In 1976 the American psychologist Julian Jaynes published a striking book titled The Origin of Consciousness in the Breakdown of the Bicameral Mind. Both the author and his argument have Wikipedia pages. The book is still in print in May 2011.
In his book Jaynes offered a very original theory of human consciousness: mainly, that it is quite a new thing, which arose only about 3,000 years ago, the human mind prior to that having been "bicameral." From this premise Jaynes developed theories of mind and of the origin of religion.
Though much pooh-poohed at the time, Jaynes's theory has been getting new attention since the "Consciousness Studies" movement got airborne in the mid-1990s. Recent understandings of the speed of recent human evolution have given still more credibility to Jaynes's ideas.
The Australian philosopher David Stove had interesting things to say about Jaynes's book in the following article he wrote for Encounter in April 1989, at — characteristically for Stove — the nadir of interest in Jaynes and his ideas. The book had long since slipped from public attention, the Consciousness Studies people had not yet arrived on the scene.
I have taken Stove's essay from the book Cricket versus Republicanism and other Essays, Ed. James Franklin and R.J. Stove (Quakers Hill Press, Sydney, 1995). I am obliged to Roger Kimball for bringing it to my attention.