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dfranke comments on Three consistent positions for computationalists - Less Wrong
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There are broadly speaking two versions of physicalism: ontological physicalism, according to which everything that exists is material, spatio-temporal, etc; and epistemological physicalism, according to which everything can be explained in physical terms. Physicalism can be challenged by the inexplicability of qualia in two ways. Firstly, qualia might be physically inexplicable because they are not physical things, which contradicts ontological physicalim. Secondly, the phsyical inexplicability of qualia might be down to their having a first-person epistemology, which contradicts epistemological physicalism. Epistemological physicalism requires that eveything be explicable in physical terms, which implies that everything is explicable in objective, descriptive, public, third-person terms. If there are some things which can only be known by acquantance, subjectively, in first person terms, then it is not the case that everything can be explained in physicalese. However, ontological physicalism could still hold.
My conclusion in the Mary's room thought experiment doesn't challenge either of these versions: something new happens when she steps outside, and there's a perfectly good purely physical explanation of what and why. It is nothing more than an artifact of how human brains are built that Mary is unable to make the same physical thing happen, with the same result, without the assistance of either red light or appropriate surgical tools. A slightly more evolved Mary with a few extra neurons leading into her hippocampus would have no such difficulty.
Incidentally, while agreeing with your main point, I feel I ought to challenge the implications of "more evolved." This has nothing to do with Mary's position on some scale of evolution; she could be "less evolved" and have those neurons, or "more evolved" and lack them.
I should have predicted that somebody here was going to call me on that. I accept the correction.
Mary still doesn't have to make anything special happen to her brain have knowledge of anything else. She can still understand photosynthesis without photosynthesising.
She can understand the sequence of chemical reactions that comprises the Calvin cycle just as she can understand what neural impulses occur when red light strikes retinal rods, but she can't form the memory of either one occurring within her body.
Which, yet again, only matters if there is something special about qualia that requires memory or instantiation in the body to be understood. She can understand the Calvin Cycle full stop.