The 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, and probably 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 Anasazi, ancestors of the modern Hopi Indians, began building stone and adobe pueblos around the year 900. These unique and amazing apartment-like structures were often built along cliff faces; the most famous, the "cliff palace" of Mesa Verde, Colorado, had over 200 rooms. Another site, the Pueblo Bonito ruins along New Mexico's Chaco River, once contained more than 800 rooms.
Perhaps the most affluent of the pre-Columbian American Indians lived in the Pacific northwest, where the natural abundance of fish and raw materials made food supplies plentiful and permanent villages possible as early as 1,000 B.C. The opulence of their "potlatch" gatherings remains a standard for extravagance and festivity probably unmatched in early American history.