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We age because we grow

Initially posted 18 May 2009 by evomed

In Chapter 5 of Principles of Evolutionary Medicine, we discussed the evolutionary biology of aging, and in Chapter 7 we explored the origins of one particular manifestation of aging in humans: the menopause. Among several hypotheses to explain aging, Tom Kirkwood’s idea of the disposable soma proposes that it is energetically less costly (and therefore evolutionarily favoured) to build new organisms from the cells of the germ line (in other words, to reproduce) than to continue to invest in maintaining the soma (the non-reproductive tissues) of the parent organism, particularly if extrinsic mortality (the risk of events such as predation, disease or accident) is high. Somatic tissue will therefore deteriorate from lack of maintenance, leading to senescence and death.

Anthropologist Hillard Kaplan and economist Arthur Robson have extended the disposable soma hypothesis to propose a new mathematical model of aging that explains the main characteristics of human demography. Their model characterises investment in bodily growth – which they call somatic capital – in terms of both quantity (the size of the body) and quality (its functional efficiency). Their model shows that it is evolutionarily optimal to build up quantity (in other words to grow large, which increases economic productivity – the amount of energy an organism can harvest from the environment over its lifetime) but to let the quality of most of the cells in the body deteriorate with time (because a bigger body takes more energy to maintain its quality). If the soma and the germ line are separate, the fidelity of reproduction – quality control of the germ line – can be maintained with little energetic cost because of the tiny number of cells in the germ line relative to the soma. The model also incorporates the idea of intergenerational transfers of capital. A simple example of an intergenerational transfer is lactation, by which energy is transferred from mother to offspring, but more broadly such transfers also include any process by which capital can flow from older to younger individuals – and for humans capital can take many forms, not just energy but also skills and knowledge. The human ability to accumulate knowledge over a lifetime and then to transfer that knowledge to the next generation means that peak economic productivity is shifted to an older age compared with other primates, causing selection for longevity – and, in females in particular, for a period of economically active post-reproductive life.

Predictions from the model fit well with observational studies of human foraging populations, allowing Kaplan and Robinson to conclude that their model explains the biological basis of the following features of human demography:

– mortality initially decreases and then increases with age;

– peak female fertility occurs at approximately the time growth ceases, thereafter declining with age;

– female fertility reaches zero at the menopause, with a substantial period of post-reproductive life;

– humans live longer than other primates.


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