Considering Longevity in Terms of Damage Versus Damage Repair
Here is a framework for thinking about aging and longevity: various forms of low-level biological damage accrue as a result of the operation of metabolism, degrading organs and tissues and ultimately causing death. Where natural selection favors longer-lived individuals, mechanisms will evolve to repair, minimize, or resist the effects of this damage. So aging is driven by damage, but genetic programs interact with that damage, evolved to try to do something about it.
Thus we could expect to be able to manipulate life span either by repairing damage or by altering the programs. The former approach should produce far more effective means of healthy life extension, however, including rejuvenation of the old. In comparison, and from what we've seen so far in longevity science, modestly slowing aging is about the best we can expect from the near future of genetic and metabolic alterations.
In spite of exciting new insights into regulatory mechanisms that modulate the aging process, the proximal cause of aging remains one of the unsolved big problems in biology. An evolutionary analysis of aging provides a helpful theoretical framework by establishing boundary conditions on possible mechanisms of aging. The fundamental insight is that the force of natural selection diminishes with age. This does not preclude senescence (age-related decrease in individual fitness) from occurring in natural populations. Senescence can develop because some genes have non-separable, but typically different or opposite, functions in reproductive-age and in old individuals. Such genes, selected according to their "youthful" function, may thus impose a distinct senescent phenotype in old age.
In general, however, unless a controversial formulation of group selection is invoked, traits that would become manifest only in old age cannot evolve. This precludes the evolutionary emergence of aging programs, which have been sometimes postulated to exist in analogy to developmental and other biological programs. (By the same token, selective pressure that diminishes with age would also prevent extreme longevity from evolving, if "extreme" denotes a potential life span much longer than that imposed by extrinsic mortality in a given environment.) This and other arguments against the existence of an aging program have been discussed previously.
The evolutionary perspective sketched out above does not specify the mechanisms that underlie aging, but it helps to narrow down the possibilities. As already discussed, an evolved deterministic aging program can be ruled out, perhaps with the exception of specific niche situations. In the absence of adaptive life-curtailing processes driven by a putative aging program, we are left with untargeted pro-aging, destabilizing phenomena which, in principle, may range from purely stochastic to side-effects of "legitimate" biochemical pathways. These destabilizing forces are counteracted by evolved, and genetically controlled, longevity assurance (or repair/maintenance) processes. The interplay of these countervailing forces determines the life span.
While I have previously presented my detailed interpretation of this model, its central tenets bear repeating: (a) the destabilizing processes that drive aging are neither evolved nor adaptive; (b) in contrast, longevity assurance mechanisms are under genetic control; (c) together, these two opposing forces determine life span; (d) the average life span of a species is set by evolving longevity assurance mechanisms so as to optimize reproductive success under environmental conditions typical for that species.
Of the eight known types of damage, is there an endogenous repair process in place, albeit imperfect?