Why living things age and die

Ageing looks like a straightforward failure of maintenance, and it is not. Bodies repair themselves constantly and could in principle keep doing so. The reason they stop is that selection loses interest in you once you have reproduced, and a force that weakens with age leaves late-acting problems unopposed.

Why ageing is a puzzle at all

Start with the observation that ageing is not universal. Some organisms show no increase in mortality with age at all, and a few appear to become less likely to die as they get older. Many trees, some fish, and certain simple animals fall into this category. So decline is not a physical necessity imposed by chemistry.

And repair is clearly possible, because it happens. Cells replace damaged proteins, mend broken DNA and rebuild tissue continuously. An organism that repaired slightly more thoroughly would last longer. The question is why almost nothing does.

Selection fades with age

Here is the key idea, and it is worth slowing down for, because everything else follows from it.

Imagine a variant that causes a serious problem at age eighty. To be removed by selection, it has to reduce the number of offspring its carriers leave. But most carriers will already have reproduced long before eighty, and in the wild most will have died of something unrelated well before then anyway. The variant is nearly invisible to selection, so it persists.

Now imagine the same problem at age five, before reproduction. Its carriers leave no descendants and it disappears immediately.

Two mechanisms, both consequences of that

Mutation accumulation. Peter Medawar's version. Harmful variants that act only late in life are barely selected against, so they build up in the population across generations. Ageing is then the combined effect of a great many late-acting problems that were never worth removing.

Antagonistic pleiotropy. George Williams' version, and the sharper one. A variant that helps early and harms late will be actively favoured, because the early benefit is fully visible to selection and the late cost is nearly invisible. On this account ageing is not neglect. It is the price paid for traits that were worth having when young, selected for deliberately.

The disposable soma idea makes this concrete as a budget. Repair costs energy, and that energy could instead go into reproducing now. An organism that spends heavily on maintenance leaves fewer offspring than a rival that spends on offspring and lets the body degrade. Given that something is going to kill you eventually regardless, building a body that would last two hundred years is investment in an outcome that will almost never be collected.

The theory makes a testable prediction

If ageing is shaped by how long organisms actually survive in practice, then species facing less external danger should evolve slower ageing, because more of their carriers are still around at later ages for selection to act on.

This holds up repeatedly. Animals that fly, and so escape most ground predators, live much longer than ground-dwelling animals of similar size. Bats routinely outlive mice of comparable body mass by a large factor. Island populations that lost their predators evolved longer lifespans. Reduce the external hazard and the shadow lifts.

Lifespan as an inherited number

In this world, longevity is one of the nineteen genes an organism carries. It is inherited, it mutates, and it sets a maximum age past which the organism dies regardless of how well fed it is.

That makes the trade-off explicit rather than emergent. A longer-lived organism has more opportunities to reproduce; but everything here pays a basal upkeep every tick simply for being alive, so a longer life is also a longer bill, and it is only worth paying if the extra time is actually converted into offspring.

Crucially, most organisms here never reach their limit. The overwhelming majority die of energy reaching zero, and most of those die before the maturity age of 100 ticks, having never had the option of reproducing at all. That is precisely the high external hazard the theory says produces short lifespans, and it means selection on the longevity gene is weak, because so few carriers ever live long enough for its value to matter. The cost table is on energy, metabolism and death.