The first question that historians ask about almost everything is “How did it get started?” Scholars of all kinds — archeologists, anthropologists, sociologists, and historians — have been arguing about the origins of civilization for a very long time, with no end in sight. Amid all the controversy, one thing seems reasonably clear: civilizations had their roots in the Agricultural Revolution. That is the reason they appeared so late in the human story, for only an agricultural technology permitted human communities to produce sufficient surplus to support large populations and the specialized or elite minorities who did not themselves produce food. But not all agricultural societies or chiefdoms developed into civilizations, so something else must have been involved. It is the search for this “something else” that has provoked such great debate among scholars.

Some historians have emphasized the need to organize large-scale irrigation projects as a stimulus for the earliest civilizations, but archeologists have found that the more complex water control systems appeared long after states and civilizations had already been established. Alternatively, perhaps states responded to the human need for order as larger and more diverse populations grew up in particular localities. Others have suggested that states were useful in protecting the privileges of favored groups. Warfare and trade have figured in still other explanations for the rise of civilizations. Geography surely played a role as well, for civilizations often took shape in biologically rich and productive environments such as wetlands, estuaries, and river basins. Anthropologist Robert Carneiro combined several of these factors in a thoughtful approach to the question.5 He argued that a growing density of population, producing more congested and competitive societies, was a fundamental motor of change, especially in areas where rich agricultural land was limited, either by geography (oceans, deserts, mountains) or by powerful neighboring societies. Such settings provided incentives for innovations, such as irrigation or plows that could produce more food, because opportunities for territorial expansion were not readily available. But circumscribed environments with dense populations also generated intense competition among rival groups, which led to repeated warfare. A strong and highly organized state was a decided advantage in such competition. Because losers could not easily flee to new lands, they were absorbed into the winner’s society as a lower class. Successful leaders of the winning side emerged as elites with an enlarged base of land, a class of subordinated workers, and a powerful state at their disposal — in short, a civilization.

Although such a process was relatively rapid by world history standards, it took many generations, centuries, or perhaps millennia to evolve. It was, of course, an unconscious undertaking in which the participants had little sense of the long-term outcome as they coped with the practical problems of life on a day-to-day basis. What is surprising, though, is the rough similarity of the outcome in many widely separated places from about 3500 B.C.E. to the beginning of the Common Era.

However they got started (and much about this is still guesswork), the First Civilizations, once established, represented a very different kind of human society than anything that came before. All of them were based on highly productive agricultural economies. Various forms of irrigation, drainage, terracing, and flood control enabled these early civilizations to tap the food-producing potential of their regions more intensively. All across the Afro-Eurasian hemisphere, though not in the Americas, animal-drawn plows and metalworking greatly enhanced the productivity of farming. Ritual sacrifice, sometimes including people, accompanied the growth of civilization, and the new rulers normally served as high priests, their right to rule legitimated by association with the sacred.