News: Glaciers blocked New World colonizers for 20,000 years.

Glaciers blocked New World colonizers for 20,000 years: study
AFP
By Mira Oberman AFP - 2 hours 12 minutes ago

http://uk.news.yahoo.com/afp/20080213/tsc-us-anthropology-research-e123fef_1.html

CHICAGO (AFP) - The first human migration into the Americas was
blocked by two massive glaciers which kept early settlers on the
doorstep of the New World for about 20,000 years, a study published
Wednesday has found.

The study, which used DNA analysis to trace genetic changes over
generations and then compared it with the geological and
archaeological record, is the latest to challenge the standard story
of how the Americas were populated.

Instead of a steady migration out of Asia and across the Bering Strait
by an initial group of a hundred or less big game hunters, this data
shows a gradual and interrupted expansion by a much larger group of
between 1,000 and 5,400 people in three distinct stages.

The early generations began to move out of Siberia into Beringia,
which is now under the icy waters of the Bering Strait, about 40,000
years ago.

They settled into the dry grasslands, which was productive enough to
support large mammals but not rich enough to support significant
population growth, after their eastward progression was blocked by two
massive glaciers which covered much of what is now Alaska and northern
Canada.

"If you think about it, these people didn't know they were going to a
new world," said study co-author Connie Mulligan, assistant director
of the University of Florida Genetics Institute.

"They were moving out of Asia and finally reached a landmass that was
exposed because of lower sea levels during the last glacial maximum,
but two major glaciers blocked their progress into the New World. So
they basically stayed put for about 20,000 years. It wasn't paradise,
but they survived."

Two passages opened up as the ice sheets started to melt about 15,000
years ago and the initial settlers expanded rapidly into the more
fertile lands in the Americas, the analysis showed.

Within about four of five thousand years the land bridge across the
Bering Strait was swallowed by the ocean, blocking further expansion
out of Asia.

"The idea that people were stuck in Beringia for a long time is
obvious in retrospect, but it has never been promulgated," said Henry
Harpending an endowed chairman of anthropology at the University of
Utah who was not involved with the research.

"But people were in that neighborhood before the last glacial maximum
and didn't get into North America until after it. It's very plausible
that a bunch of them were stuck there for thousands of years."

The researchers were able to reach these conclusions by analyzing the
DNA of geographically and linguistically diverse populations
distributed throughout the Americas.

"This was the raw material, the original genetic source for all of the
Americas," said co-author Michael Miyamoto, associate chairman of
zoology at the University of Florida.

They found two distinct population increases at around 40,000 years
ago and 15,000 years ago which were separated by a long period of
little to no growth.

The first was a gradual sevenfold population increase as the group
expanded from Central Asia. The second was a rapid sixteenfold
increase, supporting the theory of a gradual expansion out of Asia and
a rapid expansion into the Americas.

They also found evidence of genetic changes which accumulated during
the 20,000 year period when the population held steady while waiting
for the ice to retreat.

"By looking at the kinds and frequencies of these mutations in modern
populations, we can get an idea of when the mutations arose and how
many people were around to carry them," Miyamoto said.

The study was published in PLoS ONE, the journal of the Public Library
of Science.

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Bob.

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