For decades, scientists could describe the peopling of the Americas only in broad strokes, leaving plenty of mysteries about when and how people spread across the continents. Now, state-of-the-art ancient DNA methods, applied to scores of new samples from around the Americas, are filling in the picture. Two independent studies, published in Cell and online in Science, find that ancient populations expanded rapidly across the Americas about 13,000 years ago. They also emphasize that the story continued in the thousands of years since, revealing previously undocumented, large-scale movements between North and South America.

The data include 64 newly sequenced ancient DNA samples from Alaska to Patagonia, spanning more than 10,000 years of genetic history. "The numbers [of samples] are just extraordinary," says Ben Potter, an archaeologist at the University of Alaska in Fairbanks. Prior to these studies, only six genomes older than 6000 years from the Americas had been sequenced. As a result, says Jennifer Raff, an anthropological geneticist at the University of Kansas in Lawrence, "The [genetic] models that we've been using to explain the peopling of the Americas have always been oversimplified."

Eske Willerslev, an evolutionary geneticist at the University of Copenhagen who led the Science team, worked closely with the Fallon Paiute-Shoshone Tribe in Nevada to gain access to some of the new samples. The tribe had been fighting to repatriate 10,700-year-old remains found in Nevada's Spirit Cave and had resisted destructive genetic testing. But when Willerslev visited the tribe in person and vowed to do the work only with their permission, the tribe agreed, hoping the result would bolster their case for repatriation.