ARI SHAPIRO, HOST:
In Alaska, archaeologists have uncovered the bones of two infants dating back 11 and a half thousand years. Their discovery sheds light on how the first people migrated to the Americas. Here's NPR's Christopher Joyce.
CHRISTOPHER JOYCE, BYLINE: Scientists say it's a remarkable discovery.
BEN POTTER: It's incredibly rare. We only have a handful of human remains that are this old in the entire Western Hemisphere.
JOYCE: Ben Potter is an archaeologist at the University of Alaska who worked on the site called Upper Sun River. Geneticists compared DNA from one of the infants to the genes of people from around the world. They conclude that the ancestors of these infants started out in East Asia about 35,000 years ago. As they traveled east, they became genetically isolated from other Asians.
At some point during the last ice age, they crossed the frozen land bridge from Siberia to Alaska called Beringia. Potter says during this great migration, this group split again into two populations. Scientists suspected that one group stayed put. They call them the ancient Beringians. The two infants are the first hard evidence that they did indeed do that. The ice age was still on, but these people hunkered down and made the best of what was there.
POTTER: Bison, horses, mammoth - big grazers were very common.
JOYCE: The other group moved down into North and South America and are believed to be the direct ancestors of current Native Americans. David Reich, a geneticist at Harvard University who studies ancient DNA, says genetic material like this tells a more detailed story of how people came to America but not the whole story.
DAVID REICH: There were presumably many related populations like this, one of which split to form these two lineages that have diversified to Native Americans today.
JOYCE: The group that moved south spread far and wide up into Canada, the East and throughout Central and South America. They are the Native Americans of today. Meanwhile, Potter says, those who stayed behind in Beringia dealt with huge changes in the far north as the ice age drew to a close.
POTTER: They're dealing with climate change that we can only imagine now - major changes from ice ages, extinction of a wide range of mammal species, including mammoth. And these are the people that adapted in this region.
JOYCE: They lasted for a while at least. Writing in the journal Nature, the researchers say the infant remains show the Beringians lasted at least until about 11 and half thousand years ago. How their final end came is still unknown. Christopher Joyce, NPR News.
(SOUNDBITE OF THE BALLROOM THIEVES' "LA MER PEU PROFONDE")