Then, some 13,000 years ago, when America's bison began migrating north through a lush land corridor exposed by Canada's melting ice sheets, humans followed them there too.
That's the story told by bits of DNA extracted from centuries-old bones uncovered across the continent. In a study published in the Proceedings of the National Academy of Sciences this week, paleontologists, geneticists and archaeologists describe how humans may have followed bison herds north through a Canadian "ice-free corridor" after their initial colonization of what is now the United States.
The find is of interest to scholars of North America's human prehistory, who have long been locked in a debate about when and how the region's first people arrived. For decades paleontologists believed that an ancient culture known as Clovis took a north-south route through the gap between the glaciers, starting in Alaska and then making their way into the previously uninhabited expanse of the Americas. But recent discoveries of settlements far older than that ice-free corridor have challenged that theory, suggesting that people arrived here before the route was navigable. The bison bones may well be another nail in the coffin.
“This is the first strong empirical data indicating when that corridor was viable,” Michael Waters, an archaeologist at Texas A&M University who was not involved in the study, told Science. “It’s indirect evidence, but it’s still strong evidence.”
At the very end of the Pleistocene, just as the most recent ice age was coming to a close, what is now the United States was separated from Alaska and the "Berengia" land bridge by two massive glaciers that encased all of Canada: the Laurentide ice sheet, which extended from Newfoundland to the Rocky Mountains, and the Cordilleran, which covered everything west of that. This meant that bison populations that had migrated south during a previous warm period were entirely isolated from their northern counterparts.
Distance tends to make creatures drift apart. The southern bison may not have looked much different on the outside, but their distinctiveness was inscribed deep within their cells, in scraps of genetic material known as mitochondrial DNA.
In that difference, evolutionary biologist Beth Shapiro saw an opportunity to settle a debate. Scientists haven't been able to definitively date when the corridor opened, instead focusing on tracking evidence of human settlements. But human remains are incredibly hard to find. Before the spread of characteristic fluted Clovis points around 11,000 BC, we have just a handful of settlement sites, a few stone tools, and a single human coprolite (a fancy term for fossilized poop).
Bison fossils, on the other hand, are everywhere.
"We thought, why not take advantage of their genetic distinctiveness and ask when the bison individuals of [the southern] genetic type appeared in the north?" said Shapiro, an associate professor at the University of California and an author of the PNAS study. "And then we could use that as a proxy to show that this corridor was habitable at a certain point in time."
So Shapiro and her colleagues began scouring museums and research collections for late Pleistocene bison whose DNA they could test. Genetic analysis showed which specimens were southerners who migrated north, and radiocarbon dating showed when that migration occurred.
Ultimately, they concluded that northern and southern bison were mingling in Canada by 13,000 years ago. That's likely too late to make it the initial route into the Americas — stone tools all the way south in Chile indicate that humans were here as many as 5,000 years before then, and just last month researchers found evidence that humans were living in Florida 14,550 years ago. But it could have been "highway two," co-author Jack Ives, an archaeologist at the University of Alberta, told the CBC. Having already settled the lower 48, presumably after traveling down the coast by boat, humans may have migrated back north in the bisons' wake.
"It's intriguing from the perspective that as much as bison and game animals were separated, so too would have been early human populations," Ives said. "Once that corridor region opened … this would open the door for human populations to reengage."
He and Shapiro cautioned that the bison bone dates are just a proxy for human migration. We can't be sure how humans came to the Americas — and when — until we find their bones.
But as far as proxies go, bison are pretty good.
"They were definitely on the menu," Ives told the CBC. "... Any place that one would find bison, one would have to strongly suspect, that human beings could be present too."