Before the theory of evolution, ageing was seen like a machine whose parts wear out over time. Afterwards, scientists began to ask why evolution had produced highly complex, adaptable living things that are very good at surviving from birth to adulthood, but then fall into decline and death. Their search for answers led to several theories.
The first theories, in the 1880s, proposed programmed death: some evolutionary benefit from a limited lifespan, since the death of older individuals reduces competition with the young for food and other resources. Lab experiments later seemed to support this when they showed a biological limit to the number of times a living cell can divide, as if evolution had set a ceiling on lifespan.
But there is now plenty of evidence against this theory. Researchers have noticed that animals in the harsh conditions of the wild live shorter lives than those in the comfort of captivity. Of every ten wild mice, for example, nine die before they are ten months old, while mice in captivity live 24 months on average. In fact, most wild animals don't live long enough to justify an evolutionary pressure to limit lifespan.
Some studies also show that as captive animals grow old, there is no set age at which they die. For many animals, the death rate actually starts to fall at very old ages, the opposite of what we would expect if death were programmed.
The theory of accumulated mutations emerged from lab research in the late 1950s. When fruit flies were kept in dangerous conditions in which very few reached old age, natural selection favoured those that reproduced younger. After several generations, when conditions were made safer and the flies died of old age, the lifespan of this group had become shorter.
This shows that the age at which an organism reproduces is the most important factor in how long it lives. Ageing usually begins after the reproductive stage ends, when the evolutionary pressure to resist disease, cell decay and the other signs of ageing that lead to natural death weakens.
For example, a genetic mutation that kills a child will rarely be passed on to the next generation, but one that only kills people over sixty may spread through the whole population over time. In this way, many genetic faults that cause the decline seen with age build up over time.
If there is any evolutionary pressure after the reproductive years, it must be linked to the role parents play in their children's survival. This matters in species where mothers care for their young for a long time before they mature, and the lifespan of these species grows in proportion to how much parents contribute to their children's lives.
Other theories suggest that some mutations that help the young survive, and are strongly selected for that reason, may have harmful side effects in old age. Another suggests that scarce food in the wild forces creatures to spend their small energy reserves on survival and reproduction rather than on repair and maintenance. Others focus on random damage to cells from chemicals in food, environmental toxins and radiation. Some of these theories are supported by experiments showing that eating less may slow ageing and lengthen life.
Biologists don't believe any single theory can answer everything. They see ageing as having many causes, and how much each theory really contributes will remain a subject of ongoing research.
By Dr Ayham Ahmad
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