The aim of this chapter is to review and to assess our present knowledge of the biological processes that govern human ageing. In any such undertaking the nature of the relationship between ageing and evolution is a key consideration, and several basic questions need to be addressed. For example, should ageing be regarded as part of an overall developmental programme and, if so, can specific genes be identified which encode ageing? Further, if this is the case, did senescence evolve as a direct result of natural selection in order to limit lifespan?

It has been claimed that ageing is beneficial in permitting and/or promoting the turnover of generations. But until the present century only a very small proportion of humans ever succeeded in approaching the biblical age of three score years and ten. Therefore, it is difficult to perceive why genes coding for ageing, which in practice would seldom have been expressed, should have been preserved through evolutionary time. One possibility is that they could be pleiotropic in their action, with beneficial effects exerted when the individual is young or in early, reproductive adulthood. Under such conditions a small selective advantage acting early in life, when many individuals potentially would benefit, could balance detrimental late-acting outcomes in the much smaller number of persons who survive to older ages, a process known as antagonistic pleiotropy.

Despite the intellectual attraction of this line of argument, non-adaptive theories—which primarily regard senescence as an evolutionary by-product—tend to be more strongly favoured, since they are simpler in construct and do not presuppose the need for specific causative genes that drive the multifaceted ageing process. Nevertheless, this theoretical dichotomy forms the basis for many of the divisions in rationale between theories that have been advanced to explain the ageing phenotype, even though belief in the existence of a single 'magic-bullet' cause of ageing must be regarded as fallacious in any complex biological species.

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