Evolution doesn't code for just optimal. During the time that we were actually evolving, there existed pressure, for example, to optimize our use of the small amount of food that we had access to, optimizing to be as incospicuous during the dark hours, etc. Signaling processes weren't optimized for access to post-scarcity (in the first-world) availability of sugars, proteins, and fats; sleeping for as long as we do isn't optimal at all (some people have genetics that improve on this, though), and not to mention that the very nature of genetics will definitely produce humans that are sub-optimal in ways that that won't necessarily lead to sterility but will no longer be removed from the gene pool of the species.

I'm not claiming that caffeine will make you super-human but the use of substances like these that may have decreased survivability in pre-historic years (e.g., due to increased metabolism, induced bowel movements that decrease nutrient absorption) may augment several aspects of human cognition (and worsen others) in a way that is towards optimal, for say, coding but suboptimal for, say, creative writing (I don't claim this is true, just provide this as a possible example). Many contemporary college students can definitely attest to both the positive and negative aspects for drugs such as amphetamines (e.g., Adderall) in a manner that one could describe (though misleadingly) as superhuman.

I do agree that use of many of these substances can have seriously deliterious effects with prolonged use, but all studies I have come across refute this possibility for caffeine.