The steppe bison (Bison priscus), a larger version of the modern bison (Bison bison), is generally thought to be the first bison species to touch North American soil. Unlike other Pleistocene fauna, bison proved durable during an extinction event that wiped out various species, including mammoths.
Fossil evidence from these horned creatures was used by researchers to determine whether early humans utilized the Ice Free Corridor as a route to North America from Beringia, a name used for unglaciated Alaska and Yukon.
“A central question in New World biogeography and archaeology has been the role of an ‘Ice Free Corridor’ along the eastern slopes of the Rocky Mountains in facilitating biotic exchange between Beringia … and southern interior parts of the Americas,” the researchers wrote in a study appearing in Proceedings of the National Academy of Sciences.
Using radiocarbon dating and ancient mitochondrial DNA from the ancient bison, the researchers discovered that humans likely didn’t use the Ice Free Corridor as a route for initial expansion. The corridor was closed more than 20,000 years, as geological interpretations indicate that two ice sheets, the Cordilleran and Laurentide ice sheets, coalesced in the region and formed an impenetrable barrier. This lasted until the glacial retreat at the end of the Pleistocene, with the corridor opening around 13,400 years ago.
“Although bison survived the interval of glacial coalescence both north and south of the continental ice sheets, population bottlenecks and barriers to gene flow affected their mitochondrial diversity,” the researchers wrote. “By the time the glaciers began to retreat, bison populations that had been isolated to the south of the continental ice sheets were mitochondrially distinct from their contemporary northern counterparts in Beringia.”
Study author Prof. Beth Shapiro, who teaches ecology and evolutionary biology at University of California, Santa Cruz, led the DNA analyses, while Duane Froese, of the University of Alberta, handled the radiocarbon dating work.
After the corridor opened, “bison used this route to disperse from the south and by 13,000 from the north,” the researchers wrote.
Since humans expanded into North America more than 15,000 years ago, the researchers postulated that the initial southward movement was achieved via a route along the Pacific coast. Once the glacial retreat occurred though, humans and bison likely used the Ice Free Corridor to travel freely northward and southward.