During the last ice age, glaciers covered vast swathes of western North America. Researchers have proposed that an ice-free corridor served as a pathway for human migration between the far north and the rest of the continent. Now, an analysis of bison fossils revealed that humans made their ice-free way in and out of Alaska and the Yukon some 13,000 years ago. The findings are published in Proceedings of the National Academy of Sciences this week.
Our species dispersed to the North American continent via an ancient land bridge that connected Siberia with Alaska, and from there, humans went on to colonize the rest of the Americas. In the 1970s, geological studies proposed the existence of a human (and animal) migration route through an ice-free corridor along the Rocky Mountains between large continental ice sheets toward the end of the Pleistocene. The corridor likely opened and closed multiple times as the climate changed. We know, for example, that it wasn’t open 21,000 years ago at the height of the ice age because the ice sheets coalesced. Exactly when that corridor opened for human migration remained unknown.
To investigate, an international team led by University of Alberta’s Duane Froese and Beth Shapiro from UC Santa Cruz reconstructed the corridor’s chronology using radiocarbon dating and mitochondrial DNA extracted from the fossilized teeth and bones of steppe bison (Bison priscus). These giant mammals roamed freely throughout the grasslands of North America until the ice sheets cut off the bison in the north from those in the south. Over time, they became genetically distinct.
The oldest bison fossils found within the corridor region belong to the lineage that originated south of the ice sheets during a time when the corridor was closed. The team dated these bison fossils to 13,400 years ago. "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," first author Peter Heintzman of UC Santa Cruz said in a statement. The southern part of the corridor opened up first, allowing bison to move northward. Later on, the northern population moved southward, and the two met in the corridor by around 13,000 years ago. That means the corridor was open – and fully traversable for humans – by this time.
However, archaeological evidence suggests that a southward migration out of Alaska and the Yukon occurred before 15,000 years ago – which we now know predates the corridor’s opening. The researchers think the corridor wasn’t used for this initial southward dispersal – they likely took a Pacific coastal route. But the ice-free corridor did enable subsequent movement, both northward and southward. The Clovis hunting culture, for example, was widespread in the south some 13,000 years ago. The earliest known Alaskan Clovis site dates back to 12,400 years ago.