Sumerians, Babylonians, Assyrians, and More | Kids Discover | The first civilization developed in Sumer, a marshy area at the southern end of the fertile plain, between the Tigris and Euphrates Rivers. Over time, the Sumerians turned their open villages into walled cities. As cities expanded, they became city-states ruled by strong leaders. One such leader, Sargon the Great, from Akkad, conquered all of Mesopotamia and ruled from 2371 to 2316 B.C.