If however, some global disaster knocks us back to the stone age where our physical capabilities become more relevant than our cognitive abilities I would posit that we would evolve into something more fit for the environment.
Isn't that what it would take, given that we are able to modfy our environment to suit our needs? As opposed to the environment modifying us, which is essentially what drives evolution/NS? We have the ability to grow plants that we like in places where they shouldn't grow. We have the ability to import certain animals where those animals do not naturally live. We are able to live in desert and arctic climates. I really don't see anything short of a global disaster being the means with which we could evolve, or even would evolve. We have laws and societal norms which prohibit human selection and we allow fat, stupid, ugly, pretty, fit, short, tall etc. to breed.
Of course, this is all under the assumption of first/second world areas. Could there be something like, for example, a forever undiscovered Amazonian tribe that IS effected by it's environment and doesn't have the technological capabilities as 99% of the rest of the world? Sure, but unlikely so, given how fast the rest of the world chews through resources.
"Why don't you call upon your God to strike me? Oh, I forgot it's because he's fake like Thor, so bite me" -Greydon Square