Ive always liked the jaw muscle theory. It is satisfyingly random.
We hold dissimilar views to consciousness - I take all life to be conscious, as I see sentience and consciousness as the same thing - this is just how the word works on me. Conceptual consciousness, metaphysics, this is what I'd call that of which we can reasonably suspect only large brained species to be able to entertain on Earth -
however, I see consciousness as a product of delayed self-valuing, as an awareness of a continuum between moments, as an identification with a temporal process, a life.
To hold on to the identity of the slow biological organism as an immediate electrical nervous system is, or requires, what I see as consciousness - the association of the self-valuing with a process that it can never fulfill without suspending its immediate self-affirmation/creation; thus even a small rodent as a 'purpose' - even if it just just collecting nuts and staying alive to do it another time - this whole process of collecting and 'nut-ness' that is integral to the little creatures being, is all category of consciousness, as I conceive of it.
My own notion of the origin of the specifically human consciousness, is the beginning of the practice of using tools. Things one picks up outside of oneself to acquire a power with, which then becomes an auxiliary attribute to the selfvaluing, whereby true metaphysics, or meta-selfvaluing is born. I think it was Bergson who said man is a tool to create tools to create tools, etc - in a sense this is true, it is in the creation of tools, i.e. in the continuously developed identification with method, that our true creativity, and thus our true freedom-as-power lies.