome 1

has a trait referred to as teosinte branched 1(tb1) that was involved in the reduction of themany-branches of teosinte to the single stalk ofmaize (Burnham 1961; Doebley and Stec 1995).At the UMC107 marker for this locus, maizeshares a unique polymorphism with teosinte andnone with Tripsacum; whereas at the UMC140marker for tb1, maize and Tripsacum share aunique polymorphism and maize and teosinte donot (refer to Table 2 and Fig. 19). A second lo-cus on the short arm of chromosome 2, two-ranked ear (tr1) is involved in the switch fromthe two-ranked (distichous) arrangement of theteosinte and Tripsacum spikes to the many-ranked, yoked (decussate) arrangement of themaize ear (Langham 1940). At the loci linked to

tr1, maize and teosinte share a unique polymor-phism at UMC6 that is not present in Tripsacum,but at UMC 61, maize and Tripsacum share aunique polymorphism not in teosinte (refer toTable 2 and Fig. 19). A third locus on the longarm of chromosome 3, terminal ear 1 (te1) isinvolved in reduction of branch internodes thatproduces an axillary ear rather than a long, tas-sel-tipped branch (Doebley, Stec, and Gustus1995; Matthews, Grogan, and Manchester1974). For te1, maize and teosinte share twounique polymorphisms at the UMC102 locus,and maize and Tripsacum share a unique poly-morphism at BNL6.06 (refer to Table 2 and Fig.19). A fourth gene referred to as teosinte glumearchitecture 1 (tga1) is responsible for thechange in the hard fruitcase encasing the teosin-te grain to the open cup-like structure holding apair kernels of maize subtended by thin, paperyglumes (Doebley, Dorweiler, and Kermicle1992). The UMC42 marker for tga1 revealsmaize and teosinte share a unique polymorphismat this locus, and at its neighboring locusNPI386 maize and Tripsacum share a uniquepolymorphism (refer to Table 2 and Fig. 19).The fifth region of major effect is on the shortarm of chromosome 5 which affects traits forinflorescence architecture (Doebley and Stec1991). Molecular marker TDA66 which maps tothis region of chromosome 5 has three poly-morphisms unique to maize and Tripsacum andnone unique to maize and teosinte. For markerUMC27 maize shares one unique polymorphismwith Tripsacum and one with teosinte (refer toTable 2 and Fig. 19). These molecular findingsclearly show that the major traits distinguishingmaize from teosinte are a composite of allelesfrom teosinte and Tripsacum, indicating domes-ticated maize is a product of intergenomic re-combination.Comparative analysis of the DNA fingerprints

thus provides support for the hypothesis that do-mesticated maize arose from recombination be-


tween one or more ancient populations of teo-sinte and Tripsacum. A close examination com-paring the fingergprints for RFLP markersmapped to the five regions associated with themajor effects in the transition from teosinte tomaize reveals that in each of those five chro-mosome segments maize has at least one DNAfragment inherited from teosinte and one fromTripsacum. Domesticated maize is clearly acomposite of the genomes of teosinte and Trip-sacum. These findings are congruent with resultsof experimental crosses between Tripsacum anddiploid perennial teosinte and the archaeologicalrecord documenting maize evolution.

CONCLUSIONSCrossing experiments reconstructing proto-

types of ancient archaeological specimens dem-onstrate how the teosinte spike could have beentransformed into the maize ear in a few gener-ations of intergenomic recombination betweenteosinte and Tripsacum directed by human se-lection. The morphological similarities of inter-cross recombinants to macrofossils from Tehua-can, Oaxaca, and Tamaulipas support this inter-pretation. Comparative genomic analysis ofmaize, teosinte, and Tripsacum indicates maizehas unique polymorphisms most likely inheritedfrom a Tripsacum ancestor, and other uniquepolymorphisms apparently derived by descentfrom a teosinte progenitor. This supports the hy-pothesis that maize originated from natural re-combination between the genomes of teosinteand Tripsacum, and resolves discrepancies be-tween the biological evidence and the archaeo-logical record.In her profound insight of a phenomenon she

described as genomic shock, Nobel laureate Bar-bara McClintock observed: ‘‘new species canarise quite suddenly as the aftermath of acciden-tal hybridizations between two species belong-ing to different genera’’ (McClintock 1984). Thesudden appearance of maize and its rapid bio-diversity radiation within a few millennia as wit-nessed in the archaeological record (Eubanks1999a) can thus be explained by human selec-tion for mutations resulting from genomic reor-ganization in response to the shock induced byintrusion of alien DNA into the Zea genome.The chromosomal rearrangements that permitformation of fertile progeny in crosses betweenperennial teosinte and Eastern gamagrass areprecise and recur (Eubanks 2001a). Viable

