Native American Worlds, pp. 7-14
Anthropologists and historians believe that the first inhabitants of the Western Hemisphere were migrants from Asia, most of whom most probably came by land between 13,000 B.C. and 9000 B.C. across a hundred-mile-wide land bridge between Siberia and Alaska. About 3000 B.C., some Native American peoples developed better cultivation techniques and began to farm a variety of crops, most notably maize (corn), which resulted in agricultural surpluses that laid the economic foundation for populous and wealthy societies in Mexico, Peru, and the Mississippi River Valley.
Between 700 B.C. and A.D. 1500, several sophisticated native civilizations developed across Mesoamerica, beginning with the Olmecs, who were succeeded by the Mayans and then the Aztecs. Under an elite class claiming descent from the gods, Mayan society developed an advanced and complex culture of art and learning. The seminomadic warrior peoples known as the Aztecs established the base of their empire in the area of modern-day Mexico City. Ruled by priests and warrior-nobles, the Aztec empire eventually subjugated most of central Mexico and created a society of strong institutions, great wealth, and formidable military power.
The Indians who resided north of the Rio Grande-the Hopewell, Pueblo, and Mississippian peoples-lived in less complex and less coercive societies than the Mesoamerican Indians. Most of the northern peoples lived in self-governing tribes composed of clans. These native groups varied greatly in social and cultural patterns, each having a distinct language and customs, but many drew heavily from older Mesoamerican cultures. In A.D. 1500, most Indians north of the Rio Grande had resided on the same lands for generations, but the elaborate civilizations and strong city-states that had once flourished had vanished. Consequently, when the Europeans came ashore in North America there were no great Indian empires or religious centers remaining.