Shrinking of the Bering land bridge

The Bering land bridge was a land bridge roughly 1,000 miles (1,600 km) wide (north to south) at its greatest extent, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages. The term Beringia was first coined by the Swedish botanist Eric Hultén in 1937.^[1] Like most of Siberia and all of Manchuria, Beringia was not glaciated because snowfall was extremely light.^[2] The grassland steppe, including the land bridge, that stretched for several hundred miles into the continents on either side has been called Beringia. It is believed that a small human population of at most a few thousand survived the Last Glacial Maximum 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, during the Late Glacial Maximum as the American glaciers blocking the way southward melted.^[3][4][5][6]

Geography[link]

Beringia -Wisconsin glaciation

Beringia - deglaciation period

Beringia - present day

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 great ice sheets that covered North America and Europe, 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^[7] and by c. 6000 BP the coastlines had assumed approximately their present configurations.^[8]

The ice-free heartland of Beringia served as a giant ecological refugium during maximal glaciation for those tundra plants that could survive its windswept Arctic desert conditions.^[9] But Beringia constantly transformed its ecosystem 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 that there were periods when the climate was warmer and wetter. The environmental conditions were not homogenous in Beringia. Recent stable isotope studies of woolly mammoth bone collagen demonstrate that western Beringia (Siberia) was colder and drier than eastern Beringia (Alaska and Yukon), which most ecologically more diverse.^[10] 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. The extinct pine species Pinus matthewsii has been described from Pliocene sediments in the Yukon areas of the refugium.^[11]

Human habitation[link]

The Bering land bridge is significant for several reasons, not least because it is believed to have enabled human migration to the Americas from Asia about 20,000 years ago.^[12] A study by Hey^[13] has indicated that of the people migrating across this land bridge during that time period, only 70 left their genetic print in modern descendants, a minute effective founder population—easily misread as though implying that only 70 people crossed to North America. Seagoing coastal settlers may also have crossed much earlier, but scientific opinion remains divided on this point, and the coastal sites that would offer further information now lie submerged in up to a hundred metres of water offshore. Land animals were able to migrate through Beringia as well, bringing mammals that had evolved in Asia to North America, mammals such as proboscideans and lions, which evolved into now-extinct endemic North American species, and allowing equids and camelids that evolved in North America (and later became extinct there) to migrate to Asia.

A study published in 2007 in PLoS Genetics, led by University of Michigan and University College London researchers, suggests that the Bering land bridge migration occurred 12,000 years ago, that every human who migrated across the land bridge came from Eastern Siberia, and that every Native American is directly descended from that same group of Eastern Siberian migrants.^[14] The authors note a "[u]nique genetic variant widespread in natives across both continents suggests 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."

Previous connections[link]

Beringia area coverage

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.^[15] 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.

See also[link]

• Bering Strait crossing
• Geologic time scale
• Last glacial period
• Pleistocene epoch
• Yukon Beringia Interpretive Centre

References[link]

Further reading[link]

• Pielou, E. C., After the Ice Age : The Return of Life to Glaciated North America (Chicago: University of Chicago Press) 1992
• Hey, Jody (2005). "On the Number of New World Founders: A Population Genetic Portrait of the Peopling of the Americas". PLoS Biology 3 (6): e193. DOI:10.1371/journal.pbio.0030193. PMC 1131883. PMID 15898833. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1131883. 
• Hoffecker, John F.; Elias, Scott A. (2007). Human ecology of Beringia. Columbia University Press. ISBN 978-0-231-13060-8. http://books.google.com/books?id=VgomQY3o9U0C. 

External links[link]

Wikimedia Commons has media related to: Bering Land Bridge

• Shared Beringian Heritage Program
• Bering Land Bridge National Preserve
• D.K. Jordan, "Prehistoric Beringia"
• Paleoenvironmental atlas of Beringia: includes animation showing the gradual disappearance of the Bering land bridge
• Yukon Beringia Interpretive Centre
• Paleoenvironments and Glaciation in Beringia
• Study suggests 20000 year hiatus in Beringia
• Fagundes, Nelson J.R.; Kanitz, Ricardo; Eckert, Roberta; Valls, Ana C.S.; Bogo, Mauricio R.; Salzano, Francisco M.; Smith, David Glenn; Silva Jr., Wilson A. et al. (3 March 2008). "Mitochondrial Population Genomics Supports a Single Pre-Clovis Origin with a Coastal Route for the Peopling of the Americas" (PDF). American Journal of Human Genetics 82 (3): 583–592. DOI:10.1016/j.ajhg.2007.11.013. PMC 2427228. PMID 18313026. http://download.ajhg.org/AJHG/pdf/PIIS0002929708001390.pdf.