Why Ageing Occurs

Ageing affects all parts of the body and leads to increasing frailty, a declining capacity to respond to stress, increasing incidence of age-related diseases, and eventually, death. Why the body should undergo this spectrum of degenerative changes, when it is equipped with sophisticated mechanisms for self-maintenance and repair, is a question that has long puzzled biologists.

Animals in the wild do not usually live long enough to show obvious signs of ageing; they tend to die young from extrinsic hazards such as infection, starvation, or being killed by a predator. Because Darwinian fitness is strongly governed by the survival and reproductive success of young animals, genetic factors that promote growth and fecundity in early life are favored by natural selection, even though these same factors may bring deleterious consequences later on. Thus, ageing is thought to result from trade-offs. In effect, late survival is sacrificed for reproduction.

An important trade-off is that which concerns the allocation of metabolic resources, especially energy, between activities of growth, maintenance, and reproduction. Each of these activities is metabolically expensive. Natural selection requires only enough maintenance for the body remain in sound condition through the normal life expectancy in the natural (‘wild’) environment. This concept is known as the ‘disposable soma’ theory, the soma consisting of all those parts of body which do not form a part of the reproductive cell lineage, or germ-line (the germ-line must of course be maintained to a high standard, else the reproductive lineage would die out over successive generations).

Evolution theory therefore supports the view that ageing arises principally through the gradual accumulation of random (or stochastic) faults in somatic cells and tissues. This is not to deny the importance of genetic factors in specifying longevity. Genes determine the levels of action of key maintenance systems, like DNA repair, and genes affect hereditary predisposition to a wide range of age-related diseases. However, it is no longer thought plausible that ageing, such as occurs in a species like Homo sapiens, is programmed through mechanisms which exist for the specific purpose of causing death.