Wear and tear theories of biological aging propose that aging in humans and other animals is simply the result of universal deteriorative processes that operate in any organized system. According to these theories, humans age for the same reasons and because of the same processes that cause aging in automobiles and exterior paint. These theories (also called simple deterioration theories or fundamental limitation theories) are attractive to many people who are mainly concerned with human aging but have severe problems as described below.
If we were to buy a new automobile or other complex machine, we might expect some immediate problems actually referred to as "infant mortality." Following this period we would expect more or less trouble-free operation for a considerable period, the "service life." After that time we would expect many frequent problems and also notice that the problems were more similar. All the cars have deteriorated exterior paint. They all suffer from corrosion and mechanical wear. The sort of experiences we describe here are very similar to the experience of human aging. In fact, we use the same word, "aging," to describe gradual deterioration in humans, automobiles, or exterior paint, and many people think of biological aging as a "wearing out" process. Notice that in addition to mechanical wear, accumulation of oxidation and other chemical (molecular) damage are included in the "wear and tear" concept.
Entropy is often cited as "requiring" aging. Stochastic theories of aging similarly propose that aging is the result of inevitable small random changes that accumulate with time. Entropy, in this context, refers to the tendency of all matter to decay into a more random, less ordered state. For example, we can dig iron oxide out of the ground, smelt it into elemental iron, and use the iron to build a highly complex organized structure like a ship. If we then abandon the ship, it will eventually return to its original un-ordered, un-structured state: a large lump of iron oxide buried in the ground.
Darwin's survival-of-the-fittest concept says that species evolve characteristics (or traits) that help them to live longer and reproduce more but it is clear that deterioration and death from aging do not aid mammals in living longer and reproducing more so why do animals age as opposed to being immortal? An obvious answer is that immortality is impossible because it violates some law(s) of physics or chemistry and that therefore the evolution process cannot produce longer lifespans. In other words, Darwin's idea tends to support the idea that aging is the result of fundamental limitations.
Entropy does not actually "require" aging because entropy can be reversed by the application of energy. In the above example, we used energy to smelt the iron and construct the ship. Nature uses energy to grow and maintain complex structured organisms made from un-ordered raw materials. Nature can, and in some cases does, use energy to maintain living organisms indefinitely (see more below).
Despite decades of effort it eventually became clear that simple deterioration and universal fundamental limitations could not explain the life span observations if many different species were examined. In biochemically very similar species such as mammals, the cellular and molecular basis of the organism is very similar and yet mammal life spans vary over a range of more than 200:1 between Argentine desert mouse (< 1 year) and Bowhead whale (> 200 years). Why would such very similar molecules in these species deteriorate at such different rates? Even if we compare organisms of similar size, design, activity, and metabolism (parrot and crow) large differences in life span (6:1) are apparent.
Another problem is that, unlike automobiles and paint, living organisms are known to have very extensive maintenance and repair capability. Nails and hair grow, wounds heal, and dead epithelial cells are replaced. Why would these activities necessarily be limited? If a human can maintain itself for 80 years, why not a mouse? The obvious maintenance activities described here are all relatively short-term in nature (weeks). What is stopping a mouse or human from continuing those activities longer?
Some organisms apparently do not age (see Negligible Senescence). How do they avoid the supposedly inescapable deterioration?
Some organisms, rather than dying from gradual aging, die suddenly following reproduction. Wear and tear does not provide an explanation for limited life span in these species.
Because of these major problems, few biologists currently believe in simple deterioration or wear and tear theories of aging.
Note that while many scientists believe oxidation and other molecular damage are implicated in the aging process they also believe that other factors are involved in determining whether that damage is or is not repaired. In summary, the simple deterioration theories are too simple to explain the multi-species observations and a more complex explanation is needed. This led to the development of more complex aging theories. All of the modern theories involve modifications to Darwin's survival of the fittest concept!
In 1952 Sir Peter Medawar originated the now generally accepted idea that the force of evolution declines following the age at which an organism can complete a first reproduction because of attrition under wild conditions. For example there would be essentially zero evolutionary force toward having the internal ability to live longer than age X if essentially zero individuals live longer than age X in the wild because of external limitations like predators, intra-species combat, lack of food or habitat, harsh environmental conditions, or infectious diseases. This idea led to modern non-programmed aging theories to the effect that each species only evolved the necessary maintenance and repair capabilities needed to support living to a particular age that was dependent on age-at-puberty, predation, and other species-unique factors. This idea provides a good match to the huge differences in observed species lifespans.
Beginning in 1962 multiple population-benefit theories of evolution appeared to the effect that an organism trait that increased the ability of a population to avoid extinction could evolve despite being somewhat adverse to the survival and reproduction of individual members of that population. These theories provided an explanation for observations such as animal altruism and sexual reproduction that appeared to be incompatible with traditional survival-of-the-fittest evolutionary mechanics theory. By 2005 many population benefits of a purposely limited lifespan were proposed leading to modern programmed aging theories to the effect that most organisms including humans possess suicide mechanisms or aging programs that purposely limit their lifespans by causing or allowing the observed age-related diseases and conditions. Because of Medawar's modification, there existed an age at which the benefit of living longer was outweighed by the population benefit of limiting individual lifespan.
Programmed and non-programmed theories have dramatically different implications regarding the nature of aging and age-related diseases and therefore on medical research. There is no current scientific agreement regarding modern aging theories or underlying evolutionary mechanics theories. Many health researchers continue to follow the earlier non-programmed theories but substantial research based on programmed theories is now underway. see Programmed Aging Medical Research.
Proliferation of Evolutionary Mechanics Theories
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