There is simply no way that bicameralism would be the dominant phenotype only a few thousand years ago, and be completely absent in any present-day population, anywhere. We would still see it in isolated populations.
“the ideas I had about supernatural beings came to me the same way that my mathematical ideas did. So I took them seriously.” - Sylvia Nasar, "A Beautiful Mind"
I would define consciousness as that which keeps the body alive, reacting to changes in the environment and actions of other agents.
Consciousness is a perception-judgement-action-reward loop. It's a reinforcement learning kind of situation, where the agent, embodied in an environment, learns to act in such a way as to maximise its rewards. We have been programmed by evolution to favour certain kinds of rewards which are good for keeping us alive.
This definition is much more concrete and practical - it has been applied with some degree of success in designing AI agents. It gives a purpose and a mechanism to consciousness.
The consciousness loop is formed of sensing, judging, acting, learning from sensation and reward. Sensing is like deep learning - a neural net converts raw sensory signals in high level representations. Sensory learning is mostly acquired by reconstruction or self supervision. Judging is like RL - where an agent learns to predict the future rewards following its current situation and actions. It can be model based (reasoned) or model free (instinctual). Acting is like the feedback based motion planning algorithms in robotics. Reward learning happens in the background of all these processes. When you put all these systems together, and inside a body, inside an environment, you get consciousness.
In the end it's just the competition of genes for survival, self replication in a limited resource environment over billions of years.
It would be cultural and taught, not genetic.
There are non-isolated populations: I come from a religious background where the idea that God "talks" to you is quite prevalent. If you want an analysis of how could possibly be the case in our modern society, I recommend Tanya Luhrmann's "When God Talks Back".
I say Jaynes puts the transition as far too sudden compared to what it actually could have been. Individuals could have had developed his version of "consciousness" often and repeatedly, but failed to pass it on into greater culture. I think of literacy as an analogous transition; until it's actually of benefit for the average person to read+write (ie society has moved beyond majority subsistence), it's not going to happen.
[0] https://www.goodreads.com/book/show/12958803-when-god-talks-...
You're rather confident of yourself there. What if it were viral?
For comparison, we don't see any new cases of smallpox anywhere and that's only a few decades after eradication. [0]
If we can eradicate traces of smallpox, then I don't see why traces of bicameralism couldn't also be erased.