Stefan Lovgren
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 intRo the rest of the Americas
until after 16,500 years ago, when an ice-free corridor in Canada
opened up.
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.
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.
Coastal Route
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.
New Study
No comments:
Post a Comment