It should also be noted that the Wellhausen theory [the Documentary Hypothesis] found what it was looking for. The theory grew out of a movement to find rationalistic, natural explanations for the biblical text. Once one assumes that supernatural revelation cannot occur any other explanation must take precedent. The late dates and various authors assigned to the books allow for purely naturalistic sources. This is a textbook case of question begging. The underlying premise, that there can be no such thing as supernatural revelation, resulted in the conclusion that the Bible is not a supernaturally revealed document.
Don Closson in the article Did Moses Write the Pentateuch?