According to the most accepted hypotheses, the first people who arrived in North America would have passed to the continent through an old land bridge between Siberia and Alaska. They had to wait for two large layers of ice that covered what is now Canada to begin to recede, until the so-called "ice-free corridor" was created that allowed them to move south.
However, a new study published in the journal Nature debunks this theory. The international team of researchers, led by Professor Eske Willerslev, an evolutionary geneticist at the Center for GeoGenetics at the University of Copenhagen and the University of Cambridge, used ancient DNA extracted from a crucial point within this corridor to investigate how its ecosystem evolved when the glaciers began to retreat.
The scientists created a complete image showing how and when the flora and fauna emerged, covering the ice of this passage route to make it viable, a project to reconstruct prehistory like never before.
The researchers note that while humans may have traveled through this corridor around 12,600 years ago, it would have been impractical earlier, lacking crucial resources such as wood for fuel and tools, and game animals that were essential for the hunter-gatherer lifestyle.
"The bottom line is that even though the physical corridor was open for 13,000 years, it was not possible to use it until several hundred years later," says Willersley.
If this is true, then it means that the first Americans, who were already present to the south long before this date, had to make the journey by another route. The authors of the work suggest that it is likely that they migrated along the Pacific coast.
Who were those first settlers?
Who those people were is still widely debated. Archaeologists agree, however, that the earliest inhabitants of modern America included the so-called Clovis culture, which first appears in the archaeological record more than 13,000 years ago. Scientists argue that the ice-free corridor would have been completely impassable at the time.
"That means that the first people who entered what is now the US, Central and South America took a different route. If these are believed to be Clovis, or other people, they simply could not have come through the corridor." , declares the researcher.
Mikkel Winther Pedersen, a doctoral student at the Center for GeoGenetics who carried out the molecular analysis, adds: "The ice-free corridor was long considered the main gateway for early Americans. Our results reveal that it simply opened too late for that to have been possible. "
The corridor would have been about 1,500 kilometers long, and emerged east of the Rocky Mountains 13,000 years ago in what is now western Canada, when the Cordillera and Laurentide ice sheets disappeared.
An unviable corridor to survive the trip
On paper, this fits well with the argument that the Clovises were the first to disperse across America. The earliest evidence of this culture, named after stone tools found near Clovis, New Mexico, also dates from around the same time, although many archaeologists believe other people came earlier.
"What no one had examined is when the corridor became biologically viable. When the long and difficult journey through it could be survived," says Willersley.
The research focused on a 'bottleneck', one of the last parts of the corridor to be ice-free, and now partly covered by Lake Charlie in British Columbia and Spring Lake in Alberta - both in the basin of drainage of the Peace River of Canada.
The passage to the ecosystem of green areas
Around 12,600 years ago, steppe vegetation began to appear, quickly followed by animals such as the bison, the woolly mammoth, rabbits, and voles. The researchers identified a transition to a parkland ecosystem, that is, a landscape densely populated with trees, elk and bald eagles, which would have been crucial resources for human migration.
Somewhere in between, the lakes in the area became populated with fish, such as pike and perch. Finally, about 10,000 years ago, there was another moment of change, this time towards a boreal forest, characterized by fir and pine trees.
The fact that Clovis was present south of the corridor prior to 12,600 years ago means that they did not arrive by traveling through it. David Meltzer, an archaeologist at Southern Methodist University and a co-author of the study, concludes: "There is no convincing evidence that the Clovis culture was preceded by an earlier and possibly separate population. Either way, the former who came to America in the Ice Age found an impassable corridor. " The most likely scenario is that they arrived from the Pacific coast.
Eske Willerslev et al. "Postglacial viability and colonization in North America’s ice-free corridor" Nature DOI: 10.1038 / nature19085