Trade developed early and was very important to the people of Sumer because they?

The people in Sumer did not have many of the things they needed to keep them living. They had no way of getting it without the right transportation and materials so they began to trade with other areas in order to receive these materials.

I believed that the Sumerians lacked a number of natural resources that they desperately needed, and the only way to obtain them was trade. Examples include wood and minerals (stones).