Sumerian civilization took form in the Uruk period (4th millennium B.C.), continuing into the Jemdat Nasr and Early Dynastic periods. It was conquered by the Semitic-speaking kings of the Akkadian Empire around 2400 BC. Native Sumerian rule re-emerged for about a century in the third dynasty of Ur (Sumerian Renaissance) of the 21st to 20th century (short chronology).
The cities of Sumer were the first civilization to practice intensive, year-round agriculture, by 5000 BC showing the use of core agricultural techniques including large-scale intensive cultivation of land, mono-cropping, organized irrigation, and the use of a specialized labour force. The surplus of storable food created by this economy allowed the population to settle in one place instead of migrating after crops and grazing land. It also allowed for a much greater population density, and in turn required an extensive labour force and division of labour. Sumer was also the site of early development of writing, progressing from a stage of proto-writing in the mid 4th millennium BC to writing proper in the third millennium.