A recent open access commentary by researcher Michael Rose and colleagues looks at the following question: what, exactly, is aging? Much like art, we might know it if we see it, but there's plenty of room for debate over the details.
In 1991, the book Evolutionary Biology of Aging offered the following definition of aging: a persistent decline in the age-specific fitness components of an organism due to internal physiological deterioration (Rose, 1991). This definition has since been used by others a number of times. However, it was only a modest generalization of a definition proffered by Alex Comfort over three editions (1956-1979) of his key book The Biology of Senescence (Comfort, 1979): "a progressive increase throughout life, or after a given stadium, in the likelihood that a given individual will die, during the next succeeding unit of time, from randomly distributed causes."
Yet a mere definition does not necessarily tell a scientist what causally underlies the phenomenon that is so defined. The latter issue is much broader, implicitly raising fundamental scientific questions regarding mechanisms.
From here the piece heads off into a discussion of late life plateaus in aging, for example in flies where it is observed that the chance of death per unit time stops rising at advanced ages - in other words the flies stop aging by one definition of the term, left with a high but steady mortality rate. There are some arguments for this phenomenon to exist in humans, but the data is sparse and other results argue the opposite conclusion. Still, the researchers here argue that the demonstrated existence of this phenomenon in lower animals requires further thought to be directed towards how to define aging:
it appears that the cessation of aging occurs at the individual level, and is not just an artifact of population structure. Yet this is clearly paradoxical, if we think of the machinery of aging in terms of such physiological processes as steadily cumulative damage. If it is supposed that some process of cumulative damage or disharmony is supposed to underlie aging, why should that process abruptly stop at the very point, late in adult life, when it has greatly reduced the ability of the surviving individuals to sustain life and reproduction?
These results call for some fundamental re-thinking of what aging is: [that] aging is not inevitably a cumulative and unremitting process of deterioration. Instead, aging might be best conceived as a facet of [evolutionary] adaptation ... under sufficiently benign environmental conditions, individuals from species as disparate as humans and fruit flies can survive a protracted aging period and reach a subsequent late-life respite in which fitness-component deterioration stops, a phase permitted by the complete attenuation of the forces of natural selection relative to the effects of genetic drift.
The details of the way in which these researchers put forward their hypothesis to reconcile the late-life plateau in aging with existing ideas on the evolution of aging is not conceptually straightforward or easy to understand - but is interesting. As they put it:
This vision of what underlies aging may be off-putting for some, given its theoretical complexities and difficulties for experimental design. No doubt many physicists felt the same way about the destruction of the elegant late nineteenth Century version of Newtonian mechanics by the advent of relativistic and quantum mechanics, in the period from 1905 to 1945. But paradigm transitions in science are generally like that, requiring that we abandon comfortable theories in favor of those that are significantly less wrong.
It remains to be seen whether this view of aging merits comparison with the signature physics of the 20th century, but it is certainly true that a lot of new theorizing on evolution and aging is taking place these days - no doubt driven by an increasing interest in the biology of aging and the prospects of new medicine to intervene in the aging process.