Before the theory of evolution, the process of aging was seen in the same way that moving parts wear out, machines break down, and all things seem to slowly deteriorate over time.
After the development of evolutionary theory, scientists began to wonder why evolution had produced such complex and well-adapted creatures that were so successful at surviving from conception through to adulthood, but which then fell into decay and died.
The theory of programmed death
One of the earliest attempts to explain the process of aging using evolution was in the 1880s with "the theory of programmed death". This was the idea that there must be an evolutionary advantage to having only a limited lifespan. It was suggested that older members of a species are meant to die of old age so that they would no longer compete with younger generations for food and other resources.
Experiments have since confirmed that there is a biological limit to the number of times that a cell can divide, indicating that evolution has actually established an upper limit to an organism's lifespan. However, there is now also a great deal of compelling evidence against the theory of programmed death.
Researchers have observed that animals living in the harsh conditions of the wild survive for only a short period of time compared to those living in the protected conditions of captivity. For example, nine out of ten mice living in the wild will be dead before the age of ten months, whereas mice raised in captivity have an average lifespan of twenty-four months. Very few individuals of any species living in the wild survive long enough for there to be any evolutionary pressure to set an age limit.
Studies have also shown that when animals grow old in protected conditions, there is no particular age at which they begin to die off more quickly. In fact, for many animals, the rate of death starts to slow down in extreme old age, which is the opposite of what would be expected if death was programmed.
The mutation accumulation theory of aging
Laboratory experiments in the 1950s showed that when fruit flies were exposed to dangerous conditions so that few individuals survived until old age, natural selection favored those that bred at a younger ages. Many generations later, when conditions were made safer and the flies died of old age, the natural lifespan of the population was now shorter.
These kinds of experiments showed that the age at which an organism breeds is the most important factor determining its lifespan. The process of aging generally begins after the breeding period is over, after which there is little evolutionary pressure to resist disease, cell degradation, and the other signs of aging which lead to a natural death.
For example, a genetic mutation that kills children will rarely be passed on to the next generation. However, a mutation that only kills people over 60 might spread across the entire population over time, because people with this mutation have already passed it on to their offspring. In this way, throughout evolutionary history, we have accumulated many genetic defects which cause the deterioration seen in old age.
If there is any evolutionary pressure beyond the breeding period, it comes from the role that parents play in the survival of their young. This is particularly important for animals that require long periods of parental care before reaching maturity. The lifespan of these animals will increase in proportion to how much more the parents can contribute to their offspring.
There are other evolutionary theories of aging that are also worth considering.
One popular theory suggests that some mutations which help the young to survive, and are therefore strongly selected, might have negative side-affects that are harmful in old age.
Another popular theory suggests that food is generally scarce in the harsh conditions of the wild, and so our bodies have evolved to spend what little energy we have on survival and breeding rather than on repairs and maintenance.
Other popular theories focus on random cell damage caused by chemicals in food, environmental poisons, background radiation, and the gradual build-up of the body's own waste poisons. Some of these theories are supported by experimental evidence indicating that eating less can lead to a slowing of the aging process and a longer lifespan.
Evolutionary biologists do not expect any single theory to provide all of the answers, but rather see the process of aging as involving a combination of theoretical causes. The degree to which each theory contributes to the actual process of aging is the subject of ongoing research.