Using a land bridge between modern-day Siberia and Alaska, the first human beings arrived in North America and gradually moved east and south into South America, populating these two continents.

About two million years ago, for reasons not entirely understood, earth’s temperature began to fall. In the north, more snow fell in winter than melted in summer, and great sheets of ice formed on the landmasses. These glaciers went through a series of advances and retreats—sliding forward under the influence of gravity and melting back under warmer climatic conditions.

At the same time, a group of primates (monkeys, apes, and their relatives) was evolving in Africa. This group had already developed the ability to walk on their hind limbs rather than on four feet, thus freeing the forelimbs for functions other than locomotion. Climatic change had initiated a drying trend in Africa, replacing rain forests with grasslands and savannas. Several species of the two-legged primate group had successfully invaded the grassland environment and spread throughout Africa. Well into the Ice Age, late-developing species migrated north into Europe and Asia, using tools, animal skins, and fire to cope with the cold. Some members of one species, today called Homo sapiens (literally, wise human), eventually moved into frigid Siberian environments.

Eastern Siberia and western Alaska were not covered by glaciers, even at the height of glacial advance. Although the climate in these unglaciated regions was cold, a number of large mammal species (mammoths, mastodons, giant bison, and others) had invaded the northern environment ahead of the humans. The newcomers probably used many food sources, but they became especially skilled at hunting the large animals.

According to one theory, the first people to arrive in North America traveled over a land bridge between modern-day Siberia and Alaska. Once on the continent, they gradually spread southward and eastward.

Tremendous amounts of water were required to build the continental glaciers. That water came primarily from the most abundant source of water on the planet, the oceans. As a result, each advance and retreat of the glaciers was accompanied by dramatic changes in sea level—the sea rose as glaciers melted and fell with each glacial advance. Today, only about 50 miles (80 kilometers) of water separate Siberia from Alaska across the Bering Strait. The Bering Strait is less than 200 feet (60 meters) deep, and the adjacent parts of the Chukchi and Bering seas are not much deeper. Because of this, a strip of Bering Strait and adjacent sea floor 1,000 miles (1,600 kilometers) wide became dry land whenever extensive glaciation occurred. Along with adjacent parts of Siberia and Alaska, this region is called Beringia. When the glaciers were in full retreat, the Bering Strait reformed, splitting Beringia and placing a barrier between the two continents.

The sea level rose and fell throughout glacial times, and the connection between Alaska and Siberia was established and broken repeatedly. Various land organisms crossed the bridge when it was available, but exchange between the continents was blocked when it was inundated. Mammoths, mastodons, camels, horses, and many other species of animals and plants crossed throughout the Ice Age, but humans probably did not reach northeastern Siberia until the most recent glacial advance.

In North America, the last glacier (the Wisconsin) advanced until approximately sixty thousand years ago, at which time it began a retreat called the mid-Wisconsin interglacial. Fewer than thirty thousand years ago, it began its final advance (the late Wisconsin glaciation) followed by its most recent retreat, which began eighteen thousand years ago. It was during or after the mid-Wisconsin interglacial that humans from Siberia made their way across Beringia into North America.

This migration was not a directed, purposeful movement to a new continent. It is unlikely that the first Americans had any sense of their role in history or the nature of continents. The migration probably was the simple result of growing populations expanding into new regions, perhaps drawn by the presence of herds of the large mammals they were so adept at hunting.

The populations continued to expand throughout Alaska and adjacent Canada but were restricted from much of Canada by two major glacial masses. The Laurentide ice sheet covered most of Canada and much of the northern United States, from the East Coast to the Rocky Mountains. The second mass of ice resulted from the coalescence of a number of mountain glaciers into a single glacial complex, the Cordilleran glacier located between the Rockies and the coastal mountain ranges.

During glacial advance, the two ice masses probably met and blocked the migrants’ route south. However, when the glaciers melted, a corridor opened between them. The migrants moved south through Mexico and Central America, and on to the tip of South America. As the most recent glacial retreat continued, the first Americans expanded their range into all parts of Canada as well.

