Native American origins: When the DNA points two ways

This week, two teams of scientists released reports detailing the origins of Native American peoples. Both groups looked at ancient and modern DNA to attempt to learn more about the movements of populations from Asia into the New World, and about how groups mixed once they got here. Both discovered a hint that some Native Americans in South America share ancestry with native peoples in Australia and Melanesia.

But the two groups came to different conclusions when it came to how that DNA with ties to Oceania made its way into the Native American genome.

In a wide-ranging paper in the journal Science, University of Copenhagen Centre for GeoGenetics Director Eske Willerslev and coauthors studied genomes from ancient and modern people in the Americas and Asia. They concluded that migrations into the New World had to have occurred in a single wave from Siberia, timed no earlier than 23,000 years ago. They also calculated that any genes shared with Australo-Melanesian peoples must have been contributed through relatively recent population mixing.

In the meantime, Harvard Medical School geneticist David Reich and colleagues, focusing more closely on the Australo-Melanesian genes in a study published in Nature, came to a different conclusion: that the DNA had to have arrived in the Americas very long ago and that founding migrations occurred in more than one wave.

"It was crazy and unexpected and very weird and we spent the last year and a half trying to understand it," Reich said on Monday. But "it's inconsistent to a single founding population. People in Amazonia have ancestry from two divergent sources...we think this is a real observation."

David Meltzer, an archaeologist at Southern Methodist University in Dallas and a coauthor of the Science paper, said that researchers in his field had been wrestling with the early history of the Americas for centuries -- debating when the first settlers arrived here, whether there were pulses of migrations, and so on.

But where archaeologists are very good at dating physical artifacts and using them to figure out that people had to have settled in the Americas by a certain time (around 15,000 years ago), they can't suss out other details of population history that geneticists are uniquely well-equipped to explore, thanks to recent advances in DNA sequencing and analytics.

The Science paper attempted to pin down some of those details. The team calculated that Native American populations diverged from Asian groups 23,000 years ago, said co-author Yun Song, a computational biologist at UC Berkeley -- making that the earliest time they could have migrated south. 

They also estimated that North American and South American populations split between 12,000 and 15,000 years ago, and that there was "evidence of subsequent migrations after the additional wave" -- including the DNA shared with native peoples in Australia and Micronesia.

Song did not think the Science study and Nature studies were necessarily inconsistent, and wondered if one possible scenario in the Nature paper -- "a long drawn out period of gene flow from a structured ... source," amounted to the same thing as his team's notion of an initial wave with subsequent migrations.

"Maybe the confusion is semantics," he said.

John Hawks, a professor of anthropology at the University of Wisconsin-Madison who was not involved in either study, agreed that both teams' data showed a lot of similarities. He was inclined to put more stock in the Science study, he said, because it depended more heavily on ancient DNA sequences in drawing its conclusions. He added that more sampling in the future might uncover evidence of a second ancient migration, however.

Reich, who said his team conducted multiple checks to confirm its hypothesis that there were two founding groups, expected scientists ultimately to confirm the existence of the ancestral group he called "population Y" -- after Ypykuera, the Tupi word for "ancestor."

"There's a track record of predicting ghost populations," he said. "People will find this population Y."

Meltzer, a self-professed "rocks guy", said the thought excited him. Scientists don't have DNA samples from Native Americans dating from around 12,000 to 24,000 years ago. But should they secure a sample, they might be able to sequence it and search for hints of the Australo-Melanesian DNA.

"If we find that [genetic] signal, OK -- there's our answer," he said.

For more on science and health, follow me on Twitter: @LATerynbrown

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