Defining aging
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Defining aging Maël Lemoine1 Received: 8 March 2020 / Accepted: 17 August 2020 © Springer Nature B.V. 2020
Abstract Aging is an elusive property of life, and many important questions about aging depend on its definition. This article proposes to draw a definition from the scien‑ tific literature on aging. First, a broad review reveals five features commonly used to define aging: structural damage, functional decline, depletion, typical phenotypic changes or their cause, and increasing probability of death. Anything that can be called ‘aging’ must present one of these features. Then, although many conditions are not consensual instances of aging, aging is consensually described as a process of loss characterized by a rate and resulting from the counteraction of protective mechanisms against mechanisms that limit lifespan. Beyond such an abstract defini‑ tion, no one has yet succeeded in defining aging by a specific mechanism of aging because of an explanatory gap between such a mechanism and lifespan, a consensual explanandum of a theory of aging. By contrast, a sound theoretical definition can be obtained by revisiting the evolutionary theory of aging. Based on this theory, aging evolves thanks to the impossibility that natural selection eliminates late traits that are neutral mainly due to decreasing selective pressure. Yet, the results of physiolog‑ ical research suggest that this theory should be revised to also account for the small number of different aging pathways and for the existence of mechanisms counteract‑ ing these pathways, that must, on the contrary, have been selected. A synthetic, but temporary definition of aging can finally be proposed. Keywords Aging · Disease · Evolution · Death
Introduction Aging seems to be an elusive property of life. It seems to have no particular func‑ tion and is hardly subject to natural selection. It is often considered to lie at the crossroads of physical and biological explanations. It seems to be both inevitable and to result from accidents. It is even unclear whether it is a universal property of * Maël Lemoine mael.lemoine@u‑bordeaux.fr 1
Univ. Bordeaux, CNRS, ImmunoConcEpT, UMR 5164, 33000 Bordeaux, France
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lifeforms, whether it starts during development or much later, when it is healthy or pathological, and whether it can be measured at all. Any attempt to clarify these issues should begin by addressing the question: (i) What is aging? Despite the importance of this question for clarifying other related issues, there is no investigation of the concept of aging in the philosophical literature. However, phi‑ losophers of medicine have raised the question: (ii) Is aging a disease? Unfortunately, their contribution reflects commonsense rather than science. Accord‑ ing to common prescientific conceptions, diseases are “unnatural” because they are dysfunctional, whereas aging is both “natural” and dysfunctional. Boorse (1977) reconciled this apparent paradox by arguing that the ailments of age cannot be dys‑
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