By Ashley Strickland, CNN

(CNN) -- The skeletons of two ancient infants, who lived and died in Alaska 11,500 years ago, are helping tell the story of a previously unknown population called the Ancient Beringians, according to a study published in the journal Nature on Wednesday. This is the first genomic evidence that all Native Americans can be traced back to the same population that migrated using a land bridge.

Both infants found in the burial pit at Upward Sun River, in the Tanana River valley in central Alaska, were female. The local indigenous community gave them the symbolic names Xach'itee'aanenh t'eede gaay (sunrise child-girl) and Yełkaanenh t'eede gaay (dawn twilight child-girl).

The first child was between 6 and 12 weeks old when she died, and the other was premature, not having had the chance to live, at 30 weeks of development. DNA suggests that they were related, like first cousins.

Researchers were able to sequence the genome of sunrise child-girl, referred to in the study as USR1, using her DNA. She was revealed to be closely related to Native Americans, but in a distinct way.

USR1's DNA led the researchers to determine that they had found a previously unknown genetic population of Native Americans, which they dubbed Ancient Beringians, that represent the oldest known genetic lineage of Native Americans to date, according to Eske Willerslev, study co-author and professor at the University of Copenhagen.

"It changes our understanding of the timing of events that formed the genetics of Native Americans," Willerslev wrote in an email.

Beringia was the land bridge that once connected Siberia and Alaska. It has long been thought that the peopling of the Americas happened when a group migrated from East Asia 34,000 years ago, using the land bridge, to form the first population of ancestral Native Americans.

But when, and how, had been unresolved. Any evidence that there was a single founding population for the Americas was indirect.

"There are limited ancient human remains in the Americas, and only very few have undergone genomic analysis," Ben Potter, study co-author and department of anthropology chair and professor at the University of Alaska Fairbanks, wrote in an email. "This represents the first genetic data from the last Ice Age in Beringia. Prior to this study, we did not know that this Ancient Beringian population existed. We had evidence of the other branch leading to North and South Native Americans, and indirect evidence for a single founding population. So our findings show that the early history of ancestral Native Americans is more complex than previously known."

The new data allow the researchers to test and develop models for when and where population splits occurred.

Willerslev explains it like this: About 34,000 years ago, a population began separating from the Asians, but genetic exchanges continued until 25,000 years ago -- which may suggest that by then, they were in Alaska. The new data put that shift at 10,000 years earlier than previously thought.

About 25,000 years ago is also when this group met another population of people who were more related to modern Europeans than to Asians. This is how the Ancient Beringians came to be.

"By 20,000 years ago, contemporary Native American ancestors diversify from the Ancient Beringians, likely moving south of the North American ice caps and forming the two major Native American lineages all contemporary Native Americans as we know of belong to: the northern and southern branches," Willerslev said. "The northern branch moves north into Alaska and replace or absorb the Ancient Beringians sometimes after 11,500 years ago. After that time some of the northern branch members meet some Siberians and mix with them."

Researchers are still testing scenarios for where the split occurred between Ancient Beringians and other Native Americans who populated the Americas, whether it was in Asia or East Beringia/Alaska.

Potter has directed research at the Upward Sun River since 2006, where researchers are investigating who lived at the site and when, including burials and a residential camp. The ancient people who lived at the site are associated with the Denali complex of culture in Alaska, meaning they lived in Alaska, the Yukon Territory and northern British Columbia from at least 12,500 to 6,000 years ago, Potter said.

Based on what they've learned at the site, these people, including the Ancient Beringians, hunted bison, elk, hare, squirrels and birds. There is also the earliest evidence of "salmon exploitation" in the Americas. Even though they lived through climate and vegetation changes over 6,000 years, they adapted their strategy from using organized hunting parties, processing the game they caught and delivering it to centrally located base camps, like the Upward Sun River site.

But what happened to the Ancient Beringians?

"We don't know," Potter said. "We are limited by very few genetic samples of ancient North American populations, and we would need samples from peoples in the region to ascertain to what extent Ancient Beringian gene-flow occurred with neighboring peoples. It is possible that incoming Athabaskan ancestors, who are widespread throughout the region today, replaced or absorbed the Ancient Beringians inhabiting that area."

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