First posited by the 16th century Spanish naturalist, José de Acosta, it has been a long-standing theory that the indigenous human populations in North and South America arrived there at the end of the last ice age, via the Bering land bridge, before rising ocean levels cut off the connection between the Asian and North American continents. According to this theory, the migrants made their way south via an ice-free corridor that ran between the Cordilleran and Laurentide ice sheets, cutting southward through what is now the province of Alberta in western Canada.

But amongst numerous findings that run contrary to that belief is a new study on the biology of that ice-free corridor, and the history of the species that eventually moved in to populate it. This study shows that the corridor was technically impassible at the time that this migration was supposed to have taken place: "What nobody has looked at is when the corridor became biologically viable," explains evolutionary geneticist and study lead Eske Willerslev. "When could they actually have survived the long and difficult journey through it?"

This new study, the first of its kind, was conducted by the University of Copenhagen’s Centre for GeoGenetics. It drew multiple types of data from sediments from present-day Charlie Lake in British Columbia and Spring Lake, Alberta, two points in the corridor that were the last to open up to allow passage, 12,600 years ago.

The study found that the area wasn’t biologically viable at the time — basically, there wasn’t enough flora or fauna to support a population of hunter-gatherers there until roughly a millennium after the passageway opened up. Then, around 11,500 years ago, the area became forested and populated by larger animals, necessary for a human population spreading through the area.

This is problematic, though, as 11,500 years ago is far too late a date to explain the evidence of human habitation at sites further south at earlier dates, such as in Florida 14,500 years ago, and in Chile 18,500 years ago.

"There is compelling evidence that Clovis was preceded by an earlier and possibly separate population, but either way, the first people to reach the Americas in Ice Age times would have found the corridor itself impassable," explains study co-author and archaeologist David Meltzer, from Southern Methodist University. Meltzer is referring to the Clovis people, a prehistoric Paleo-Indian culture that appeared roughly 13,000 years ago, but may have been predated by an even earlier group of humans. The study’s researchers hypothesize that these early inhabitants may have traveled southward along the Pacific coast, at a much earlier date.