His book is called 'The Origin of Consciousness in the Breakdown of the Bicameral Mind.' It's precisely the maladaptive aspects of bicameralism in the encounters with increasingly complex social environments that Jaynes postulates as the reason for its evolutionary deprecation.

Man is a function of his environment, instead of defining the environment.

Neither of these is true (I realize you are arguing against the first position). We are in constant interaction with our environments, neither simply acted-upon nor possessed of god-like powers of manipulation from whose consequences we are immune. I feel you must not have read Jaynes' book, since he postulated bicameralism at work in tasks such as agricultural planting, which by definition alter the environment. If you haven't read it, then I urge not to rely on the summary explanation that can fit in a blog article or even a longish essay. Jaynes' articulation of his theory is elaborate and discursive, but also rigorous and well-qualified (in terms of acknowledging limitations and the difficulties of falsifiability).