By Will Dunham
WASHINGTON, Feb 27 (Reuters) - When it comes to camp-outs, this one was epic.
On the way from Asia into the Americas, the ancestors of Native Americans hunkered down for about 10,000 years during a particularly frigid period of the Ice Age in territory called the Bering Land Bridge that once linked Siberia to Alaska.
That's the argument advanced on Thursday by scientists who said that fossil evidence showing that shrubby lowlands there could have supported human habitation fits nicely with DNA data about the ancestors of today's Native American population.
Perhaps several thousand people lived in the territory now submerged under the Bering and Chukchi Seas from about 25,000 years ago to 15,000 years ago before crossing into Alaska and dispersing throughout North and South America, they said.
"It's staggering when you think about people living in temporary shelters - probably something like a tent - in the Arctic, especially in winter," paleoecologist Scott Elias of Royal Holloway, University of London said in a phone interview.
"These are extremely rugged people. I'm sure they were very well adapted to living in the cold in terms of their physique, their physiology, their ability to withstand temperatures that would make most of us be absolutely miserable or die," he said.
Previous DNA research indicated that the ancestors of Native Americans became isolated from rest of the human race long enough to acquire their own distinctive genetic blueprint.
As a result, most Native Americans share genetic traits not seen in the Asian populations from which their ancestors arose.
Elias and two colleagues, archaeologist John Hoffecker of the University of Colorado and anthropologist Dennis O'Rourke of the University of Utah, wrote in the journal Science that evidence suggests this isolation took place on the land bridge.
They cited their own work and the findings of other scientists to back up their ideas.
During the Ice Age, this massive land bridge - twice the size of Texas - connected the two continents at a time when ocean levels were substantially lower than they are today.
People trekking onto the land bridge were blocked from entering North America by huge ice sheets covering large parts of the continent during one of the coldest periods on Earth.
As the planet warmed, ice sheets retreated, opening up routes into North America. The land bridge disappeared amid rising sea levels caused by surging global temperatures.
Some experts have felt that the conditions on the land bridge would have been too harsh and barren to sustain people. But Elias said the central part of the land bridge may have provided a proper refuge.
Though it is now under water, scientists have been able to analyze what life may have been like on the land bridge. The U.S. Geological Survey drilled into the seafloor in the 1970s and 1980s to check for oil and gas deposits, and those core samples included the surface of the submerged land bridge.
This revealed fossil pollen, plant and insect material that indicated the region had a tundra environment with woody plants and trees like birch, willow and alder. This wood supply, the scientists said, may have been a source of fuel for campfires.
People could have used wood to start a fire, then placed large mammal bones on it to ignite fats inside the bones, the scientists said. Large mammal leg bones could have burned for hours, keeping people warm through frigid winters, they said.
"A wood supply may have been critical as a supplement to bone fuel," Hoffecker said by telephone.
With a meat-heavy diet, people may have fished and hunted water birds, elk, deer and caribou in their shrubby lowlands, Hoffecker said. A nearby steppe-tundra region supported bigger grazing mammals like mammoths, bison and horses but lacked the shrubs and trees needed for long-term human habitation.
Hoffecker said that people may have made temporary hunting forays into those regions during warmer months.
These researchers said the weakest link in their case is a lack of archaeological evidence that people were actually on the land bridge at the time. After all, the ancient land bridge is now under 160 to 200 feet (50 to 60 meters) of water.
O'Rourke said that archaeological research in the few coastal areas in Alaska or Siberia that have remnants of the ancient region might yield evidence of human habitation.
(Editing by Cynthia Osterman)
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