Vol. 274 #4
The Peopling of the Americas
New genetic data suggests that the earliest Americans came from Asia in one or two waves-not more-challenging an earlier synthesis of linguistic, dental, and genetic data.
Six years ago, D. Andrew Merriwether was a master's student in the lab of geneticist Douglas Wallace at Emory University in Atlanta, learning to use genes to trace the ancestry of native American peoples. When he left to pursue doctoral studies at the University of Pittsburgh, Merriwether continued that research, expecting to bolster the conclusion coming from the Wallace lab: that genetically distinct groups of prehistoric people migrated to the Americas in three separate waves. But this year, Merriwether found himself publicly contradicting his mentor, in a series of papers suggesting that there was only a single migration. Although they remain personally friendly, mentor and student clearly are divided on this issue. Says Merriwether, now at the University of Michigan: "I feel badly about it because Wallace is the one who inspired me to go into this field. It's awkward."
Chalk up one more disagreement to one of the most contentious issues in human prehistory: the question of who settled the Americas. A decade ago, intellectual battles raged over a bold synthesis of linguistic, genetic, and dental data named after co-creator Joseph Greenberg, a Stanford University linguist. The Greenberg theory suggested that the first Americans arrived from Asia in at least three separate waves, each wave giving rise to one of three linguistic groups. Linguists opposed putting the diverse languages of most native Americans into one "Amerind" group, but the theory fit dental and genetic evidence from several labs, including Wallace's.
But now the pillar of support from genetics is showing cracks, thanks to new data from Merriwether and others, including a European team whose review is published in the October issue of the American Journal of Human Genetics. Additional DNA samples and better resolution show that native Americans aa diverse as the Eskimos of Alaska and the Kraho and Yanomami of Brazil share more gene types than previously thought, which suggest that they are descended from the same founding populations in Asia-and that their ancestors entered North America in only one or two migratory waves, says Oxford University evolutionary geneticist Ryk Ward. Scientists are already searching for those ancestors' closest kin in Siberia and Mongolia.
Not surprisingly, not everyone supports the new interpretations. Greenberg, for example, says that given the flip-flopping conclusions from the DNA data, he's simply ignoring it until geneticists reach consensus. Others caution against putting too much weight on any one type of genetic data, and Wallace still concludes that native Americans arrives in three migrations. He's philosophical about the new work, saying that "testing new hypotheses is what research is all about." But if the new studies are right, the notion of three or more separate migrations is unlikely, and the whole idea of marrying linguistic and genetic data comes into question. "This tends to confirm our conclusion that there isn't a relationship between genetic signatures of migrations and language," says Ward.
The Greenberg hypothesis, although controversial, had great appeal because it synthesized so many independent lines of evidence. And no matter what one was comparing-languages, teeth, or genes-the magic number was three. There were three linguistic groupings: Amerind (spoken by American Indians), Eskimo-Aleut (spoken by Eskimos and Aleutian Islanders), and Na-Dene (spoken by people of the Northwest coast of Canada and the United States). There were also three types of molar shapes and three genetically distinct populations. "Every time you come around, it's three," says Arizona State University bioarchaeologist Christy Turner, co-author of the hypothesis.
The dates also meshed with existing archaeological data. Using the degree of difference among languages, Greenberg calculated that the first language arrived in Alaska about 12,00 years before the present. That fits in with the first widely accepted archaeological evidence of culture in the Americas, the 11,500-year-old sites of the Clovis people.
At first, the genetic evidence seemed to tie in, too. Indeed, in the early 1990s, some of the best supporting evidence for the Greenberg hypothesis came from genes (Science, 15 January 1993, p.312). In general, the more similar genes are among two populations, the more closely the populations are related. To trace these similarities in native American DNA, Wallace, working with geneticist Antonio Torroni, now at the University of Rome, and graduate student Theodore Schurr, assembled hundreds of blood samples from 24 tribes from Alaska to Argentina. They analyzed the DNA coiled in the mitochondria, the energy factories of the cell. Most anthropological studies use this mtDNA because it mutates faster than nuclear DNA, allowing researchers to distinguish populations that recently separated. The mtDNA is also inherited only from mothers and so avoids the gene shuffling that can obscure the evolutionary trail of most nuclear genes.
