“It behooves us, however, to remember that exercise is a truly odd sort of medicine. It is largely medicinal because the absence of physical activity is unhealthy. Further, exercise not only is an abnormal behavior from an evolutionary perspective but also never evolved to be therapeutic. Instead, we evolved to spend energy—much more than our ape cousins do—on physical activity primarily out of necessity and for other social reasons and otherwise sensibly reserve scarce calories for the chief thing natural selection cares about: reproductive success. To use energy frugally, many of the genes that maintain our bodies thus depend on the stresses caused by being active.”