Two new studies have now, finally, put an end to the long-held theory that the Americas were populated by ancient peoples who walked across the Bering Strait land-bridge from Asia approximately 15,000 years ago. Because much of Canada was then under a sheet of ice, it had long been hypothesised that an “ice-free corridor” might have allowed small groups through from Beringia, some of which was ice-free. One study published in the journal Nature, entitled “Postglacial Viability and Colonization in North America’s Ice-Free Corridor” found that the corridor was incapable of sustaining human life until about 12,600 years ago, or well after the continent had already been settled.

An international team of researchers “obtained radiocarbon dates, pollen, macrofossils and metagenomic DNA from lake sediment cores” from nine former lake beds in British Columbia, where the Laurentide and Cordellian ice sheets split apart. Using a technique called “shotgun sequencing,” the team had to sequence every bit of DNA in a clump of organic matter in order to distinguish between the jumbled strands of DNA. They then matched the results to a database of known genomes to differentiate the organisms. Using this data they reconstructed how and when different flora and fauna emerged from the once ice-covered landscape. According to Mikkel Pedersen, a Ph.D. student at the Center for Geogenetics, University of Copenhagen, in the deepest layers, from 13,000 years ago, “the land was completely naked and barren.”

“What nobody has looked at is when the corridor became biologically viable,” noted study co-author, Professor Eske Willerslev, an evolutionary geneticist at the Centre for GeoGenetics and also the Department of Zoology, the University of Cambridge. “The bottom line is that even though the physical corridor was open by 13,000 years ago, it was several hundred years before it was possible to use it.” In Willerslev’s view, “that means that the first people entering what is now the U.S., Central and South America must have taken a different route.”

A second study, “Bison Phylogeography Constrains Dispersal and Viability of the Ice Free Corridor in Western Canada,” published in the Proceedings of the National Academy of Sciences, examined ancient mitochondrial DNA from bison fossils to “determine the chronology for when the corridor was open and viable for biotic dispersals” and found that the corridor was potentially a viable route for bison to travel through about 13,000 years ago, or slightly earlier than the Nature study.

Geologists had long known that the towering icecaps were a formidable barrier to migration from Asia to the Americas between 26,000 to 10,000 years ago. Thus the discovery in 1932 of the Clovis spear points, believed at that time to be about 10,000 years old, presented a problem, given the overwhelming presumption of the day that the ancient Indians had walked over from Asia about that time. In 1933, the Canadian geologist William Alfred Johnston proposed that when the glaciers began melting, they broke into two massive sheets long before completely disappearing, and between these two ice sheets people might have been able to walk through, an idea dubbed the “ice-free corridor” by Swedish-American geologist Ernst Antevs two years later.

Archaeologists then seized on the idea of a passageway to uphold the tenuous notion that Indians had arrived to the continent relatively recently, until such belief became a matter of faith. Given the recent discoveries that place Indians in the Americas at least 14,000 years ago, both studies now finally lay to rest the ice-free corridor theory. As Willerslev points out, “The school book story that most of us are used to doesn’t seem to be supported.” The new school book story is that the Indians migrated in boats down along the Pacific coast around 15,000 years ago. How long that theory will hold up remains to be seen.