New Land-Bridge Evidence Adds to Mystery of 1st Americans

Adrianne Appel
for National Geographic News
October 18, 2006
The long-gone land bridge between Asia and Alaska—a route possibly followed by the first humans to reach the Americas—flooded about 12,000 years ago, a new study suggests.

That's about a thousand years earlier than previously thought, adding to evidence that humans may have reached the Americas by other means.

"I think we're on the verge of rewriting the whole history of the region," said study leader Lloyd Keigwin of Woods Hole Oceanographic Institution in Massachusetts.

The new evidence from the Arctic also suggests that the thousand-mile-wide (1,609-kilometer-wide) bridge was available for a much shorter time than previously believed.

Unconventional Wisdom

The commonly held theory is that humans migrated across the Pacific during the last ice age, which ended around 11,500 years ago.

During the Ice Age much of Earth's water was locked up in glaciers, resulting in lower sea levels that exposed land previously underwater. The land bridge north of the Bering Sea that once linked Siberia and Alaska, is one example (see an Alaska map including the Bering Sea).

As the Ice Age ended and glaciers melted, sea levels rose, eventually swamping the Bering land bridge—but when?

Keigwin's team collected data suggesting that the bridge flooded not 11,000 years ago, as is widely believed, but closer to 12,000 years ago.

The new results appear in the October issue of the journal Geology.

The report adds to a growing body of research that challenges the idea that the only route the Americas was a single land bridge from Asia around 12,000 years ago.

(Related: "Americas Settled by Two Groups of Early Humans, Study Says" [December 12, 2005].)

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.

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