How would this hypothesis explain the fact that people with low-functioning autism are maladapted to the environment, in the sense that they would have low chances of surviving a few thousand years ago?
It seems to me that it is nearly impossible that this form would be naturally selected, unless current levels of dietary iron were always common and are therefore not excessive to begin with.
There has been selection towards autism and low functioning autism may even be something else. Anyway, large part of it will be because of growing up among "normal" people. Maybe that (low functioning) autistic people would have no trouble using a langauge, it just wouldn't be the same as our languages. You can see it even with high functioning autism who sound odd (as, among possibly other things, tone is heard with the word, while as far as I understand it, neurotypical people hear tone as something completely separate)
It seems to me that it is nearly impossible that this form would be naturally selected, unless current levels of dietary iron were always common and are therefore not excessive to begin with.