After 4000 B.C.E., organized drainage and flood-control projects in the Fertile Crescent allowed cultivation of river deltas and led to large population increases. A. Before the rise of large cities with complex economies, there had been no need for any kind of administrative structure to organize workers, and thus religion tended to be the only source of authority. B. By 3000 B.C.E., a number of agricultural societies had emerged as civilization characterized by large cities, centralized administrations, specialization of labor, class divisions, metal technology, and writing. C. Although river-valley societies had to import the metals for making bronze, the first phase of civilization is known as the Bronze Age because of the importance of metal technology in these societies. D. Large cities functioned as administrative centers, creating a concentration of wealth and resources that stimulated technological advances, such as the invention of writing and the creation of bronze. E. For a culture to be considered a civilization, it must have independently invented its own form of writing and become generally literate, thereby moving from prehistory into the historic period. F. The creation of bronze made it possible for civilizations based on large-scale agriculture to be located far away from the river valleys.