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A narrow strip of land, an isthmus, connects two wider sections of land. The isthmus of Panama, which connects South America to Central/North America, and the Sinai peninsula, which connects Africa to Asia, illustrate better than any other examples.

Moving plates create many isthmi (isthmuses). The earth's outer shell, the crust, breaks into sections, plates, that slowly slide around the earth. When two continents collide, they can build an isthmus connecting the two continents (Sinai), or when a continent collides with a sea floor plate, enough volcanism can result to build land (Panama region).

During ice ages, glaciers hold much water in the form of ice and cause sea level to fall as much as 427 ft (130 m). With sea level down, more isthmi appear connecting, for example, the British isles to each other and to continental Europe. Rising oceans later flood these temporary bridges.

Remnants of a former isthmus linking Africa to Europe lie at Gibraltar. The collision of Africa with Europe around eight million years ago closed the Strait of Gibraltar creating a dam which cut off the Mediterranean Sea's main source of water, the Atlantic Ocean. While the dam remained intact, the Mediterranean evaporated in the desert climate leaving a large, arid basin. Eventually, the Atlantic eroded the isthmus, and the Mediterranean refilled.

In the early 1900s, when Alfred Wegener proposed his continental drift theory—that continents move around Earth—geologists of the time rejected his fossil evidence based on their belief in submerged isthmi. Wegener stated that the distribution of fossil animals in South America and Africa could only be explained by the two continents being formerly one bigger continent. Opponents countered: those organisms used now-submerged land bridges (isthmi) to cross the Atlantic from South America to Africa, explaining why geologists discover fossils of animals that could not swim that far on both continents. After World War II, when improved technology mapped bridgeless sea floors, the submerged isthmus belief perished, vindicating Wegener.

While an isthmus exists, organisms can travel across it freely—mixing, breeding, evolving, preying, and hiding. Eliminating the connection lets organisms develop, evolve, and die out separately. The isolation of Australia from other continents millions of years ago, for example, produced animals and plants found nowhere else in the world.

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