Abstract: Almost everyone knows that physical activity (PA) is generally healthy. However, there is less awareness of the extent to which lifelong PA —particularly during middle and older age— promotes health. We also lack an ultimate, evolutionary explanation for why humans are less likely to remain healthy as they age in the absence of regular PA including its modern manifestation, exercise (discretionary PA undertaken for the sake of health and fitness). Here I show that humans were selected to be more physically active than other apes throughout our unusually long lifespans, including the several decades after we cease reproducing. I then propose and review how metabolic and physiological stresses induced by PA promoted selection to allocate energy away from harmful overinvestments in fat storage and reproductive tissues and towards repair and maintenance processes that slow senescence and reduce vulnerability to many forms of chronic diseases. As a result, extended human health spans, hence lifespans, are both a cause and consequence of habitual PA, helping explain why lack of lifelong PA can increase disease risk and reduce longevity. I will conclude by considering how an evolutionary anthropological perspective on physical activity can help encourage more people to exercise without being exercised about it.