[CFLAddLoader]

Straightforward intelligence boosting genes are valuable enough that unless they are very recent, they should already have spread everywhere.

It's more likely that developing brains have a huge number of potential tradeoffs to make for what tasks they are good at and how much energy they use. Genetics and epigenetics are merely one of the tools a brain uses to decide what shape it takes, and aren't even close to the whole picture.

[graham127851]

You could say the same about genes in tons of other areas, and you'd be completely wrong. That's now how evolution works. "Beneficial" genes don't just get to spread everywhere quickly, otherwise parapatric speciation could never occur. Read the wikipedia article for Parapatric Speciation. What does "incomplete geographical barriers" remind you of?

Evolutionarily our intelligence IS very recent.