Jaynes asserted that until the times written about in Homer's Iliad, humans didn't have the interior monologue that is characteristic of consciousness as most people experience it today. Instead, he believed that something like schizophrenia was the typical human mental state as recently as 3000 years ago.
Jaynes described this state as a bicameral mind by analogy with bicameral legislatures and parliaments. Jaynes believed that preconscious humans effectively had a split brain which allowed one part of the brain to appear to be "speaking" to another part. Jaynes built his case for this theory by citing evidence in the Iliad, studies of schizophrenics, and other diverse sources.
According to Jaynes, the breakdown of this bicameral mind into an integrated system led to modern consciousness, by allowing the creation of a continuous internal narrative.
This theory is not taken seriously by scientists, although it created great controversy when first published, and provided impetus for many other scientists and philosophers to investigate the matters it dicussed in detail in order to attempt to refute its arguments.
It's also been great fodder for cyberpunk authors; Neal Stephenson's first several books (The Big U, Zodiac, Snow Crash, The Diamond Age) involve the bicameral mind theory, as does Bruce Sterling's Distraction.