DNA studies on the skeletal remains of two babies buried in Alaska around 11,000 years ago yield strong evidence the first Americans spent thousand of years living there before migrating down into North and South America, researchers say.

The findings suggest Native Americans, originally migrating from Asia, were settled and living on the Bering land bridge for a long time before moving south on the continent and further down into South America, they say.

The babies, found in the Upward Sun River site in what is now central Alaska, had different mothers, University of Utah researchers say. One was a 6- to 12-week-old baby, while the other was a preterm fetus, possibly stillborn.

DNA studies of the remains strongly support a hypothesis known as the "Beringian standstill" theory, which holds that the ancestors of modern Native Americans spent a long enough period of time isolated from other populations that their DNA eventually differentiated from its Asian roots and developed uniquely American lineages.

Beringia is the name scientists use to describe the vast land bridge that linked Siberia with North America during the last ice age, from around 28,000 to 18,000 years ago.

Rising seas and melting glaciers began to cover the land bridge, at the same time melting ice opened areas for expansion, and around 15,000 years ago the ancestors of today's Native Americans began heading south.

The standstill theory was borne out by the DNA of the Alaskan babies, which is identical to lineages that are present in many Native American populations today in both North and South America, the researchers report in the journal Proceedings of the National Academy of Sciences.

"You don't see any of these lineages that are distinctly Native American in Asia, even Siberia, so there had to be a period of isolation for these distinctive Native American lineages to have evolved away from their Asian ancestors," says study senior author Dennis O'Rourke, an anthropology professor at the University of Utah. "We believe that was in Beringia."

The Alaskan babies are among human remains found at only eight sites in North American dated as older than 8,000 years, the researchers say.

"Here is a case of ancient DNA coming in and helping to inform archaeology," says study first author Justin Tackney, a doctoral student in anthropology.

The people living at the Upward Sun River site were most likely a remnant of those who moved rapidly south into the Americas around 15,000 years ago, the researchers say.

Because most of what was Beringia is today under water and inaccessible, "this is the closest we might ever get to seeing what the Beringians were like genetically," Tackney notes.

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