DNA Research Confirms Native Americans Descended from Single Ancestral Group

For 20 years, researchers have been using a growing quantity of genetic data to argue 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 carefully comparing DNA samples from people in dozens of modern-day Native American and Eurasian groups, an international team of scientists believe they have solved the puzzle: Virtually without exception the new evidence supports the single ancestral population hypothesis.

"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 Dr. Kari Britt Schroeder, a lecturer at the University of California, Davis (USA), and the first author of the article describing the study, which was published in the May 2009 issue of the journal Molecular Biology and Evolution. "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," remarked Dr. Schroeder, who was a Ph.D. student in anthropology at the university when she did the research.

The investigators research 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. Named the "9-repeat allele," the variant (which does not have a biologic 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 clear-cut 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 probably 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 infallible. There were two other conceivable 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, specific circumstances could have prompted it to cross into the other groups and become widespread. If there was a second allele--one placed very close to the 9-repeat allele on the DNA strand--that bestowed a strong advantage to humans who carried it. Natural selection would carry this allele into new populations and because of the processes of inheritance, long stretches of DNA surrounding it, including the functionless 9-repeat allele, would be carried along with the beneficial allele.

To exclude these possibilities, the research team, which was led by Dr. Noah Rosenberg, from the University of Michigan (Ann Arbor, USA), examined DNA samples of people from 31 modern-day Asian populations, 19 Native American, one Greenlandic, and two western Beringian populations.

The scientists discovered that in each sample that contained the 9-repeat allele, short stretches of DNA on either side of it were distinguished 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," Dr. Schroeder noted. "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 study's findings also ruled out the multiple mutations theory. If that had been the case, there would have been myriad DNA patterns surrounding the allele instead of 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," Dr. Schroeder said. "But this is the first definitive evidence we have that comes from DNA that is carried by both sexes."

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University of California, Davis
University of Michigan