About 3000 B.C. the Sumerians moved into the area of Mesopotamia in the area of the lower Tigris Euphrates River valley. There independent city states, such as Ur, Erech, and Kish. Each city state had its own government and ruler. Each city state also had its own gods that they worshiped. Sumerians believed that they were dependent on their gods for their protection and food. So their survival depended on keeping the gods happy. Only the priests knew how to do this and acted as intermediaries between the gods and people. Therefore priests began to gain political power until these city states were run by priest-kings.