The last glacial episode between 20,000 and 10,000 years ago marks the transition between the end of the Pleistocene epoch, about 10,000 years ago and the beginning of the Holocene, the epoch in which we live.
A wide land bridge, called the Bering Land Bridge, that linked present-day Alaska with present-day northeastern Siberia was exposed and provided a corridor between Eurasia and North America. The Land Bridge and lands immediately to the east and west of it have acquired the name Beringia and is an extremely important geographical entity in the ecological history of North America. At the height of the last glacial maximum so much of the world’s water was locked up in continental ice sheets that the sea level was several hundred feet below its present level.
Global climates were steadily cooling during the Pliocene Epoch (5.3 to 1.8 million years ago). During cold periods, snow falling on mountains and plateaus fails to return back to the sea. Glaciers grew into ice caps and sea levels fell. When the ice caps melts, sea levels rise. This happened at least 10 times during the past 80,000 years. After reaching a temporary maximum width and area 18,000 years ago, the Bering Land Bridge rapidly narrowed as ice caps shrank and sea level rose. By 9,500 years ago, the last land connection between the western tip of St. Lawrence Island to the southeastern corner of the Chukchi Peninsula was severed by the by the continuing post-glacial rise in sea level. Eastern Bering includes most of modern mainland Alaska and portions of the Yukon Territory and Northwest Territories. It was bounded on the east by the Mackenzie River valley beyond which lay continental ice.
During the glacial maximum, Alaska was basically closed off from the rest of North America by the great cordilleran ice sheets that extended from the eastern part of the Yukon down into the central United States. The resulting effects were monumental. One example of the evolutionary process resulting from that time is the Dall Sheep (Ovis dalli) for which Denali National Park was originally established. Dall sheep are the only wild white mountain sheep in the entire world. Bighorn sheep and A=Dall sheep are a product of one sheep species being pinched into two isolated segments for a long period of time during the last glacial episode. Dall sheep, being white, adapted to the mostly white (snowy) habitat.
Origins of the Land Bridge
Tilt of the earth’s axis during the Pleistocene. Not a constant 23 ½ degrees. May have been much less tilt (Malenkovich Cycles of axis variation).
Beringia was ice-free and considered a refugia. The Bering Land Bridge was about 620 miles wide, north to south. This land bridge connecting North America to Asia allowed for the important movement of people, flora and fauna. Today North America and Asia come close to meeting at the Arctic Circle.
People of the Bering Land Bridge
It is now accepted that the Bering Strait is the route of entry of the Native Americans. Humans were in the Americas at least 12,000 years ago and perhaps much earlier. Groups of people moved across the Bering Land Bridge hunting game and migrated southward through western North America. Language similarities, particularly between the Navajo and the Athabascans are one of many examples of those shared beginnings.
The Holarctic flora (Holarctic is a term used by zoologists to define the ecozone covering much of Eurasia and North America which has often been connected to the Bering Land Bridge) is known as the “mammoth steppe”, abundant grasslands which allowed for numerous grazers such as mammoth, bison, horse and wapiti. Examination of pollens laid down in lakes and other areas at that time show there were no spruce or alder.
By far the greater part of the migration of mammals between continents was from Eurasia to America. Only 6 forms of fauna have been found moving from America to Eurasia (beaver, camel, horse…) and 22 mammal species migrated to America. These included mammoth, musk ox, caribou, moose, grizzly bear, polar bear, and saiga antelope. The Bering Land Bridge is now submerged by for than 200 feet of water. The grasslands of Interior Alaska have been replaced by forests and bogs.
The peculiar trans-Beringian migration paths of many of today’s populations of cranes, ducks, geese and songbirds must have been established in land-bridge times. Some ornithologists believe bird genes retain a memory of a landscape in which Chukotka and Alaska were joined. North American waterfowl cross to Chukotka (north easternmost Siberia) and a variety of songbirds migrate in the Bering Strait also provides an avenue for seasonal north-south migrations, most notably by walrus, bowhead whales, bray and beluga whales.