Coming to America Through Beringia

Twelve thousand years ago, if humans or animals felt like walking from Asia to North America, all they had to do was head over the Bering Land Bridge. (It’s a little harder now.)

A MYSTERIOUS, DISAPPEARING BRIDGE

From the edge of Alaska’s Seward Peninsula, Siberia is just 53 miles away. And at a point just south of the Arctic Circle, the two continents are separated only by a narrow channel of water that links the Arctic Ocean to the Bering Sea. That channel is called the Bering Strait, and scientists long wondered if primitive people used it to cross from Asia to North America.
But frequent, severe storms and massive ice floes would have made it difficult for primitive people to make their way across the Bering Strait by boat. And so, for centuries, archaeologists speculated that perhaps there had once been a piece of land that stretched across the strait. If so, people could have walked from one continent to the other in less than three days.
The idea first appeared back in 1590, when a Jesuit priest named Jose de Acosta noticed a resemblance between the native people of South America and those of Asia. He was the first to propose that the first people in the Americas had traveled there from Asia— he just didn’t know how. In the 1800s, archaeologists expanded that hypothesis, saying that at least some of the indigenous people of North and South America had migrated from Asia to America over the Bering Strait, walking on a bridge of land that was above sea level. They called the theoretical region the Bering Strait Land Bridge, or “Beringia.” And unlike many early theories of how the world worked, the land bridge idea has held up against modern scientific examination.

THE WIDEST BRIDGE EVER

Beringia existed during the Pleistocene epoch, which lasted from about 2.5 million years ago to 11,500 years ago. At times during the Pleistocene, the earth was going through ice ages, and its water was frozen in massive glaciers. Since the planet has always had the same amount of water on it, what was trapped in glaciers meant there wasn’t as much in the oceans. Water levels during the Pleistocene were about 300 feet lower than they are now, and the Bering Strait is only 165 feet deep at its lowest point. So when there were large glaciers on the continents, the bottom of the Bering Strait was well above sea level, and it connected Alaska to Siberia.
Beringia is called a “land bridge,” but it wasn’t a narrow path. The ocean around the Bering Strait is so shallow that when water levels fell, they exposed a landmass with a width of about 1,000 miles, almost the distance from San Francisco to Denver.
Russian and American scientists have studied the Bering Strait shoreline and dated sea cores (long cylinders of sediment) from the ocean floor. Traces of pollen, plant material, and insects in the cores help them date when Beringia existed as dry land. That happened at various times for more than two million years, and the land bridge stayed above water for thousands of years at a time. During one of the more recent appearances, the land bridge popped up about 30,000 years ago, staying above water for at least 15,000 years. It was during that time that humans living in Siberia had the opportunity to walk over into the Americas.

THOSE RESTLESS BERINGIANS

The sea cores also give clues about what Beringia was like: flat, dry, treeless tundra, where the soil was frozen much of the year. It had little rain, bitterly cold winters, and a thin cover of snow that melted in the cool, dry summers. Since it was too dry to have glaciers (which covered much of North America at the time), Beringia would have seemed like a comparatively good place to live. It was cold and harsh, but it still had grasses, herbs, and shrubs that could support wildlife. Caribou evolved on the land bridge, and woolly mammoths and mastodons grazed there… so did some yaks, musk oxen, bison, deer, rabbits, camels, and horses. Predators followed the grazing animals, and soon saber-toothed tigers, bears, and wolves were also living and drifting between the two continents. And humans would likely have followed the animals.
Research shows that many indigenous peoples in North and South America share DNA, physical characteristics (a similar formation of teeth and jaws), culture, and even language with Siberian natives of northeastern Asia. Instead of a mass migration, though, it’s likely that small bands of hunters and their families followed prey animals from Asia into Beringia and then onto the North American continent. Then when the climate began to change and the water rose, access to Asia disappeared and the people who had crossed the bridge were “stuck” in America.

 

Or so the scientists believe. Studies of the first humans in the Americas are still ongoing, and so far, only sparse evidence exists about their migrations. There are also some pretty big problems the migrants would have had to overcome. For one thing, when the land bridge was exposed, much of what’s now Canada and the United States were covered with glaciers. People would have had trouble actually moving into North and South America. However, there were two ice-free “corridors” that might have provided pathways for migration southward. An inland corridor ran from Alaska along the eastern side of the Canadian Rockies to Montana. And a coastal corridor lay in the Pacific Northwest. When one corridor was iced over, the other was usually open. So most archaeologists believe that the first people in the Americas traveled along the West Coast and continued south, always looking for new opportunities in various parts of the new continents.

THANKS FOR JOE CAMEL

The human mysteries may still be hard for archaeologists to piece together, but they do know that Beringia was important to wildlife. Many animals— including dinosaurs, rabbits, and bears— moved from continent to continent, making each a more diverse place. And when it came to saving wildlife from extinction, Beringia made a big difference. For example, camels, which have adapted so well to the deserts of the Middle East, had forebears that lived in North America. Most of the animals that made their way across Beringia came from Asia to America. But the camels went the other way— from America into Asia and then to the Middle East. In the Americas, camels became extinct. Only the ones that crossed into Asia survived.
Horses also evolved in North America, and they also went extinct on that continent. Had prehistoric horses not crossed the land bridge into Asia and kept traveling until they reached the plains of Mongolia, the animals might never have survived to become human companions.