Why did the sumerian civilization develop as a number of independent 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".

the answer to fill in the blank in this question would be the word resource, it is defined as the money, material or people (since human resource is a very valuable good) that a company or a person has at disposition (meaning they can use the resource as they see fit), to be able to complete a task or achieve a goal.  

for hobos it was a very hard time most hid on the sides of the railroad tracks causing people to get bulls and get them off sometimes causing in fatal death they couldnt get people to give them money because the   people were also in need of money