What are the main scientific theories of aging? Aging theories fall into two main categories, programmed and non-programmed. A programmed theory of biological aging also known as adaptive aging, active aging, or aging-by-design proposes that organisms are designed to age and have a limited life span and that aging is purposely caused or allowed by a genetic program similar to the one that controls the development and growth of an organism. According to this concept, aging is an adaptation in that it is an organism design feature that was selected by the evolution process because it benefited the organism. Because a limited life span conveys benefit, organisms can possess potentially complex evolved systems that actively regulate life span.
Non-programmed, also known as non-adaptive or passive aging theories contend that aging is the passive result of an organism's inability to better withstand deteriorative processes and that aging has no evolutionary purpose or benefit. Aging is a defect not a feature of an organism's design. Some non-adaptive aging theories contend that aging is an adverse side-effect of some beneficial function such as reproduction or cancer prevention that aids the organism in early life at the expense of aging in later life.
Isn’t aging known to result from generic deteriorative processes such as oxidation, wear, or other accumulated molecular damage? Processes similar to those that cause gradual deterioration in machinery or exterior paint also operate in living organisms. However, it is known that living organisms possess many maintenance and repair functions that act to counteract deterioration. In addition, generic deteriorative processes cannot be the entire explanation for aging because very similar species, possessing very similar biochemistry (with presumably similar exposure to deterioration), have very different life spans. According to one programmed aging concept, the aging mechanism purposely discontinues or slows the maintenance and repair functions at a species-specific age to result in the observed species-specific life spans.
Isn’t programmed aging incompatible with Darwin’s theory of evolution? Doesn’t evolution favor development of characteristics that benefit an organism’s ability to live longer and breed more? Yes, programmed aging is incompatible with traditional evolutionary mechanics and requires an evolutionary mechanics theory that allows for a slightly expanded definition of “benefit.” However, a number of other observed organism characteristics also appear to conflict with classical theory. This led to development of four different types of proposed adjustments to classical evolutionary mechanics theory since 1962. These alternative theories support programmed aging. The alternatives are: group selection theories, kin selection theories, evolvability theories, and gene-centered evolutionary mechanics theories. Note that there is no scientific disagreement with the idea that evolution has occurred and that current species are descended from different earlier species. The scientific disagreement centers around details of how evolution works, the evolutionary mechanisms and processes.
What possible “benefit” could a purposely limited life span produce? Different theorists following the programmed aging concept have developed theories proposing many different benefits in the following broad categories: First, that a purposely limited life span increases the chance for group survival (e.g. kin, herd, larger population, even species) and thereby reduces the chance of extinction. Second, that a limited life span enhances the ability of a species or population to evolve or adapt or allows it to adapt more rapidly to changes in its environment producing a competitive advantage. Aging or other design-imposed life span limitation is obviously adverse from the viewpoint of an individual organism. Theorists generally agree that an organism’s evolutionary need for additional life span declines following the age at which it is first capable of reproduction.
How do programmed aging theories compare to non-programmed theories of aging? Non-programmed (non-adaptive) theories (mostly developed prior to 1962) fit with classical evolutionary mechanics while programmed (adaptive) theories provide a better fit with observational evidence but require one of the newer alternative evolutionary mechanics theories. Recent observations such as “aging genes” have added to supporting empirical evidence and resulted in increased interest in programmed theories. The choice of non-programmed vs. programmed aging theory is consequently essentially a choice between believing traditional evolutionary mechanics theory against observational evidence or believing observational evidence against traditional mechanics but with the support of the newer alternative mechanics theories. There are multiple programmed aging theories that differ in detail and there are also multiple competing non-programmed theories.
Is programmed aging biological suicide? Programmed aging could be considered a form of biological suicide. However, some animals and plants die suddenly following reproduction rather than from gradual deterioration and represent more explicit instances of biological suicide. Examples include octopus, salmon, and even some mammals, the male marsupial mice. Such instances of acutely self-limited life span would also fit programmed aging theories that propose that a species-specific limited life span generally conveys benefit. Some programmed aging theorists contend that gradual aging conveys additional evolutionary benefit relative to sudden death.
What is the history and status of the programmed vs. non-programmed aging argument? At some level this controversy has existed for the 150 years since Darwin’s evolutionary mechanics theory was published in 1859. Prior to Darwin, there was no reason to suspect that life span was a characteristic whose origin differed from that of other characteristics that varied widely between species. The conflict between the survival-of-the-fittest concept and life span observations was immediately noticed following publication of Darwin's book On the Origin of Species.
The first formal programmed aging theory was a “programmed death” theory published in 1882 by German biologist August Weismann. Additional formal programmed aging theories based on group selection and evolvability appeared following publication of those alternative evolutionary mechanics theories in 1962 and 1995.
Recently, the trend has been toward increasing scientific evidence (such as aging genes) supporting programmed aging. Concurrently, scientific confidence in traditional evolutionary mechanics has declined. However, most gerontologists and other medical researchers still believe in non-programmed (non-adaptive) aging.
Why should we care? About 75 percent of all deaths in the U.S. and other developed countries now result from age-related conditions such as cancer, heart disease, and stroke. In effect, aging is the most important cause of these diseases. Understanding the aging process is therefore essential to understanding, preventing, and treating the age-related diseases. Although highly associated with aging, these diseases kill or injure many relatively young people and are the target of much or even most medical and pharmaceutical research. Programmed aging theories suggest different and additional approaches in prevention and treatment of age-related conditions relative to non-programmed theories.
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