Mesoamerica comprises most of the lower region of Mexico and upper Central America. It consists of a broad range of ecological environments, from deserts, grasslands, and coniferous forests of the northern and central highlands to the dense tropical jungles and savannahs of the lowlands. Much of its ecological diversity is due to great changes in altitude, which can vary from sea level to nearly 6,000 feet within a few hundred miles. The regions profound differences in terrain fostered great cultural heterogeneity. In 1519, at the time of the Spanish Conquest, Mesoamerica was the habitation of more than twenty distinct language groups. However, the environmental diversity also meant that not a single population could become utterly self-sufficient. Early on, highland and lowland people became reliant upon each other for the most basic commodities, which lead to the establishment of an extensive trade.
             Similar to most inhabitants of the western hemisphere between 15,000 and 5,000 B.C., Mesoamerican people were hunters and gatherers until the domestication of plants, especially corn, beans, and squash, which provided them with a surplus that allowed year-round settlements to thrive. Most historians agree that towns, cities, and other basic cultural attributes associated with civilization emerged from these villages, but the exact nature of the shift to full-time agriculture production remains the subject of considerable debate. The traditional view, based on investigations in Mexico's Tehuacan Valley, is that agriculture first emerged in the central highlands sometime between 5,000 and 3,000 B.C. as a response to the need for stable food sources during a period of severe environmental deterioration. Agriculture spread to the tropical lowlands and other parts of Mesoamerica only much later.
             It is assumed that the disparity amongst rulers and ruled developed with the conjoined social powers by chiefs who controlled society by coor...