The Sumerian civilization was the earliest known civilization that emerged in southern Mesopotamia around 4000 BC. Some of its notable cities included Ur, Uruk, and Kish, which centered around temples dedicated to the city gods and were surrounded by agricultural lands. The Sumerians had advanced for their time, with extensive trade networks and the world's first writing system. They were highly religious and believed gods controlled all aspects of life, with kings acting as agents to the gods and enforcing religious rule. Their class system divided people into commoners and elite, and artworks were mostly sculptures made of clay depicting religious scenes and figures.