Sumer developed into a civilized nation for what reason?

The region of Sumer was long thought to have been first inhabited around 4500 BCE. This date has been contested in recent years, however, and it now thought that human activity in the area began much earlier. The first settlers were not Sumerians but a people of unknown origin whom archaeologists have termed the Ubaid people (from the excavated mound of al-Ubaid where the artifacts were uncovered which first attested to their existence) or the Proto-Euphrateans (which designates them as earlier inhabitants of the region of the Euphrates River).

Ibelieve the answer to be e