It seems likely that Teotihuacán's natural resources-along with the city elite's ability to recognize their potential - gave the city a competitive edge over its neighbors.The archaeological evidence clearly indicates, though, that Teotihuacán was the center that did arise as the predominant force in the area by the first century A.D. With Cuicuilco eliminated as a potential rival, any one of a number of relatively modest towns might have emerged as a leading economic and political power in Central Mexico. Around this time, the largest of these centers, Cuicuilco, was seriously affected by a volcanic eruption, with much of its agricultural land covered by lava.
Prior to 200 B.C., a number of relatively small centers coexisted in and near the Valley of Mexico. This last factor is at least circumstantially implicated in Teotihuacán's rise.The exact role of other factors is much more difficult to pinpoint - for instance, Teotihuacán's religious significance as a shrine, the historical situation in and around the Valley of Mexico toward the end of the first millennium B.C., the ingenuity and foresightedness of Teotihuacán's elite, and, finally, the impact of natural disasters, such as the volcanic eruptions of the late first millennium B.C. How did this tremendous development take place, and why did it happen in the Teotihuacán Valley? Among the main factors are Teotihuacán's geographic location on a natural trade route to the south and east of the Valley of Mexico, the obsidian¹ resources in the Teotihuacán Valley itself, and the valley's potential for extensive irrigation.Moreover, the city had economic and perhaps religious contacts with most parts of Mesoamerica (modern Central America and Mexico). Clearly, much planning and central control were involved in the expansion and ordering of this great metropolis. It had over 2,000 apartment complexes, a great market, a large number of industrial workshops, an administrative center, a number of massive religious edifices, and a regular grid pattern of streets and buildings. 150 and 700, it probably had a population of more than 125,000 people and covered at least 20 square kilometers. The city of Teotihuacán, which lay about 50 kilometers northeast of modern-day Mexico City, began its growth by 200 –100 B.C.