Anthropologists and archaeologists call these first Americans (or their immediate descendants) Paleo-Indians. Many details of relationship and pathways of descent are not known, but the Paleo-Indian culture gave rise to another widespread culture, called the Archaic, around 7000 b.c.e. Approximately two thousand years ago, the Archaic culture began to give way to the mound-building culture of eastern North America (the Adena, Hopewell, and Mississippian), the agricultural groups of the southwestern deserts (the Hohokam, Mogollon, and Anasazi), and other cultures. Some time before 1500 c.e., these prehistoric cultures gave rise to the Native American tribes that were later displaced by European settlement. Some archaeologists argue that a similar sequence of cultural replacement took place in Mexico and Central and South America, culminating in the Inca, Aztec, and Mayan civilizations decimated by the Spanish conquistadores in the 1500’s.

One of the most vituperative arguments in the history of science centers on the question of when the first Americans arrived. A few students of the question argue for dates earlier than the mid-Wisconsin interglacial, and some argue for entry times more than thirty thousand years ago (during the mid-Wisconsin interglacial) or around 18,000 b.c.e., during the last glacial maximum, but many favor from 15,000 to 11,000 b.c.e.

The basis for the most popular position is the widespread occurrence of a particular type of spear point found at archaeological sites all over North America, sites determined to be between 11,500 and 10,500 years old. These sites constitute the first recognized North American Paleo-Indian culture, now called the Clovis culture because it was established on the basis of finds in Blackwater Draw near Clovis, New Mexico. Because the culture was so widespread, archaeologists assume that Native Americans must have been on the continent some time before the Clovis dates. Discoveries at sites such as Meadowcroft Rockshelter in southwestern Pennsylvania and Cactus Hill and Saltville in Virginia support the idea that other cultures preceded the Clovis culture. At these sites, archaeologists found artifacts that are very different from and are significantly older than those found at Clovis. Although many archaeologists believe that the first immigrants spread from Beringia to Tierra del Fuego and throughout both continents, the discovery of sites that appear to predate the Clovis culture in South America have caused others to believe that South America may have been populated separately, perhaps even earlier than North America.

Evidence based on Native American languages, tooth anatomy, and genetics suggests that there were at least three migrations of different Siberian peoples into North America. The first group of migrants gave rise to most Native American groups. One of the later migrant groups was ancestral to the Navajo, Apache, and some western Canadian tribes; the Eskimo (Inuit) and Aleut peoples derived from the other group. Each migration probably involved movement of many subgroups through an extended time period. Some archaeologists believe that marine travelers, along the coast or across open seas, may have contributed to the colonization as well.

Although the timing and details of the colonization of North America are unsettled, most archaeologists agree on its basic character. Northern Asiatic people crossed Beringia into North America and spread fairly rapidly throughout North and South America. These people, with possible contributions from earlier and later (and perhaps seagoing) immigrants, developed into the multitude of Native American groups present when Europeans “discovered” the continents. By the most conservative estimates, ancestors of the Native Americans who met the European explorers and colonists more than five hundred years ago had occupied the Americas for more than twelve thousand years.

• Adovasio, J. M. The First Americans: In Pursuit of Archaeology’s Greatest Mystery. New York: Random House, 2002. An examination of the first human beings to populate the Americas. Bibliography and index.
• Dixon, E. James. Bones, Boats, and Bison: Archeology and the First Colonization of Western North America. Albuquerque: University of New Mexico Press, 1999. Dixon looks at the archaeological evidence surrounding the peopling of North and South America. Bibliography and index.
• Dixon, E. James. Quest for the Origins of the First Americans. Albuquerque: University of New Mexico Press, 1993. An archaeologist discusses the first Americans in the context of his own research. Illustrations, index, bibliography.
• Fagan, Brian M. Ancient North America: The Archaeology of a Continent. 3d ed. New York: Thames and Hudson, 2000. A consideration of the first Americans in the context of North American archaeology. Illustrations, index, bibliography.
• Jablonski, Nina G., ed. The First Americans: The Pleistocene Colonization of the New World. Memoirs of the California Academy of Sciences, no. 27. San Francisco: California Academy of Sciences and University of California Press, 2002. A collection of papers from the fourth Wattis Symposium held in October, 1999, discussing various aspects of the peopling of the Americas. Bibliography and index.
• West, Frederick Hadleigh, and Constance F. West, eds. American Beginnings: The Prehistory and Palaeoecology of Beringia. Chicago: University of Chicago Press, 1996. This collection of essays examines topics concerning the Bering land bridge, including excavations in the surrounding area. Bibliography and index.