The Old Testament was based on the Torah, which is generally accepted to be derived from the true history of the Semitic peoples. The Sumerians were one of the Semitic races, and their Epic of Gilgamesh is thought to be a direct antecedent of the Torah. The Epic is about a thousand years older (in written form - it almost certainly goes back even further as an oral tradition), and has many striking points of similarity with the OT.
Here are some of the salient points of two stories from the Epic - see if they remind you of anything!
The Flood
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