Bison fossils shed light on human migration in the Ice Age

Radiocarbon dating and DNA analysis of bison fossils has revealed that an ice-free corridor along the Rocky Mountains was fully open by around 13,000 years ago, during the late Pleistocene.

Geological studies in the 1970s led to the corridor being posited as a route for the colonization of the Americas by humans moving south from Alaska. But the new study, published in Proceedings of the National Academy of Sciences, suggests that the pathway was not the one taken during the initial migration.

It was, however, a likely route for later dispersal of people and animals.

“The opening of the corridor provided new opportunities for migration and the exchange of ideas between people living north and south of the ice sheets,” said first author Peter Heintzman, a postdoctoral researcher at UC Santa Cruz who led the DNA analysis. (We like to think the bison were sharing ideas, too).

The most up-to-date thinking on the subject is that the Cordilleran and Laurentide ice sheets coalesced at the height of the last ice age, around 21,000 years ago, and closed the corridor much earlier than any evidence of humans south of the ice sheets. A Pacific coastal route is the more likely candidate for the first movement southwards, around 15,000 years ago.

In fact, according to Study coauthor Beth Shapiro, professor of ecology and evolutionary biology at UC Santa Cruz, archaeological evidence suggests that the predominant human migration within the Rocky Mountains corridor was from south to north.

“When the corridor opened, people were already living south of there. And because those people were bison hunters, we can assume they would have followed the bison as they moved north into the corridor,” Shapiro said.

Southern bison to start moving northward as early as 13,400 years ago, as the south began to open first, before the corridor fully opened and the two populations overlapped by 13,000 years ago after northern bison had also started to move southward.

Distinctive hunting tools

“The radiocarbon dates told us how old the fossils were, but the key thing was the genetic analysis, because that told us when bison from the northern and southern populations were able to meet within the corridor,” Heintzman said.

“Bison fossils are the most widespread Quaternary mammal in western North America and of interest because they survived the extinctions at the end of the Pleistocene, unlike most other North American large mammals,” explained coauthor Duane Froese of the University of Alberta. “We were able to sample bison fossils, largely from museum collections, including critical ones from central Alberta that dated to the initial opening of the corridor.”

Shapiro had done previous work to show that by the time the Rocky Mountains corridor opened, bison populations north and south of the ice sheets were genetically distinct.

One of the cultural patterns under focus was the Clovis hunting culture, which had distinctive fluted point stone tools. Associated sites were widespread south of the corridor around 13,000 years ago and decline in abundance from south to north in the region of the corridor.

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Image credit: Government of Yukon

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