Hunters migrating from Asia across the Bering Land Bridge.When glaciers form, they freeze up vast quantities of water that would otherwise return to the oceans. This causes water levels to go down, and as the waters recede, land that was once under water becomes exposed. Scientists believe that glacial periods beginning about one hundred thousand years ago created a sea level that was 300 to 400 feet (91 to 122 meters) lower than present-day sea level. They theorize that the low water level exposed a vast land bridge spanning the distance across the Bering Strait, from Siberia in northern Russia to the northwest tip of North America (present-day Alaska). The land bridge, called Beringia, probably remained exposed until about twelve thousand years ago. Then it vanished beneath the rising waters as the Great Ice Age ended.
Since the Europeans first arrived in the Western Hemisphere, people have been interested in the origins of the natives of the land. In 1589 José de Acosta (1539–1600), a Jesuit missionary (a member of the Roman Catholic Society of Jesus, a religious order dedicated to spreading the Roman Catholic religion) stationed in South America, theorized that the first Americans had migrated on a route by land from Siberia. At that time the Bering Sea had not yet been discovered. In the late 1800s scientists expanded this theory, suggesting that human hunters might have followed big game from Siberia out onto a land bridge spanning the Bering Sea between Siberia and Alaska and then continued across present-day Alaska and farther into the Americas.
Until the 1920s most people thought the land bridge migration had taken place between four thousand and six thousand years earlier. Then, in 1926, a cowboy in Folsom, New Mexico, discovered a stone spear point embedded in the rib cage of a type of bison that has been extinct for nearly ten thousand years. This proved that human hunters had been living in North America for at least ten thousand years. Ten years later, in Clovis, New Mexico, stone spear points were found in the remains of a mammoth. These remains were discovered in a layer of earth that was deeper than the site of the Folsom spear points, indicating that they came from an even earlier age. The older spear points, which came to be known as Clovis points, were radiocarbon-dated (tested for carbon 14 level to see how old they were) and found to be about 11,500 years old. After the first of these spear points were found, many more Clovis points and tool kits were discovered throughout North America. The sharpened rocks, attached to lances or spears for hunting, could be easily identified because they were fluted or grooved at the base and showed remarkable craftsmanship. They proved that humans had been living throughout the area that is now the United States and parts of Central America since at least 9000 B.C.E. For several decades most scientists thought that the Clovis points signaled the earliest period of human life in the Americas. This later became known as the “Clovis First” theory.
From the late 1950s to the recent past, most scholars accepted the theory that the first people in the Americas migrated from northeast Asian areas such as China, Siberia, and Mongolia. People from these regions were very specialized ice age hunters, who had developed a nomadic lifestyle, following big game wherever the animals roamed. According to the theory, within about one thousand years, the American continents—from Canada to the southernmost tip of South America—were populated with these big game hunters, who gradually developed into what is now called the Clovis culture. (A culture is the arts, language, beliefs, customs, institutions, and other products of human work and thought shared by a group of people at a particular time.)