In about 3500 B.C., a people known as the Sumerians migrated from Asia into Mesopotamia. These people founded a city located in the Sumer Valley.
By around 3000 B.C., the Sumerians had formed a number of complex city-states. A city-state is a large city along with any surrounding territory that is ruled by a single entity or ruler. It is believed that there were at least 12 Sumerians City-States in the Fertile Crescent.
The people of these city-states shared a common culture and religion with one another. However, their rulers had complete autonomy from one another. They ruled their city-states without any interference from the leaders of other city-states.
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Crude oil was being drilled for in ancient China. In the 6th century it was recorded that hollow bamboo rods were driven into the ground in search of brine to provide salt for cattle. In the process, they also came across natural gas and flammable petroleum, which they used themselves or sold for fuel.