The Bering land bridge was a land bridge roughly 1,000 miles (1,600 km) north to south at its greatest extent, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages. It was not glaciated because snowfall was extremely light due to the southwesterly winds from the Pacific Ocean having lost their moisture over the fully glaciated Alaska Range. The grassland steppe including the land bridge and stretching for several hundred miles either side of it has been called Beringia. It is believed that a small human population of at most a few thousand survived the last ice age in Beringia, isolated from its ancestor populations in Asia for at least 5,000 years, before expanding to populate the Americas sometime after 16,500 years ago, as the American glaciers blocking the way southward melted.
The rise and fall of global sea levels has exposed and submerged the land bridge in several periods of the Pleistocene. The bridging land mass called "Beringia" is believed to have existed both in the glaciation that occurred before 35,000 BC and during the more recent period 22,000-7,000 years ago. By c. 4000 BC the coastlines had assumed approximately their present configurations.
Beringia constantly transformed its ecology as the changing climate affected the environment, determining which plants and animals were able to survive. The land mass could be a barrier as well as a bridge: during colder periods, glaciers advanced and precipitation levels dropped. During warmer intervals clouds, rain and snow altered soils and drainage patterns. Fossil remains show that spruce, birch and poplars once grew beyond their northernmost modern range today, indicating there were periods when the climate was warmer and wetter. Mastodons, which depended on shrubs for food, were uncommon in the open dry tundra landscape characteristic of Beringia during the colder periods; in this tundra, mammoths flourished instead.
A new study published November 26, 2007 (see PLoS Genetics), which was led by University of Michigan and University College London researchers, seems to suggest that the Bering land bridge migration occurred during one specific time period which was 12,000 years ago, that every human who migrated across the land bridge all came from Eastern Siberia during that time period, and that every native American is directly descended from that same group of Eastern Siberian migrants. The claim suggests that a "unique genetic variant widespread in natives across both continents — suggesting that the first humans in the Americas came in a single migration or multiple waves from a single source, not in waves of migrations from different sources".
While there is considerable evidence for faunal interchange of dinosaurs in the Campanian and Maastrichtian phases of the Late Cretaceous, mammals, however, seem not to have dispersed so easily, perhaps because of their relatively small size; at any rate, there is no direct evidence supporting mammalian faunal exchange in the Cretaceous. Fossils in China demonstrate a migration of Asian mammals into North America around 55 million years ago. By 20 million years ago, evidence in North America shows a further interchange of mammalian species. Some, like the ancient saber-toothed cats, have a recurring geographical range: Europe, Africa, Asia, and North America. The only way they could reach the New World is through the Bering land bridge. Had this bridge not existed at that time, the fauna of the world would be very different.
Molecular phylogenetics is now being used to trace the history of faunal exchange and diversification, through the genetic history of parasites and pathogens of North American ungulates. An international Beringian Coevolution Project is collaborating to provide material to assess the pattern and timing of faunal exchange and the potential impact of past climatic events on differentiation.