> There must be a link back from consciousness to physics.

Yes, and it is trivial to prove. One second lapse of attention and you could get bitten by a snake, or run over by a car. If the top level (consciousness) fails, the neurons die too. Cells depend on this centralized decision point to exist. There is no way to have humans without consciousness because ... they would not be able even to put the thing into the other thing to make more of us without it ... excuse the language, but the argument is solid.