Deep inside a cave in Siberia's Altai Mountains, Christy Turner and his Russian colleagues may have found an answer to a question that has hounded him for more than three decades.
As a young anthropologist, Turner spent time in Alaska's Aleutian Islands in the 1970s, working at several archaeological sites and occasionally gazing westward toward Siberia.
"I thought, 'That's the place that Native Americans came from,' " he says now from his laboratory at Arizona State University in Tempe.
But why, he wondered then as he still wonders today, did it take them so long?
The Bering Land Bridge that the first Americans crossed into the New World from Siberia had been there for thousands of years before those first immigrants arrived, most likely around 12,000 years ago. Archaeological evidence suggests the bridge surfaced repeatedly for at least 40,000 years as seawater became trapped in glaciers during the last Ice Age.
North America was one of the last places on the planet to be populated by humans, and "there has to have been a series of things that kept people out of the New World until very, very late," Turner says.
The evidence he and his colleagues have uncovered, he says, suggests that one player in that drama may have been a most unlikely, and yet terrifying, villain.
Ancient hyenas were larger than their relatives found today in Asia and Africa, and even the modern hyena has a jaw so powerful it can crush the leg of a rhinoceros, Turner says. Modern hyenas tend to be fearless in the presence of humans, and they have been known to drag a human hunter from a tent in Africa and crush his bones like toothpicks.
Could it be that the human migration into the Americas was held up by a nasty beast that preyed on people in the darkness of night, forcing them to remain far south of the land bridge that would have taken them to a new world?
Turner is the first to admit he doesn't know the answer to that. Not enough evidence is in yet to draw any strong conclusions, so at this point this is all scientific theorizing. But the clues so far are tantalizing.
Turner made his first of many expeditions to Siberia in 1979, and as one of the pillars of anthropology in the United States, he has become good friends with many of his counterparts in the former Soviet Union. He has been able to share in their research, including the artifacts they have uncovered in the far North.
"In all of the excavations that my Russian colleagues have been doing across Siberia, they can find almost no human remains," he says. "That's very interesting because if you go anywhere else with an equally good climate, almost always you find a little bit of human bone here and there."
Farther south, at about the latitude of Mongolia, "there are hundreds and hundreds of [human] archaeological sites that go back 50,000 to 60,000 years ago," he says, but just a few degrees north and the sites are no older than 12,000 years.
Clues in Crushed Bones
So Turner began to suspect that perhaps hyenas, running in packs of 40 to 50, may have been intimidating enough to keep those early humans well south of the region where hyenas roamed in great numbers.
Part of the evidence comes from a remarkable cave that was occupied solely by hyenas for about 40,000 years. Turner, who is also a dental anthropologist, examined bones found in the cave and concluded that all of the animals in the cave were dragged there by hyenas.
Most animals gnaw at a bone, or rip it open with slicing molars, but a hyena just crushes it. Even a bear can't do that. The bones found in the cave, Turner says, were clearly there because of the hyenas.
But one set of bones especially intrigues Turner.
"We found a true dog skull," he says. "We've dated the skull to about 14,000 years ago, and it's a domesticated dog," so much smaller than a wolf that it would not have survived if it had not been domesticated. The dog, he adds, was dragged into the cave, where it was devoured by hyenas.
It's the oldest dog ever found in Siberia, Turner says, and it was domesticated just before humans started their migration north, leading them eventually to the Americas.
"The coincidence is so remarkable," he says. "Once we get the dog, then we get people in the new world almost immediately."
Dogs Save the Day
Although at this point it's largely guesswork, Turner thinks it's quite possible that those early Siberians domesticated the dog in an effort to protect themselves from hyenas. A dog will bark at anything that approaches its territory, so barking dogs might have helped keep hyenas away from hunting camps.
At the very least, it would have alerted humans to an approaching horde of bone-crushing beasts.
That, Turner theorizes, might have finally given humans the edge, allowing them to encroach further into land thick with hyenas.
Eventually, the humans found the bridge across the Bering Sea, about 2,000 years before the hyenas themselves, along with many other larger animals, died out.
There are many uncertainties and gaps in the archaeological record, because the hyena has been largely ignored by anthropologists, Turner says.
But if he's right, those nasty critters kept us out of here for thousands of years, and dogs finally let us in.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.