Cosma Shalizi points to this article in the inaugural issue of Stucture and Dynamics: eJournal of Anthropological and Related Sciences. It tries to answer why a complex system of city-states emerged in Sumeria earlier than elsewhere in the world.
“[The] emergence of early cities in the southern Mesopotamian alluvium must be understood in terms of the unique ecological conditions that existed across the region during the fourth millennium, and the enduring geographical framework of the area, which allowed for the efficient movement of commodities via water transport and facilitated interaction between diverse social units alongside natural and artificial river channels. . .
More specifically, my contention is that by the final quarter of the fourth millennium the social and economic multiplier effects of trade patterns that had been in place for centuries – if not millennia – had brought about substantial increases in population agglomeration throughout the southern alluvial lowlands. Concurrent with these increases, and partly as a result of them, important socio-economic innovations started to appear in the increasingly urbanized polities of southern Mesopotamia that were unachievable in other areas of the Ancient Near East where urban grids of comparable scale and complexity did not exist at the time. Most salient among these innovations were (1) new forms of labor organization delivering economies of scale in the production of subsistence and industrial commodities to southern societies, and (2) the creation of new forms of record keeping in southern cities that were much more capable of conveying information across time and space than the simpler reckoning systems used by contemporary polities elsewhere.”