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.
