It is thought that Sumerian society really started to take shape between 4000 B.C. and 3100 B.C. This time is known as the Uruk period, and it saw the land becoming more urbanized. Towards the end of this era, Sumer had split into multiple city-states.
Canals were constructed and boundaries were then raised to distinguish between these city-states. Each one had a patron deity, whom the population would worship in temples built in their honor. Kings or governors known as lugals or ensis ran the cities.
Although some believe the earliest cities emerged in India or China, it is broadly thought that the Sumerian cities were the first. Eridu is believed to be the oldest of these, and it can still be seen today as ruins. About seven-and-a-half miles away from Eridu lies evidence of the city of Ur.