Consciousness is deliberately suppressed in young humans.(Yes, I know you don't think that's true, but my personal memories falsify all other theories. Yes, they could be false memories. If so, they are the only false memories I have that aren't specifically labelled as such. Also, past memories of remembering these memories, such as when I was eight, are consistent with present recollections of the memory. Also, they are precisely following exponential decay.)
Consciousness exists at least in the species that pass the mirror test.
Emotion and choice and thus consciousness can be inferred in very low life forms, such as fruit flies and house flies.
Consciousness is therefore not only a very useful adaptation, having been conserved across nearly every neuron-using organism, but can be deliberately affected by changes in genetic expression over an organisms lifecycle. Put another way, a gland that specifically enables consciousness is plausible, much like the pituitary gland enables puberty.
In other words, it's not an essential epiphenomenon of the computation of the brain.
I just realized this.
This means I got a theory, and now I'm accidentally stumbling across supporting evidence.