One theory states that Genesis, Exodus, Leviticus, Numbers and Deuteronomy were written by Moses around 1400 B.C.

The alternate theory to that of single authorship by Moses is the compilation theory (officially known as the Documentary Theory), which indicates that the book of Genesis, along with other books of the Old Testament, were assembled over a long period of time from the traditional tales of Israel by several different authors. There could have been a great many sources for the stories of the Bible, but there are several very distinctive writers whose work appears throughout the Old Testament. For example, the author now known as J (the Jahwist, sometimes referred to as the Jehovist because of his consistent identification of God as Jehovah) is considered to have written a good deal of the material contained in Genesis, but also to have written material included in Exodus, Kings, and possibly Judges and I and II Samuel. J is the earliest known source of biblical material (in written form, as the traditions must go back much farther) and his work is dated to about 900 BC. J was an elegant and powerful writer whose work did much to define modern consciousness. Genesis also includes passages written by another source known as E (the Elohist, called so because of his consistent references to God as El or Elohim). Some bits from the P source are also included, which is a priestly tradition of writings that also makes up most of Leviticus. The book of Deuteronomy, originally considered to be another book of Moses, is now believed to have been written by an entirely different group (the D source) from the authors who wrote Genesis. How did these threads come together into the single narrative we see today? A compiler called the Redactor pulled together these seperate documents into one. Some scholars believe the Redactor was Ezra, writer of the book of the same name from a much later period. The Documentary Theory may sound strange if you've never heard it, but it actually makes an awful lot of sense. This theory accounts for why some stories in the bible are repeated and seem to contradict each other. If you read each book as though one author wrote it, it will appear as though an event has occurred twice - however, if the book was a compiled version of several different viewpoints on the same story, then the event only actually occurred once and the duplicated version is an alternate way of looking at the story. For example, there are two seperate stories of Creation in Genesis - one and then another. Why does it start over and tell the story again differently? Were there two creations? No, of course there was only one. One version of the story is told by J, and is the traditional story from Judah, while the other version is from E, the traditional story from Israel. Moses is not considered to be a likely author of any of the written tradition of the Bible - not in the least because the Bible itself does not indicate that he did so. If he did write a biblical contribution it was lost long ago. There are many reasons for why scholars believe this (and have since the 17th century). One of the more convincing reasons is that Moses is said to have been a humble and modest man, and it seems unlikely that he would write long passages about how great he was. Or indeed, about how humble and modest he was. Biblical scholarship goes on a great deal about this subject and it is fascinating to read about. Richard Friedman writes very accessible books on the subject. One book I recommend is "The Book of J" by Harold Bloom, which isolates the "J" text to demonstrate the ways in which it is a complete work in itself. As a literary critic, Bloom points out that the J writer, all Divine Inspiration aside, was a literary genius almost unmatched in human history. He also believes that J may have been a woman, but that's a whole other can of worms. ;)