Evolution doesn't think long-term. It thinks generation-to-generation and no farther than that; in fact I'd go as far to say as it does absolutely no thinking whatsoever.
Evolution only favors traits that do not inhibit reproduction. Everything else is incidental. There is no other driving force.
Strength is not necessarily favorable in the eyes of natural selection. If our biggest asset is our intellect, then it's more likely that physically weaker specimens will produce physically weak children without them being murdered by falling rocks and huge hulking bara tigers. Because what's stopping a weak forgeter from making babies? The answer is nothing. There you go: strength is no longer naturally selected, because it's not necessary for survival.
It is thus: Offspring is created. Offspring creates offspring. Occasionally offspring mutates, which often kills it. Sometimes offspring mutates in a way that makes it better at not loving dying. It creates offspring. Does the helpful mutation get passed down? Maybe. If it does, that helpful allele will probably thrive in the gene pool and out-compete previous 'strains' of specimen.
