The CF gene, furthermore, seems to only be present in the white population. Only in Caucasians. It seems to have been around for a long time. This implies that there must be some sort of advantage that it confers on carriers. The idea of just what that advantage is came to me one day while reading about the cholera epidemic in Rwanda.
Cholera causes loss of fluids.
It causes major losses of these electrolites. Since populations all over the world, and probably throughout all of history, have had things like cholera, anything that gave one population an advantage, a resistance to water loss, and upsetting electrolite balances, would give an advantage in the form of a more stable nervous system. And in thousands of competitions between warriors, for example, the steadier nerves, all other things being equal, will give a small advantage. And in such a situation, a small advantage is everything. A small advantage means victory.