Sediments as Evidence
To determine the dates of the flooding, Keigwin's team collected cylindrical samples of Hope Valley. The valley was once part of the bridge but is now part of the floor of the Arctic Ocean's Chukchi Sea.
The scientists radiocarbon-dated and otherwise tested fossilized shells in the layered seafloor sediment. These readings helped the team determine what the climate was like during the time periods represented by the different layers in any given sample.
The now submerged Hope Valley, the researchers found, was flooded close to 12,000 years ago.
By Land and by Sea?
Some scientists now believe early humans migrated to the Americas from Asia in boats or by land over a much longer period of time.
"They may have done both, depending on the timing," said Vance Holliday, a geoscientist and anthropologist at the University of Arizona in Tucson.
"It's good to have this sort of data. This whole story is surprisingly poorly reported—of when the ice-free corridor was opened and closed," Holliday said.
Many scientists began to question the prevailing land-bridge theory in 1997. In that year researchers verified that human artifacts in Monte Verde, Chile, date back to an average of about 12,500 years ago.
Some scientists believe it is unlikely that people could have crossed the Arctic bridge during the time it was passable and still have made it south to Chile by 12,500 years ago.
Also, archaeological sites in the high Arctic have been dated at 29,000 years ago—long before the period of heavy glaciation that exposed the land bridge.
Such sites may have been home to populations that could have migrated to Chile by 12,500 years ago, according to Ted Goebel, associate director of Texas A&M University's Center for the Study of the First Americans in College Station.
Before the land bridge had become fully exposed, people might also have migrated from Asia by boat, Goebel says.
"There may have been enough land 14,000 years ago that people could have leapfrogged [by boat] from landmass to landmass," Holliday said.
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES