Genetic and environmental evidence indicates that after the ancestors of Native Americans left Asia, they spent 10,000 years in shrubby lowlands on a broad land bridge that once linked Siberia and Alaska. Archaeological evidence is lacking because it drowned beneath the Bering Sea when sea levels rose.
University of Utah anthropologist Dennis O'Rourke and two colleagues make that argument in the Friday, Feb. 28, issue of the journal Science. They seek to reconcile existing genetic and paleoenvironmental evidence for human habitation on the Bering land bridge – also called Beringia – with an absence of archaeological evidence.
O'Rourke says cumulative evidence indicates the ancestors of Native Americans lived on the Bering land bridge "in the neighborhood of 10,000 years," from roughly 25,000 years ago until they began moving into the Americas about 15,000 years ago once glacial ice sheets melted and opened migration routes.
O'Rourke co-authored the Science Perspective column – titled "Out of Beringia?" – with archaeologist John Hoffecker of the University of Colorado at Boulder, and Scott Elias, a paleoecologist at the University of London.
"Nobody disputes that the ancestors of Native American peoples came from Asia over the coast and interior of the land bridge" during an ice age called the "last glacial maximum," which lasted from 28,000 to at least 18,000 years ago, O'Rourke says,
The ice sheets extended south into the Pacific Northwest, Wyoming, Wisconsin and Ohio. Large expanses of Siberia and Beringia were cold but lacked glaciers.
The absence of archaeological sites and the inhospitable nature of open, treeless landscape known as tundra steppe mean that "archaeologists have not given much credence to the idea there was a population that lived on the Bering land bridge for thousands of years," he adds.
O'Rourke and colleagues say that in recent years, paleoecologists – scientists who study ancient environments – drilled sediment cores from the Bering Sea and Alaskan bogs. Those sediments contain pollen, plant and insect fossils, suggesting the Bering land bridge wasn't just barren, grassy tundra steppe but was dotted by "refugia" or refuges where there were brushy shrubs and even trees such as spruce, birch, willow and alder.
"We're putting it together with the archeology and genetics that speak to American origins and saying, look, there was an environment with trees and shrubs that was very different than the open, grassy steppe. It was an area where people could have had resources, lived and persisted through the last glacial maximum in Beringia," O'Rourke says. "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."