Evidence from bison fossils has enabled researchers to shape a more accurate timeline for the so-called ‘ice-free corridor’ — a route for Pleistocene human and animal migrations between Beringia in the far north (near Alaska and the Yukon) and the rest of North America.
This map shows the location of the ice-free corridor and Paleo-Indian sites. Image credit: Roblespepe / CC BY-SA 3.0.
In the 1970s, studies suggested that an ice-free corridor along the Rocky Mountains might have been the pathway for the first movement of humans southward from Alaska to colonize the rest of the Americas.
More recent evidence, however, indicated that the Cordilleran and Laurentide ice sheets coalesced at the height of the last Ice Age, around 21,000 years ago, closing the corridor much earlier than any evidence of humans south of the ice sheets.
The initial southward movement of people into the Americas more than 15,000 years ago now seems likely to have been via a Pacific coastal route, but the ice-free corridor has remained of interest as a potential route for later migrations.
A team of scientists led by Prof. Duane Froese from the University of Alberta and Prof. Beth Shapiro from the University of California, Santa Cruz, combined radiocarbon dating and DNA analysis to track the movements of bison (Bison sp.) into the corridor, showing that it was fully open by about 13,000 years ago.
Their findings indicate that the corridor could not account for the initial dispersal of humans south of the ice sheets, but could have been used for later movements of people and animals, both northward and southward.
“What we’ve done is put some fairly strong constraints on when this area was viable, and when it was connected to the north and the south in terms of it being a corridor,” Prof. Froese said.
Previous work by Prof. Shapiro had shown that the bison populations north and south of the ice sheets were genetically distinct by the time the corridor opened.
Paleo-Indian hunters scout a herd of Bison antiquus. Image credit: Mark Marcuson / University of Nebraska State Museum.
By analyzing bison fossils from within the corridor region, the team was able track the movement of northern bison southward into the corridor and southern bison northward.
“To infer the chronology of the corridor linking Beringia and interior North America, we generated radiocarbon dates from 78 North American bison fossils, 49 of which were recovered from the corridor region,” they explained.
“Sites included Clover Bar and Charlie Lake Cave in western Canada, both of which previously yielded dates in the time frame of interest (11,500-13,500 years before present).”
“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,” said study first author Dr. Peter Heintzman, from the University of California, Santa Cruz.
The results, published in the Proceedings of the National Academy of Sciences, showed that the southern part of the ice-free corridor opened first, allowing southern bison to start moving northward as early as 13,400 years ago, before the corridor fully opened.
Later, there was some movement of northern bison southward, with the two populations overlapping in the corridor by 13,000 years ago.
“There were established indigenous populations both in central Alaska and south of the ice masses before 14,000 years ago; this new information shows that before 13,000 years ago, bison — and therefore, likely people — could move between these two regions, previously separated by a glacial barrier,” said co-author Dr. Prof. John Ives, from the University of Alberta.
The amount of northward movement was a surprise to researchers, who expected to find predominantly southward migration patterns.
“This removes this very limited idea of this corridor as simply a route for the very first North Americans, to now suggest a much richer history — that it was used multiple times by populations likely moving in both directions,” Prof. Froese said.
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Peter D. Heintzman et al. Bison phylogeography constrains dispersal and viability of the Ice Free Corridor in western Canada. PNAS, published online June 6, 2016; doi: 10.1073/pnas.1601077113
