c. 9000 BCE

Beringia Land Bridge

While most of the Earth’s continental land masses are above sea level, significant fractions of many continents extend below sea level as well, such as the continental “shelf” that extends underwater from the east coast of North America. Not all continental margins exhibit shelves; but where they do occur, they average about 50 miles (80 kilometers) in width, and about 500 feet (152 meters) below sea level.

The shallow nature of continental shelves means that historically, when sea level drops significantly, formerly underwater continental shelves can become dry land above sea level. This is precisely what occurred during the last glacial period around 15,000 to 25,000 years ago along one of the largest continental shelf regions on Earth. The Siberian shelf, in the Arctic Ocean, stretches for more than 900 miles (1,450 kilometers) along the northern margin of the Eurasian continent, where it eventually merges to the east with the Chukchi and Bering shelves along the Eurasian/North American boundary north of Kamchatka and Alaska. During the last glacial maximum period, sea level dropped by more than 160 feet (50 meters) as water accumulated in continental glaciers, exposing the Chukchi and Bering shelves and creating a “land bridge” between Eurasia and North America. Geologists refer to this past land bridge as “Beringia,” because it is now underneath the Bering Strait.

Archaeologists broadly agree that modern humans have their roots in Africa and the Middle East, radiating outward from there into Europe and Asia. Prior to the last glacial maximum, however, there would have been no simple way for humans to get to North or South America because of the Atlantic Ocean to the west and the Pacific Ocean and Bering Sea to the east. Once Beringia was exposed, however, it became possible to simply walk from Eurasia to North America. A new era of human migration had begun.

As the continental glaciers began to melt at the beginning of the Holocene epoch, the sea level rose and covered the Beringia land bridge. While it was still possible to cross during iced-over periods, the closing of the Beringia land bridge would ultimately lead to the isolation and genetic divergence of the indigenous Eurasian and American populations.

SEE ALSO Homo sapiens Emerges (c. 200,000 BCE), Invention of Agriculture (c. 10,000 BCE), End of the Last “Ice Age” (c. 10,000 BCE)

Prior to about 11,000 years ago, the sea level was low enough that Alaska and Siberia were part of a continuous strip of land that geologists now refer to as “Beringia.”