The earliest complex societies found in the Western Hemisphere developed under very different environmental conditions. The Maya, for instance, emerged in the tropical lowlands of the Yucatan Peninsula, a region with high seasonal rainfall and rich biodiversity. The Puebloans, in contrast, developed in the semiarid region of what is today Arizona and New Mexico, an area with limited rainfall and biodiversity. Vernon Scarborough will discuss two important archaeological sites from these different ecological and cultural zones—Tikal in Guatemala and the Chaco Canyon in New Mexico—to illustrate how the availability of water and climate influence the evolution of societies and what we can learn from these historical precedents.