The initial sequencing of archaeological cultures in Mesoamerica occurred in the Valley of Mexico, where Gamio (8) was able to show that “Archaic” was stratigraphically earlier than “Teotihuacan,” which was stratigraphically earlier than “Aztec.” Eventually, a “Toltec” horizon was placed between Teotihuacan and Aztec, thus establishing a basic framework for the interpretation of central Mexican archaeological remains. However, outside the Valley of Mexico, it was not always clear how sites, peoples, and cultures related to each other. In the Maya area an independent chronological sequence was established stratigraphically at Uaxactun, Guatemala in the 1920s (9, 10). However, the resulting artifactual groupings, or “styles,” were positioned in time only relative to each other (11). How far back they went, how long they persisted, and how they related to other archaeological regions in Mesoamerica was largely unknown. Even in areas where dendrochronology was available, most defined archaeological assemblages could not be positioned confidently on an absolute temporal scale. Thus, dating remained a paramount challenge for archaeological research and a problem that was not resolved fully until the 1950s. Radiocarbon dating had a profound effect on archaeology because it provided secure time depth and finally permitted ancient materials to be related to each other dependably in absolute time