Maize (Indian corn) Their advanced agricultural practices, based primarily on the cultivation of maize, which is Indian corn, fed large populations, perhaps as many as 20 million in Mexico alone. Agriculture, especially corn growing, accounted for the size and sophistication of the Native American civilizations in Mexico and South America. About 5000 B.C. hunter-gatherers in highland Mexico developed a wild grass into the staple crop of corn, which became their staff of life and the foundation of the complex, large-scale, centralized Aztec and Incan nation-states that eventually emerged. Cultivation of corn spread across the Americas from the Mexican heartland. Everywhere it was planted, corn began to transform nomadic hunting bands into settled agricultural villagers, but this process went forward slowly and unevenly. Corn planting reached the present-day American Southwest by about 1200 B.C. and powerfully molded Pueblo culture. Corn cultivation reached other parts of North America considerably later. The timing of its arrival in different localities explains much about the relative rates of development of different Native American peoples. Throughout the continent to the north and east of the land of the Pueblos, social life was less elaborately developed—indeed "societies" in the modern sense of the word scarcely existed. No dense concentrations of population or complex nation-states comparable to the Aztec empire existed in North America outside of Mexico at the time of the Europeans' arrival one of the reasons for the relative ease with which the European colonizers subdued the native North Americans.