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) 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. Entropy does not "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).
Problems with Wear and Tear Theories of Aging
It was soon obvious that simple deterioration could not explain the life span observations if many different species were examined. In 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 100:1 (between Argentine desert mouse and human). Why would such very similar molecules 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 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 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.
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.
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