Then they discovered Sumerian records 5,000 years old. It was obvious the Bible's oldest stories were so similar to these most ancient written records that they had the same source, or were simply alternate versions of the same stories, most likely written down at the earliest possible times by Sumerians ~ they, after all, invented writing.
The ancient Hebrews had very good contact with Babylon to the East, Assyria to the North and Egypt to the South, and writing had already been in full bloom in those areas for many centuries before 500 BCE. It's beyond the realm of conjecture to believe that all of the surrounding and neighboring civilizations had written records with the Jews still sitting around working out a primitive oral tradition.
You help my point. The Hebrews adopted much of the Biblical creation myth and much of Genesis from the Babylonians while in captivity circa 500 BCE.