Wallace's group used particular enzymes to cut the DNA into standard pieces, then looked for variations in the length of those segments-called restriction fragment length polymorphisms (RFLPs)-to indicate the presence of mutations. They got intriguing results, finding that native Americans carry only four variants of mtDNA, called haplogroups A, B, C, and D, with each group characterized by a different set of mutations. These variants were found in some East Asians and Siberians (but not in Europeans or Africans), which indicated that the mutations originally came from Asia. Not every indigenous group seemed to carry all four, however. Wallace's team found, for example, that although most Amerind speakers carried all four haplogroups, Na- Dene speakers carried just one (haplogroup A), and the Eskimo- Aleut speakers carried two (haplogroups A and D). So the team concluded that Amerind speakers descended from women who carried all four types, while the other two groups descended from women who carried just one or two. And this suggested that they came to the New World in three distinct waves from Asia, just as Greenberg had proposed.
But just as it seemed a consensus was emerging, geneticists cast their net wider, testing DNA of more native Americans and Asians and, in some cases, looking directly at DNA sequences. Merriwether, for example, was analyzing DNA samples of 1300 American Indians and other native groups, and he kept finding people, such as the Yanomami of Brazil, whose genes didn't fit in the four lineages identified by Wallace's team. Not only were there more than four genetic variants, but the four original types showed up on all three major language groups. Merriwether's work was confirmed by a group of South American researchers, led by Nestor A. Bianchi of the Multidisciplinary Institute of Cellular Biology in Argentina, who analyzed mtDNA from 25 populations and observed the same results. The presence of all four markers in each of the three linguistic groups makes it unlikely that the groups' ancestors came in different migrations thousands of years apart, says Merriwether. "Think of the source population as a bowl of colored marbles," says Smithsonian Institution molecular anthropologist Connie Kolman. "You won't pick out the ›same combination of| rare types three or four times of you reach randomly." So Merriwether and University of Pittsburgh geneticist Robert Ferrell joined forces with Francisco Rothhammer of the University of Chile to propose just a single migration, in which the first women to set foot in America carried all four haplogroups.
Once in America, this first wave of settlers spread out. Some pushed south, but others stayed in the northwest, where their numbers were drastically reduced-perhaps by bitter cold during the last glacial period that ended about 11,500 years ago. As a result, the northern populations, the ancestors of the Na- Dene and Eskimo-Aleuts, lost their original genetic diversity. Their numbers eventually bounced back, but with fewer copies of haplogroups B, C, and D than carried by their southern relatives.
The latest word on the settling of the Americas comes from Europe-and it too challenges the three-migration theory. AN interdisciplinary group tackled the problem by re-analyzing recent data on mtDNA sequences, pooling studies of DNA from a total of 574 native Americans and Siberians, says molecular biologist Peter Forster, a graduate student working with Hans- Jurgen of Hamburg University, Rosalind Harding of the Institute of Molecular Medicine at Oxford, and Torroni in Rome.
Instead of looking for markers that may accompany mutations, this team looked directly at the DNA sequence-a slower but more certain way to find variations. They entered mtDNA sequences into a computer and searched for matches between American tribes, Asians, and Siberians. Ironically, they found all four of the original variants in almost all the Amerinds-supporting the part of the Greenberg hypothesis that linguists dislike most, namely that ancestors of all the Amerinds came in one wave. Overall, their more powerful method detected nine founding mtDNA sequences in native American peoples, and some of these sequences were only in Na-Dene speakers, Eskimos, and coastal Siberians, suggesting that those groups emerged from a common ancestral population, not from separate groups, as Wallace had proposed.
The team put this data together and proposed that the ancestors of the Amerinds came in the first wave from northeastern Siberia and carried all the variants, some of which were lost in northern Asians and Americans, perhaps due to climate. Later, the survivors rebounded, probably in Beringia, and gave rise to the Na-Dene and Eskimos. This scenario allows for either one or two migrations into North AMERICA, depending on whether the homeland of the surviving northerners was in North America or Siberia. Foster says: "We call it a re-expansion. It's a matter of taste whether you call it a separate migration."
