Summary: Why Do We Age? Scientists answer the question with the latest evolutionary theory of aging, the disposable soma theory, which supports the longevity benefits of calorie restriction. With commentary by leading geroscientist, Tom Kirkwood. [This article first appeared on the website LongevityFacts.com. Author: Brady Hartman. ]
The process of aging, or growing old, presents an apparent contradiction to people who believe in Darwin’s theory of evolution. Aging increases the vulnerability of an organism, which ultimately leads to its death. How could evolution favor a process that gradually increases mortality and decreases the ability to reproduce?
Leading scientists have found an answer to this puzzling contradiction and offer us new theories to explain why we age and die using evolutionary theory – the idea that aging confers an evolutionary advantage.
The newest theories of aging – such as the Antagonistic Pleiotropy and Disposable Soma Theories – are based on old concepts. August Weismann – a German biologist and one of the founders of the science of genetics – was one of first to argue that aging conferred an evolutionary advantage. In 1891, Weismann initially proposed that aging benefitted the species by replacing worn out individuals with younger ones, a theory known as group selection. Weismann dropped this concept later and suggested instead that aging evolved because organisms must invest additional resources to reproduce instead of maintaining their bodies.
Antagonistic Pleiotropy Theory
Origins of the Antagonistic Pleiotropy Theory
In 1957, George C. Williams came up with the origins of the idea of antagonistic pleiotropy – a theory explains aging from an evolutionary perspective. Williams was an American evolutionary biologist and a professor of biology at the State University of New York at Stony Brook and is best known for his vigorous critique of the theory of group selection. Professor Williams argued that since natural selection is weaker at later ages, then perhaps some genes are beneficial at earlier ages but harmful at later ones. George Williams called these pleiotropic genes, a term for genes with opposite effects. Antagonistic pleiotropy occurs when a gene that is beneficial to survival in early life becomes harmful later.
Many genes don’t operate according to antagonistic pleiotropy but age us none the less. For example, adolescents enjoy an upsurge in hormone levels from puberty until they are fully developed. This upsurge is beneficial to growth, but ends after full development, because, if left unchecked, these hormones could cause cancers.
Professor Williams argued that gene which increases survival to and during reproductive age will be favored by natural selection. Moreover, harmful late-acting genes can remain in a population if they are beneficial early in life, by increasing fitness at early ages or increasing reproductive success.
We are programmed by evolution to stay alive and healthy decades or more after we reach sexual maturity. However, evolution doesn’t care if a gene causes harmful effects in our senior years after we have successfully reproduced.
For example, male peacocks display feathers that are crucial to attracting females and reproducing. These feathers are handicaps in later life, hindering the ability of the males to escape predators.
Antagonistic Pleiotropy Theory Gets a Name
In 1991, Michael R. Rose, an evolutionary biologist, improved on William’s work, and coined the phrase ‘antagonistic pleiotropy.’ Michael Rose, Ph.D. is a Professor in the Department of Ecology and Evolutionary Biology at the University of California, Irvine. Dr. Rose’s primary area of work is the evolution of aging, approached both theoretically and empirically via the technique of experimental evolution. To prove his theories, Rose ran experiments with fruit flies. Dr. Rose observed that by breeding only longest-lived flies in each generation, he could produce longer-lived editions of the fruit fly. Rose quadrupled the original lifespan in some flies.
Rose used the flies to test the theory of antagonistic pleiotropy. Based on the theory, Dr. Michael Rose predicted that the long-lived flies would have much lower fertility in the earlier parts of their life and that their peak reproduction would occur later in their lifespans. The prediction of Dr. Rose proved wrong, and the experiments showed that the long-lived flies produced more eggs at every stage of life. However, observers attribute this increased fertility to the fact that the flies lived in an environment conducive to fertility – a warm and protected laboratory. The observers speculated that if these flies were exposed to the natural elements, they might just revert to the regular and less long-lived type. Moreover, the long-lived lab flies displayed weaknesses that would have made them poor survivors in the wild. Faced with these facts, Rose developed the theory of late-life immortality, the antithesis of antagonistic pleiotropy.
