Another hypothesis, which is probably mostly wrong but still extremely interesting, is that put forth by Julian Jaynes: that the Incas, and most other early peoples, were not conscious (by Jaynes' very specific definition of "conscious"), but instead were unconscious agents guided by hallucinations, and that attempts to reason about them with modern models of volition and awareness are doomed.

See: http://en.wikipedia.org/wiki/Bicameralism_(psychology)