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What route did Ice Age peoples take to first reach what is now the United States? A new study concludes that a land bridge between Siberia and Alaska—the leading theory—would be “biologically unviable.”

To do those, those first people would have had to wait until two huge ice sheets that covered what is now Canada started to recede, creating the so-called “ice-free corridor” which enabled them to move south.

“What nobody has looked at is when the corridor became biologically viable,” says coauthor Eske Willerslev, an evolutionary geneticist at the Centre for GeoGenetics at the University of Copenhagen and University of Cambridge. “When could they actually have survived the long and difficult journey through it?”

For the new study in Nature, the researchers used ancient DNA extracted from a crucial pinch-point within the corridor to investigate how its ecosystem evolved as the glaciers began to retreat. They created a comprehensive picture showing how and when different flora and fauna emerged and the once ice-covered landscape became a viable passageway. No prehistoric reconstruction project like it has ever been attempted before.

The researchers conclude that while people may well have travelled this corridor after about 12,600 years ago, it would have been impassable earlier than that, because the corridor lacked crucial resources, such as wood for fuel and tools, and game animals which were essential to the hunter-gatherer lifestyle.

An empty corridor

If this is true, then it means that the first Americans, who were present south of the ice sheets long before 12,600 years ago, must have made the journey south by another route. The study’s authors suggest that they probably migrated along the Pacific coast.

Who these people were is still up for debate. Archaeologists agree, however, that early inhabitants of the modern-day contiguous United States included the Clovis culture, which first appeared in the archaeological record over 13,000 years ago. But, the new study argues, that the ice-free corridor would have been completely impassable at that time.

“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,” says Willerslev. “That means that the first people entering what is now the US, Central and South America must have taken a different route. Whether you believe these people were Clovis, or someone else, they simply could not have come through the corridor, as long claimed.”

“The ice-free corridor was long considered the principal entry route for the first Americans. Our results reveal that it simply opened up too late for that to have been possible,” adds PhD student Mikkel Winther Pedersen, who conducted the molecular analysis.

Molecular fossils from the ‘bottleneck’

The corridor is thought to have been about 1,500 kilometers (932 miles) long, and emerged east of the Rocky Mountains 13,000 years ago in present-day western Canada, as two great ice sheets—the Cordilleran and Laurentide—retreated.

The research focused on a “bottleneck”, one of the last parts of the corridor to become ice-free, and now partly covered by Charlie Lake in British Columbia, and Spring Lake, Alberta—both part of Canada’s Peace River drainage basin.

The team gathered evidence including radiocarbon dates, pollen, macrofossils, and DNA taken from lake sediment cores, which they obtained standing on the frozen lake surface during the winter season. Willerslev’s past work demonstrated that it is possible to extract ancient plant and mammalian DNA from sediments, as it contains preserved molecular fossils from substances such as tissue, urine, and feces.

Having acquired the DNA, the group then applied a technique called “shotgun sequencing.” “Instead of looking for specific pieces of DNA from individual species, we basically sequenced everything in there, from bacteria to animals,” Willerslev says. “It’s amazing what you can get out of this. We found evidence of fish, eagles, mammals, and plants. It shows how effective this approach can be to reconstruct past environments.”

Plants, fish, animals

This approach allowed the team to see, with remarkable precision, how the bottleneck’s ecosystem developed. Crucially, it showed that before about 12,600 years ago, there were no plants, nor animals, in the corridor, meaning that humans passing through it would not have had resources vital to survive.

Around 12,600 years ago, steppe vegetation started to appear, followed quickly by animals such as bison, woolly mammoth, jackrabbits, and voles. Importantly 11,500 years ago, the researchers identified a transition to a “parkland ecosystem”—a landscape densely populated by trees, as well as moose, elk, and bald eagles, which would have offered crucial resources for migrating humans.

Somewhere in between, the lakes in the area gained fish, including several identifiable species such as pike and perch. Finally, about 10,000 years ago, the area transitioned again, this time into boreal forest, characterized by spruce and pine.

Source: University of Copenhagen