Programmed Aging Theory Information Disposable Soma Theory of Aging

 

In 1977, a statistician named Thomas Kirkwood (now a biologist and professor of medicine at the University of Newcastle) published his disposable soma theory of aging [1, 2]. Kirkwood’s idea was that organisms only have a limited amount of energy that has to be divided between reproductive activities and the maintenance of the non-reproductive aspects of the organism (soma).

 

The disposable soma theory is one of those based on the idea (Medawar's and Williams' earlier modifications to Darwin's survival-of-the-fittest idea) that the evolutionary value of additional life declines following the age at which an organism achieves reproductive capability. This concept in turn suggests that aging might be the result of a tradeoff between some quality that produces a benefit in an organism’s youth (in this case reproduction) that happens to be rigidly linked to a quality that only causes evolutionary disadvantages in old age (aging). Rigid linkage means that the evolution process cannot work out a way to achieve the benefit without incurring the adverse side-effect (aging). See Evolutionary Value of Life. The disposable soma theory requires a belief in the modifications to Darwin's theory made by Medawar and Williams (1952 – 1957) but does not require accepting any of the post-1962 population benefit theories that in 1977 did not exist or were widely rejected.

 

Problems with the Disposable Soma Theory

 

It is widely accepted that indeed many deteriorative processes exist in nature and that organisms do expend much energy and material resources in combating these processes. Dead cells are replaced, hair and nails grow, infections are combated, and wounds are healed. However, disposable soma requires a subtle assumption: that an organism can reduce maintenance and repair at time t but the adverse fitness effects will not occur until some later time t + ∆t. The energy and resources associated with organism maintenance seem to be largely very short-term in nature. For example, in humans, hair, nails, epithelial cells, and red blood cells are frequently replaced (weeks).  If we added up the weight of all of these items produced during an average human life span it would likely exceed the weight of the person. On the other hand, nerve and brain cells are very infrequently replaced. The total mass of material and associated energy associated with replacing these cells or other long-term maintenance is therefore negligible by comparison. Our need for sleep (widely seen as associated with maintenance) is very short-term. These observations make it extremely difficult to see how there could be a tradeoff between the major maintenance and repair effort (short-term) and aging (long-term). A young animal cannot afford to cease short-term maintenance and repair because doing so would damage its ability to reproduce. Ceasing long-term maintenance and repair (to the extent that it exists) would not save significant resources. The disposable soma concept works best in cases where the beneficial reproductive effect is immediately prior to mortality such as in the case of the semelparous male marsupial mouse that is said to fatally exhaust itself in extremely extended copulation.

 

Some organisms are not observed to age and yet are able to continue reproducing. Some display increased reproductive capacity with age. These observations obviously conflict with the idea that there is some unavoidable biological tradeoff between reproduction and aging.

 

Modern Programmed Aging Theories

 

Programmed (or adaptive) aging, the idea that organisms possess a mechanism that purposely causes deterioration and death in order to obtain an evolutionary advantage, was originally proposed in 1882 but almost universally rejected because of conflicts with Darwin’s survival-of-the-fittest concept. Beginning in the 1990s modern programmed aging theories reemerged with a substantial theoretical basis. This basis was in turn the result of many intervening discoveries (mainly in genetics) that altered the way we think about the evolution process. Programmed theories are based on Medawar’s and Williams’ ideas regarding the declining value of life and in addition, on the idea that aging, per se, creates a population benefit that compensates for its individual disadvantage. Limiting the lifespans of individual members of a population increases the probability that the population will avoid extinction.

 

Kirkwood is still a vocal proponent of non-programmed aging and his own disposable soma theory and published a paper in 2011 attacking programmed aging theories and other non-programmed theories [3]. Programmed aging proponents were surprised to see that this article essentially concedes the evolutionary basis for programmed aging by conceding that non-individual benefit can influence the evolution process. It also concedes that "circumstances could exist" in which aging could be programmed and further, does not argue against the specific circumstances proposed by specific programmed aging theories. See On the Programmed/ Non-Programmed Aging Controversy [4] for specific counter-arguments to the 2011 paper. V. Skulachev also wrote an article [5] responding to Kirkwood's article and making arguments for programmed aging.

 

Programmed Disposable Soma Theory

 

The disposable soma theory essentially requires a program or a biological mechanism that would cease or down-regulate myriad different maintenance and repair mechanisms at the appropriate age for a particular species living in particular circumstances. This article [7] explains why such a program would be very similar to the programs suggested by programmed aging theories. The nature of the aging program, whether “adaptive” or “not adaptive,” is obviously critical to research on highly age-related diseases and conditions. The "programmed disposable soma theory" leads researchers in the same directions as "programmed aging" theories!

 

Programmed aging is increasingly seen as the correct theory and there are now substantially funded research activities based on programmed aging including the NIH/NIA Interventions Testing Program and the Google Calico/ AbbVie Aging Research Program both of which are searching for anti-aging agents.

 

1. Kirkwood TB. Evolution of ageing. 1977. Nature 170(5635) 201-4

2. Kirkwood T.B.L. & F.R.S. Holliday. The evolution of ageing and longevity. 1979. Proceedings of the Royal Society of London B 205: 531-546

3. Kirkwood T, Melov S. On the Programmed/Non-Programmed Nature of Ageing within the Life History. Current Biology 21, R701-7  2011

4. Goldsmith T. On the Programmed/ Non-Programmed Aging Controversy 2012. Biochemistry (Moscow) Phenoptosis 77-7. (A programmed aging response to 2011 Kirkwood article above.)

5. Skulachev V. 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").Aging (Albany NY). 2011 Nov;3(11):1120-3.

6. Goldsmith T. Arguments against non-programmed aging theories. Biochemistry (Moscow) 78:9 971-978 2013

7. Goldsmith T. Emerging Programmed Aging Mechanisms and their Medical Implications. Med Hypotheses. 2015 Oct 23. doi: 10.1016/j.mehy.2015.10.015

 

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