About fifty thousand years ago, as the northern hemisphere was locked in a global deep-freeze and the continental glaciers of the Pleistocene were at a maximum, a large land bridge that connected Asia and North America existed in the general region now occupied by the Bering Sea and Alaska, the so-called “Beringia” region. Across that corridor many mammals migrated from Asia over the millennia, including North America’s ancestral brown bears and, much more recently, the first humans.
One early influx of bears arrived in North America from Asia less than fifty thousand years ago. Some of these ancestral Alaskan brown bears apparently became isolated in island and coastal habitats by the last of the great glaciers, and the polar bear evolved from them. A later influx of bears from Asia produced the modern brown and grizzly bears (Ursus arctos).