- Scientists studied bison fossils to pinpoint when human used Rocky Mountains for migration
- Results suggest Rocky Mountain migration came after migration along the Pacific
(CNN)Exactly when the first humans in North America left Alaska and moved south into the rest of the continent has always been subject to some debate.
Scientists have long thought that humans first migrated south 15,000 years ago along a coastal Pacific path. But they always wondered about a second route, along the Rocky Mountains. People used that path for migration too, but scientists had never determined if it was trod before or after the Pacific route.
A new study suggests use of the Rocky Mountain path occurred as many as 2,000 years after the Pacific route was used. How do scientists know that? Bison fossils.
Ice on the path
In the study, published Monday in Proceedings of the National Academy of Sciences, scientists studied fossils to find out when bison began to use the Rocky Mountain route.
If you remember your geological history from high school, a little something called the ice age was going on during this time, so bison north or south of the ice sheets were blocked from using the Rocky Mountain route for thousands of years. During that time the bison populations in the north (what's now modern-day Alberta) and south (Washington and Oregon) became genetically different from each other. As the ice receded and the Rocky Mountain path began to open up, bison began to move up and down the corridor.
And since the southern bison were different from the northern bison, scientists -- using radiocarbon dating and DNA analysis of the fossils -- were able to determine when the groups of bison on both ends began to use the path and when they meet up in the middle, pinpointing when the path was clear of ice and fully accessible. And the bison weren't alone on this route, because eventually bison hunters began to follow them.
Follow the bison
"When the corridor opened, people were already living south of there," said Beth Shapiro, one of the study's authors and a professor of ecology and evolutionary biology at the University of California, Santa Cruz. "And because those people were bison hunters, we can assume they would have followed the bison as they moved north into the corridor."
So that means the path was being used by migrating humans about 13,000 years ago, much later than the Pacific route.
"It shows the interior path was not available early enough to have been used by the first humans," Duane Froese, an associate professor at the University of Alberta who also coauthored the study, told CNN. "So it's back to the Pacific coast theory" as the first route used by humans.
Another reason scientists believe humans used the Pacific coast route first? The archeological evidence suggests humans mainly migrated from south to north along the Rocky Mountain route, Shapiro said, suggesting they had already established themselves south of the path -- after migrating along the Pacific.