Land bridges are something like Internet company start-ups: once they were everywhere but today there are relatively few. However, those still left are important.
Bridging Continents with Land
A land bridge is a connection between landmasses that comes and goes. The land bridge we all think of today connects Alaska with Siberia when the sea level is low, as it did during the recent ice ages. When polar ice caps take water from the ocean, much of the Bering Sea, including the Bering Strait, becomes dry land. Geologists have named the land Beringia.
Similar land bridges are postulated between Britain and Europe, between New Guinea and Australia, between the Philippines and Indonesia, between Sri Lanka and India, between the Southeast Asian mainland and the Indonesian islands, and between South America and the Falkland Islands.
A land bridge is like a real bridge in that not everything can cross it. Beringia, for instance, is too cold to serve as a highway for palm trees or any other tree. Carnivores won't move where the animals they feed on can't go. The main species of interest when we consider the ice-age land bridges is Homo sapiens, which invaded many areas during late glacial times. The earliest humans in Australia, the Philippines, Japan and the Americas probably arrived on land bridges.
These shallow, short-lived land bridges are a far cry from the ones geologists used to talk about. But at least they actually existed. The ones envisioned early in the last century were a different creature.
The Heyday of Land Bridges
A hundred years ago it was unthinkable that the continents move. But fossils showed that in the distant past, the same species lived in places now separated by wide oceans. Today we know those continents were once together and drifted apart. But back then the best hypothesis was that land bridges arose, then subsided. That was the explanation offered for mountains, too.
Eduard Suess claimed in 1885 that the ancient broken continent he named Gondwanalandfragments that make up India, Africa, Antarctica and South Americahad once been connected by land bridges. Other workers invoked them whenever a mystery needed explaining.
Land Bridges Submerged
This eventually got ridiculous. When Alfred Wegener considered Gondwanaland in 1912, he rejected land bridges and called for the motion of these and all other continents. His theory solved the fossil puzzles and much more, including the problem of mountains.
George Gaylord Simpson, focused more exclusively on fossils, didn't go that far. But in 1940 he too objected to researchers who raised land bridges willy-nilly; one person he cited had "an individual Late Oligocene route from Haiti to west-central Africa for insectivores, one diagonally across this from Brazil to northwestern Africa in the Late Eocene for certain rodents, one in the Early Miocene straight across the Atlantic from the United States to Spain for a genus of horses, Anchitherium, one at the same time parallel to but south of this from northern Africa to Florida for the mastodonts, and so on."
With the advent of plate tectonics in the 1960s, this type of thinking ended. Simpson's 1940 paper, "Mammals and Land Bridges," is a look beyond it to the methods we use today in the study of the migrations of ancient life, or paleobiogeography.
A Land Bridge Remnant
One of those old-time land bridges is still certified and still celebrated today: the Isthmus of Panama. When this landmass rose above the sea during the Pliocene, it created a connection between North and South America. When the animals of the two continents met and mixed, the result was a classic example of Darwinian competition. Read more about it in Simpson's paper.