Afro-Eurasia-America is a continuous landmass that exists during periods of low sea level induced by ice ages, and comprises about 85% of the Earth's land area when present in the current era. It consists of the supercontinents of Afro-Eurasia, and the Americas, which are in turn comprise Africa and Eurasia, and North and South America, respectively. These are joined by the Bering land bridge, or Beringia, during an ice age's glacial maximum, when the sea level is low enough for the area to become dry land. When this happens, some wildlife is exchanged.
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The Bering Strait, the Chukchi Sea to the north and the Bering Sea to the south, are all shallow seas (see maps). During cycles of global cooling, such as the most recent ice age, enough sea water became concentrated in the ice caps of the Arctic and Antarctic, so that the subsequent drop in eustatic sea levels exposed shallow sea-floors that have subsequently re-flooded. Other land bridges around the world have emerged and disappeared in the same way. Approximately 14,000 years ago, mainland Australia was linked to both New Guinea and Tasmania; the British Isles formed an extension of continental Europe via the dry beds of the English Channel and North Sea; and the dry basin of the South China Sea linked Sumatra, Java and Borneo to the Asian mainland.
The rise and fall of global sea levels exposed and submerged the bridging land mass called "Beringia" in several periods of the Pleistocene. The Beringian land bridge is believed to have existed both in the glaciation that occurred before 35,000 Before Present (BP) and during the more recent period 22,000-7,000 years BP. The strait reopened about 15,500 BP[1] and by c. 6000 BP the coastlines had assumed approximately their present configurations.[2] When extant, Beringia joins the Afro-Eurasian and American continents.
Biogeographical evidence demonstrates previous connections between North America and Asia. Similar dinosaur fossils occur both in Asia and in North America. For instance the dinosaur Saurolophus was found in both Mongolia and western North America. Relatives of Troodon, Triceratops, and even Tyrannosaurus rex all came from Asia.
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.[3] 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 was by the Bering land bridge. Had this bridge not existed at that time, the fauna of the world would be very different.
Researchers have started to use molecular phylogenetics 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.
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