BOSTON, July 21 (UPI) -- Two new studies published Tuesday both clarify the timeline of the ancient settlement of the Americas and raise new questions.
Both studies indicate people of the Brazilian Amazon are genetically linked to Australasians, though they differ on how this occurred.
One analysis, published in the journal Science, suggests the Americas were settled by a massive wave of people from Siberia, who came over no earlier than 23,000 years ago. Further melting of polar glaciers eventually allowed inroads into the interior of the Americas and facilitated the genetic split into two distinct groups -- Amerindians and Athabascans.
"There is some uncertainty in the dates of the migration and the divergence between the northern and southern Amerindian populations," study author Yun Song, a statistician and computer scientist at the University of California, Berkeley, explained in a press release. "But as we get more ancient genomes sequenced, we will be able to put more precise dates on the times of migration."
The study dispels the notion that the two groups of native peoples are the product of multiple waves of migration. While the research found a link to Australasian peoples, the authors posit smaller scale contact contributed to the gene flow, possibly through Aleutian Islanders living off the coast of Alaska.
But another study raises the possibility of a previously undiscovered migration to the Americas, separate from those who crossed the Bering land bridge 15,000 years ago.
Genetic analysis by Pontus Skoglund, a researcher at Harvard, also shows a handful of native peoples in Central and South America share a unique genetic link with indigenous tribes of Australasia. The findings suggest a "ghost population," dubbed Population Y, descended from the north to settle parts of the Americas and Australasia.
The study linking the forefather of Brazilian tribes with those of Papua New Guineans and Aboriginal Australians was published in the journal Nature.
"It's incredibly surprising," senior study author Dr. David Reich, Harvard Medical School professor of genetics, said in a press release. "There's a strong working model in archaeology and genetics, of which I have been a proponent, that most Native Americans today extend from a single pulse of expansion south of the ice sheets -- and that's wrong. We missed something very important in the original data."
The findings suggest a unique group of people migrated from Siberia around the same time or before the group Reich refers to as the First Americans, only to be mostly replaced by the larger wave of migrators 15,000 years ago. The genetic evidence of these forefathers is missing from most people of North, Central and South America, and is also absent from Asia -- found only among indigenous Australasians, as well as the Tupi-speaking Surui and Karitiana, and the Ge-speaking Xavante of the Amazon.
"We've done a lot of sampling in East Asia and nobody looks like this," said Skoglund. "It's an unknown group that doesn't exist anymore."
Researchers of the two studies say they're not necessarily in disagreement, but they they offer two interpretations of similar datasets -- both rejecting old theories in favor of a new reality.
David Meltzer, an anthropologist and archaeologist at Southern Methodist University and one of the authors of the Science paper, told the New York Times that the divergent analyses are "not an irresolvable problem."
But Reich was not as conciliatory, telling the Times his team went deeper into the exploration of this unique Australasian genetic signature.
"We have overwhelming evidence of two founding populations in the Americas," he said.