Why do we age at all, if evolution rewards survival?

Because evolution does not reward survival for its own sake: it rewards passing on genes, and its grip loosens as the years go by. In most species the chance of reproducing peaks in young adulthood and then declines, partly because outside dangers such as predation or illness keep killing individuals along the way. A harmful effect that only shows up late in life is therefore barely visible to natural selection.

Biologists have three main explanations, and each fits a different piece of the puzzle. In 1952 Peter Medawar proposed that harmful mutations acting late in life are too weakly selected against to be removed, so they pile up over generations. Another idea, antagonistic pleiotropy, says some genes help early and hurt late, which evolution tolerates because the early benefit counts for more. A third, Thomas Kirkwood's disposable soma theory from 1977, says bodies have limited resources and that spending them on growth and reproduction leaves less for repair.

A famous example comes from Huntington's disease. J. B. S. Haldane pointed out that it only shows up after about age 30, so in pre-modern societies selection against it would have been low. Nature offers clues too: bats, which fly away from predators, live on average three times longer than similarly sized mammals, and established queens of social insects, which face almost no outside risk, age far more slowly than their nestmates.

None of the three is thought to be the whole answer. Experiments find that death rates level off well before reaching 100%, which mutation accumulation alone does not explain. The three are believed to contribute in some way to senescence, the biological word for ageing.