One of my favorite weird scientific theories says that prior to a few thousand years ago, this internal narrative voice was mistaken for the voice of the gods, and explains why so many old texts are full of gods saying and doing things with people. The theory says that as we became fully conscious in the way that modern humans are, this narrator--which is actually the linguistic centers in the left hemisphere--finished integrating into the rest of the brain, and we started recognizing that it was actually just our internal monologue, not the gods; this was supposed to be the catalyst for modern human mentality.
It's almost certainly false and pretty fringe, but I've always really loved it as a theory. It's called "the bicameral mind."