The evolution of the earliest complex state-level societies and cities from small sedentary communities took place in southern Mesopotamia between 8000 and 5000 cal yrs BP during the ‘Ubaid and Uruk periods. Attempts to explain this transition often discount the role of environmental change and tend to evaluate available archaeological evidence for urban-based state development either within a static environmental context or assuming conditions similar to those of the present. This practice is no longer tenable given newly available paleoenvironmental records for the region. Post-glacial sea-level rise resulted in the inundation and creation of the Arabo-Persian Gulf, and, as the marine transgression slowed in the Middle Holocene, rich coastal and aquatic habitats formed in southern Mesopotamia. These habitats favored the establishment and growth of ‘Ubaid Period communities and the efficient transport of goods, ideas, and people throughout the region. High water tables also promoted early experimentation with irrigation agriculture and the expansion of these systems as populations grew and the humid conditions of the Early Holocene gave way to increasing aridity. We argue that the critical confluence of eustatic and climatic changes unique to this circumscribed region favored the emergence of highly centralized, urban-based states.
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