A land bridge, in biogeography, is an isthmus or other land connection between what at other times are separate land masses which allows animals and plants to cross and colonise new lands. Land bridges are commonly created by regression, in which sea levels fall exposing previously submerged sections of continental shelf. Land bridges are also formed by: (a) upthrust at the edge of continental plates; and (b) glacial retreat alleviating pressure on shallow marine formations (e.g. the emergence of Oland, Sweden)
The most recent significantly low sea levels were about 20,000 years ago (during the Upper Paleolithic) when worldwide sea levels were about 120 meters below today's level. By 10,000 years ago, the sea level had risen to 20 meters below today's level. Sea level rise can occur as a result of global warming, or apparent sea level rise may occur as a result of glacial depression or certain tectonic movements.
 Examples Perhaps the best-known example is the Bering land bridge, which joined present-day Alaska and eastern Siberia at various times during the Pleistocene ice ages, enabling humans to migrate from Eurasia to the Americas (see Models of migration to the New World).
 Land bridge theory Before the theory of plate tectonics, it was believed that land bridges could explain the occurrence of species in separate continents and the resemblances of geologic formations on different continents. Many land bridges were hypothesized, criss-crossing large areas of ocean, the most prominent of them being Lemuria. However, when the sea bed of the Atlantic Ocean was mapped using echo sounding between 1924 and 1927, no remains of land bridges could be found. Though this was a strong argument for Alfred Wegener's theory of continental drift, it would take about 50 more years until mainstream geology fully acknowledged the motion of continents.