"The Origin of Consciousness in the Breakdown of the Bicameral Mind" (1976) by Julian Jaynes.
"When Julian Jaynes . . . speculates that until late in the twentieth millennium B.C. men had no consciousness but were automatically obeying the voices of the gods, we are astounded but compelled to follow this remarkable thesis." -- John Updike, The New Yorker. Julian Jaynes Society
-- Quotes and Reviews
-- from the home page:
"Jaynes asserts that consciousness did not arise far back in human evolution but is a learned process based on metaphorical language. Prior to the development of consciousness, Jaynes argues humans operated under a previous mentality he called the bicameral ('two-chambered') mind. In the place of an internal dialogue, bicameral people experienced auditory hallucinations directing their actions, similar to the command hallucinations experienced by many people who hear voices today. These hallucinations were interpreted as the voices of chiefs, rulers, or the gods."