Human beings are a marvel of evolved complexity. Such systems can be difficult to improve upon. When we manipulate complex evolved systems which are poorly understood, our interventions often fail or backfire. It can appear as if there is a "wisdom of nature" which we ignore at our peril. Sometimes the belief in nature's wisdom -- and corresponding doubts about the prudence of tampering with nature, especially human nature -- manifest as diffusely moral objections against enhancement. Such objections may be expressed as intuitions about the superiority of the natural or the troublesomeness of hubris, or as an evaluative bias in favor of the status quo. This paper explores the extent to which such prudence-derived anti-enhancement sentiments are justified. We develop a heuristic, inspired by the field of evolutionary medicine, for identifying promising human enhancement interventions. The heuristic incorporates the grains of truth contained in "nature knows best" attitudes while providing criteria for the special cases where we have reason to believe that it is feasible for us to improve on nature. We apply this heuristic to suggested repairs of ageing damage, examining when it gives us a green light, where caution might be needed and where we need more data.