There is no consensus about how consciousness arises from the brain specifically, or how it works. We're pretty sure that if the brain breaks, the expression of consciousness will also break (occasionally in predictable ways). Further than that, you can make no definite statements about it and call yourself scientific. It's still mostly a philosophical problem with no clear solutions.
"how consciousness arises from the brain specifically"
This seems like the most important question in the universe, and one not nearly studied enough.
For instance, the idea that consciousness is a fundamental feature of the universe that is simply manifested in brains sounds very woo - but hasn't been falsified.
The question of when an animal goes from "unconscious, animate matter" to "thinking self-aware being" seems unclear as well (and is very relevant to medical ethics w.r.t embryo experimentation, etc.)
By comparison to another out-there idea, if we're willing to take the Many-Worlds Interpretation seriously (and enough well-informed scientists do) then I'm not sure it's a _completely_ settled matter that we have a good sense that humans are "fully physical, embodied minds".
However, lead is bad for brains. I think we know that much.
"This seems like the most important question in the universe, and one not nearly studied enough."
If anything, I think the opposite is true. We study it far more than we actually should in some sense, relative to the tools we have.
Bear with me, this is going to seem a strange diversion: One of the biggest problems in particle physics is that the smallest possible tools we can use to examine the universe, like quarks, electrons, photons, etc. are all radically larger than what seems to be the smallest size of the universe itself. Compared to the Planck length, all of those things are enormous. I liken it to trying to determine whether a planet has life on it, when your only tool is the ability to smash other planets into it, and examine the resulting blobs six months later, from a distance so great they're just dots in your telescope. It's really hard to determine how the universe works at such small scales when your smallest tools are orders of magnitude too big.
We have the opposite problem with consciousness research. We have tools that can measure a neuron somewhat. Honestly, we're not even all that great at that. (We can measure voltages fairly well, but our visibility into the deep biochemistry is very weak. Honestly even a single neuron is beyond us in some non-trivial ways.) We have some tools that can take extremely broad averages across massive chunks of neurons. Meanwhile, we have every reason to believe that whatever consciousness is, it lies a good ten+ orders of magnitude above that (we can see this from the fact that it seems to require a human brain to be human conscious, and even the animals that appear to possess a lot of it like dolphins and birds still have brains far, far beyond anything we can treat quantitatively). We're ants trying to determine the nature of the entire planet we're sitting on, and the tools we've got to investigate it are that we can move these grains from here to there.
Occasionally ants get up on their tiny little soap boxes and declare "Damn it, it just shouldn't be that hard to understand our planet with these tools!" Sometimes they declare "It's obviously just all topsoil!" or other equally silly things. It doesn't particularly help; it just shows the smallness of the ant in question.
There's also no real consensus on what differentiates animate from inanimate matter, but vitalism is no longer a viable take on it. Dualistic theories of consciousness will soon follow vitalism into the dust bin of history.
A lack of definite knowledge isn’t an excuse to fantasize. Every time people have insisted that the gaps in our knowledge imply gods or ghosts, they’re rudely disabused. Every time we learn more about how things work, it turns out magic isn’t a factor at all. Meanwhile we really desperately want to believe in magic. It’s probsbly a good idea to be skeptical of God of The Gaps arguments that appeal to what we wish were true in the face of what appears to be more likely.
I am not trying to appeal to any supernatural explanation. The closest (crude) example I can think of is seeing the effect of a magnet on iron filings for years and then finally determining there's a magnetic field involved. Without study I suppose somebody could conclude that iron filings just really liked magnets and creating interesting formations. I am curious as to the mechanism behind the... subjective experience of being, I guess.
Ultimately, I was simply unsure about the assertion that all parties would naturally agree (without support) to "once you accept that humans are fully physical, embodied minds in a sea of whatever chemicals can cross the blood-brain barrier".
This raises a number of questions, which other people have raised before (Does consciousness require a wet brain or just the information processing that goes on within (or something else)? is a computer that simulates the processes of a brain conscious? Is a computer that simulates 1% of the processes of a brain conscious? I am conscious now, and if my brain were dissolved in acid I would not be, but what structures and processes must be destroyed or interrupted to bring about that end to consciousness, and is it a discrete or continuous phenomenon? Are computers now conscious but only infinitesimally so? Is there a degree of consciousness in any structure that processes information? How would we even know? What is consciousness?)
Careful. Much of the consciousness is more than the brain argument arrises from pro-life religious groups. They argue that brain death, or lack of a physical brain, does not preclude consciousness. This is to support anti-abortion rules and prevent the removal of life support from the brain dead. These arguments are a minefield of religious dogma.
Maybe where you are, but not where I am from. Here, you are considered a religious extremist if you do not support abortion (and I think that this is the reasonable stance, but that is not relevant right now). I'm thus not very concerned with what secondary agenda you associate this argument, and I will definitely not be careful of having an uncertain stance just because this is the case.
Huh, have heard a lot of arguments from pro lifers lately and this wasn't one of them. A discussion of the nature of consciousness and its mechanisms would have been refreshingly elevated, to be honest. Incidentally we're having a vote today on that very topic.
