I've been reading a crazy book called The Origin of Consciousness in the Breakdown of the Bicameral Mind.
Pdf here: http://s-f-walker.org.uk/pubsebooks/pdfs/Julian_Jaynes_The_Origin_of_Consciousness.pdf
It posits the theory that our consciousness / inner monologue / ego is a relatively recent development, evolving with language only 8 - 12,000 years ago..
Before that, people did indeed hear voices in their heads, but because they had no sense of 'self', believed them to be gods instructing them what to do.
Obviously pretty out there, but very well, and convincingly written