Since the 1970s, researchers have hypothesized that humans first colonized the Americas by crossing a land bridge between present day Russia and Alaska known as Beringia. Once they made it through Alaska, however, early humans found themselves blocked from the rest of the continent by the large Cordilleran and Laurentide ice sheets covering the Yukon and western Canada.
Researchers suspected that about 13,000 years ago, during the late Pleistocene, an ice-free corridor through the Rocky Mountains opened up, allowing humans to move south and spread out across North and South America. But a new study published in Proceedings of the National Academy of Sciences disputes that claim, and their evidence is as strong as a bison.
According to Alan Yuhas at The Guardian, researchers from the University of California, Santa Cruz, analyzed 78 now-extinct steppe bison fossils from the area of the Rocky Mountain ice-free corridor. The team collected mitochondrial DNA and dated the fossils. Previous research showed the animals had been separated for so long before the corridor opened, the north and south populations were genetically distinct.
The DNA analysis shows that the two groups of bison began intermingling about 13,000 years ago, meaning the ice-free corridor must have opened about that time. Dating of the fossils, however, shows that the southern corridor was the first to open and evidence of human activity in the south abounds. These traces decrease northward, suggesting that humans migrated from the south to the north—opposite the direction previously believed.
There's other convincing evidence that people made it south of the ice sheets a thousand years before the Rocky Mountain corridor opened, including a 15,000-year-old human settlement in Monte Verde, Chile and a recent discovery showing humans hunted mammoth in Florida 14,500 years ago.
"When the corridor opened, people were already living south of there,” coauthor Beth Shapiro says in a press release. “And because those people were bison hunters, we can assume they would have followed the bison as they moved north into the corridor.”
But how did people migrate south before the ice opened up? The only explanation is that humans tromped around the Pacific coast instead of traveling through the mountains. “It’s really hard to think of any other ideas,” Pete Heintzman, lead author of the study, tells Yuhas. “14 to 15,000 years ago, there’s still a hell of a lot of ice around everywhere. And if that wasn’t opened up you’d have to go around the ice, and going the coastal route is the simplest explanation.”
The idea that the Americas were settled by humans moving down the Pacific coast is plausible, and the Pacific Coast Migration model has been around for a few decades. The problem is finding evidence. Erosion and tides have likely wiped out many potential archaeological sites along the coast, Heintzman points out to Yuhas. Finding more sites and improving dating techniques, he says, will help bring the path of migration into sharper focus.