Sure I read somewhere about "enthusiasm" coming from the Greek to have a god acting within/through you as happens in the Illiad and Odyssey. Rather than a split consciousness I always assumed it was referring to the more animal - pre-ego consciousness where perhaps it does just feel like cosmic powers acting through your specificity (man...) and those early urban societies were in transition from the preponderance of one sort to another.
ETA Of course perhaps that's what your man meant by split.
Not quite. I think Jaynes was wrong about most things, but that he was wrong in interesting ways. Firstly, I think he was wrong to think that a split mind such as he imagined to exist in the Bronze Age ('bicameral mind' was his term) was not conscious. It may have lacked a unified 'I' - some people still do of course, and Jaynes goes into that too - but that doesn't mean no consciousness. A consciousness does not need to have a specific kind of content. It certainly doesn't need to contain words, nor does it need to contain a single unified perceiver. That's the biggest mistake in his thesis, imo.
Something he does go into a fair bit as well is the origin of words, specifically of words referring to the mind and consciousness, such as psyche, that originate in physical, non-conscious phenomena. In the case of psyche, Jaynes says that its origin is from 'breathe' (this is disputed). So the recognition of life's physical manifestation came before the recognition of its mental manifestation, essentially. And the origin of words, which arise from a process of metaphor, reflects and is evidence of that. I think Jaynes is onto something with this part of his argument. It isn't his main argument, but it is his strongest, I think. It links in with the idea that we recognise our own autonomous existence only after recognising the autonomy of others and recognising that they are 'not I', that we come to our own minds via the minds of others.
Specifically regarding 'enthusiasm', that it is a motivation of mysterious origin makes it a natural thing to attribute to the gods, I would think. And plenty of people still think in these kinds of terms a lot, or at least part, of the time. Jaynes' idea was more specific than that - confronted with a novel situation, your decision as to what to do presents itself to you as a voice. But decisions are also of rather mysterious origin really, so again, not so strange to think of them as being attributed to gods. Whether that was always strictly as Jaynes imagined - a voice heard as real rather than a 'silent' internal voice such as we all still have - is another matter. It's not so controversial to think of people's thoughts manifesting as voices that are taken to be real and external, though. Religious scripture from Moses to Mohammad is filled with stories of exactly that happening. But by what reasoning do we take our own internal 'silent' voices to be
us and not external. I would suggest that it's not immediately obvious that we should do so without some external confirmation.