The ancestors of Native Americans came to the New World by walking over a land bridge across the Bering Strait. But there's a rather glaring 10,000 year gap in the story — one that could be explained by a migratory pause that lasted for millennia.
Genetic evidence proves that Asian populations made the trek across Beringia roughly 25,000 year ago (though some still hold out for the so-called the Solutrean hypothesis in which humans made the trek from southwest Europe). But a recent genetic analysis of mitochondrial DNA by University of Utah anthropologist Dennis O'Rourke and colleagues shows that these populations didn't actually make it to North America until about 15,000 years ago. Quite obviously, it shouldn't take a group of paleolithic-era humans 10,000 years to trek across a 51 mile (82 km) stretch. So what happened?
According to O'Rourke, they may have been stuck there, unable to cross over a massive glacial barrier that separated the two continents. And he makes the argument without presenting any actual archaeological evidence. But that doesn't mean he and his team didn't didn't offer some compelling clues.
A Home, Not a Bridge
Indeed, because the "land bridge" is now submerged, anthropologists have not been able to uncover actual archaeological evidence hinting at prolonged human habitation. But other evidence exists, such as fossilized insects, plants, and pollen extracted from Bering Sea sediment cores.
The Bering Land Bridge, it would seem, is perhaps misnamed. It wasn't just a barren, grassy tundra steppe, but rather a rich and viable ecosystem populated by brushy shrubs, and even trees such as spruce, birch, willow, and alder. It was also home to various animals, such as bison, mammoths, elks, birds, moose — and even camels (who may have been traveling in the other direction, making the trek from North America to Asia).
What's more, it was a huge landmass. During the last glacial maximum, which lasted from 28,000 to about 18,000 years ago, thick glacial ice sheets extended south into what is now Alaska. Back then, sea levels were 400 feet lower, creating a sizable swath of land between and south of Siberia and Alaska, at the present sites of the Chukchi Sea, the Bering Strait and the Bering Sea, respectively. At its largest, Beringia measured 1,000 miles (1,600 km) from north to south and as much as 3,000 miles (4,800 km) from Siberia's Verkoyansk Range east to the Mackenzie River in in Canada.
In addition, much of Beringia, particularly the lowlands, had average summer temperatures nearly identical (or only slightly cooler in some regions) to those in the region today.
"The local environments likely were not as daunting as many have assumed for years," noted O'Rourke in a statement. "They probably hunkered down pretty good in the winter though. It would have been cold."
Given its size, rich ecosystem, and reasonable climate, no human or animal would have perceived it as a "bridge." But for migrating humans, the glacier would would have represented an insurmountable barrier.
At some point, argues O'Rourke, the genetic blueprint that defines Native American populations had to become distinct from that Asian ancestry. But the only way for that to happen was for the population to be isolated.
"Most of us don't believe that isolation took place in Siberia because we don't see a place where a population could be sufficiently isolated," he says. "It would always have been in contact with other Asian groups on its periphery. But if there were these shrub-tundra refugia in central Beringia, that provided a place where isolation could occur."
The Great Melt
When the glacial maximum period came to end, so too did the barrier separating the two continents. With the glaciers gone, the isolated humans spilled into North America some 15,000 years ago. At least that's what's suggested by these clues; O'Rourke admits that archaeological sites must be found in Beringia if this theory is to be confirmed. This will prove easier said than done given that these areas are underwater. But some evidence of human habitation in shrub tundra may be present above sea level in low-lying areas of Alaska and eastern Chukotka in Russia.
But until then, genetic and environmental clues will have to do.
"We're putting it together with the archaeology and genetics that speak to American origins and saying, look, there was an environment with trees and shrubs that was very different than the open, grassy steppe. It was an area where people could have had resources, lived and persisted through the last glacial maximum in Beringia," O'Rourke says. "That may have been critical for the people to subsist because they would have had wood for construction and for fires. Otherwise, they would have had to use bone, which is difficult to burn."
Read the entire study at Science: "Out of Beringia?"
Image: William Manley, Institute of Arctic and Alpine Rsearch, University of Colorado.
Follow me on Twitter: @dvorsky