It would be a shame to tell you Julian Jayne's hypothesis because the most important thing is reading his book, "The Origin of Consciousness".
It is a wonderful book but most of all if you read it, your mind will be enlarged. If I merely tell you, you will have a bit of extra information in your mind, but your mind will remain the same.
And this is what education, learning, is. It is enlarging the mind rather than filling it with information.
Enlarging the mind is a bit like stretching a muscle - at first it might hurt a little but after a while it is more satisfying.
And reading Julian Jayne's book is an experience - you might fall in love with the author or you might fall in love with his idea. And after a while you might start to become criical and see the flaws in the book and even in Jayne's himself - at this point you are becoming disenchanted.
So only if you read the book, you may become enchanted. And only if you become enchanted, is there the possibility of becoming disenchanted.
And becoming enchanted and disenchanted is a way of eating the book and digesting the book and making it your own.
If I told you what the book was about, I would be robbing you of this experience.
All true, idealistically. (So i can empathize.)
But many of us have lives outside of this -- other time commitments.
Therefore it helps to know the gist of the book and how/why it should be read, so we can prioritize it accordingly.
Realistically, marketing the product the way you have discourages readership even among those who would have enjoyed and benefited from the title. So while your goal might have been to encourage long-term absorption of the knowledge, it would make more sense to give the info and get 15-20% of the people who hear it to read it on their own... rather than having 0% of people interested by not giving them anything to go with.
Jaynes proposed the theory that about 2,000 ish + years ago, the right side of the human brain was the dominant hemisphere of brain, and somewhere during the course of development the left hemisphere become dominant. The right side of the brain is currently regarded as the "silent" part of the brain: in non-abnormal humans it does absolutely NO language processing or speech generation.
Yipes. 2000 years? That's, um, not really a lot of time at all, evolutionary-wise, for such a major shift/leap of processing to perpetuate throughout the entire human species. I mean, how could it? At that point the species was already very differentiated.

The change would have rippled through a few particular strands, if any, leaving others untouched, or only made it into large populations through a small (not large) shift, which wouldn't have really changed things much.
I would say it is important to read his book not because he may be right but because the book is a cultural tour de force. It is a cultural experience to read it. An experience that can't be gained from reading Wikipedia.
I think this is an important distinction that should have been made up front. Withholding information / motivations could lead to a lot of people not trusting your recommendations in the future; you need to be clear about why you're promoting a particular product. Because your case from the start seemed to insinuate it was the ideas that were worth exploring ("to open our minds"), not just that the book was an experience to be savored.
Here's a page that's
pro-jaynes but at least it presents some of the arguments in more detail (including the one I raised here), for further exploration and/or challenge.
Well, the topic will go on my "future exploration" list now that I know about it and have a little bit of an overview. It's interesting but right now I don't know what to make of it. (I don't know if I agree about his assessment of the Bible either, that it doesn't possess "introspection." By the NT, Paul definitely showed a lot of introspection in his style of logical thinking; and in the OT, there are many examples of people who were introspective. (Up front, the Psalms were very introspective, but that's only the tip of the icerberg.) The purpose and style of the writing also has to be taken into account.
I think I basically do agree that language is necessary for conscious/self-aware thought.