The European group also explored an even more controversial issue, the timing of these migrations, by using the amount of genetic difference among populations as a molecular clock. The Amerind speakers show the most diversity, so the team concluded that they arrives in the first wave, 20,000 to 25,000 years ago. The predates the Clovis culture but matches dates from Wallace's group and from several new South American archaeological sites (Science, 19 April, p. 346). The re-expansion, they say, happened about 11,300 years ago-the time of the Clovis people.
In search of a homeland
If all of today's native Americans do go back to a single population in Asia, which one? The multiple-migration advocates put their founders in Siberia, as does the European team, because Siberians share some founding variants with Na-Dene speakers and Eskimos-and live close to the land bridge to the Americas. Merriwether and Kolman are skeptical, however, because all modern Siberians tested so far lack haplogroup B. In separate papers, Merriwether, and Kolman and her Smithsonian colleague Eldredge Bermingham have proposed that the founders may have been Mongolians, because they carry all four haplogroups.
Even on the number of migrations, there is no consensus. Satoshi Horai of the Native Institute of Genetics in Mishima, Japan, for example, notes that his analyzes of the genetic distance among native American peoples suggests that there are four groups that have been isolated for a relatively long period of time. He concludes that there were four separate migrations. All this disagreement prompts Greenberg to simply ignore the new mtDNA data. He says: "Every time, it ›mtDNA| seems to come to a different conclusion. I've just tended to set aside the mtDNA evidence. I'll wait until they get their act together."
Even some geneticists are reluctant to claim they have solved the problem of the peopling of the Americas. "I am worried that too much weight is being given to mitochondrial DNA," says Stanford University geneticist Luigi Luca Cavalli- Sforza. He notes the mtDNA reflects only the movements of women. Because women in some hunter-gatherer societies join their husbands' families and move more then men, their mtDNA may not reveal the migrations of the whole population. So Cavalli-Sforza and colleague Peter Underhill, as well as other teams, are studying markers on the paternally inherited Y chromosome. So far, their results don't rule out additional migrations.
Indeed, researchers warn that more data from several genetic lineages will be needed to provide a picture of the peopling of the Americas. However, that time may not be so far off. One of the other authors of the Greenberg hypothesis, University of Arizona geneticist Stephen Zegura, is taking the new studies seriously and has taken a sabbatical this year to try to sort out the findings: "I'm trying to decide if after 10 years, is the time right to do a new synthesis?" The answer from a new wave of young geneticists, at least, is a resounding yes.
***** Science ***** Vol. 274 ***** 4 October 1996 *****
Greenberg and Others Respond
The "Greenberg Hypothesis"
Joseph H. Greenberg
Geoffrey K. Pullum
I would like to comment on the article "The peopling of the Americas" by Ann Gibbons, whose reporting I respect (Research News, 4 Oct., p. 31), concerning what
is therein called the "Greenberg hypothesis"that the Americas were peopled by three waves of migrations. It is said that this hypothesis is challenged by new genetics data suggesting two waves, or one wave. In this case, it appears that pertinent evidence was not taken into account.
Regarding the dating of the Amerind language, the impression may have been given that the linguistic age based on glottochronology is wedded to the Clovis culture of about 12,000 years before the present (B.P.). What we stated in 1986 (1) was that glottochronological dating has major problems, that long dates are seriously underestimated, and that "for Amerind we are dealing with a time period greater than 11,000 B.P. and beyond the limits of glottochronology."
Regarding genetic dates, Peter Forster, who is cited at the end of the article in a context of suggesting doubt regarding the three-migration scenario, has stated (2), "I was very excited to find that my results match your findings so closely."
If we evaluate the various lines of evidence, Eskimo-Aleut must be very recent and separate, given the mutual intelligibility of Alaskan and Greenlandic Eskimo and the undisputed acceptance of Eskimo-Aleut, even by conservative linguists. Eskimo and Na-Dene have entirely separate linguistic relatives in the Old World, Na-Dene being most closely related to Ket in Siberia, and Eskimo to quite other groups in northern Asiafor example, Chukchi.
