Bridge over northern waters.

The Bering land bridge, thought to have provided a passageway from northeastern Asia to North America for ancient New World settlers and large mammals, existed until about 11,000 years ago, according to a report in the July 4 Nature. That's much later than some prior studies had suggested.

Although the land bridge has often been thought of as an arid grassland capable of sating the appetites of mammoths and other large beasts, vegetation there 11,000 years ago consisted mainly of hardy plants and shrubs adapted to tundra conditions, assert Scott A. Elias of the University of Colorado in Boulder and his coworkers. Such plants could not have supported extensive grazing by large mammals, the scientists argue.

"This new evidence tells us that the big mammals probably did not linger on the land bridge," Elias contends.

The researchers analyzed 20 core samples from the floor of the Bering and Chukchi Seas. The cores contain layers of organic peat and silt that encase plant, pollen, and insect remains. Radiocarbon dates for this material indicate that vegetation typical of the modern Alaskan tundra characterized the now-submerged region from 14,000 to 11,000 years ago.

A previous radiocarbon study of seafloor sediment placed the land bridge inundation at about 14,400 years ago. But dates for those samples may have been inaccurate because of contamination by other substances in the soil, Elias holds.