As research into the mechanisms of healthy life extension through calorie restriction (CR) continues, the skeptics are finding their feet: "Calorie restriction is known to increase lifespan in many but not all species and may perhaps not do so in humans. ... Given the variety of physiological responses to variation in food supply that are possible, evolutionary life history theory indicates that an increased investment in maintenance in response to resource shortage will not always be the strategy that maximises Darwinian fitness. Additionally, for the well-studied species in which life extension is observed, there is considerable variation in the response. This suggests that it is not an ancient ancestral response, which has been conserved across the species range." For all that, the positive benefits on human health and resistance to common age-related diseases have been quite amply demonstrated.