In Alaska, scientists have uncovered something they say is remarkable: the remains of two infants dating back more than 11,000 years.
Their discovery is evidence of the earliest wave of migration into the Americas.
"It's incredibly rare," says Ben Potter, an archaeologist at the University of Alaska who is among the researchers on the project, at a site called Upward Sun River in central Alaska. "We only have a handful of human remains that are this old in the entire Western Hemisphere." The findings were published Wednesday by the journal Nature.
The remains were in such good condition that geneticists were able to extract DNA from one of them. They compared the sample with 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 a frozen land bridge from Siberia to Alaska called "Beringia."
Potter says during this great migration, either before or after they crossed the land bridge, this group (which the researchers call the founding population for all Native Americans) split again, into two populations. Scientists had suspected this and surmised that one group stayed put in and around Beringia. They call them 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 in this arid, frigid landscape, says Potter. "Bison, horses, mammoth. Big grazers were very common."
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. "There were presumably many related populations like this," he says, "one of which split to form these two lineages that have diversified into Native Americans today."
That group, the one that moved south, eventually spread far and wide: up into Canada, the East and throughout Central and South America. Their descendants are the Native Americans of today.
Two Native American groups cooperated with the researchers in the excavation in Alaska.
For the Beringians who stayed behind, Potter says, it would have been a rough living as the last ice age drew to a close. "They're dealing with climate change that we can only imagine now — major changes from [the] ice age, to extinction of a wide range of mammal species, including mammoth," he says. "And these are the people that adapted in this region."
For a while at least. Writing in Nature, the researchers say the infant remains show the Beringians lasted at least until about 11,500 years ago. How their end came is still unknown.