Is Mortality Necessary? Part Two: the demographic theory of senescence

In an earlier posting I raised the question of whether death is theoretically necessary for life—especially in a way that would thwart desires to extend healthful human longevity (“Is Mortality Necessary?” Aug 23, 2020). I pointed out that to answer this question would involve an evolutionary understanding of why mortality exists, and how the senescence of the multi-celled organism relates to that of its cells. In this second article I pursue these questions further, in relation to the prospect for life extension.

Evolution depends on natural selection, which depends on the passing of generations: in other words, on death. Each individual life is a trial, favoring those with traits better adapted to the current environment in a way that increases reproductive success. It seems counterintuitive that mortality itself could be an adaptive trait, though without it evolution by natural selection could never have taken place. However, the pace of natural evolution of the human species is less relevant now that our technological evolution exponentially outstrips it. Perhaps, then, evolution’s incidental shortcomings (mortality and aging of the individual) can and should be overcome. At the least, this would mean disabling or countering the mechanisms involved.

The history of culture amounts to a long quest to improve the human lot by managing environmental factors and creating a sheltering man-made world. (See my book, Second Nature, archived on this site.) This has been an accelerating process, so that most of the increase of human longevity has occurred only in the past couple of centuries, largely through medicine, sanitation, and technology. Infant and child mortality in particular had heavily weighted average lifespan downward for most of human existence. Their recent mitigation caused life “expectancy” to double from 40 to 80 or so years. Because it is a statistic, this is misleading. It is not about an individual lifespan, which (barring death by external causes) has not changed much over the centuries. Nevertheless, diseases of old age have replaced diseases of childhood as the principle threat to an individual life. These diseases may not be new, but formerly most people simply did not live long enough to die of them. Because they typically occur after reproductive age, there was no way for natural selection to weed them out. Heart disease, cancer, diabetes, Alzheimer’s, etc., are now deemed the principle causes of death. While these may be in part a product of our modern lifestyle, increasing vulnerability to such diseases is considered a marker of aging.

Yet, some scientists are coming to view aging itself as the primordial disease and cause of mortality. It was the triumph against external causes that resulted in the huge gain in the longevity statistic over the past century. However, to increase it much further would require eliminating internal causes. There has been a lot of research about particular mechanisms associated with aging, with optimism that these can be manipulated to increase an individual lifespan. Yet, there remains the possibility that aging is deeply built-in for evolutionary reasons. If so, aging might resist mere relief of its associated disease symptoms and might require different strategies to overcome.

The intuitive notion that the aged must die to make place for the young dates from antiquity. However, it goes against the modern idea that natural selection maximizes the fitness of the individual. (Shouldn’t a more fit individual live longer?) To make place for new generations, a built-in mechanism would be needed to cause the “fit” organism to senesce and die, if it had not already succumbed to external causes. This mechanism would have to prove advantageous to the species or to a sub-population, else it would not be selected. Such mechanisms have been identified at the cellular level (apoptosis, telomere shortening), but not at the higher level of the organism as an individual or a population. If there is some reason why individuals must die, it is not clear how this necessity relates to the cellular level, where these mechanisms do serve a purpose, or why some cells can reproduce indefinitely and some telomeres either do not shorten or else can be repaired.

Some creatures seem programmed to die soon after reproducing. Unlike the mayfly and the octopus, a human being can live on to reproduce several times and continue to live long after. Humans can have a post-reproductive life equal at least to the length of their reproductive stage. But the other side of that question is why the reproductive phase comes to an end at all. If evolution favored maximum proliferation of the species, shouldn’t individuals live longer in vigor to reproduce more? This gets to the heart of the question of why there might be an evolutionary reason for aging and mortality, which—though not favoring the interests of the individual—might favor the interests of a population.

