Aging, or senescence, can be described as a reduction in the chance of survival and/or fertility of an organism over time. Senescence is characterized by a decline in organ functions, ultimately resulting in death. This may be caused by many mechanisms, such as inappropriate redistribution of DNA over daughter cells after cell division and accumulation of damage caused by, for instance, chemical radicals over a lifetime. These mechanisms may indeed explain senescence, but they do not explain the origin of senescence and death. Although many evolutionary theories of aging provide explanations for the life history of an organism, they again do not explain the origin of senescence and death per se. Consequently, senescence remains largely an enigma to evolutionary biology. Complete repair of damage and immortality are not impossible. Indeed the germ-cell line is in essence immortal. The existence of single-cell organisms demonstrates immortality indirectly as they give rise to descendants by splitting into two nearly identical daughter cells, and there are even some multicellular organisms that are allegedly immortal. I argue that senescence and mortality are the result of less-than-optimal circumstances causing immortality to become essentially impossible, because extrinsic (from outside the organism) factors will cause death even in the presence of perfect cellular repair mechanisms.