Answer:
Surplus food
After the Sumerians developed irrigation systems, they grew more wheat and barley than they needed at one time.
Division of labor
Large teams of Sumerian workers cultivated the fields, and irrigated and harvested crops. Some continued to farm, but others became carpenters, metal-workers, potters, weavers, merchants, soldiers, doctors, etc.
Cities
People clustered together in villages and towns to find work, sell their goods, store their grain, and bargain for food. Villages and towns grew into cities as more and more people came together. Ur, Uruk, Kish, Lagash, and Umma are examples of Sumerian cities.
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