Bones of contention

That's why many paleoanthropologists are starting to believe that the first migration from Siberia to the New World must have happened at a much earlier date than was previously believed, maybe around 40,000 years B.P. Other, more radical theorists are beginning to think that there might have been an initial movement across Beringia (if not into the interior of North America) much, much earlier, even back to the days of archaic humans -- an idea that would delight the Native Americans who claim to have "always" been here. New research on the Siberian side of the land bridge, particularly in Northern China, seems to be supporting the way-back dates by demonstrating that even in very early times, humans were able to adapt to extraordinarily inhospitable environments.

Turd worms

In the early 1980s, Brazilian parasitologists Luiz Ferriera and Aduato Arazjo discovered what they believed were hookworm eggs in human coprolites -- a.k.a. fossilized poop -- from archaeological sites dating well before the arrival of Columbus. The life cycle of hookworms requires a period of incubation in warm, or at least temperate, soils. If the New World was populated via the Beringian land bridge, the cold earth of the tundra supposedly would have acted as a kind of cold-weather filter keeping out hookworms and other tropical parasites. So if those parasites can be shown to have infected Native Americans before Europeans arrived, the worm must have come straight from a tropical climate in Asia or Africa to the tropics of the New World, not passing through the Frozen North.

Even leaving aside the possibility that the worm eggs Ferriera and Arazjo found might not be human parasites at all, or were misidentified (as at least one researcher has contended in a published response to their claims), there was another possibility. Depending on when we want to assume that humans and their intestinal worms might have crossed Beringia, the weather up there could have been far from frozen.

When the glaciers were melting at the end of the last Ice Age, a time-frame considered likely for a Beringian migration (even by those who believe that some Paleoindians or pre-modern humans may have come across much earlier), "there were periods warm enough to bring tropical fauna north as far as Maine," says Greg Laden, an anthropologist at the University of Minnesota. "I myself once recovered a tropical form of a razor clam in Boston in sediments that would date to some time in this period."

Laden also points out that the possibility of individuals harboring intestinal parasites across a warmer, wetter Beringia -- or perhaps even carrying them in semi-dormant or precursor forms amidst the dirt of their fireside baggage -- is no more difficult to contemplate than the idea that the parasite made it across the endless Pacific in an open boat, among people who were probably dumping their wastes into the briny deep.

No exit

The most romantic scenario in the Beringian Walk theory has the intrepid First Americans trekking down the Mackenzie River valley of Canada around 12,000 years B.P., in view of towering walls of blue-white ice on either side (a story with an entertaining hint of Moses and the Red Sea). Without this "ice-free corridor" snaking along between the mountainous glaciers, the Paleoindians couldn't have moved down from Beringia to make their arrowheads in Clovis at the right time.

Alejandra Duk-Rodkin, a geologist with the Canadian Geological Survey, and her colleague Don Lemmen researched the geological record along the so-called "Mackenzie corridor" (which wasn't in today's Mackenzie River valley) and concluded that there was no navigable route from north to south at any time between about 30,000 B.P. and 11,000 B.P. If that path didn't exist, Dewar notes triumphantly, the first New World immigrants would have had to have come from somewhere other than Siberia.

But it ain't necessarily so. If the earliest Americans were already over the land bridge and into the Americas by around 40,000 B.P., as some Beringian Walk theorists think, "there would have been plenty of room and time for Beringian people to squeeze through the slowly expanding ice sheets," says Cinq-Mars. Duk-Rodkin might be a good geologist, but even experienced anthropologists can't make definitive statements about what terrain and climates a sturdy, mobile and cold-adapted people might have been able to live in or negotiate.

And, Cinq-Mars points out, "the geological record is very spotty," so the chronology of the glaciers and their interaction over the corridor is "imprecise." The only formal theory that the Duk-Rodkin research really calls into question is the Clovis First model of migration. The Beringian Walk itself, Cinq-Mars says, remains a "sine qua non."

The 3,000 faces of Eve

The NAGPRA regulations that the U.S. Army Corps of Engineers cited in spiriting away the Kennewick Man skeleton from James Chatters's laboratory require that an effort be made to identify the "affiliation" of ancient remains to a contemporary tribe, so the deceased can be appropriately "repatriated" to his descendants. The problem with this provision, of course, is that it is often devilishly difficult to make that determination, and was especially so in the case of Kennewick Man.

One relatively new branch of research that undertakes to prove or disprove tribal connections is the examination of mitochondrial DNA (mtDNA), a kind of genetic material that can only be inherited from the mother's line. Beguiling Biblical echoes turn up in the theory that everyone alive on on earth today contains some small element of mtDNA from a single female ancestor who lived in Africa about 200,000 years ago. She's known as the "Mitochondrial Eve." Molecular biologists have been working for about 15 years to sort out the threads of mtDNA inheritance, hoping to be able to trace the genetic history of mankind and, not incidentally, track the dispersal of humans across the planet.

Next Page: Why Native Americans have "Mongolian" characteristics

• View blog reactions