There is a great old book on 'voices in the brain' : https://www.amazon.com/Origin-Consciousness-Breakdown-Bicameral-Mind/dp/0618057072/

Basically he argues that consciousness came in 1600 (Renaissance) and before that we thought inner speech was gods talking to us. And he argues that some people never learned better, even today.

From a computational cognitive neuroscience point of view this makes sense, just like eidetic (photographic) memory, in some or many people.