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A land bridge is an isthmus or some other land-based connection between two otherwise disconnected islands or continents. Land bridges are important ecologically because they allow the exchange of plants and/or animals that are otherwise separated, sometimes by millions of years of independent evolution. Land bridges are often transient, appearing and disappearing over geological time due to rising and falling sea levels.
The most famous land bridge in Earth's history is probably the Bering land bridge, which existed from present-day eastern Russia to Alaska about 20,000 years ago, during the last glacial period. During this time, sea levels were approximately 120 meters (394 ft) below today's levels, due to the huge quantities of ice locked up in continental glaciers. The Bering land bridge allowed humans to migrate from Asia to the Americas. After they crossed over, it is thought that they migrated down the coastline.
Though the Bering land bridge is the most famous, there were several other land bridges in the world at the same time. These include the large tract of low-lying tundra in what is today the south North Sea, which has been dubbed Doggerland, after the Dogger Bank, a large sand bank in the area today. Doggerland connected together England with continental Europe, and a separate land bridge connected England to Ireland. The earliest inhabitants of these islands made it across via land bridges.
There were extensive land bridges between islands in present-day Indonesia, connected them to the southeast Asian mainland. These made it possible for early humans to travel from Africa to islands like Borneo. In what is the first confirmed instance of humans traveling over a significant stretch of open ocean, early man built rafts and made it across the present-day Wallace Line, a deep sea channel in central Indonesia that separates the fauna of west Indonesia (which is more Asian) from east Indonesia (more Australian). From the east side of the Wallace Line, these people reached New Guinea and Australia, which were also connected by land bridges.
Further back in the past, about 2 million years ago, one of the world's most ecologically significant land bridges formed -- that between North and South America. Unlike the other land bridges discussed, this was formed by continental drift rather than lowered sea levels. As South America had previously been isolated for tens of millions of years, this exposed the continent to the flora and fauna of essentially the rest of the world (Europe, Africa, Asia, and North America were all connected). Generally speaking, the North American fauna dominated the South American fauna, though some South American animals did thrive in North America, for instance, the opossum.
Reading about a land bridge makes me think about the Swiss Family Robinson movie.
In the movie they are stranded on a tropical island that just happens to have all kinds of African animals on it, many of which had no business being on a smallish island, since they were too large to establish a real population there.
Some of the animals came with the Robinsons, of course, but not all of them.
And the explanation in the end is that the island was recently attached to the mainland by an isthmus or land bridge or was perhaps the remains of a land bridge that had since disappeared.
It still didn't make any sense, since the animals were all from different areas of Africa and India but at least it tried to give somewhat of an explanation as to why they kept coming across monkeys and tigers and ostriches.
@pleonasm - Well, we are raised to see extinction as always being a bad thing, which it usually is for us, since the extinctions we're witnessing right now are happening way too fast.
But when the two continents of North and South America were joined by a land bridge, the extinctions in South America (and, I'm sure, in North America as well) would have happened slowly and naturally.
All the animals wouldn't have rushed across at once. A land bridge theory of migration would have them move slowly across and slowly fan out over the new land.
As they gradually out-competed the natives, the new comers would also be adapting and new species would appear.
I mean, if the dinosaurs hadn't eventually gone extinct, we wouldn't have the birds and mammals we have today. The fauna and flora of the Americas would look radically different if they never formed a land bridge.
I know the world is constantly changing and there's no way to stop that, but it does make me sad that there must have been a huge extinction of plants and animals following the establishment of the "land bridge" between North and South America (what is now known as Central America).
I mean, South America must have been really amazing back then, much the way that Australia and New Zealand are completely unique now.
It's still pretty amazing, of course, in terms of unique wildlife, but I guess extinction always makes me sad, whether it is from man made events or natural catalysts.