Civilizations of the Fertile Crescent: The Sumerians
The earliest people to grow into a flourishing civilization in the Fertile Crescent (or anywhere) were the Sumerians. Just northwest of the Persian Gulf, between the Tigris and Euphrates Rivers, the Sumerians built a series of city-states that were later incorporated into an empire. Impressively, the Sumerians invented the wheel, irrigation, writing, trade, the sailboat, organized government, and a number system based on 60 (which we still use to measure time).
The Sumerians were a polytheistic society, believing in a few principal gods and thousands of lesser gods and goddesses. The had one well-known creation myth involving a great flood and a hero named Gilgamesh. This story may have been the inspiration for the Noah's Ark chapter of the Bible.
Some great sources for information about the Sumerians:
The British Museum
National Geographic Article