An evolutionary hypothesis based on an 'antagonist pleiotropy' or 'disposable soma' mechanism is put forward to explain differences in longevity between species, strains, and sexes. Data from several congenic mouse strains and mammalian species suggest that there may be an association between cleavage rate of concepti and longevity, in such a way that concepti from species, strains or the sex (male) with the fastest cleavage rates have shorter life spans. The major histocompatibility complex (MHC) and, in particular, the conceptus development gene (Ped) together with several Y-linked genes that are expressed during the preimplantation stages of development may play an important role in determining or modulating longevity in mammals. Notwithstanding, effects of other loci as well as environmental factors on conceptus development and longevity cannot be ignored.