Julian Jaynes was an American psychologist, best known for his book The Origin of Consciousness in the Breakdown of the Bicameral Mind, in which he argued that consciousness is a cultural development based on metaphorical language that occurred 3,000 years ago. Prior to the development of consciousness, humans operated under a different mentality Jaynes calls "the bicameral mind." Jaynes argues that vestiges of the bicameral mind are still prevalent throughout the modern world.
Jaynes defines "consciousness" more narrowly than some philosophers, and his definition of consciousness is essential to understanding his theory. Jaynes' definition of consciousness is synonymous with what philosophers now call "meta-consciousness" or "meta-awareness" i.e. awareness of awareness, thoughts about thinking, desires about desires, beliefs about beliefs.
For more information on Julian Jaynes's theory, please visit the Julian Jaynes Society at julianjaynes.org