Actually, Aaron, his brother wrote Genesis based on the verbal story given to him by Moses. The story was based on The Epic of Gilgamesh, which was taught to him when he was young.
When Moses was in high places in Egypt he learned one version. When he went into the Arabian peninsula he heard a slightly different version from his father-in-law. The Jews had a 3rd version. As he was educated in the best of Egypt, he probably heard the versions from other lands.
All these versions were similar but had slight variances. Noah and the Ark is the best example where the histories in various ancient cultures were extremely similar, but never exactly the same.
So what was needed was an editor to pull together the various versions and determine which one was accurate.