Palaeoanthropology: Tracking the first Americans
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/jour...
by TOM D. DILLEHAY
04 September 2003
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.
Questions of which human populations first arrived in the Americas, and when, where and how this happened, have been debated by scientists for
decades(1). It has long been presumed that the first people entering the New World were the direct ancestors of present-day Native Americans and that
they arrived in America from northeast Asia about 12,000 years ago(2). But this theory has been challenged by new archaeological discoveries and by
findings of early human remains bearing anatomical similarities to the people of south Asia and the southern Pacific Rim(3,4). Writing on page 62 of
this issue, Gonz?lez-Jos? et al.(5) add more fuel to this heated debate. They present a comparative study of early historic human skulls from Baja
California, Mexico, and their findings lend weight to the view that not all early American populations were directly related to present-day Native
Americans.
Human skeletal remains have long been used by palaeoanthropologists to model early human migration. The conventional view is that different skeletal
populations with similar craniofacial features (skull form) shared a common ancestry and were genetically related, whereas different features reflect
different ancestry. Migration histories and evolutionary forces explain the similarities or differences.
Piecing together the ancestry of the Americas has been difficult, as early human remains dating from about 10,000 years ago (the end of the last ice
age) are fragmentary and scarce. Scientists have typically reconstructed the missing pieces of the most ancient skulls by extrapolating backwards from
later, more complete skeletons. Ancient American skulls reconstructed in this way were anatomically indistinguishable from early northeast Asians and
also from present-day Native Americans(2). So a theory arose, supported by dental and other archaeological data(6), that the first humans entering the
Americas were northeast Asians who arrived in three successive migrations beginning around 12,000 years ago. These founding colonizers were thought to
be big-game hunters, equipped with so-called Clovis spears(7), who rapidly populated the Western Hemisphere and gave rise to present-day Native
Americans.
But more recent archaeological discoveries suggest that there were several different founding populations, arriving from different places, each with
different lifestyles and technologies(8). Some populations not only hunted big game but also exploited a wide range of plant and animal life. To
complicate matters further, it is no longer certain that the first colonizers arrived about 12,000 years ago ? some archaeological sites in South
America date from 12,500 years ago, which suggests that the first humans arrived at least 15,000 years ago.
A similar pattern of diversity is emerging from statistical analyses of cranial and facial measurements of some of the oldest skeletons found in the
Americas. The archaeological and skeletal data have led to a new model, in which the Palaeoamericans ? the proposed first arrivals in the New World ?
were not northeast Asians. They came instead from south Asia and the southern Pacific Rim, and they probably shared ancestry with ancient Australians
and other southern populations(3,9). A second group of humans then arrived from northeast Asia or Mongolia, and it was this second population that
adapted to the warming climate after the Ice Age and gave rise to the modern Amerindians (an ancient population of Americans whose skeletal remains
make up most of the human material found in the New World) and the present-day Native Americans. So according to this theory, the Palaeoamericans are
unrelated to most modern Amerindians and to the Native Americans.
Gonz?lez-Jos? et al.(5) now propose a more complex view of American ancestry. These authors analysed the skeletal remains of 33 modern Amerindians
from early historic times, excavated from the tip of the Baja peninsula in Mexico. Surprisingly, the craniofacial features of these Baja Amerindians
show closer affinity to the Palaeoamerican skulls than to other modern Amerindian remains. The Baja Amerindian and Palaeoamerican skulls have similar
long and narrow braincases and relatively short, narrow faces, implying a common ancestry with the ancient inhabitants of south Asia and the Pacific
Rim. Gonz?lez-Jos? et al. confirm that modern Amerindian skulls from other areas are similar to ancient northeast Asian remains. Their new data add to
accumulating evidence of morphological differences between early humans from different areas of the Americas(8,9).
The authors consider several potential explanations to account for the presence of Palaeoamerican traits in the Baja Amerindian skulls, but they
suggest that the best explanation is that the Palaeoamericans were the direct ancestors of the Baja Amerindians. After the Ice Age, the increased
aridity could have geographically isolated the founding Palaeoamerican population in the Baja area, and limited its gene flow with other modern
Amerindian groups.
Do the new findings tell us anything more about when the first humans arrived in the Americas? The authors do not fully discuss the chronological
implications of their work, but their interpretation of shared ancestry between the Palaeoamericans and the Baja Amerindians might best fit a model of
Palaeoamerican arrival about 11,000?12,000 years ago. There is no direct evidence to support this view, but if the Palaeoamericans had arrived 15,000
years ago or earlier, the Baja population would have remained isolated for much longer. This seems unlikely, given the rate of population growth and
movement that probably occurred after initial colonization and then after the Ice Age when the climate warmed.
But could the similarities between the ancient Palaeoamericans and the later Baja Amerindians instead reflect the influence of other evolutionary
forces, such as gene flow or natural selection and convergent adaptation of different populations to similar local environments? Answering this
question will depend upon finding more isolated prehistoric populations showing ancient Palaeoamerican traits, and then establishing whether parallel
evolutionary forces were acting on them and whether they were derived from a single ancestry. But this will be a difficult task. Human remains from
the end of the Ice Age are scarce and often fragmentary, so we have only a vague notion of the skeletal characteristics of the ancient
Palaeoamericans. And we have a poor understanding of the migration history of different American populations and what kind of evolutionary forces
might have influenced them(10).
Given these limitations, the findings of Gonz?lez-Jos? et al. do not allow us to draw firm conclusions about the relationship between the ancient
Palaeoamericans and the later Baja Amerindians. But the importance of this and other studies(7,11) is that they suggest a different view of the
origins and interactions of early human populations in the Americas. What we really want to know is what took place within and between these
populations, how they changed over time, and how quickly they changed. These issues can be resolved only by obtaining more skeletal data11 and by
combining them with regional archaeological records, which should provide information on the social and cultural histories of the different
populations. Slowly, we are realizing that the ancestry of the Americas is as complex and as difficult to trace as that of other human lineages around
the world.
References:
1. Dixon, E. J. Quat. Sci. Rev. 20, 277-299 (2001).
2. Dalton, R. Nature 422, 10-12 (2002).
3. Neves, W. A. & Pucciarelli, H. M. J. Hum. Evol. 21, 261-273 (1991).
4. Dillehay, T. D. The Settlement of the Americas: A New Prehistory (Basic Books, New York, 2001).
5. Gonz?lez-Jos?, R. et al. Nature 425, 62-65 (2003).
6. Turner, C. G. Mem. Calif. Acad. Sci. 27, 123-158 (2002).
7. Meltzer, D. J. Mem. Calif. Acad. Sci 27, 27-58 (2002).
8. Lahr, M. M. The Evolution of Modern Human Diversity (Cambridge Univ. Press, 1998).
9. Neves, W. A. et al. Homo 50, 258-263 (1999).
10. Steele, D. G. & Powell, J. F. Mem. Calif. Acad. Sci. 27, 93-122 (2002).
11. Jantz, R. E. & Owsley, D. W. Nevada Hist. Soc. Q. 40, 62-84 (1997).
|
![[*]](images/xmbforum/default_icon.gif){kind=icon}
