The east Asian EDAR mutation is screwy: it does too many things. It causes increased scalp hair thickness, shovel-shaped incisors, an increase in the number of eccrine sweat glands, and smaller breasts. Which of these seem likely to confer a few-percent advantage? Why, none of them, of course.
Normally, an advantageous mutation does something useful and not a whole lot of anything else. That’s because change in a trait is usually bad – it moves you away from the Darwinian optimum. Coming up with an improved version of one trait is unlikely enough, and usually happens only in a new environment, where the selective pressures have changed. Coming up with multiple significant changes that are all neutral is most unlikely.
Now if the side effect is in a trait that doesn’t matter anymore, you might get away with it. For example, a mutation that has a favorable effect X while making you pale works in northern Europe. The paleness itself may even be advantageous.
Pardis Sabeti thought that extra sweat glands might have been the advantage. Joshua Akey suggested sexual selection – a preference for thick hair and small breasts. Kamberov thought different effects of EDAR might have been favored at different times.
They are all wrong. Am I really supposed to believe that you needed to sweat more in ice age China than in Africa? Breast size already varies – if for some mysterious reason human sexual fashion came to favor smaller ones, selection on standing variation could have done the job without changing your hair and teeth and sweat glands – changes that would almost certainly be disadvantageous.. The idea that different effects conferred advantage at different times is also a nonstarter: any single positive change is very unlikely, and I’m suppose to believe in several?
It’s gotta be something else, an effect that we don’t know, and it must confer a substantial advantage.