Why do we age?

Aging feels like a law of nature, but it is not one. Physics permits a self-repairing system to keep repairing itself indefinitely, and biology has built things close to that: hydra show no measurable decline, Greenland sharks live for centuries, and bristlecone pines outlast civilizations. Humans, meanwhile, run down on a schedule so reliable that actuaries bet on it. Why bodies that can heal wounds and replace cells for decades eventually stop keeping up is a question with many good answers and no settled one.

Why it matters

Aging is the largest single risk factor for heart disease, cancer, stroke, and dementia combined. Medicine mostly treats those diseases one at a time; understanding aging itself would mean treating the thing upstream of all of them. The question is also evolutionarily strange in a way worth taking seriously. Natural selection built the eye and the immune system, yet seems to let every body it builds fall apart. Explaining why selection permits aging is as deep a puzzle as explaining how aging works.

What has been tried

The evolutionary side has three classic theories. Medawar argued in 1952 that selection is blind to late life, so late-acting mutations accumulate unchecked. Williams added antagonistic pleiotropy: genes that help youth get selected even if they harm old age. Kirkwood’s disposable soma theory framed it as budgeting, with organisms spending on reproduction rather than endless repair. On the mechanistic side, researchers have catalogued hallmarks of aging: shortening telomeres, accumulating senescent cells, mutating mitochondria, drifting epigenetic marks, misfolding proteins. Interventions that slow aging in lab animals exist, including caloric restriction and drugs like rapamycin, and partial cellular reprogramming has reversed some aging markers in mice.

Where the edge is

The open question is architecture. Is aging one process with many symptoms, or many independent processes that merely share a clock? Epigenetic clocks can now estimate biological age from a blood sample with unsettling accuracy, but whether the clock is a driver or a readout of aging is unresolved. Nor is it settled why lifespans across mammals span two orders of magnitude on broadly similar biochemistry, or whether the evolutionary theories, mechanistic hallmarks, and clocks are describing one phenomenon or several.

What would count as an answer

A causal account linking the evolutionary logic to specific mechanisms, precise enough to predict a species' lifespan from its biology and to say, before an intervention is tried, whether it will extend healthy life. The proof would be an intervention that works in people because the theory said it would.