Evolution of aging

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, shoes get wrecked with use 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 lifespans. Furthermore, this initial theory failed to explain why most organisms maintain themselves so efficiently until adulthood and then, after reproductive maturity, begin to succumb to age-related damage.

August Weismann was responsible for interpreting and formalizing the mechanisms of Darwinian evolution in a modern theoretical framework. In 1889, he theorized that ageing was part of life's program because the old need to remove themselves from the theatre to make room for the next generation, sustaining the turnover that is necessary for evolution. This theory again has much intuitive appeal, but it suffers from having a teleological or goal-driven explanation. In other words, a purpose for ageing has been identified, but not a mechanism by which that purpose could be achieved. Ageing may have this advantage for the long-term health of the community; but that doesn't explain how individuals would acquire the genes that make them get old and die, or why individuals that had ageing genes would be more successful than other individuals lacking such genes. (In fact, there is every reason to think that the opposite is true: ageing decreases individual fitness.) Weismann later abandoned his theory.

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