"Now there seems to be consensus among those studying mitochondrial DNA and [chromosome records] of modern native Americans that it happened pretty late, after the last glacial maximum, maybe as late as 15,000 calendar years ago," Goebel said.
(Related: "First Americans Arrived Recently, Settled Pacific Coast, DNA Study Says" [February 2, 2007].)
Meanwhile, archaeologists for years had considered sites belonging to the so-called Clovis culture, which dates back 13,000 years, to represent evidence of the first Americans.
The Clovis culture was named after flint spearheads found in the 1930s at a site in Clovis, New Mexico. Clovis sites have been identified throughout the contiguous United States as well as in Mexico and Central America.
But several sites, from Wisconsin to Monte Verde in Chile, have been discovered in recent years that predate Clovis by at least a thousand years.
"There probably has to have been some time before Clovis in which people were here, but they didn't leave much of a record behind because there just weren't that many people," Goebel said.
Archaeological evidence shows that there were people occupying the Asian side of the Bering land bridge area as early as 30,000 years ago.
"That tells us that once early modern humans spread out of Africa around 50,000 years ago and colonized temperate Eurasia, it wasn't very long before they had developed the technology and the skills needed to be able to make a go of it in the Arctic," Goebel said.
Modern humans spread across the land bridge about 22,000 years ago, according to the new article.
But then the group got stuck for up to 5,000 years, blocked by thick ice sheets across Canada.
It was only when the ice had melted sufficiently that humans began to spread south, either along the coast or though an interior corridor in western Canada, the authors say.
"That might have been the bottleneck that kept people from draining south from Alaska into temperate North America," said Goebel, adding that geological evidence suggests the Pacific coastal corridor would have become ice-free perhaps as early as a thousand years before the interior corridor.
"This suggests that the first Americans may have spread through the New World along a coastal route," he said.
Henry Harpending is an anthropologist and population geneticist at the University of Utah in Salt Lake City who was not involved in the study.
He agreed that there is a consensus emerging among researchers studying the first Americans.
"But there are still outstanding questions," he said.
For example, there are some "puzzling anomalies" in the Alaskan archaeological record dating back to before the glacial melt, he pointed out.
And there are several possible reasons other than ice why people did not venture south earlier, including a "ferocious army of predators" living in North America that might have had a role in keeping humans away.
"We all have open minds, and we will leave them open," Harpending said.
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