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Biological Aging Theories

Simple Deterioration Theories

Many people believe that biological aging is simply the result of universal deteriorativeprocesses 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 organismsroutinely 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 ofdamage. 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 bodyis able to avoid these problems for many decades, but then permits the damage to occurin 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 explanation.

Few scientists still believe in simple deterioration or "accumulated damage" theoriesalthough deteriorative processes such as oxidation and other molecular damage arepart

of most aging theories. Seelonger 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

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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 evolvedorganism design. Evolution theory tells us how organisms acquire their design

characteristics. Therefore scientists turned to evolution theory and developed

evoluti onary theories of agingin their efforts to explain human aging. Understanding thecurrent status of aging theory therefore requires understanding the current situationsurrounding evolution theory.

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 worksand defines the types of organism characteristics that can and cannot develop viathe 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 sinceits introduction in 1859. Currently there is noscientific disagreement with the idea thatevolution 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 significantscientific disagreement

regarding details of evolutionary mechanics. This disagreement results from apparent

conflicts between some observations of living organisms and the traditional mechanicstheory (SeeEvolution Theory Problems). In particular, modern discoveries in genetics

led togenetics issues with traditional mechanics. These conflicts resulted in the

eventual development of multiple alternati ve evoluti onary mechanics theor ies. It turnsout 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!

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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 evolutionprocess 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 onaging 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 atime 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 supportprogrammed aging have been developed and extensive new observational evidence

favoring programmed theories or adding to observed conflicts with traditional mechanicshas been discovered. SeeAging Theory Timeline.

Medawar's Hypothesis

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 decreasesfollowing the age at which the organism is first capable of reproducing. Although

theorists now disagree regarding details ofMedawar's hypothesiseverybody agrees that

an organism that died of old age prior to reaching puberty would not make logical senseand 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

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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 theoryand 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-activelylimits 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:

Mutation accumulation theory Antagonistic pleiotropy theory Disposable soma theory

Developmental (DevAge) or Life-history theories of aging contend that aging is anadverse 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."

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