The new study can’t pinpoint exactly where Native Americans emerged from the meeting of those two peoples. The ice age was at its peak 24,000 years ago, and so different populations across Siberia and surrounding regions may have retreated into refuges where wild game still survived.

Anne Stone, an anthropological geneticist at Arizona State University who was not involved in the new study, speculated that the Native American population may have emerged in one such refuge on the land bridge that linked Siberia and Alaska between about 34,000 and 11,000 years ago.

But testing that idea will be hard, she warned. “I think it’s going to be frustratingly slow,” she said. “Finding human remains of this age is truly daunting.”

Making the task even more difficult is the fact that melting glaciers drowned the land bridge at the end of the ice age, submerging any human remains that might hold more DNA.

Yet the disappearance of the land bridge did not stop the movement of people between the continents. Later waves of people moved across the Bering Sea.

Teasing apart this traffic is proving difficult for scientists — and has led to debates about how the migrations shaped the origins of living Native Americans.

In its research on ancient DNA, Dr. Willerslev’s team found evidence that a second wave of Ancient Paleo-Siberians reached Alaska sometime between 9,000 and 6,000 years ago. They made contact with Native Americans there and interbred.