Why did the Sumerian civilization develop as a number of independent city states

The people were to give/reserve their lives to propitating the gods to prevent terrible events from happening to the community. The political structure of Sumer was independent city-states. The map shows the important communities. Note that in Sumerian times the Persian Gulf extended to the area of the city-states.

Sumerians are believed to be the first of the humans in the world history to have formed civilization.

Explanation:

These people lived in the southern part of the Mesopotamia. The region was spread between the "Tigris" and the "Euphrates River" in the "Middle East". The cities were first formed in "Sumer" around "5000 BC". Because of the fertile lands the nomads initially moved into small villages. These villages later turned into towns and eventually developed into the "civilization of Sumer". Eventually these cities became independent city-states with separate government. Most powerful cities of this civilization were "Eridu, Bad-tibura, Shuruppak, Uruk, Sippar and Ur".