Gregson Schachner & Reuven SinenskyAbstract:Ancestral Pueblo communities in the American Southwest underwent dramatic transformations in the mid-1st millennium AD, including rapid population growth and the widespread adoption of social structures that remained in place over the next millennium. We explore to two key moments in this process: the widespread adoption of sedentary agriculture in the mid-6th century and the founding of the earliest aggregated villages during the 8th century. While the former transition is marked by abrupt changes in architecture, storage facilities, and technological traditions region-wide, the latter displays remarkable diversity. Drawing on recent excavations and high quality chronometric and paleoclimate data, we suggest that a severe climatic downturn brought about by a series of massive volcanic eruptions is in part responsible for the abrupt changes associated with the onset of the Neolithic Demographic Transition in the northern Southwest during the mid-AD 500s, while a myriad of entangled economic and social factors contributed to the formation of early population aggregates a little over a century later.