The Demographic Theory of Senescence is the intriguing idea that built-in aging and mortality serve to stabilize population level and growth. (See the writings of Joshua Mitteldorf.) Without them, populations of predator and prey could fluctuate chaotically, driving one or both to extinction. Built-in senescence dampens runaway population growth in times of feast, without inhibiting survival in times of famine (brought on, for example, by overpopulation and resource depletion). In fact, individuals hunker down, eating and reproducing less and living longer under deprivation, to see a better day when reproduction makes more sense. In other words, mortality is a trait selected for at the group level, which can override selection against it at the individual level. Mortality also favors the genetic diversity needed to defend against ever-mutating disease and environmental change (the very function of sexual reproduction). For, a population dominated by immortals would disfavor new blood with better disease resistance. Yet, theorists have been reluctant to view senescence as a property that could be selected for, because it seems so contrary to the interests of the individual. In western society, sacrifice of individuality is a hard pill to swallow—even, apparently, for evolutionary theorists. Yet the whole history of life is founded on organisms giving up their individuality for the sake of cooperating toward the common good within a higher level of organization: mitochondria incorporated into cells, cells into differentiated tissue and organs, organs into organism, individuals into colonies and communities.

Apoptosis (programmed cell death) has been documented in yeast cells, for example, as an “altruistic” adaptation during periods of food scarcity. In the multi-celled organism, it serves to sculpt development of the embryo, and later to prevent cancer, by pruning away certain cells. In the demographic theory, it is also a mechanism naturally selected to stabilize population. Since telomerase is available in the organism to repair telomeres as needed, the fact that it can be withheld might also be an adaptation to reduce life span for the sake of stabilizing population. Pleiotropy (when a gene serves multiple functions that are sometimes contradictory) is selected, then, because it promotes aging and thus acts against runaway population growth.

If this is the right way of looking at it, on the one hand specific mechanisms are selected that result in mortality as a benefit to populations though at a cost to individuals. It might be possible to counteract these mechanisms in such a way as to prolong individual life. On the other hand, following this path successfully would defeat the evolutionary function of built-in mortality unless intentional measures are taken to insure genetic diversity and to control population growth. The lesson is that if we are to deliberately interfere with aging and mortality (as we are certainly trying to do), we must also deliberately do what is required in their place: limit human population while providing artificially for the health benefits of genetic diversity. If the goal is a sustainably stable population, people cannot be born at current rates when they are no longer dying at natural rates.

These are global political and ethical issues. Who would be allowed to reproduce? Who would be kept artificially alive? The production and rearing of children could be controlled, indeed could become a state or communal function, divorced from sexual intercourse. The elderly could be educated to let go of an unsatisfying existence—or be forced to. The diversity issue is a technological problem, which genetic medicine already proposes to engage; it too raises the unsavory prospect of eugenics. All this brings to mind the transhumanist project to assume total conscious control of human nature, evolution, and destiny.

At present, of course, we do not have the institutions required for humanity to take full charge of its future. Science and technology attempt to control nature for piecemeal, often short-sighted purposes, or as rear-guard actions (as we have experienced in the pandemic). But far more would be involved to wrest from nature control over evolution and total regulation of the earth’s biosphere (or of an artificial one somewhere else). Immortality would require a lot more information than we now have. To be sustainable, it would also require a level of organization of which humanity is currently incapable. It would need a selfless spirit of global cooperation to act objectively for the common good, which does not yet exist and perhaps never will. (Imagine instead a tyrannical dictator who could live forever!) Immortality would require a serious upgrading of our values. At the very least, it would require rethinking the individualism on which modern humanism and science itself were founded, let alone the consumer culture. Science, politics, religion, and philosophy will have to finally merge if we are to take charge of human destiny. (Plato may have grasped this, well ahead of our time.)

Ecological thinking is about populations rather than individuals. Communism was perverted by the societies that embraced it and rejected by the societies that cling to individual property rights—in both cases out of individualist self-obsession. In the current pandemic, however, we are forced to think of populations as well as individuals. Yet, the strategy so far has been oriented toward “saving lives at any cost.” If the demographic theory of senescence holds any water, our social development will have to keep pace with strategies to increase lifespan if we are not to breed ourselves to oblivion. Individuals will have to voluntarily redefine personal identity and their relation to the collective. And, of course, their relationship to aging and death.