As a result, the Bering Sea was hundreds of meters below its current level, and a land bridge, known as Beringia, emerged between Asia and North America. At its peak, Beringia is thought to have been some 1,500 kilometers wide. A moist and treeless tundra, it was covered with grasses and plant life, attracting the large animals that early humans hunted for their survival. The first people to reach North America almost certainly did so without knowing they had crossed into a new continent.
They would have been following game, as their ancestors had for thousands of years, along the Siberian coast and then across the land bridge. Once in Alaska, it would take these first North Americans thousands of years more to work their way through the openings in great glaciers south to what is now the United States. Evidence of early life in North America continues to be found. Little of it, however, can be reliably dated before 12,000 B. C.
; a recent discovery of a hunting lookout in northern Alaska, for example, may date from almost that time. So too may the finely crafted spear points and items found near Clovis, New Mexico. Similar artifacts have been found at sites throughout North and South America, indicating that life was probably already well established in much of the Western Hemisphere by some time prior to 10,000 B. C. Around that time the mammoth began to die out and the bison took its place as a principal source of food and hides for these early North Americans. Over time, as more and more species of large game vanished — whether from overhunting or natural causes — plants, berries and seeds became an increasingly important part of the early American diet.
Gradually, foraging and the first attempts at primitive agriculture appeared. Indians in what is now central Mexico led the way, cultivating corn, squash and beans, perhaps as early as 8,000 B. C. Slowly, this knowledge spread northward. By 3,000 B. C.
, a primitive type of corn was being grown in the river valleys of New Mexico and Arizona. Then the first signs of irrigation began to appear, and by 300 B. C. , signs of early village life. By the first centuries A. D.
, the Hohokum were living in settlements near what is now Phoenix1, Arizona, where they built ball courts and pyramid-like mounds reminiscent of those found in Mexico, as well as a canal and irrigation system. MOUND BUILDERS AND PUEBLOSThe first Indian group to build mounds in what is now the United States are often called the Adenans. They began constructing earthen burial sites and fortifications around 600 B. C. Some mounds from that era are in the shape of birds or serpents, andprobably served religious purposes not yet fully understood.
The Adenans appear to have been absorbed or displaced by various groups collectively known as Hopewellians. One of the most important centers of their culture was found in southern Ohio, where the remains of several thousand of these mounds still remain. Believed to be great traders, the Hopewellians used and exchanged tools and materials across a wide region of hundreds of kilometers. By around 500 A.
D. , the Hopewellians, too, disappeared, gradually giving way to a broad group of tribes generally known as the Mississippians or Temple Mound culture. One city, Cahokia, just east of St. Louis, Missouri, is thought to have had a population of about 20,000 at its peak in the early 12th century. At the center of the city stood a huge earthen mound, flatted at the top, which was 30 meters high and 37 hectares at the base. Eighty other mounds have been found nearby.
Cities such as Cahokia depended on a combination of hunting, foraging, trading and agriculture for their food and supplies. Influenced by the thriving societies to the south, they evolved into complex hierarchical societies which took slaves and practiced human sacrifice. In what is now the southwest United States, the