Enquiry into the evolution of ageing aims to explain why almost all living things weaken and die with age.
There is not yet agreement in the scientific community on a single answer. The evolutionary origin of senescence remains a fundamental unsolved problem in biology.
Historically, ageing was first likened to 'wear and tear': Our bodies get weak for the same reason that a knife gets dull or metal rusts.
But this idea was discredited in the 19th century when the second law of thermodynamics was formalized.
Entropy (disorder) must increase inevitably within a closed system, but living beings are not closed systems.
In fact, it is a defining feature of life that we take in free energy from the environment and unload our entropy as waste. Living systems routinely repair themselves, and, in fact, can build themselves up from seed.
There is no thermodynamic necessity for senescence.
Ageing is believed to have evolved because of the increasingly smaller probability of an organism still being alive at older age, due to predation and accidents, both of which may be random and age-invariant.
It is thought that strategies which result in a higher reproductive rate at a young age, but shorter overall lifespan, result in a higher lifetime reproductive success and are therefore favoured by natural selection.
Essentially, ageing is therefore the result of investing resources in reproduction, rather than maintenance of the body (the "Disposable Soma" theory), in light of the fact that accidents, predation and disease will eventually kill the organism no matter how much energy is devoted to repair of the body.
Various other, or more specific, theories of ageing exist, and are not necessarily mutually exclusive.