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.
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
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.
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.
study is published in the May issue of the journal Molecular Biology
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.
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.
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
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.
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.
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.
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."
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
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."
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.
work was supported by NIH grants to Rosenberg and Smith and an NSF
Graduate Research Fellowship to Schroeder.
and distribution on website at: http://www.sciencedaily.com/releases/2009/04/090428223836.htm