The competing "Documentary Hypothesis" that Genesis was written much later by scribes who falsely attributed it to Moses is strongly refuted by, among other things, the summation of each account with a phrase popular in clay tablets in Mesopotamia around the time of Abraham (around 1800-1700 BC). Perhaps some place names were updated, as was a common practice then and today, but the essence of the text is from great antiquity.
There is no doubt that there are similarities and common threads in many ancient stories from Mesopotamia and the words of Genesis. It would be a book in itself to lay all this out, and one better written by an archaeologist, but I see signs that those stories are distorted versions and mutations of the account in Genesis, and not vice-versa. Are the pagan versions the only ones found when we excavate temples and palaces? They simply had the advantage of being the "party line" of ruling dynasties who could give their version of events from a state sponsored platform. The accounts we have in early Genesis were those from the family history of the line leading back to Adam. And in the Christ-centered model that was not everyone.
One difficulty with the Tablet Theory is that writing seems to have only been around in a crude form for five thousand years or so. And that was in cuniform not the proto-Hebrew/Canaanite script likely used by Moses (which is basically Hebrew). So far as we know, early Hebrew didn't come into use until about the time of Abraham.
Unfortunately the answer may be lost in the mists of time. We are therefore forced into speculation within the bounds of what is reasonable or possible. One possibility is that the clan of Adam had a system of writing among themselves which resembled Canaanite (who were of common ancestry with the Hebrews after all) for a very long time and we simply did not find any samples of it until around the time of Abraham. Another possibility is that the original tablets were written in an even more ancient script. In such a case someone would have to know that ancient language and translate them.
Perhaps it was Moses, with all the learning of Egypt at his disposal. Perhaps he didn't have to because Abraham translated the old tablets into new ones using the then-current language of the land. It is mighty suspicious that of the eleven "generations" or accounts in Genesis, none are from father Abraham, greatest of the patriarchs. This lack would be explained if he were the translator of them all. None were called the "generations of Abraham" because it was Abraham's family library. His job was to put the old accounts into proto-Hebrew, not write one with his own name on it.