Other research showing that humans might have arrived in the Americas at least 14,500 years ago -- and perhaps a couple of thousand years before that -- had already begun to undermine the ice sheet corridor hypothesis, forcing experts to look more closely at the possibility of a coastal route.
Pedersen and colleagues now appear to have closed the door on the inland route for good.
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The innovative methods they used for reconstructing the late Ice Age ecosystem were crucial.
Rather than hunt for DNA traces of specific plants or animals buried in sediment -- the standard approach -- Pedersen's team used what is called a "shotgun" method, cataloguing every life form in a given sample.
"Traditionally, we have been looking for specific genes from a single or several species," he explained.
"But the shotgun approach really gave us a fantastic insight into all the different trophic" -- or food-chain -- "layers, from bacteria and fungi to higher plants and mammals."
The researchers chose to extract sediment cores from what would have been a bottleneck in the inland corridor, an area partly covered today by Charlie Lake in British Columbia.
The team did radiocarbon dating, and gathered samples while standing on the frozen lake's surface in winter.
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Up to 12,600 years ago, the environment was almost entirely bereft of life, they found.
But the ecosystem evolved quickly, giving way within a couple of hundred years to a landscape of grass and sagebrush, soon populated by bison, woolly mammoth, jackrabbits and voles.
Fast-forward a thousand years, and it had transitioned again, this time into a "parkland ecosystem" dense with trees, moose, elk and bald-headed eagles.
The findings open "a window onto ancient worlds" and are a cornerstone in a "major reevaluation" of how humans arrived in America, said Suzanne McGowan of the University of Nottingham, commenting in Nature.
They also make the coastal passage scenario much more likely, she added.
Other scholars agree.
"If there ever was an ice-free corridor during the Last Glacial Maximum," James Dixon of the University of New Mexico wrote in a recent study, "it was not in the interior regions of northern North America, but along the Northwest Coast."
A "biologically viable" passage stretched along that coast from the Bering Land Bridge to regions south of the glaciers starting about 16,000 years ago, he reported in the journal Quaternary International.
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