plants are produced as a result of precise geno-mic reorganization involving chromosome trans-locations and fusions that yield the same chro-mosome number as Zea. Eubanks (1999b) hasdemonstrated the mutations that give rise to nov-el phenotypes seen in early archaeological maizeare rapidly fixed and stably inherited in subse-quent generations. Therefore, human selectionand cultivation of plants with the recombinantgenomes of ancestral populations of these wildgrasses could have led to the quantum speciationof domesticated maize within a few generationsas proposed by Gould (1984). This synthesis ofthe biological evidence is consistent with the ar-chaeological picture in which the oldest remainshave all the characteristics that distinguish maizefrom its wild relatives. It is supported by mac-rofossil evidence from Tehuacan, Oaxaca, andTamaulipas. The variability in archaeologicalcob morphology is consistent with intergenomicrecombination and introgression during the earlystages of maize domestication. Natural hybridsbearing the primordial genes of the grain thatwould revolutionize human subsistence strate-gies in American prehistory first appeared inMexico over 6000 years ago. By integrating to-tal evidence from molecular systematics, exper-imental breeding, morphology, and archaeologyin future research, it should be possible to moreaccurately predict: (1) where and when maizefirst emerged; (2) who the original cultural andbiological players were; (3) if there were poly-phyletic origins involving more than one speciesof teosinte and/or Tripsacum; and (4) how andin what directions maize agriculture spread.Humanity’s symbiotic partnership with nature

that led to the domestication of maize thousandsof years ago has ultimately transformed theAmerican biocultural and physical landscape.The closer we come to solving its myterious or-igin, the greater our ability to sustain and reaprewards from our interdependent relationshipwith America’s golden grain will be.

ACKNOWLEDGMENTSResearch support was provided by the National Science Foundation

grant nos. 9660146 and 9801386 to M. W. Eubanks, and DEB-94-15541and IBN-9985977 to the Duke University Phytotron. I thank JamesCoors, Arnold Hallauer, Major M. Goodman for providing seed of themaize inbreds and land races, Hugh H. Iltis and the USDA GermplasmResearch Initiative Network, Ames, IA for seed of teosinte species, andRay Schnell and W. C. Wasik of the USDA-ARS South Atlantic AreaSubtropical Horticultural Research Station, Miami, FL for providingclonal material of the Tripsacum accessions. I thank Walton C. Galinatfor challenging me to think critically about the significance of the (teo-sinte � gamagrass) recombinants for maize evolution and modern corn


breeding. I am also grateful to Dr. Galinat for providing a copy of hisdrawing that, based on his study of the archaeological cobs from theTehuacan Valley, reconstructs what the earliest maize looked like. I wantto express my gratitude to colleagues who read versions of the manuscriptand provided helpful feedback: Walton C. Galinat, H. Garrison Wilkes,John Willis, Paul Manos and James Knutson. I thank Donald H. Pfisterand Susan Rossi-Wilcox for enabling study and photography of the Ta-maulipas specimens in the Harvard University Herbaria. I thank Kent V.Flannery for providing digital images of the maize specimens from Oa-xaca. I am indebted to M. C. Aurora Montufar Lopez and Ruth ZavalCampos for their kindness in permitting me to study the Tehuacan col-lections in the Instituto Nacional de Antropologıa y Historia Laboratoriode Paleobotanica. Thank you to Margaret Houston for assistance with theTehuacan materials. The photograph illustrating the evolutionary se-quence of maize from the Tehuacan Valley is courtesy of the Robert S.Peabody Museum of Archaeology, Phillips Academy, Andover, MA. Lastbut not least, I owe a tremendous debt of gratitude to the late Richard S.MacNeish for the letter he wrote me after reading my 1995 article inEconomic Botany. Scotty was a visionary who was light years ahead ofmost of us. He immediately recognized the profound and far-reachingsignificance of the (teosinte � gamagrass) derivatives for the origin ofMesoamerican agriculture. Through his persistent nudging of this willingstudent of archaeology struggling with the complexity of maize genetics,I was eventually able to uncover the pathway that permits experimentalrecovery of prototypes of early maize when I investigated a possiblematernal effect on the expression of key genes involved in speciation anddomestication. Thank you, Scotty, posthumously.

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