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September, 27, 2016

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Human DNA points to a single exodus from Africa

NEW YORK--The genetic ancestry of people living outside Africa can be traced almost completely to a single exodus of humans from that continent long ago, new studies suggest.

Still, a tiny legacy from an earlier exit may persist in some native islanders in the southwestern Pacific Ocean.

That's the conclusion from three studies of modern DNA from around the world, released Wednesday by the journal Nature.

Our species, Homo sapiens, arose about 200,000 years ago in Africa. From there, it colonized the world, and scientists are still trying to understand the timing of that expansion.

The new work takes advantage of the fact that human DNA accumulates tiny changes over time. That can be used like a clock to estimate how long ago two populations split off from each other. The approach can't reveal every migration out of Africa, just those that left a genetic legacy that has been handed down to this day.

Scientists have long traced one such exit to a single population that left around 40,000 to 80,000 years ago, probably over time rather than all at once. But some other work has turned up potential signs of a previous migration as early as 120,000 to 130,000 years ago.

One of the new papers says it found a trace of an earlier migration in native people of Papua New Guinea, which lies north of Australia. At least 2 percent of their DNA may come from a population that split off from Africans about 120,000 years ago, reported researchers from the Estonian Biocentre in Tartu, Estonia, and other institutions. The study analyzed the DNA of 483 people from 148 populations worldwide, including six Papuans.

The two other papers concluded that if there was a genetic contribution from an earlier migration, it must be tiny. One studied the DNA of 300 people from 142 diverse populations, while the other examined the genetic codes of 25 Papuans and 83 aboriginal Australians.

Overall, the evidence shows that the vast majority of modern human ancestry outside of Africa comes from a single exit from Africa, said David Reich of Harvard Medical School, an author of the 142-population paper.

Joshua Akey of the University of Washington in Seattle, who co-wrote a Nature commentary on the papers, said linking that vast majority of ancestry to just one departure seems settled.

Todd Disotell of New York University, who didn't participate in the research, said the reported hint in Papuans of an earlier migration might actually be due to something else. The analyses in all three papers are very complex, he noted.

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In this Feb. 20, 1962 file photo, an elder warrior with a stone ax over his shoulder stands over the Baliem Valley in the central mountain range of Papua New Guinea. (AP)

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