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": living bodies get weaker just as with use a knife's edge becomes dulled or with exposure to air and moisture iron objects rust. 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. It is a defining feature of life that it takes in free energy from the environment and unloads its entropy as waste. Living systems can even build themselves up from seed, and routinely repair themselves. There is no thermodynamic necessity for senescence. In addition, generic damage or "wear and tear" theories could not explain why biologically similar organisms (e.g. mammals) exhibited such dramatically different life spans.