Jaynes believes that language needed to exist before what he has defined as consciousness was possible. So he decides to read early texts, including The Iliad and The Odyssey, to look for signs of people who aren’t capable of introspection—people who are all sea, no rime. And he believes he sees that in The Iliad. He writes that the characters in The Iliad do not look inward, and they take no independent initiative. They only do what is suggested by the gods. When something needs to happen, a god appears and speaks. Without these voices, the heroes would stand frozen on the beaches of Troy, like puppets.
Speech was already known to be localized in the left hemisphere, instead of spread out over both hemispheres. Jaynes suggests that the right hemisphere’s lack of language capacity is because it used to be used for something else—specifically, it was the source of admonitory messages funneled to the speech centers on the left side of the brain. These manifested themselves as hallucinations that helped guide humans through situations that required complex responses—decisions of statecraft, for instance, or whether to go on a risky journey.
The combination of instinct and voices—that is, the bicameral mind—would have allowed humans to manage for quite some time, as long as their societies were rigidly hierarchical, Jaynes writes. But about 3,000 years ago, stress from overpopulation, natural disasters, and wars overwhelmed the voices’ rather limited capabilities. At that point, in the breakdown of the bicameral mind, bits and pieces of the conscious mind would have come to awareness, as the voices mostly died away. That led to a more flexible, though more existentially daunting, way of coping with the decisions of everyday life—one better suited to the chaos that ensued when the gods went silent. By The Odyssey, the characters are capable of something like interior thought, he says. The modern mind, with its internal narrative and longing for direction from a higher power, appear. Source
I've always found this theory fascinating, even if untenable. Neuroscience is lacking in romance, where it doesn't align itself laughably with futurism.