The Earth has seven continents: North America, South America, Asia, Europe, Australia, Antarctica and Africa. Twenty millennia to 25 millennia ago, a land bridge -- a piece of land that connects two other pieces of land -- connected Asia and North America, but rising ocean levels destroyed this connector. Today, no true land bridges connect two continents.
The Bering Strait
The Bering Sea is next to the Arctic Ocean between Asia and North America. The strait separates the two continents at their closest point and is 53 miles wide at its widest point. Twenty thousand to 25,000 years ago, glaciers used up so much water between the Arctic and Pacific oceans at this point that ocean levels were 300 feet lower than they are now. This lower water level exposed the Bering Land Bridge, which connected Asia and North America.
North and South America are connected by a strip of land known as Central America. North America ends with the southern tip of Mexico, but Central America -- which is part of neither North nor South America -- continues with Belize, Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica and Panama. The Pan-American Highway stretches from northern Mexico all the way to southern Argentina. However, the Panama Canal cuts off Central America just as it reaches South America, so Central America cannot be considered a true land bridge.
Africa and Asia
Northeastern Africa and eastern Asia are connected a large piece of land, the Sinai Peninsula. Before November 1869, the Sinai Peninsula could have been considered a land bridge between Africa and Asia; in 1869, however, the Suez Canal -- a waterway connecting the Mediterranean Sea at Port Said with the Red Sea -- made Africa an official island. This waterway prevents Sinai from being a true land bridge between Israel and Egypt. It is possible, however, to travel by land between the two, through a tunnel beneath the Suez Canal.
Other continents are separated by water. Asia and Europe, for example, are connected by a waterway called the Bosphorus Strait, which connects the European and Asian parts of Istanbul, Turkey. Australia is an island, surrounded by water. The Americas, of course, are separated from Asia by the Pacific Ocean and Europe and Africa by the Atlantic Ocean. Europe and Africa are separated by the Mediterranean Sea.