Rose also proposed the controversial idea that aging can stop in the oldest of humans, usually after 93 years of age. Rose based this concept on studies of women over 93, in which some stopped aging. Of course, all the women eventually died. Dr. Rose proposed that if natural selection gradually comes to an end, then aging should also. At some point, the aging process seems to no longer be necessary and stops. According to Rose, very old people do not die of aging, or of aging diseases, but they just wear out. This controversial idea has been put to the test, and experiments seem to suggest that aging seems to stop once the forces of natural selection peter out.
The Disposable Soma Theory
Proposed in 1977 by Thomas Kirkwood, the disposable soma theory is one of the few mainstream theories of aging. The theory presumes that the body must budget the amount of energy available to it, allocating its resources to metabolism, reproduction, as well as repair and maintenance. Kirkwood’s disposable soma theory argues that organisms maintain a balance between the resources they invest in maintenance of the body and reproduction.
The disposable soma theory came into existence when Thomas Kirkwood took up Weismann’s ideas and proposed that we age and die because we have outlived our reproductive usefulness. Thomas B.L. Kirkwood CBE is an English biologist who made his contribution to the biology of aging by proposing the concept of the disposable soma. Kirkwood is currently the Associate Dean for Ageing in Newcastle University and heads up the Institute for Ageing and Health in its school of clinical medical sciences.
The disposable soma theory is a refinement to the evolutionary theory of aging and argues that there is a trade-off between reproduction and maintenance of an aging body. The disposable soma theory argues that all living things are optimized to carry their genetic code to the next generation. However, that need is balanced with the expenditure of resources used to maintain the body or so-called soma.
Kirkwood and Austad updated the disposable soma theory in a 2000 paper, arguing that aging occurs due to the accumulation of damage during life due to inadequate repair mechanisms. Instead of investing costly resources in repair, the body invests more in reproduction.
Can’t Dispose of the Disposable Soma Theory
Evidence supporting the disposable soma therapy comes from animals that die almost immediately after reproducing. However, many animals maintain their bodies after reproduction, except not to the level that ensures immortality, showing that humans, weren’t born to simply to pass on our genes.
Disposable Soma and Fighting the Aging Process
Thomas Kirkwood does not regard the disposable soma concept as an impediment to the goal of exceeding the maximum human lifespan. In fact, Kirkwood believes this evolutionary weakness can be turned to our advantage. In Kirkwood’s view, geroscientists can overcome this force of nature by identifying the underlying factors that make us disposable in the first place and then manipulate them. This is along the lines of addressing the nine hallmarks of aging, and conquering them one at a time or all at once.
“The ageing process is driven by the gradual lifelong accumulation of faults in the cells and tissues of our body,” says Professor Kirkwood, who continues “That helps us to understand the relationship between ageing and age related diseases. For many of these it’s an accumulation of molecular and cell damage that underpins the diseases.” Kirkwood adds “It also helps us to understand why things like healthy nutrition and exercise can be beneficial for health in old age, because they are all about enhancing the body’s capacity to maintain and repair itself.”
Kirkwood believes that if we could improve the maintenance of our bodies, we could significantly slow down the aging process. Aubrey de Grey is more optimistic and believes that if science could maintain youthful repair and maintenance processes, then we could delay aging, possibly forever.
Scientific advancements have already extended our average lifespans by a significant amount, as modern medicine and improved sanitation practices enable humans to live much longer than their period of reproductive usefulness.
Calorie Restriction and Disposable Soma Theory
Proponents of the disposable soma theory suggest that the practice of calorie restriction or intermittent fasting invokes a protective response, which causes our bodies to hunker down and invest more resources in maintenance as opposed to growth and reproduction.
The fact that calorie restriction extends lifespan ties in well with the disposable soma theory. In periods of food shortage, reproduction is curtailed, because it carries a heavy energy cost. The disposable soma theory predicts that lifespan should increase when reproduction is deferred or eliminated. Reproduction is expensive regarding bodily energy consumption and uses resources that could be better employed for maintenance and repair of the body, such as repairing DNA damage or restoring lost tissues using stem cells.
The disposable soma theory seems to make the most sense. It blends in well with the damage theories, such as the network theory of aging. The fact that it goes well with these theories is no coincidence, after all, they are both championed by Tom Kirkwood.
Geroscientists such as Kirkwood, Aubrey de Grey, Felipe Sierra and many others believe that we could significantly slow down aging and improve the maintenance of our bodies.
At present, calorie restriction seems to be the only way to extend lifespan. Plenty of evidence shows it works in lab animals. However, we don’t know if it extends lifespan in humans.
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