Biological Aging Theories
Simple Deterioration Theories
Many people believe that biological aging is simply the result of universal deteriorative processes such as oxidation or wear and tear that cause aging in machinery, exterior paint, and other inanimate objects. These theories are superficially attractive if only human aging is considered but fail if life span characteristics of other species are also examined.
Some might say that entropy requires increasing disorder and therefore deterioration. However, entropy can be reversed by application of energy and living organisms routinely reverse entropy using food energy in order to grow and maintain the condition of adult organisms.
There is no question but that
many aspects of aging look like the accumulation of damage. Examples are
oxidative damage, mutations, and the protein cross-linkages that cause our
collagen to lose flexibility with age. But the essential mystery is why the
body is able to avoid these problems for many decades, but then permits the
damage to occur in old age. And why do some animals age so much more slowly
than other, very similar animals? (Mouse-sized naked mole rats live 30 years, while mice
live 2 years.) These are reasons that aging requires a more complex
Few scientists still believe in simple deterioration or "accumulated damage" theories although deteriorative processes such as oxidation and other molecular damage are part of most aging theories. See longer description of wear and tear theories.
Evolutionary Theories of Aging
Examination of life span characteristics of different species quickly revealed that life span was a characteristic that was extremely unique to individual species. In fact, there are species that are essentially identical but have very different life spans (such as different varieties of salmon). Life span must therefore be part of an organism's design or at least determined as a result of (a dependent property of) some aspect of evolved organism design. Evolution theory tells us how organisms acquire their design characteristics. Therefore scientists turned to evolution theory and developed evolutionary theories of aging in their efforts to explain human aging. Understanding the current status of aging theory therefore requires understanding the current situation surrounding evolution theory.
The theory of evolution as described by Darwin in 1859 consisted of two distinct parts:
Species descendency theory -- says current species are descended from earlier, different, species.
Evolutionary mechanics theory -- describes how the evolution process works and defines the types of organism characteristics that can and cannot develop via the evolution process, e.g. survival of the fittest.
Evolution Theory Controversies and their Effect on Aging Theories
Everyone is aware that there have been religious objections to evolution theory ever since its introduction in 1859. Currently there is no scientific disagreement with the idea that evolution of life on Earth has in fact occurred (the evidence for this part of Darwin's theory is overwhelming). Religious objections and resulting pseudo-scientific arguments against evolutionary descendency do nevertheless affect science by introducing confusion and social opposition.
However, most people are not aware that there is significant scientific disagreement regarding details of evolutionary mechanics. This disagreement results from apparent conflicts between some observations of living organisms and the traditional mechanics theory (See Evolution Theory Problems). In particular, modern discoveries in genetics led to genetics issues with traditional mechanics. These conflicts resulted in the eventual development of multiple alternative evolutionary mechanics theories. It turns out that seemingly arcane and academic details of the evolutionary mechanics theory logically essentially determine the choice between programmed and non-programmed aging theories, the two classes of evolutionary theories of aging!
Traditional evolutionary mechanics mandates non-programmed (non-adaptive) aging.
Empirical evidence and alternative mechanics theories favor programmed (adaptive) aging.
Scientists agree that survival of the fittest (natural selection) is central to the evolution process but have been arguing about finer details for 150 years. The endless academic arguments have been ignored by society at large but are now potentially significantly impeding medical research and need to be resolved. As with the descendency theory, religious opposition and pseudo-science (e.g. intelligent design) produce confusion surrounding evolutionary mechanics.
For a more extensive discussion of evolutionary mechanics theories and their impact on aging theories see: Evolution Controversies and the Theory of Aging.
History of Evolutionary Mechanics Theories and Dependent Aging Theories
The historical sequence in which aging theories were developed is important to understanding the current situation. The non-programmed theories originated during a time when there were no scientific alternatives to traditional evolutionary mechanics theory. Consequently, traditional mechanics was a "given" in developing these theories. Since then, three different categories of alternative mechanics theories that support programmed aging have been developed and extensive new observational evidence favoring programmed theories or adding to observed conflicts with traditional mechanics has been discovered. See Aging Theory Timeline.
Peter Medawar, a famous and eventually Nobel-prize-winning British zoologist, published an idea in 1952 that is important to all subsequent evolutionary theories of aging. He suggested that evolutionary force toward achieving a longer life span decreases following the age at which the organism is first capable of reproducing. Although theorists now disagree regarding details of Medawar's hypothesis everybody agrees that an organism that died of old age prior to reaching puberty would not make logical sense and that therefore Medawar's hypothesis has at least some validity. Medawar's hypothesis was widely embraced because it provided a major explanation for the gross variation in life spans seen in animals. Life spans of various species do correlate loosely with age of sexual maturity. The age at which Medawar's hypothesis would become active varies with species details. Some species would need to live longer in order to protect, nurture, and rear their young.
Non-Programmed Aging Theories
Non-programmed aging theories are based on traditional evolutionary mechanics theory and Medawar's hypothesis, which together hold that organisms can and do evolve myriad complex biological mechanisms directed at achieving a life span at least somewhat beyond age of reproductive maturity. Since traditional theory does not support the idea that a limited life span, per se, could convey evolutionary benefit, non-programmed theories do not support the evolution of a mechanism (i.e. program) that pro-actively limits life span.
Some non-programmed theories contend that aging is an unavoidable adverse side-effect of some beneficial function. Because of Medawar's hypothesis, a beneficial function that contributed to an animal's early life in even a minor way could offset even catastrophic disadvantage (e.g. death of old age) in later life.
Non-programmed theories compete with each other, have apparent logical flaws, and have difficulty in explaining many observations.
The following articles contain descriptions of each of the principal non-programmed (non-adaptive, passive) theories of mammal aging including discussion of their apparent logical flaws:
Developmental (DevAge) or Life-history theories of aging contend that aging is an adverse side-effect of the development or growth process. From an evolutionary viewpoint these are versions of "aging is an adverse side-effect of some beneficial function."
Programmed Aging Theories
Programmed (adaptive) aging theories are based on one or another of the alternative evolutionary mechanics theories, which allow for an evolutionary benefit in limiting an organism's life span to a species-specific value also loosely based on Medawar's criteria. An organism needs to achieve a species-specific life span but also needs to limit its life span. According to this thinking, an organism might have myriad evolved mechanisms for achieving the life span suggested by Medawar's hypothesis but can also evolve complex biological mechanisms for pro-actively limiting life span to that value. Programmed theories fit observations better than non-programmed theories. These are the programmed aging theories listed by underlying alternative evolutionary mechanics theory:
Proponents of non-programmed theories have produced Objections to Programmed Aging Theories.
Living organisms possess extensive maintenance and repair capabilities. These capabilities are central to programmed and non-programmed Maintenance Theories of Aging.
Aging Theories and Medical Research
Programmed and non-programmed theories of aging lead to very different concepts regarding biological mechanisms of aging, which in turn lead to very different approaches in attempting to prevent and treat age-related diseases. See: Medical Implications of Aging Theories.
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