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Land Bridge Theory: Fact or Fiction
When it comes to human settlement, the New World is an also-ran. No Neanderthals ran here. The Homo genus spent millions of years evolving in East Africa and the rest of the Old World. But the first accepted date for humans in the Americas is 12,000 years back.
Looking at a globe, the reason is obvious: there's no dry route from there to here. A ticket to ride could mean a ticket to drown.
But we've long known that a land bridge joined present-day Alaska and Russia in a kind of pre-cold war geologic detente. The bridge "rose" from the ocean as vast amounts of ocean water became tied up in the enormous glaciers of the last ice age. That exposed the broad continental shelves now covered by the Bering Strait and created the land bridge.
The bridge last arose around 70,000 years ago. For years, scientists thought it disappeared beneath the waves about 14,500 years ago, toward the end of the last ice age. Unfortunately, that was about 2,500 yearsbeforethe first accepted date for human settlement in the new world.
Let's make a date
So either the date was wrong, or the whole theory about the land bridge was bunk. In recent years, paleobiologist Scott Elias and his colleagues at the Institute for Arctic and Alpine Research at the University of Colorado have pinned the problem on the date.
These researchers analyzed samples of sediment taken from beneath the Bering Strait with an advanced version of the carbon dating method that had produced the 14,500 years date. Their more accurate analysis showed that terrestrial plants and animals were living on the land bridge 11,000 years ago, meaning that the land bridge existed until after the oldest proven human settlements in the New World were started.
The technology, called radiocarbon dating, relies on the fact that cosmic rays transform nitrogen in the atmosphere into a radioactive form of carbon called carbon-14. Carbon-14 has six protons and eight neutrons -- for a total of 14 nuclear particles. It is unstable and immediately starts decaying to nitrogen 14. Living plants take in carbon-14 and store it in their tissues. When they die, the radioactive form eventually decays and disappears. Thus the ratio of the specimen's carbon-14 content to the carbon-14 content found in modern plants reveals how long ago the plant stopped taking in carbon dioxide -- in other words, when it died.
It's a handy technique, but it depends on pure samples. And that's where the earlier dating fell down, Elias says. Previously, scientists needed large carbon samples. But the large samples taken from the land bridge contained bits of ancient coal, which threw off the original carbon dating process.
With a newer analytical technique using a smaller sample, Elias skirted this difficulty and deduced that the land bridge was still taking tolls (just kidding) 11,000 years ago.
The area probably resembled today's North Slope of Alaska, Elias says. It was a landscape with stunted willows, birches and small clumps of sedges. And why did it have no glaciers when ice as much as two miles thick was crushing much of the eastern two-thirds of North America? Because the regional climate was too dry, Elias says. "It was cold enough for glaciers, but without moisture, you can't form them."
Questions remain
Although the new figures do "permit" the migration that peopled the Americas, they have not answered some intriguing questions. Physical evidence, after all, gives only indirect evidence about people's motivations and thoughts.
Why did the migrants come in the first place? Perhaps to follow herds of bison or other game that were migrating across the land bridge. Perhaps due to some change in living conditions on the Asian side of the bridge. Perhaps because they thought the grass was greener on the other side of the Bering Strait. Nobody knows.
Nor do we know if the wanderers spent years living on the land bridge, if they crossed in one quick motion, or whether the migration comprised several distinct movements. "We only reconstructed the physical environment, for what people faced in the area," says Elias. "We have no data on migration." For more on his findings, see "Bridge to the Past" in thebibliography.
And we don't know yet if the original Americans used boats to cruise the Pacific coast to South America, where archeological sites are nearly as old as those in Alaska. There's now evidence that an ice shelf at the south of the land bridge may have been an ideal place for migrating, since the migrants would have had access to sea mammals and fish as they moved.
Nonetheless, knowing exactly when the migration could have occurred may shed light on one important dispute in prehistory. Fifteen thousand years ago, the American menagerie looked much like that of East Africa, with North American lions, cheetahs, saber-toothed cats, mammoths, and relatives of camels all awaiting the first postcard photographer. Within a millennium or so, they were all gone. Were they, as some scientists think, exterminated by a huge wave of hunting by the first Americans? Or did a change in climate or something else account for their sudden disappearance?