Four to five thousand years after the Neolithic Revolution, the first civilizations arose in Mesopotamia, Egypt, and the Indus Valley. Mesopotamian civilization began in city-states like Ur and Uruk in southern Mesopotamia. Sumerian culture featured centralized rule by priest-kings and a social hierarchy topped by kings and priests. Egypt developed a highly centralized society ruled by divine pharaohs. The Indus Valley civilization spanned over 900 miles along the Indus River and featured planned cities built on grids with advanced sanitation systems. All three civilizations exhibited early advances in areas like writing, mathematics, metalworking, and social organization.