Excerpts from Books and Wikipedia
The Sumerian civilization, writing, and the first cities began in about 3500 BC in Mesopotamia where the Tigris and Euphrates Rivers converge. The cities of Ur and Uruk traded by means of the Persian Gulf with civilizations to the East. About 50,000 people lived in Uruk. A temple to the moon god Nanna stood in the center of Ur. [National Geographic Almanac, p. 44-6]