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,