Tracking the First Americans Tom D

news and views Tracking the first Americans Tom D. Dillehay

A study of 33 ancient skulls excavated from Mexico invites us to reconsider our view of the ancestry of the early Americans. Unlike most other early American remains, the skulls resemble those from south Asian populations.

uestions of which human popula- different ancestry. Migration histories and Figure 1 The skull of an tions first arrived in the Americas, evolutionary forces explain the simi- early American. The Qand when, where and how this hap- larities or differences. Kennewick Man, pened, have been debated by scientists for Piecing together the ancestry excavated from decades1. It has long been presumed that of the Americas has been diffi- Washington the first people entering the New World cult,as early human remains state, lived were the direct ancestors of present-day dating from about 10,000 about 9,300 E. THOMPSON/AP PHOTO Native Americans and that they arrived in years ago (the end of the years ago. His America from northeast Asia about 12,000 last ice age) are fragmentary craniofacial years ago2. But this theory has been chal- and scarce. Scientists have features, which lenged by new archaeological discoveries typically reconstructed the do not resemble and by findings of early human remains missing pieces of the most living Native bearing anatomical similarities to the ancient skulls by extrapo- Americans, are typical people of south Asia and the southern lating backwards from of the skulls that are Pacific Rim3,4 (Fig. 1). Writing on page 62 later, more complete skel- increasingly being found of this issue, González-José et al.5 add more etons. Ancient American in ancient American sites. fuel to this heated debate. They present a skulls reconstructed in comparative study of early historic human this way were anatomically indistinguishable But more recent archaeological discover- skulls from Baja California, Mexico, and from early northeast Asians and also from ies suggest that there were several different their findings lend weight to the view that present-day Native Americans2. So a theory founding populations, arriving from differ- not all early American populations were arose, supported by dental and other ent places, each with different lifestyles and directly related to present-day Native Amer- archaeological data6, that the first humans technologies8. Some populations not only icans. entering the Americas were northeast Asians hunted big game but also exploited a wide Human skeletal remains have long been who arrived in three successive migrations range of plant and animal life.To complicate used by palaeoanthropologists to model early beginning around 12,000 years ago. These matters further, it is no longer certain that human migration. The conventional view founding colonizers were thought to be big- the first colonizers arrived about 12,000 is that different skeletal populations with game hunters, equipped with so-called years ago — some archaeological sites in similar craniofacial features (skull form) Clovis spears7, who rapidly populated the South America date from 12,500 years ago, shared a common ancestry and were geneti- Western Hemisphere and gave rise to present- which suggests that the first humans arrived cally related,whereas different features reflect day Native Americans (Fig.2a). at least 15,000 years ago. A similar pattern of diversity is emerging Present day from statistical analyses of cranial and facial abNative Native Americans Americans measurements of some of the oldest skel- etons found in the Americas. The archaeological and skeletal data have led to a new Modern Baja Modern model,in which the Palaeoamericans — the Amerindians Amerindians Amerindians proposed first arrivals in the New World — were not northeast Asians. They came Arrival from Arrival from instead from south Asia and the southern Arrival from Palaeoamericans Arrival from northeast northeast Pacific Rim, and they probably shared northeast Arrival from south Asia/ northeast Asia/Mongolia Asia/Mongolia ancestry with ancient Australians and other Asia/Mongolia southern Pacific Rim Asia/Mongolia 10,000 southern populations3,9. A second group of to humans then arrived from northeast Asia or Three successive migrations 15,000 Two migrations from northeast Asia years ago Mongolia, and it was this second population that adapted to the warming climate Figure 2 Tracing American ancestry. Analysis of skeletal remains has led to at least two models to after the Ice Age and gave rise to the modern explain the origin of early human populations in the Americas. a, It was originally thought that Amerindians (an ancient population of the first colonizers were the direct ancestors of present-day Native Americans, who arrived from Americans whose skeletal remains make up northeast Asia and possibly central Asia in three successive migrations about 12,000 years ago. b, most of the human material found in the More recent analyses of the craniofacial features of skulls dating from the end of the Ice Age suggest New World) and the present-day Native that the first arrivals were from south Asia or the Pacific Rim. These ‘Palaeoamericans’ were thought Americans (Fig. 2b). So according to this to be unrelated to the majority of modern Amerindian remains — a later group of colonizers from theory, the Palaeoamericans are unrelated northeast Asia were thought to have given rise to these late-prehistoric populations. Now, González- to most modern Amerindians and to the José et al.5 have found that a group of early historic Amerindian skulls from the Baja peninsula in Native Americans. Mexico bear a strong resemblance to the early Palaeoamericans, suggesting that the colonization González-José et al.5 now propose a of the Americas was more complex than had previously been suspected. more complex view of American ancestry.

