The incidence of aging is different between mammals and their closer ancestors (e.g. reptiles and amphibians). While all studied mammals express a well-defined aging phenotype, many amphibians and reptiles fail to show signs of aging. In addition, mammalian species show great similarities in their aging phenotype, suggesting that a common origin might be at work. The proposed hypothesis is that mammalian aging evolved together with the ancestry of modern mammals. In turn, this suggests that the fundamental cause of human aging is common to most, if not all, mammals and might be a unique phenomenon. Experimental procedures capable of testing these theories and how to map the causes of mammalian and thus, human aging, are predicted.