Modern theories of biological aging in humans and other mammals fall into two categories: Programmed theories, also known as adaptive or active theories contend that organisms are designed to have a genetically programmed limited life span because aging and consequently limited life span serves some evolutionary purpose. According to this concept, aging is a necessary biological function, and like other biological functions can be directed by a complex life span regulation system involving hormones, signaling, genes, sensing of external conditions, and other characteristics typical of evolved biological functions. A life span longer than some species-specific value conveys disadvantage and therefore produced evolutionary motivation to develop the aging function or other means for purposely limiting life span.
Non-programmed theories, also known as non-adaptive or passive theories, contend that aging does not serve a valid "selectable" evolutionary purpose and that therefore an evolved mechanism whose primary purpose is to cause aging or otherwise purposely limit life span can not exist, at least not in mammals. These theories contend that aging is an unavoidable adverse side-effect of some useful biological function or that organisms do not have an evolutionary need to live longer than some species-specific life span and therefore did not develop or lost the maintenance and repair capabilities needed for living longer. A life span longer than a species-specific value conveys little benefit but no disadvantage. See: Evolutionary cost/benefit of life span.
According to both categories of aging theories, age of initial reproductive capability is an important factor in determining life span. Both, unlike earlier generic deterioration, "entropy", or "wear and tear" theories of aging, provide an explanation for the very wide variation in life spans (~100:1) seen in different mammal species despite their very similar biochemistry. Both categories also recognize that deteriorative processes cause deterioration of any organized system but that living organisms have extensive capability for maintenance and repair of damage. There are therefore versions of maintenance and repair theories of aging to fit both categories.
In general, programmed theories provide a substantially better fit for various organism observations beyond the inter-species life span variation (see Summary Theory Comparison).
Whether aging and limited life span theoretically can or cannot have a directly useful evolutionary purpose that could result in selection of an evolved mechanism that purposely limits life span hinges on evolutionary mechanics theories, that is, theories that describe how the evolution process works. Traditional evolutionary mechanics theory (see Glossary) mandates non-programmed aging. Multiple newer alternative evolutionary mechanics theories support programmed aging and suggest that a purposely limited life span has multiple selectable evolutionary benefits.
Traditional theory and consequent non-programmed aging theories predate the alternative evolutionary mechanics theories and modern programmed aging theories (see Aging Theory Historical Timeline and Detailed Chronology).
The programmed vs. non-programmed aging controversy has existed in some form for 150 years! Non-science factors have contributed to the persistence of this issue. The first formal programmed aging theory was proposed by August Weismann in 1882.
Recent developments: In 2011, Tom Kirkwood, author of the non-programmed disposable soma theory (1975) and vocal critic of programmed aging theories published a paper with S. Melov titled On the programmed/ non-programmed nature of ageing within the life history attacking various programmed aging theories and their authors. Programmed aging proponents responded with articles strongly criticizing the Kirkwood-Melov article and summarizing the case for programmed aging: These include: On the programmed/ non-programmed aging controversy (PubMed) (T. Goldsmith 2012) and Aging as a particular case of phenoptosis, the programmed death of an organism (A response to Kirkwood and Melov "On the programmed/non-programmed nature of ageing within the life history") (V. Skulachev 2011). See also Aging by Design (2012) for a summary of issues discussed in these papers.
This controversy has significant public health implications because the two categories of aging theories suggest very different ways of approaching the treatment and prevention of age-related diseases such as cancer and heart disease that now cause about 75 percent of all deaths in developed countries. See Medical Implications of Aging Theory Controversies.
See Aging Theories for descriptions of programmed and non-programmed aging theories and their respective underlying evolutionary mechanics theories. See also: Aging Theory Frequently Asked Questions.
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