Of all the sciences concerned, archaeology has the most assured dating. In a recent massive work (3), the editor Frederick West of the Harvard Peabody Museum points out (3, p. 525) the "striking parallelism" of the archaeological evidence to the three-migration theory." No one has seriously discredited Turner's dental evidence. The massive material on population genetics assembled by Cavalli-Sforza shows essential agreement, as does the early mitochondrial DNA evidence of Wallace and Torroni. A scientist should abandon an incorrect theory, but a fair appraisal of evidence from four independent sources, including new archaeological evidence, shows that this would be premature in the case of the "Greenberg hypothesis."
Joseph H. Greenberg
Department of Linguistics and Anthropology,
Stanford, CA 94305, USA
1.J. Greenberg, C. Turner, S. Zegura, Curr. Anthropol. 27, 477 (1986).
2.P. Forster, personal communication.
3.F. H. West, Ed., American Beginnings (Univ. of Chicago Press, Chicago, IL, 1996).
Gibbons's description of the objections to Greenberg's hypothesis (that virtually all the indigenous languages of the Americas have a common ancestry) has linguists saying that "it's impossible to trace the family tree of Amerind languages back to one 12,000-year-old ancestor, because written records go back only 5000 years." But tracing linguistic family trees is not based on written records. For most Amerindian languages, there are no such records. Language families are established by systematic comparison of phonetic data from currently spoken languages. Sometimes (although not in Amerindian linguistics), written records have been peripherally relevant to substantiating the validity of the methods used (as when 4000-year-old Hittite inscriptions turned out to confirm certain conclusions about early Indo-European).
What linguists typically disagree with Greenberg about is whether phonetic comparison of present-day languages could ever provide a warrant for suggesting a relationship going back 12,000 years. It is extremely unlikely. Languages appear to change fast enough that over that sort of time scale the phonetic similarities within a group of languages would be irretrievably obscured. That conclusion is (contra Greenberg) fairly secure, and is quite independent of the existence of writing. The languages of the Americas could, of course, have had a common northeast Asian ancestor spoken tens of millennia ago. Historical linguists don't dislike that idea; they just feel obliged to point out that linguistic evidence cannot confirm it.
Geoffrey K. Pullum
Department of Linguistics,
University of California,
Santa Cruz, CA 95064, USA
Response: The article's brief mention of written records was not intended to imply that these were used to trace the origin of American Indian languages, but simply to point out that, even in the best cases, where written records exist, many linguists think that it is impossible to trace languages back to a 12,000-year-old ancestral language.
Volume 274, Number 5292 Issue of 29 Nov 1996, pp. 1447 - 1451
©1998 by The American Association for the Advancement of Science.
Copyright © 1998 by the American Association for the Advancement of Science.
Mother Tongues Trace Steps of Earliest Americans
From 12 to 17 February, some 5400 people descended on Philadelphia for the annual meeting of the American Association for the Advancement of Science (AAAS, which publishes Science), celebrating its 150th anniversary this year. President Bill Clinton addressed a packed hall, unveiling Neil Lane as his next science adviser and Rita Colwell as the next NSF director (Science, 20 February, p. 1122). But there were more reasons to celebrate: symposia on everything from the earliest Americans to martian life-forms, two of the topics featured in this special news section.
When several prominent archaeologists reached a consensus last year that humans lived in South America at least 12,500 years ago, their announcement struck a lethal blow to what had been a neat picture of the peopling of the Americas--that the first settlers were big-game hunters who had swept over the Bering land bridge connecting Asia and North America about 11,000 years ago. But this revised view of prehistory, based on 2 decades of study of the South American site called Monte Verde in Chile, has spawned a new mystery: When did the ancestors of Monte Verde's inhabitants first set foot in North America? Archaeologists trying to address that question have come up empty-handed, as there are few reliably dated digs in America older than the Chilean site.
At the AAAS meeting, however, a possible answer emerged from another field--linguistics. Using known rates of the spread of languages and people, Johanna Nichols, a linguist at the University of California, Berkeley, estimates that it would have taken about 7000 years for a population to travel from Alaska to Chile.
