Shortest possible answer... A collective brain is what you'd have if human communication prior to the incident associated with the tower of Babel were basically telepathic (which is one interpretation of Julian Jayne's findings), if Jaynes were correct in thinking that consciousness as we know it did not exist 4000 years ago, and if consciousness in those days were basically general rather than individual.
I was in college right about the time Jaynes' big book - name escapes me - came out. It came up in a theology class (Catholic University), and the discussion became so heated, people almost came to blows. As I remember it, much of it revolved around humans not being self-aware (or something), prior to a certain time in the neolithic era.
I thought it was idiotic then and thirty years later, nothing has changed my mind, at least that would suggest our "self-awareness" happened sometime in the 1K-2K BC range, especially considering some of the archeological finds that have been proven to predate even Mesopotamia.