Well, what a blast from my college past. I vividly recall all the late night manic chat sessions trying to decode Patricia and Paul Churchland's Neurophilosophy and Daniel Dennets Conciousness Explained.
Anyway, after years of rumination, to me it's clear that:
Quantum mechanics are definitely a part of neurobiology, and hence a critical building block of conciousness. We couldn't think without quantum mechanics. But plants couldn't photosynthesize without quantum mechanics either.
The quantum mechanical properties of neurophysiology apply just as much to clams as it does to humans. And it's just as applicable to those in a coma as to those engaged in a peak experience of some sort. So quantum mechanics definitely don't explain the conciousness of humans and in lesser degrees of other species.
Conciousness is an emergent property of the brain. Most of our evolutionary ancestors weren't concious in the sense we mean it today. Our massive brains are evolutionarily adaptive. Humans pay a big biological cost in having these big brains; very difficult childbirth, very long period of helpess infancy, wide pelvises to accomodate these giant heads, and a whole lot of extra calories and oxygen needed. But we're obviously breeding like rabbits as a species, and the primary limitation on further explosions of population are conciousness-driven (deciding not to have children, and having developed the means to do so).
Conciousness is, pretty much by definition, a really thorny thing to think about and almost perfectly designed to drive philosophers and cognitive scientists into mental loops. Since conciousness can also be described as self-insight, you get into a deep virtualization question in trying to have accurate insight into how you have insight :)!
So the trickiest part about conciousness is figuring out our own conciousness! It's a lot more easy and productive to try and consider someone else's conciousness than our own. Thinking about our own conciousness can easily get to the "eye of the universe question" - even if one has a good biological theory of conciousness, why do *I* have an experience of unique selfhood? That winds up being one of those unsolvable Big Questions, like "why is there something instead of nothing." Whether the existence of existence is explained via the Big Bang or theology, there's still the unanswerable question of what was the first mover. What started the cosmological ball rolling for there to be a universe in the first place?
Well, that was my moment of peak nerditry for the day! I'm going to go kiss a pretty girl for a while as penance...