There is a theory that before humankind evolved into a problem solver species, they had incredible inner faculties such as insight/intuition/instinct. As these inate qualities began being replaced in the evolutionary process with problem solving skills such as learning to cooperate for protection, for finding food, shelter, and the like, basically to come together to survive as a species, memory became of greater importance. As homo-sapiens continued to evolve so did their capacity to remember. As the civilized groups moved into new territories and began mingling with other groups, memories were shared.
One thought is this, Abraham's era, was when the Hebrew people were "chosen" as the "salt of the earth", a.k.a. the preservers. They passed down from generation to generation, from tribe to tribe, what was "known."
As humankind continued to evolve this great memory weakened with strife and war. Some began filling in the forgotten spots. Eventually the traditions began to change in their telling. Embellishment was added to make the stories memorable so they wouldn't be completely lost, and so it went until written language was developed and the stories gradually became written down and memory deteriorated from lack of need.
Think how just in the last few decades we've forgotten phone numbers, how to read printed maps, and instill in our memories details we know are only a Google away from recall.
Moses' era was the last of the great memories. He is often considered the author of the Pentateuch, or first 5 books of the Old Testament. The Book of Numbers alone reads as though someone wanted to make absolutely sure a record was made. Genesis aligns with so much archeological discovery of modern times that it puts one in absolute awe, even when it's not "absolutely" accurate, it's amazing.