RENEE MONTAGNE, HOST:
Here is some conventional wisdom. The first people to set foot in the Americas apparently came from Siberia during the last Ice Age. But as NPR's Christopher Joyce reports, there's also new evidence that the first Americans may have come from different places at different times.
CHRISTOPHER JOYCE, BYLINE: The human race has walked or paddled or sailed until it covered the globe. We can trace those migrations from the stuff they left behind, tools or dwellings or burial grounds. But now geneticists can do that by examining the genes of living people and comparing them to each other as well as the genes extracted from ancient bones. Rasmus Nielsen of the University of California at Berkeley and a team of scientists have done just that for native people of the Americas. And they've figured out how the very first people got here.
RASMUS NIELSEN: They came in a single migration wave into the Americas, people that diverged from people originally in Siberia and East Asia about 23,000 years ago.
JOYCE: Now, that confirms the standard view that people first got here across a frozen land bridge between Siberia and Alaska. But there's been this puzzle. Some ancient skulls found in the Americas look rather like Europeans or even Polynesians. Did another group come from somewhere else? Writing in the journal Science, Nielsen says no. Genetically, no Native Americans match up with Europeans or Polynesians. Instead, he says, any diversification of Native American groups must have evolved on its own after the first bunch got here.
NIELSEN: This diversification of modern Native Americans happened in the Americas. It's not because different people came from all over the world into the Americas.
JOYCE: But there's often a twist when you're teasing apart the threads of ancient history with genetic tweezers. David Reich, who's a geneticist at Harvard Medical School, says his research, published this week in the journal Nature, suggests a more complicated story. When he studied groups of indigenous people in the Americas, he found that some of them had a distinctive genetic fingerprint. He searched the world for other copies of that fingerprint. He found it a faraway place in modern Australasia.
DAVID REICH: What we found was that Native American people from Amazonia, from present-day Brazil, are more closely related to some populations in Asia than are other Native Americans, for example, from Mexico or from Western South America and many parts of North America.
JOYCE: The Amazonians' modern relatives, the Australasians, are native Australians and people of New Guinea and the Andaman Islands in the Pacific. Reich says what may have happened is this. Members of a now extinct population of people in Southeast Asia - Reich calls them Population Y - crossed the land bridge as well, either before or after the first wave of people made it to the Americas. They kept going. Some of those Population Y-ers got all the way to Brazil. The ones who did not leave Asia populated what is now Australasia. But the two groups still are linked genetically.
REICH: We now have the possibility of there being two different streams of ancestry penetrating south of the ice sheets, and so that's a very exciting new observation.
JOYCE: And one that adds yet another branch or root to the American family tree. Christopher Joyce, NPR News.
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