The history of Sumer spans the 5th to 3rd millennia BCE in southern Mesopotamia, and is taken to include the prehistoric Ubaid and Uruk periods. Sumer was the region's earliest known civilization and ended with the downfall of the Third Dynasty of Ur around 2004 BCE. It was followed by a transitional period of Amorite states before the rise of Babylonia in the 18th century BCE.
The oldest known settlement in southern Mesopotamia is Tell el-'Oueili. The Sumerians claimed that their civilization had been brought, fully formed, to the city of Eridu by their god Enki or by his advisor (or Abgallu from ab=water, gal=big, lu=man), Adapa U-an (the Oannes of Berossus). The first people at Eridu brought with them the Samarra culture from northern Mesopotamia and are identified with the Ubaid period, but it is not known whether or not these were Sumerians (associated later with the Uruk period).