Americas Settled 15,000 Years Ago, Study Says
for National Geographic News
|March 13, 2008|
A consensus is emerging in the highly contentious debate over the colonization of the Americas, according to a study that says the bulk of the region wasn't settled until as late as 15,000 years ago.
Researchers analyzed both archaeological and genetic evidence from several dozen sites throughout the Americas and eastern Asia for the paper.
"In the past archaeologists haven't paid too much attention to molecular genetic evidence," said lead author Ted Goebel, an archaeologist at Texas A&M University in College Station.
"We have brought together two different fields of science, and it looks like they are coming up with the same set of answers."
The article, which is published in tomorrow's issue of the journal Science, shows that the first Americans came from a single Siberian population and ventured across the Bering land bridge connecting Asia and North America about 22,000 years ago.
The group got stuck in Alaska because of glacial ice, however, so humans probably didn't migrate down into the rest of the Americas until after 16,500 years ago, when an ice-free corridor in Canada opened up.
(Related: "New World Settlers Took 20,000-Year Pit Stop" [February 14, 2008].)
Clovis Not First
Scientists have long agreed that the first Americans came from northeast Asia, according to Goebel.
But the new article—which analyzed genetic and archaeological evidence from 43 sites, including a dozen sites in Asia—better pins down the makeup of the first Americans.
Genetic evidence, for instance, points to a founding population of less than 5,000 individuals.
Some geneticists had also previously suggested that the migration across the land bridge could have occurred as early as 30,000 years ago.
"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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