Wait, what? Sounds cool and I'd love a source on that.
https://en.wikipedia.org/wiki/Bicameralism_(psychology)#The_...
Basically, before 3,000 years or so, it was common for people to hear the gods talk. Somehow, that changed and it became far more rare, with only a few (prophets and such) claiming to actually hear the gods' voices.
The bicameral theory is that there are still such people today, schizophrenics, and it is one half of their brain talking to the other - but they perceive it as an external agency. And that somehow this was the common state of humanity in the past, but changed. We became self-aware - we recognized the internal voice as part of ourselves.
This does actually fit many historical facts well. Still, it is likely wrong for other reasons.
Still, it lives on. Not many incorrect ideas do. Perhaps it's because it is a more concise and elegant answer to several unresolved questions (why did self-awareness arise? why did religions change at that particular time?) in ways we don't have any other good answers for.
It's a brilliant idea, that sadly happens to be wrong.