Because that would put the first Americans' arrival squarely in the middle of the last major glacial advance, Nichols proposes that "the first settlers began to enter the
New World well before the height of glaciation"--earlier than 22,000 years ago.
That date is early but is in accord with recent genetic studies suggesting that the diversity of DNA across American Indian populations must have taken at least
30,000 years to develop (Science, 4 October 1996, p. 31). In addition, Nichols's extensive analysis of Northern Hemisphere languages also suggests that several
groups of Asians entered the New World, where they adapted rapidly to a range of habitats and adopted diverse ways of hunting and gathering.
This picture is winning favor with linguists. "I believe that her general analysis of the linguistic situation in the Americas is essentially right," says linguist Victor Golla of
Humboldt State University in Arcata, California. "We need a much longer period of diversification among American linguistic stocks than the 11,500 years" allowed
by the old view, he says. And although not totally embracing the linguistic findings, archaeologists acknowledge that, combined with other recent findings, Nichols's
results indicate that the old, simple view of the peopling of the Americas is dead. "The bottom line," says University of Kentucky, Lexington, archaeologist Tom
Dillehay, who excavated Monte Verde, is that "the picture is a lot more complex than it was."
To try to get a better fix on how long it would have taken people entering the New World to get to Monte Verde, Nichols surveyed 24 language families that had spread over vast distances, such as Eskimoan languages that traveled from Alaska to Greenland and Turkic tongues that migrated from Siberia to central Europe. She found that the fast-moving languages that spread on foot--the only way the first American settlers could travel--moved 200 kilometers per century on average.
With this yardstick, Nichols calculated that even if early Americans made a beeline, taking the shortest routes over the 16,000 kilometers of varied terrain from Alaska to southern Chile, the trek would have taken at least 7000 years. This would have put the Monte Verdeans' ancestors in Alaska when glaciers made it "probably impossible" to enter the continent, she says. Instead, Nichols argues, the evidence "strongly suggests" a migration before a major glacial advance began 22,000 years ago.
Nichols checked her result against those obtained by other methods. For example, the New World has 140 language families--almost half of the world's total--and she estimated how long it would have taken this rich diversity of tongues to develop. Nichols began by surveying nearly all the language families of the Northern Hemisphere, from Basque to Indo-European, to see how often new language families have split off from an ancestral stock. She found that, on average, 1.5 new language families arose in each ancestral stock over the last 6000 years. Plugging that rate into computer models--which included an allowance for new migrations that carried in new languages after the glaciers retreated--yielded 40,000 years as the minimum time required to produce so many language families.
Nichols also found that languages along the coasts of the Pacific Rim, from Papua New Guinea north to Alaska and then down the west coast of the Americas, share
a remarkable set of grammatical and phonological features, such as the sound "m" in the second-person pronoun (the singular "you" in English), verb order, and -numerical classifiers--words used in some languages when a number modifies a noun. These features set apart the coastal language families from those farther inland, indicating that coastal tongues were probably imported by later settlers.
These kinds of features prompted Nichols to propose the following scenario: The first immigrants from Asia crossed the Bering land bridge "well before" 22,000 years ago and made it to South America. After the glaciers retreated, some people spread north, where they gave rise to the Southwest's Clovis culture, perhaps, and to other peoples. Meanwhile, human beings were again on the move along the Pacific Coast in Asia, with some language families heading south to Papua New Guinea and others north over the land bridge into Alaska--where they could have crossed once the ice sheets melted 12,000 years ago. Yet another group arrived at least 5000 years ago, she argues, giving rise to the Eskimo-Aleut family of languages.
These early dates from linguistics and genetics are prompting archaeologists to reexamine and take more seriously their earliest sites of human occupation, including possible signs of a human presence at Monte Verde as early as 33,000 years ago, says Dillehay. "These findings of great antiquity from linguistics and genetics help us out, but in the end, we have to get the actual time dimension from the archaeological record." To linguists, however, a thousand words are worth a fossil.
Volume 279, Number 5355 Issue of 27 Feb 1998, pp. 1306 - 1307
©1998 by The American Association for the Advancement of Science.