Aging is an inevitability, or so we have to assume: the processes of evolution blindly but efficiently explore the space of possible living creatures, and have been doing so for a very, very long time. We might think that surely a very long-lived or ageless species would have a great advantage in evolutionary competition, its individual members able to produce descendants for far longer than competitors in more short-lived species. Yet virtually all species - with only a very few exceptions - age in easily measured ways. The species that age are also the species that have won in evolutionary terms, and therefore prospered and spread. Why is this?
Living organisms shouldn't age, at least if that could be helped (many of use would certainly like that, but our wishes are not a valid argument). Evolution works in a way that any species whose representatives have any distinct disadvantage will be driven to extinction. It makes sense then to assume that, if aging could be avoided, species that showed senescence as the individuals grow older should be replaced by others where aging does not happen (or happens at a much slower rate). Senescence increases mortality and an individual who dies of old age will leave, in average, a smaller number of descendants than another individual that does not age and manages to live and reproduce for a longer time. And yet many known living organisms show senescence. The time it takes for an individual to show signs of old age varies greatly among species, but aging seems so natural that many people fail to realize there is an apparent contradiction between senescence and evolution.
Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model for aging based on assumptions that are compatible with evolutionary theory: i) competition is between individuals; ii) there is some degree of locality, so quite often competition will between parents and their progeny; iii) optimal conditions are not stationary, mutation helps each species to keep competitive.
When conditions change, a senescent species can drive immortal competitors to extinction. This counter-intuitive result arises from the pruning caused by the death of elder individuals. When there is change and mutation, each generation is slightly better adapted to the new conditions, but some older individuals survive by random chance. Senescence can eliminate those from the genetic pool. Even though individual selection forces always win over group selection ones, it is not exactly the individual that is selected, but its lineage. While senescence damages the individuals and has an evolutionary cost, it has a benefit of its own. It allows each lineage to adapt faster to changing conditions.
We age because the world changes.
And there is illustrated one of the present competing viewpoints on the origins of aging.