By about 3000 B.C. some of the Sumerian city-states had grown to great size. For example, more than 60,000 people may have lived in the city-state of Uruk. This large population growth was made possible by the success of agriculture. Sumerian farms produced enough food to create a surplus, or extra supply, to feed the people who came to settle in Sumer.
Having a surplus led to a division of labor. Some people became craftworkers in stone, clay, cloth made of wool, and leather. Others became metalworkers, using copper and tin and later combining those metals to make bronze. With the addition of new products, some Sumerians became managers, people who were skilled at directing the work of others. Others became merchants. Merchants are people who buy and sell goods to make a living.
Sumerian merchants traded with merchants throughout the Fertile Crescent, even as far away as the Mediterranean Sea. The Sumerians traded what they had in surplus wheat, barley, and copper tools such as axheads and plowheads. In return they got resources that they needed, including wood, salt, precious stones, and raw copper and tin.
What effect did a surplus of food have on life in Sumer?