These authors analysed the skeletal understanding of the migration history of populations. Slowly, we are realizing that remains of 33 modern Amerindians from different American populations and what the ancestry of the Americas is as complex early historic times, excavated from the tip kind of evolutionary forces might have and as difficult to trace as that of other of the Baja peninsula in Mexico. Surpris- influenced them10. human lineages around the world. ingly,the craniofacial features of these Baja Given these limitations, the findings of Tom D. Dillehay is in the Department of Amerindians show closer affinity to the González-José et al. do not allow us to draw Anthropology, University of Kentucky, Lexington, Palaeoamerican skulls than to other mod- firm conclusions about the relationship Kentucky 40506, USA. ern Amerindian remains. The Baja Amer- between the ancient Palaeoamericans and e-mail: [email protected] indian and Palaeoamerican skulls have the later Baja Amerindians. But the impor- 1. Dixon, E. J. Quat. Sci. Rev. 20, 277–299 (2001). similar long and narrow braincases and tance of this and other studies7,11 is that they 2. Dalton, R. Nature 422, 10–12 (2002). relatively short, narrow faces, implying a suggest a different view of the origins and 3. Neves, W. A. & Pucciarelli, H. M. J. Hum. Evol. 21, 261–273 (1991). common ancestry with the ancient inhabi- interactions of early human populations in 4. Dillehay,T.D.The Settlement of the Americas: A New Prehistory tants of south Asia and the Pacific Rim. the Americas.What we really want to know is (Basic Books, New York, 2001). González-José et al. confirm that modern what took place within and between these 5. González-José, R. et al. Nature 425, 62–65 (2003). Amerindian skulls from other areas are populations, how they changed over time, 6. Turner, C. G. Mem. Calif. Acad. Sci. 27, 123–158 (2002). 7. Meltzer,D.J.Mem. Calif. Acad. Sci 27, 27–58 (2002). similar to ancient northeast Asian remains. and how quickly they changed. These issues 8. Lahr, M. M. The Evolution of Modern Human Diversity Their new data add to accumulating can be resolved only by obtaining more (Cambridge Univ. Press, 1998). evidence of morphological differences skeletal data11 and by combining them 9. Neves, W. A. et al. Homo 50, 258–263 (1999). between early humans from different areas with regional archaeological records, which 10.Steele, D. G. & Powell, J. F. Mem. Calif. Acad. Sci. 27, 8,9 93–122 (2002). of the Americas . should provide information on the social 11.Jantz, R. E. & Owsley, D. W. Nevada Hist. Soc. Q. 40, The authors consider several potential and cultural histories of the different 62–84 (1997). explanations to account for the presence of Palaeoamerican traits in the Baja Amer- indian skulls, but they suggest that the best Earth science explanation is that the Palaeoamericans were the direct ancestors of the Baja Amerindians.After the Ice Age,the increased Just add water aridity could have geographically isolated Albrecht W. Hofmann the founding Palaeoamerican population in the Baja area, and limited its gene flow with A new model could explain why Earth’s upper mantle is depleted of other modern Amerindian groups. many trace elements. At a certain depth, minerals might release water, Do the new findings tell us anything creating a molten filter that traps trace elements in the mantle beneath. more about when the first humans arrived in the Americas? The authors do not fully andwiched between Earth’s thin crust gases and other ‘incompatible’ elements. discuss the chronological implications of and its metallic core lies a layer of The primitive layer serves as a reservoir for their work, but their interpretation of Spressurized rock at high temperature these elements (which were depleted from shared ancestry between the Palaeoameri- — the mantle. Convection in this layer the upper mantle when Earth’s crust was cans and the Baja Amerindians might best drives plate tectonics and sea-floor spread- formed) and it is occasionally sampled by fit a model of Palaeoamerican arrival about ing, but we know little about the pattern of deep-mantle plumes (Fig.1a). 11,000–12,000 years ago. There is no direct circulation. Indeed, current thinking about Over the past several years, however, seis- evidence to support this view, but if the mantle dynamics is in a state of turmoil. As mic tomography has given us increasingly Palaeoamericans had arrived 15,000 years we cannot observe convection directly, we detailed images of apparently cold ‘slabs’ ago or earlier, the Baja population would must piece together indirect evidence from (characterized by fast seismic velocities) have remained isolated for much longer. seismology, geochemistry, mineral physics, descending into the deep mantle right This seems unlikely, given the rate of fluid dynamics and numerical simulations through the 660-km boundary, effectively population growth and movement that of convection. But the evidence is contradic- cutting to shreds the simple picture of the probably occurred after initial coloniza- tory and has led to at least two conflicting mantle convecting in two nearly isolated tion and then after the Ice Age when the views about mantle movement. On page 39 layers. If cold ‘slabs’ descend into the deep climate warmed. of this issue, Bercovici and Karato1 propose mantle, there must be a corresponding But could the similarities between the a new model that might resolve this conflict. upward flow of deep-mantle material to ancient Palaeoamericans and the later Baja They suggest that water dissolved in the shallow levels (Fig. 1b). No matter what Amerindians instead reflect the influence of mantle might create a thin layer of melt at a specific form the exchange across the 660-km other evolutionary forces, such as gene flow depth of 400 km, causing an unexpected boundary takes, in this ‘whole-mantle’ or natural selection and convergent adapta- pattern of circulation in the mantle. model of mantle convection,it would within tion of different populations to similar local The two conflicting models for mantle a few hundred million years destroy any environments? Answering this question will convection (Fig. 1a, b, overleaf) are usually compositional layering that had possibly depend upon finding more isolated prehis- described as ‘layered’ convection (suppor- been inherited from early in Earth’s history. toric populations showing ancient Palaeo- ted by geochemists) and ‘whole-mantle’ Meanwhile, the geochemical arguments american traits, and then establishing convection (supported by seismologists). for a separate deep reservoir have not disap- whether parallel evolutionary forces were Geochemists have long insisted on the two- peared. Primordial noble gases are still acting on them and whether they were layered model, in which the mantle consists preferentially associated with ‘hot spots’2,at derived from a single ancestry. But this will of a relatively primitive layer below a depth of least some of which seem to come from deep- be a difficult task. Human remains from the 660 km — containing primordial noble mantle plumes3. And much of the upper end of the Ice Age are scarce and often frag- gases, trapped 4.5 billion years ago when the mantle remains highly depleted of incom- mentary, so we have only a vague notion Earth formed — and an upper layer that is patible trace elements, including the heat- of the skeletal characteristics of the ancient highly depleted of heat-producing elements producing thorium,uranium and potassium Palaeoamericans. And we have a poor (uranium, thorium and potassium), noble — also suggesting the presence of a less