And they developed things as basic as the wheel. The first wheel that was ever discovered was 3500 BCE in Sumeria. They are famous for their architectural structure. This is a picture here of a ziggurat, which was at the center of many of the Sumerian, and as we'll see, Mesopotamian cities. This would have required an incredible amount of labor to produce. And at the top of these, they had temples to their gods. And their gods, they had a polytheistic religion.