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.