Brains for Phenotypic Advantage Over Environment?

I had a thought in January 2004 while re-reading Daniel Dennett's Darwin's Dangerous Idea and thinking about Jared Diamond's postulated "Great Leap Forward" in human capacity 50K years ago.

Consider that humans "suddenly" exploded all over the face of the earth 50K years ago. This is a big deal because all other species are constrained to a bounded habitat. This event was almost certainly the result of an increase in human brain capability.

Dennett says we are allowed to take the "intentional stance" when trying to understand evolutionary phenomena. That is, we can ask what something is "good for". Indeed, he says that the intentional stance is necessary for any real reverse engineering effort.

So, taking the intentional stance, we can say that the mutation that spread through human brains 50K years ago was "good for" spreading human phenotypes into new habitats.

Now, genes don't care about environment. Environments impact genes' ability to survive. Phenotypes care a *lot* about the environment, but they are stuck with the hand they are dealt.

I think it makes sense to think about the mutation that caused the "Great Leap Forward" as a mutation for allowing (fixed) phenotypes to extend their habitat (by thinking, having better mental maps of cause/effect relationships, whatever). Phenotypes which could have the option of moving to better environments could presumably breed more, embedding the mutation in the gene pool.

Humans can survive in difficult environments (e.g. Siberia) by thinking and by passing down "good" thoughts ("memes") via cultural transmission. It seems to make sense that our distant ancestors did not have that capability, and so were limited to more forgiving environments for their phenotypes (e.g. equatorial Africa).

This difference in phenotypic capability is certainly modelable, but I am not sure whether the model could be tested against any hard evidence. At best, one could show that the modeled capability is one possible mechanism that would allow humans to survive in Siberia (a mutation that gave you blubber being another!). That's OK, since there are many possible paths through Darwinian design space and I think the best anyone can show would be a valid path, which might differ from the single path actually trodden by humans without further evidence from genetics/archaeology/etc.