Scientists have just come to a major conclusion about early human history based entirely on the discovery of an ice-age bison skulls and bones in North America.

Using evidence from bison fossils helps scientists determine when there was an ice-free corridor on the Rocky Mountains in the late Pleistocene, and this indicates when human and animal migrations between the far north and the rest of North America could have begun, something that has been a mystery for scientists, according to a University of California Santa Cruz statement.

Using bones from giant steppe bison and clues from ice age hunters, researchers were able to concludde that people colonized North America south from Alaska along the Pacific coast instead of through the Rocky Mountains.

Scientists think that the first ancient people in North America crossed a land bridge from Siberia, but exactly when the crossing was made has remained a history.

The new research indicates that rather than a single mass migration 13,500 years ago, there may have been multiple migrations. They found the Rocky Mountain corridor probably was open until about 21,000 to 23,000 years ago. Using DNA analysis of 78 bison fossils, the research team found two distinct populations in the north and south that showed that northern and southern bison began mingling in the pass about 13,000 years ago.

That means the mountains would have been clear more than a thousand years after the south had already been colonized, indicating that humans must have settled via the Pacific Coast instead.

“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. “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. … 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,” said 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.”