The validity of the rate of living theory of aging in mammals has been seriously questioned over the last two decades because it does not account for the life span of many mammalian species. However, though this concept is an oversimplification and inapplicable in general, this does not mean that aging is unrelated to cellular metabolic processes. In general agreement with previous discussions, it can be stated that the rate of aging is proportional to the difference between the rate of cellular entropy production (which is by necessity roughly proportional to the rate of cellular biochemical processes and thus specific metabolic rate) and the cellular ‘counterentropic’ mechanisms (such as cellular repair, antioxidant protection, etc.). These counterentropic mechanisms may have evolved to a different degree in some mammals. This could be effected indirectly by natural selection of certain traits, particularly those expressed in differences in the rates of embryonic and postnatal development. These rates, relatively to basal metabolic rate, determine a species’ rate of becoming, which is proposed to be a predictor of mammalian life span. Data from 22 species, from shrew to elephant, with representatives from the main mammalian orders (including many exceptions to the rate of living concept), agree with this hypothesis. A mechanism underlying such natural selection, proposed elsewhere, is based on differential selection pressures among orders for which the different life-styles (particularly with respect to the birth of young) are responsible.