was the site of some of the earliest and most advanced civilizations in the western hemisphere. The Mayan culture, according to archaeological research, attained its greatest development about the 6th century AD. Another group, the Toltec, established an empire in the Valley of and developed a great civilization still evidenced by the ruins of magnificent buildings and monuments. The leading tribe, the Aztec, built great cities and developed an intricate social, political, and religious organization. Their civilization was highly developed ....