Stress targets energy carriers. Genes for stress resistance are selected that convey high metabolic efficiency enabling adaptation to the energetically restrictive and hence stressful environments of natural populations. Data from experimental organisms and from humans are consistent with a primary role for stress resistance underlying life span, which provides a hitherto neglected procedure for assaying longevity in natural populations. Taking into account the metabolic consequences of stressful environments, the free-radical theory of aging becomes a general stress theory of aging. A recent derivative, the deprivation-syndrome theory of aging, highlights resource and hence energy shortages. Energy balances under the stress theory of aging are primary for an understanding of the evolutionary limits of longevity of organisms in their habitats. In contrast, well-nourished humans of the modern era, and laboratory, domesticated and island populations are exposed to more benign conditions which appear to provide the background for other evolutionary theories of aging, especially the mutation accumulation and antagonistic pleiotropy theories. In modern human populations where selection for stress resistance is relaxed compared with earlier harsher conditions, substantial future evolutionary extensions to maximum life span may be difficult to attain because of the mutation accumulation process. However there is an urgent need for comparative empirical studies of life-history traits including longevity under benign and harsh environments.