I'd make a population based argument against this. If the idea is that cooking allows more nutrients which allows the brain to expand, then it would have to be available to the entire human population for the genes to spread. Meanwhile going to a fucking hot spring to dip your meat in would seem to be a scarce resource.
Well, if the founding population of human species only consist of hundreds of beings that lived around a region with lots of natural hotsprings, it's believable.
Reminds me of that hypothesis that modern humans stayed in Africa until very late (60,000kya) because we were relying on one lake with clams that have protein needed for brain synthesis, until somebody get the mutation to make it ourself. I forgot the exact details, though...
Also a population of hundreds is insufficient - it means a million years of evolution would have to take place in a tiny genetic background insufficient to yield much genetic heterogeneity - the other necessary piece to produce evolution.
