Why Do We Age?
Evolution explains so much about life, but can it explain aging and death?
Is there an evolutionary advantage to aging?
Natural selection is one of the main drivers of evolution. It is the process by which certain heritable traits of a species provide a survival or reproductive advantage to individuals because they are better suited to the environment. Such traits could include a larger brain, opposable thumbs or webbed feet depending on the species and environment. Aging, in this context, seems paradoxical. What is the advantage of a process of slow, gradual deterioration of the body leading to loss of function and eventually death? Biologists have wrestled with this question since the theory of evolution was proposed.
Alfred Russell Wallace, who jointly published his independently-conceived theory of evolution with Charles Darwin’s in 1858, speculated in his notes that there might be a programmed death mechanism. The initial thinking was that old and worn-out individuals died to make space for the next generation. Biologist August Weismann championed the position until he realized the argument was circular. Individuals are only worn-out because of the aging process. In this line of thinking, it remains unclear why aging itself occurs.
Aging is incredibly variable across and even within species
Aging is not universal. Some creatures, like the tiny, yet fascinating hydra, do not seem to age. Hydras also have an incredible ability to regenerate. If cut in half, both halves will grow back whatever was lost in the separation. The immortal jellyfish, Turritopsis dohrnii, can repeatedly revert back to a sexually immature polyp state, restarting its development cycle each time. This jellyfish can essentially age in reverse.
Video of a hydra eating and reproducing
The oldest organism on the planet may be a clonal colony of Quaking Aspen. Clonal colonies are considered a single organism sharing a vast root system that sprouts genetically identical clone trees above ground. A colony in Utah named Pando is believed to be over 80,000 years old. Individual Bristlecone Pine trees can also get very old. The oldest living specimen is estimated to be over 4800 years old.
Animals have shorter lifespans than such ancient trees, but they can still get surprisingly old. A Quahog Clam dredged up of the coast of Iceland was believed to be over 500 years old. Scientists estimated the age of one Greenland Shark to be somewhere between 272–512 years, making it the oldest vertebrate on the planet. On land, giant tortoises live longest. A captive Aldabra Giant Tortoise was believed to be 255 at the time of its death.
Slow and steady wins the race. (Photo by Jude Infantini)
The most striking thing about the comparative biology of aging is not that some animals can get very old, but that aging across the animal kingdom is so incredibly variable. Closely related species can have very different lifespans. For example, the little brown bat lives six times longer than the evening bat. The Canada goose can live over 23 years, while the emperor goose only lives around six. Even within species, there can be surprising differences. Queen bees can live for years, while genetically identical worker bees survive just a few months.
Are aging and death genetically programmed?
A big debate in evolutionary biology has been whether there is a biological trigger for aging. Programmed theories of aging suggest that aging occurs on a fixed schedule triggered by genetic factors. Programmed death is an attractive theory for scientists because it indicates that there could be a genetic switch that turns on aging. If we were able to find the switch and prevent it from being activated, we could stop aging. Recent findings that the lifespans of animals in labs can be manipulated genetically seemed to support this theory. However, while the majority of evolutionary biologists accept that genes play a role in aging, they don’t support the theory of programmed death. There are several convincing arguments against programmed death, one of which is that aging and death are highly variable across individuals, while other genetically controlled processes such as growth and development are much more predictable.
Is death the price that we pay for sex?
Reproduction seems to be a key influencing factor in aging. Species that can reproduce asexually appear less affected by aging, while sexual reproduction seems to be linked to aging. Some species of bamboo can live for 120 years reproducing asexually but die soon after sexual reproduction. Some species of salmon are semelparous, which means that they spawn (reproduce) a single time and then die shortly thereafter.
A sockeye salmon going to spawn (Photo by Ingrid Taylar)
Aging is likely to be a trade-off between survival and reproduction. Semelparous species like some salmon and marsupial mice invest so much into a single burst of reproduction that they can’t survive for long afterward. Through natural selection, reproduction is prioritized over maintenance of the body.
Aging is probably a side-effect of natural selection
J. B. S. Haldane realized that the force of natural selection declines as an individual reaches and later exceeds reproductive age. Haldane was trying to understand why the genetic mutation that caused Huntington’s disease was not eliminated by natural selection. The debilitating disorder commonly begins to manifest when carriers of the mutation reach their late thirties and early forties, often after they have had children and passed on the disease to the next generation. Like aging, Huntington’s is a late-onset trait, that usually only appears after successful reproduction. Natural selection occurs because traits that enhance survival and reproduction are passed on to future generations. Late-onset traits do not affect natural selection because they appear too late to make a difference.
Evolution doesn’t care about you past your reproductive age. It doesn’t want you either to live longer or to die, it just doesn’t care.— Felipe Sierra
Haldane’s insight led to the evolutionary theories of aging supported by the majority of biologists in the field today. They view aging as a form of evolutionary negligence. Deleterious genetic mutations that are only expressed late in life are ignored by natural selection and accumulate, leading to aging and age-related disease. Worse yet, some genes might have a dual function, beneficial at a young age (e.g. enabling reproduction) but damaging at older ages (e.g. disease). Natural selection would favor the benefits early in life while ignoring the disadvantages late in life.
From an evolutionary perspective, the body is disposable
Natural selection favors traits that promote reproduction, which may lead to trade-offs when allocating energy to other functions of the body, such as repair and maintenance. Thomas Kirkwood’s disposable soma theory suggests that the body is “disposable”, needing to last just long enough to reproduce. The semelparous salmon, whose body begins to shut down shortly after spawning, lends credibility to this argument.
As we go about our lives, our bodies are subject to many sources of damage: cuts, scrapes, exposure to sunlight, infection by bacteria and viruses, random DNA mutations, and many more. The body has natural maintenance and repair mechanisms to deal with this damage. But it appears that these mechanisms are not perfect. As we evolved, they only had to be good enough to survive to reproductive age, pass on our genes, and care for our children. Our current understanding suggests that aging most likely results from a continuous accumulation of randomly occurring damage (e.g. DNA damage, stiffening of the arteries, accumulation of plaques in the brain) resulting from interactions with the environment or natural chemical reactions within the body. There is no genetic switch that sets a programmed aging process into motion. Aging is a side effect of our imperfect repair and maintenance. It is a long, slow process of deterioration resulting from the accumulation of damage that occurs naturally throughout life.
If aging is the result of damage, it has implications for the treatment of aging and further extension of human lifespans. We are unlikely to find a simple genetic switch that allows us to turn off the aging process. We will need to find ways to maintain the body for longer by preventing and reversing the damage that accumulates throughout life.