I'll mention a couple of things that should help people connect a few dots:
1) Usrai mentions in the Talmud cobbling together stories that the Jews brought back from Babylon with stories possessed by the Samaritans that remained in Israel and Judah, and editing them all for coherence, to form the modern Torah, which includes Genesis.
2) There is evidence of substantial redaction within the book of Genesis itself. For one thing, there are at least three distinct names of God mentioned--Elohim, YHVH, and El Shaddai. Elohim is a feminine singular noun--Elo--with a masculine plural terminative--him. It therefore probably means either "The Goddess and the Gods" or "The Goddesses and Gods," but it may also imply a mystical bisexuality to a single entity--though given the fact that the name predates the axial age, the former explanation is to be preferred. All this presents an anachronism, however. Adam used the name YHVH, whereas this same God reveals himself to Mosheh in the burning bush and says specifically that this was a name he had revealed to no one prior to then.
3) Other signs point to redaction within Genesis. The numerical equivalents of "Messiach" and "Nechesh" (the serpent in the Garden) point to a probable redaction by a later unknown person, perhaps the "great redactor" himself, of the story of 2 and 3 Genesis. The occurrence of the story of the Tower of Babel in the middle of the lineage of Shem, and the careful play on words in the story itself, also lend credence to the notion of redaction. And there are scores of other such examples.
4) Still other signs show that it is a highly synthetic book--the resemblance of the story of Noah with the story of Utnapishtim from the Epic of Gilgamesh is simply too great for the former not to have been based on the latter, which was itself written at least a millenium prior to the earliest possible date for a written version of Genesis. The presence of the name El Shaddai, a Sumerian deity of storms, as the God of Abraham is further evidence.
Finally, this:
That is incorrect. The Dead Sea Scrolls disprove that theory. They contain almost a complete Genesis which is identical with the one we have today. The Scrolls date back to before the Babylonian Empire.
is false. The Babylonian Empire (well, the second Empire) dates from something like 626 B.C., whereas the earliest of the Dead Sea scrolls dates from the 3rd century B.C. Ergo, the earliest scrolls are three centuries later than the Babylonian Empire.
In any case, the stories of Genesis bear strong resemblance to the stories of Sumeria, not Babylonia, which makes sense since Abram was from Sumeria originally.