Imprensa: "Native Americans Descended From A Single Ancestral Group, DNA Study Confirms"

Thu Apr 30 02:39:57 UTC 2009

University of California, Davis (2009, April 29). Native Americans
Descended From A Single Ancestral Group, DNA Study Confirms.
ScienceDaily. Retrieved April 29, 2009, from
http://www.sciencedaily.com/releases/2009/04/090428223836.htm

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Native Americans Descended From A Single Ancestral Group, DNA Study Confirms

ScienceDaily (Apr. 29, 2009) — For two decades, researchers have been
using a growing volume of genetic data to debate whether ancestors of
Native Americans emigrated to the New World in one wave or successive
waves, or from one ancestral Asian population or a number of different
populations.

Now, after painstakingly comparing DNA samples from people in dozens
of modern-day Native American and Eurasian groups, an international
team of scientists thinks it can put the matter to rest: virtually
without exception, the new evidence supports the single ancestral
population theory.

“Our work provides strong evidence that, in general, Native Americans
are more closely related to each other than to any other existing
Asian populations, except those that live at the very edge of the
Bering Strait,” said Kari Britt Schroeder, a lecturer at the
University of California, Davis, and the first author on the paper
describing the study.

“While earlier studies have already supported this conclusion, what’s
different about our work is that it provides the first solid data that
simply cannot be reconciled with multiple ancestral populations,” said
Schroeder, who was a Ph.D. student in anthropology at the university
when she did the research.

The study is published in the May issue of the journal Molecular
Biology and Evolution.

The team’s work follows up on earlier studies by several of its
members who found a unique variant (an allele) of a genetic marker in
the DNA of modern-day Native American people. Dubbed the “9-repeat
allele,” the variant (which does not have a biological function),
occurred in all of the 41 populations that they sampled from Alaska to
the southern tip of Chile, as well as in Inuit from Greenland and the
Chukchi and Koryak people native to the Asian (western) side of the
Bering Strait. Yet this allele was absent in all 54 of the Eurasian,
African and Oceanian groups the team sampled.

Overall, among the 908 people who were in the 44 groups in which the
allele was found, more than one out of three had the variant.

In these earlier studies, the researchers concluded that the most
straightforward explanation for the distribution of the 9-repeat
allele was that all modern Native Americans, Greenlanders and western
Beringians descend from a common founding population. Furthermore, the
fact that the allele was absent in other Asian populations most likely
meant that America’s ancestral founders had been isolated from the
rest of Asia for thousands of years before they moved into the New
World: that is, for a period of time that was long enough to allow the
allele to originate in, and spread throughout, the isolated
population.

As strong as this evidence was, however, it was not foolproof. There
were two other plausible explanations for the widespread distribution
of the allele in the Americas.

If the 9-repeat allele had arisen as a mutation multiple times, its
presence throughout the Americas would not indicate shared ancestry.
Alternatively, if there had been two or more different ancestral
founding groups and only one of them had carried the 9-repeat allele,
certain circumstances could have prompted it to cross into the other
groups and become widespread. Say that there was a second allele — one
situated very close to the 9-repeat allele on the DNA strand — that
conferred a strong advantage to humans who carried it. Natural
selection would carry this allele into new populations and because of
the mechanics of inheritance, long stretches of DNA surrounding it,
including the functionless 9-repeat allele, would be carried along
with the beneficial allele.

To rule out these possibilities, the research team, which was headed
by Noah Rosenberg at the University of Michigan, scrutinized DNA
samples of people from 31 modern-day Asian populations, 19 Native
American, one Greenlandic and two western Beringian populations.

They found that in each sample that contained the 9-repeat allele,
short stretches of DNA on either side of it were characterized by a
distinct pattern of base pairs, a pattern they seldom observed in
people without the allele. “If natural selection had promoted the
spread of a neighboring advantageous allele, we would expect to see
longer stretches of DNA than this with a similarly distinct pattern,”
Schroeder said. “And we would also have expected to see the pattern in
a high frequency even among people who do not carry the 9-repeat
allele. So we can now consider the positive selection possibility
unlikely.”

The results also ruled out the multiple mutations hypothesis. If that
had been the case, there would have been myriad DNA patterns
surrounding the allele rather than the identical characteristic
signature the team discovered.

“There are a number of really strong papers based on mitochondrial DNA
— which is passed from mother to daughter — and Y-chromosome DNA —
which is passed from father to son — that have also supported a single
ancestral population,” Schroeder said. “But this is the first
definitive evidence we have that comes from DNA that is carried by
both sexes.”

Other authors of the study are David G. Smith, a professor of
anthropology at UC Davis; Mattias Jacobsson, University of Michigan
and Uppsala University in Sweden; Michael H. Crawford, University of
Kansas; Theodore Schurr, University of Pennsylvania; Simina Boca,
Johns Hopkins University; Donald F. Conrad and Jonathan Pritchard,
University of Chicago; Raul Tito and Ripan Malhi, University of
Illinois, Urbana-Champaign; Ludmilla Osipova, Russian Academy of
Sciences, Novosibirsk; Larissa Tarskaia, Russian Academy of Sciences,
Moscow; Sergey Zhadanov, University of Pennsylvania and Russian
Academy of Sciences, Novosibirsk; and Jeffrey D. Wall, UC San
Francisco.

The work was supported by NIH grants to Rosenberg and Smith and an NSF
Graduate Research Fellowship to Schroeder.

Journal reference:

Schroeder et al. Haplotypic Background of a Private Allele at High
Frequency in the Americas. Molecular Biology and Evolution, 2009; 26
(5): 995 DOI: 10.1093/molbev/msp024

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