The story of the ancestors in Genesis serves as a prequel to that of Moses in Exodus. Originally, however, each were self-standing accounts of Israel’s origin. They were combined for the first time by the Priestly author in the post-exilic period.This makes some sense to me, mainly because I can see a case for distinguishing J and E in Genesis, but after that the distinction seems to evaporate. I have commented briefly on that here.
Although I think elements of traditional source criticism remain sound, I am skeptical that we are able to make real progress on the question with the information currently available. As it stands now, there are many attempts to refine the state of the question, but none command anything like a consensus.
I have posted more thoughts on Pentateuchal source criticism here and links.
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