The ancient peoples whom researchers have long believed migrated from Asia to North America over a long-since submerged land bridge across the Bering Strait apparently stayed on that bridge for quite a while, scientists now say.

Genetic and environmental evidence, scientists assert, show that after the ancestors of Native Americans left Asia, they spent approximately 10,000 years living upon the shrubby strip of land that once linked Siberia and Alaska.

Dennis O'Rourke, an anthropologist at the University of Utah, and two other research colleagues offered new arguments in favor of the 17-year-old theory in Feb. 28 issue of the journal Science.

Cumulative evidence suggests the prehistoric travelers lived on the Bering land bridge an estimated 25,000 years ago until about 15,000 years ago, according to O'Rourke, when they began pushing into the Americas along routes opened by the retreat of glacial ice.

From about 28,000 until 18,000 thousand years ago, ice sheets extended south into the Pacific Northwest, Wyoming, Wisconsin and Ohio, the research team said in a press release. On the other hand, large portions of Siberia and Beringia, or, the land that connected the two continents, were cold but didn't have glaciers.

The scientists say they ultimately want to reconcile existing genetic and environmental evidence associated with the period with the current lack of any archeological evidence that supports the theorized human habitation on the bridge.

The assumption, of course, is that archaeological traces of settlements on the bridge were washed over when the levels of the Bering Sea rose.

Based on the absence of archaeological sites, coupled with the considerably inhospitable nature of the arid landscape the bridge was presumed to have, "archaeologists have not given much credence to the idea there was a population that lived on the Bering land bridge for thousands of years," said O'Rourke.

But in recent years, noted O'Rourke, scientists who study ancient environments have discovered indications the bridge actually boasted a wide variety of plant, trees and animal life -- quite possibly enough to support human existence.

"We're putting it together with the archaeology and genetics that speak to American origins and saying, look, there was an environment with trees and shrubs ... It was an area where people could have had resources, lived and persisted through the last glacial maximum in Beringia," O'Rourke said. "That may have been critical for the people to subsist because they would have had wood for construction and for fires. Otherwise, they would have had to use bone, which is difficult to burn."

The theory humans inhabited the land bridge for 10,000 years or so helps explain how Native Americans ended up with a genetic makeup unique from their presumed Asian ancestors.

"At some point, the genetic blueprint that defines Native American populations had to become distinct from that Asian ancestry," O'Rourke said. "The only way to do that was for the population to be isolated."

Scientists don't think that period of isolation took place in Siberia because they haven't found a place in the region where a population could have been isolated enough to allow such genetic divergence.

The area in the central part of the land bridge, however, could "provide a place where isolation could occur," due to distance from Siberia, O'Rourke said.

"A substantial population existed somewhere, in isolation from the rest of Asia, while its genome differentiated from the parental Asian genome," he continued. "The researchers suggested Beringia as the location for this isolated population, and suggested it existed there for several thousand years before members of the population migrated southward into the rest of North and, ultimately, South America as retreating glaciers provided routes for southern migration."

Added O'Rourke: "Several other genetic-genomic analyses of Native American populations have resulted in similar conclusions."