Settlement of Americas a 3-act play

As for Beringia, sea levels rose about 10,000 to 11,000 years ago as the peak of the ice age waned, submerging the land and creating the Bering Strait that now separates the New World from Siberia with at least 60 miles of open, frigid water.

The expansion into the New World may have occurred by land after the ice sheets covering what is now Canada began to retreat 14,000 to 17,000 years ago, the scientists noted. However, they added that glaciers on the northwest Pacific coast of North American also might have receded by about 17,000 years ago, thus presenting a viable coastal route by sea to the continent. "It doesn't have to be an either-or thing. They could have used both routes," Mulligan said.

Innovative work
Anthropologist and population geneticist Henry Harpending at the University of Utah, who did not participate in the research, said he found the work innovative.

"The idea that people were stuck in Beringia for a long time is obvious in retrospect, but it has never been promulgated," said Harpending, who found it very plausible that people were stuck in Beringia "for thousands of years."

Although this new theory could resolve some of the conflicting results scientists have come up with over the years, "I don't pretend this inclusive approach will make us any friends, just more critics," Mulligan told LiveScience.

For instance, one criticism Mulligan anticipates is the fact that "20,000 years in Beringia is a long time. Some people will say, 'Get real — where's the evidence?' We would say that there's no one that does arctic underwater archaeology."

Researcher Andrew Kitchen added, "Our theory predicts much of the archaeological evidence is underwater. That may explain why scientists hadn’t really considered a long-term occupation of Beringia."

Mulligan noted one might also contend that any evidence of such a 20,000-year stay in Beringia might have left evidence in Siberia or Alaska. "But we envision a small population there that probably left a relatively light footprint on the landscape," she explained. "And the areas we're talking about — Siberia, Alaska — there's no way to argue that researchers have covered those areas thoroughly, with their incredibly harsh climates."

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