It seems I've been writing a lot lately about interesting theories on human evolution. Here is another one. Several years ago I read a book entitled The Origin of Consciousness in the Breakdown of the Bicameral Mind. It's worth reading just to be able to brag about having read a book with such an impressive title.
The author, Julian Jaynes, has an interesting hypothesis on the origin of modern consciousness. He believes that soon after acquiring language, human beings began to hallucinate. Back then, most of our intelligent thought took place in the subconscious. It probably still does today in the sense that we can be guided by our dreams or by sudden insights that appear from nowhere. But early man had far less awareness of his thought processes. According to Jaynes, once language had been developed, the subconscious used this ability to communicate directly with the conscious mind. In other words, people experienced auditory hallucinations that guided them by compelling them to act in certain ways. Through the development of religion, these voices came to be regarded as messages from the gods and the dead. It is very interesting to go back and reread the Iliad or Odyssey from this perspective, seeing the constant visitations from the gods as a neurological phenomenon.
This theory sees schizophrenia as a kind of throwback to that earlier mental state. In both cases, people hear voices which are so compelling and full of authority that they often cannot resist, even if they are told to do something terrible. The problem with modern schizophrenia is that the voices are almost always hostile and destructive. This may be due to the fact that to a fully conscious modern person, it becomes a struggle for control. The author does mention, though, that in giving talks about his theory, he sometimes encounters ordinary people who admit to him that they often hear voices, and have no problem with this fact. One woman told him that every morning when she does the housework she converses with her dead mother. So apparently there are some closet voice-hearers out there.
It's a very interesting theory, and I'm not sure that my post really does the book justice. Despite the title and length, it really is an easy and fascinating read. While the evidence is far from supplying incontrovertible proof, it is nevertheless well researched, and if nothing else, demonstrates that there was likely some very interesting transition state between when humans were mentally chimplike and when we started thinking like modern humans.