Why do we age?
Evolutionary theories of ageing
Why do we age at all? If there are processes in our cells that can extend our lifespan, why haven't organisms evolved mechanisms to do so?
The lifespan of different animals varies widely, from a few hours in mayflies to five hundred years in Icelandic mussels. Some more primitive animals, such as sea anemones and the freshwater polyp Hydra, do not appear to age at all, while the longest-lived vertebrate species is the Greenland shark, which can live up to 400 years and does not reach sexual maturity until it is 150 years old. However, ageing is not the only strategy evolved during evolution: some animals, such as the Pacific giant octopus, male ants or the males of the small marsupial Antechinus agilis, die immediately after reproduction. So although most organisms age, the questions of ‘why do we age’ and ‘what determines the differences in longevity between species’ are much less understood.
From an evolutionary perspective, ageing is a paradox. Ageing makes us less healthy, so why should such a deleterious process evolve? The answer is that ageing evolved not because it is beneficial, but as a side effect of something else [Johnson et al. 2019]. This conclusion stems from two popular theories of ageing proposed by evolutionary biologists Peter Medawar and George Williams back in the 1950s and 1960s.
Peter Medawar's ‘mutation accumulation theory’ states that the force of natural selection remains high until the first reproduction [Medawar 1952]. It then declines with age. Therefore, deleterious mutations whose effects occur late in life can accumulate because they are not selected against. This is known as the ‘selection shadow’. This means that the primary goal of an organism is reproduction, and until that time natural selection ensures the maintenance of cellular processes that are essential for survival. After reproduction, there is no evolutionary pressure to ensure the continued survival of the organism. Cellular processes decline, the organism ages and eventually dies.
George Williams’ theory of ‘antagonistic pleiotropy’ states that natural selection can favour gene variants with beneficial effects early in life, even if the same variants have deleterious effects later [Williams 1957]. Because the deleterious effects of these genes occur in old age, after the reproductive phase, they have little evolutionary impact. Nature cannot directly select against a gene or its mutation that causes the death of an individual in old age if its deleterious effects do not occur before the end of the reproductive phase.
Another conclusion that can be drawn from these theories is that the individual rate of ageing of an organism is expected to evolve in accordance with the degree of external hazard. That is, the more likely an animal is to die from predation or lack of food, the shorter its life expectancy tends to be. Animals that have developed strategies to avoid danger tend to be longer-lived. For example, birds and bats that can escape a dangerous situation by flying are often long-lived; other strategies are social organisation or protection by poison or armour.
Another conclusion that can be drawn from these theories is that the individual rate of ageing of an organism is likely to evolve in accordance with the level of external threat. That is, the more likely an animal is to die from predation or lack of food, the shorter its life expectancy will tend to be. Animals that have developed strategies to avoid danger tend to live longer. For example, birds and bats that can fly away from a dangerous situation are often long-lived; other strategies include social organisation or protection by poison or armour.
In summary, ageing has evolved as a side effect rather than a planned developmental process, i.e. no genes have evolved to cause damage and death. This may also explain why it is such a variable process within and between individuals.
More FAQs on ageing
What is ageing?
How do we age? The hallmarks of ageing
Why do we age?
How to stay healthy in old age?
Can ageing be slowed down?
When does ageing start?
What happens in our bodies as we age?
What is biological age?
What do the terms life expectancy, lifespan, longevity and health span mean?
What is the epigenetic clock?
Do our genes determine how old we get?
Why do women outlive men?
What are Blue Zones?
References:
- Johnson AA, Shokhirev MN, Shoshitaishvili B (2019) Revamping the evolutionary theories of aging. Ageing Res Rev 55(100947).
- Medawar PB (1952) An Unsolved Problem of Biology: An Inaugural Lecture Delivered at University College, London, 6 December, 1951. H.K. Lewis and Company.
- Williams GC (1957) Pleiotropy, Natural Selection, and the Evolution of Senescence. Evolution 11(4):398-411.