of ageing.
There are two complementary approaches to the biology of ageing. In the populational approach, the focus is on the properties of ageing populations, some of which can be extrapolated to individuals. In the physiological approach, one investigates the mechanisms of ageing at the level of organisms, organs, tissues, cells, or even molecules.
A populational perspective on ageing
Incidence of death is the proportion of individuals of a population who die within a period of time. In the human population, as well as other populations, it is known to increase exponentially from the age of maturity to maximum lifespan (see Figure 1.1). Such curves are called Gompertz models. They have been known and studied since the nineteenth century.1 Were the curve to depend only on extrinsic factors of death like accidents, predation, starvation, or war, it would not show any regular, time‐dependent pattern (a recent example is the naked mole‐rat,2 as shown in Figure 1.2). In contrast, the typical shape of the curve is commonly interpreted as an effect of intrinsic factors of death. These chiefly consist of diseases. They are both more frequent and deadlier with time because they are somehow catalysed by ageing. Ageing is, therefore, the primary intrinsic factor of death. It has also become the leading cause of death in most human populations and populations of domestic animals, well before infectious diseases, accidents, and starvation.
Focusing on the right‐hand part of the curve and what happens at the end of life, three facts must be emphasised. The first is that after the age of 80, the incidence of death still increases, but more slowly – and it continues to slow with time. The incidence of death even seems to reach a maximum. This so‐called ‘late‐life mortality plateau’, i.e. the time of life when the incidence of death ceases to increase and becomes constant, has been documented for Melanogaster 3 and in several other species, but not for mammals. For humans, it remains a matter of debate. The discussion revolves around the small number of people reaching extreme old age and the reliability of the data.4 If the human population reached a late‐life mortality plateau, the incidence of death would stabilise around .5 after age 105–110. In other words, so‐called ‘supercentenarians’ would have a 1 in 2 chance of dying before the end of the year, whether they were 110 or 122 (age of Jeanne Calment at death). More importantly, the process of ageing seems to slow and perhaps even stop at some point. If this process does stop, then there is no intrinsic maximum lifespan for the human species. Hazard alone must then explain why the very few people reaching extreme old age are exceedingly unlikely to live another 15 years.
Figure 1.1 A Gompertz curve showing the increase in the incidence of death with age.
Figure 1.2 The ‘non‐Gompertzian’ curve of the incidence of death in the naked mole‐rat. Source: Ruby, Smith, and Buffenstein2.
The second important fact is the compression or rectangularization of the survival rate curve (see Figure 1.3). In practice, it shows that medicine delays the death of most individuals of a given generation until they are closer to maximum lifespan, but it does not change the maximum lifespan much. Fries, who introduced this notion 30 years ago, interpreted this ‘compression of mortality’ as the result of a ‘compression of morbidity’5: it follows from the facts that there is a maximum lifespan (be it intrinsic or extrinsic); that even without chronic disease, individuals would still die; and that chronic diseases can be prevented. Related to this fact is the emergence of the concept of healthspan, or duration of life in good health.
Figure 1.3 The rectangularization or compression of mortality (dotted line) that occurs with medical intervention.
The third fact is a change in our views on medical intervention in old age. Healthspan and cognate concepts such as ‘successful ageing’6 and ‘healthy ageing’ are intended to be descriptive. However, they remain biologically vague. Most healthy old people undergo low‐grade chronic inflammation, progressive endothelial dysfunction, decreased glomerular filtration rate, and other phenomena associated with particular diseases. The only difference may be that the progression is slower: it is therefore confusing to define successful ageing as being ‘healthy’. The right function for these concepts is not descriptive; rather, it is prescriptive of a consensual norm of intervention for medicine7: keep as many people as possible as healthy as possible for as long as possible. This is also the acknowledged goal of anti‐ageing intervention, which has recently gained traction and credibility.
The evolutionary roots of the phenomenon of ageing
All humans age, develop biological dysfunctions, and eventually die. If this process is universal, it is reasonable to assume that it is also necessary. But what kind of necessity is it? Since Gompertz, there has been a consensus that it is an intrinsic rather than extrinsic necessity. Indeed, the reduction of extrinsic challenges is known to translate the curve on the y‐axis (incidence of death), not on the x‐axis (time of death). Accidents, starvation, and extreme living conditions do not make a human population grow old more quickly; they only make the population extinct sooner, as a now‐famous study on the curve of incidence of death for a population of Australian prisoners in prisoner‐of‐war camps has shown.8
A different question is whether ageing is a biological or physical necessity. It is sometimes said that ageing is the necessary realisation of the second law of thermodynamics: i.e. the irreversible progress of any closed system toward maximum entropy, which (as Schrödinger originally put it) for the living, is death.9 However, Schrödinger mentioned no physical reason why a particular organism could not perpetually draw ‘negative entropy’ from its environment, as long as the Sun provides free energy. The reason why ageing is ineluctable must, therefore, be biological.
Many species undergo senescence, as evolutionary biologists call it (note that molecular biologists use the word in another sense). The dominant explanation of the ‘origin and evolution’ of senescence was first proposed by Medawar10 and then refined by Williams,11 Hamilton,12 Charlesworth,13 and Kirkwood.14
Imagine a population that does not undergo functional decline with age. It is not immortal, because of extrinsic causes of death, but the number of individuals in a generation will still decline with time. Mechanically, older individuals will have fewer offspring than younger generations, although they all have an equal chance of survival and reproduction, younger or older. If genetic variations produced deleterious effects that manifest themselves only after some time, they will be eliminated by natural selection only in proportion to the number of individuals that reach the age of appearance. In other words, deleterious traits appearing in an age class with few individuals cannot be eliminated as efficiently as in an age class with many individuals. A body organisation that does not lead to late functional decline is no fitter than one that does, when most individuals die before functional decline may occur. On the other hand, natural selection will tend to postpone all deleterious effects