Genesis 5: lineage to Noah. People lived a long time when there weren't so many and we needed a boost. Methuselah is believed to have died when the flood starts.

Genesis 6: Nephelim mentioned. Not as a huge deal, but more in passing. It was probably something people just accepted as reality at the time of Genesis' writing that corrupted lines of people existed who were deeply involved in wicked things. The stories must have been well known and remembered. Interestingly I learned of the Chaldean Genesis, which contains another telling of this story and is allegedly older than Moses'. Curious if anyone has thoughts on this .