The question is a common but inaccurate belief. Humans are primates, we didn't evolve "from" them. Secondly, the question assumes that if evolution were to occur, the entire species would have to evolve instead of allowing a species to branch off. This isn't the case. The common belief in the scientific community is that we are most closely related to chimpanzees, not that we evolved from chimpanzees. We had a common ancestor. Two species branched off from this ancestor and each branch continued to evolve and branch until arriving at what we see today. Evolution is not about a constant increase in "complexity," which is commonly assumed. If this were the case than there would be only a single species on the entire planet, us, yet single-celled organisms still vastly outnumber us. Evolution is about an increase in diversity. If an available niche is open, a species will evolve to fill it, even if it must become "less complex" to be best suited to it. The human line branched off and evolved in a different direction than that of the other primates, but we didn't evolve "from" them.