But there is no evidence that any pill can raise the average person's IQ by 10 points

Please read this short review of the state of the art of chemical intelligence enhancement.

We probably cannot reliably guarantee 10 added points for every subject yet. Quite far from it, in fact. But there are some promising leads.

if some simple chemical balance adjustment could have such a dramatic effect on fitness

Others have made these points before, but I will summarize: fitness in a prehistoric environment is a very different thing from fitness in the world of today; prehistoric resource constraints (let's pick, for instance, the scarcity of refined sugars) bear no resemblance to those of today; certain refinements may be trivial from the standpoint of modern engineering but inaccessible to biological evolution, or at the very least ended up unreachable from a particular local maximum. Consider, for example, the rarity of evolved wheels.