The average human lifespan has significantly increased in the last couple of hundred years, prompting suspicion of a potential evolutionary trend towards living longer.
I have before heard the argument, that human interference in the natural progression of disease and disability is affecting the “Darwinian, survival of the fittest”, and consequently is likely to influence evolution in the favor of a genetically ‘weaker’ human species. That argument has merit only when predicated on the inaccurate assumption that the individual or group (population) is the fundamental unit of selection. [Dawkins, 1976; Dawkins, 1982]
Can we however reasonably expect our descendants to keep getting older every generation, or is there likely to be an upper limit for maximum age for humans?
To postulate an answer to the question at hand, we will have to delve into the technicalities regarding the evolution of longevity, though I suspect that my (possibly feeble) attempt at a thought experiment, might only answer questions to which we already know the answers.
First, I would like to suggest that we define the term longevity for its use herein, as the length of time that extends beyond reproductive age, as a proportion of total life expectancy, or lifespan, used interchangeably with ‘length of time beyond reproductive age’.
Is it likely that genes ‘for’ longevity will have an increased selection coefficient compared to their rival allele(s)?
For there to be an increase in life-expectancy, due to selection in favor of an increased longevity proportion, we have to assume for the purpose of this argument, that average time to reproduction remains constant, in order to isolate for the selection coefficient regarding a ‘longevity’ gene/allele. We also assume the existence of an allele that confers some sort of increased lifespan due to increased longevity (increased proportion of life-expectancy after reproduction).
Selection pressure that would favor the propagation of a gene ‘for’ increased longevity, is linked to the phenotypic effect that this gene is likely to have on the propagation of copies of itself into future generations. The presence of an allele ‘for’ longevity, should in principle favor the increase of such an allele in future populations, at the expense of its rival.
Since we have stated that the average age of reproduction is not affected, and this gene is strictly for increased lifespan beyond reproductive age, we can safely assume that the gene ‘for’ longevity has no direct influence over copies of itself being present in its progeny. The only way by which such a gene can increase the inclusive fitness of copies of itself, is by insuring an increased survival probability of such a gene (and therefore individuals who carry a copy of this gene) in future generations. If there is more time after reproduction in which the principle investment of energy goes to ensuring survival of progeny, then such a gene will benefit from an increased longevity fraction. If energy is no longer invested in reproduction, then it makes evolutionary sense to invest energy in ensuring the survival of offspring, which carries copies of the genes of its parents.
Selection in favor of a gene ‘for’ longevity will have a higher selection coefficient than its rival allele, all other things being equal.
However, it is safe to assume that there is an evolutionary stable state for longevity, based on costs of developing such a trait. In principle, if there were no increased costs associated, then organisms would tend to evolve to gain immortality. But lifespan after reproduction is likely to be optimized for ‘minimum time beyond reproduction required to insure survival of copies of genes into the next generation’. This is necessarily bound to reproductive age. A gene ‘for’ longevity will increase the probability of its survival, if it can insure that its progeny survives until reproductive age. It will thereafter have exactly half the benefit if it can can assist grandchildren of itself to reach reproductive age. Whatever arbitrary value of inclusive fitness a gene ‘for’ longevity can have, will be halved in each subsequent generation, reducing the evolutionary benefit of survival after reproductive age, with the passing of each generation.
The chance that his children will contain this ‘increased longevity’ gene is ½, and that for his grandchildren is only ¼. The value of a parent assisting his grandchildren to survive is only half the value for that of his own children. A gene ‘for’ longevity will gain double the advantage of assistance, if both its parents and grandparents are assisting it in reaching sexual maturity, and has therefore twice the (arbitrary) fitness value for surviving into the next generation, compared to its rival alleles which incur no such advantage. From the offspring point of view, it might seem beneficial to survive for parents to survive for long periods of time after reproduction.
However, the advantages of increased longevity is balanced by the costs of diverting resources away from reproduction, in order to increase lifespan thereafter. Off the top of my head, it would require the evolution of better policing systems, genetically speaking, to ward of age related diseases (cancer, Alzheimer’s etc.). It would also require the co-evolution of better copying fidelity for somatic cell genes. Resources for reaching maximal physical health at reproductive age would have to be diverted to ensure better mechanical tenacity at advanced age. We can stop here, by assuming the list is very incomplete, and whatever other factors that need be considered, will add to the burden of costs. We can also assume that costs will also increase rapidly with time, whereas benefit will decrease radically with each passing generation.
What then if we assume, that there is an established equilibrium for longevity, governed by the average reproduction age? This has already been shown to be the case, both in previous research and for reasons mentioned in the above argument. It has been described more accurately here.
Would an increased average time to reproduction lead to an increase in lifespan? Would the artificial selection pressure induced by humans, reserving the capacity to procreate until much later than the average, lead to an increased average reproductive age, and therefore increased lifespan?
At first glance, the argument above would suggest that increased average reproductive age would indeed lead to increased longevity. Though I would like to get into the details of selection governing such a potential increase, this communication is already at its limits with respect to readability due to my ramblings over technicalities.
I will simply state, that, artificial selection for ‘increased age of reproduction’, is required to have an increased propagation potential (selection coefficient), compared to shorter reproductive cycles, for such an allele to stabilize itself in a population. Waiting longer to have children in this instance would have to increase the number of descendants from longer reproductive cycles, relative to the number of those produced from shorter reproduction intervals. This has associated with it a number of costs, such as increased resource requirements to reach reproductive age. It has been suggested for this reason, that life-expectancy is negatively correlated with reproductive age.
I would like to add another component to this line of thought. The Constructal Law. Recently published in this here article, is a mathematical model, describing the correlation between, body size, distance traveled during a lifetime and off course life-expectancy. Whether, reproductive cycles and life-expectancy is a product of organism size, or organism size is a product of either one, or a combination, of the aforementioned components, remains a discussion best reserved for a future opportunity.
The Constructal Law essentially states that, the larger a moving body, the longer its lifespan and distance traveled during its life. If my logic serves correct at this juncture, then increased life-expectancy will be associated with increased average human size, though I am certain that we have speciated our way to within the current limits of our (human) body size distribution, very long ago.
If you are wondering whether there is a reasonable chance that humans will one day live to exceed 100 years on average, then the answer should be no.
And if and you might be still be inclined to answer yes, then the following is something to consider. If such a genetic mutation does happen to occur, one that causes a change in the very roots of embryology, one that will increase body size, increase time to reproduction and therefore increase life-span, it is likely that they would not be referred to as humans (by our current criteria), as a result of speciation. It would be the evolutionary equivalent of primates having predicted that humans would evolve a more intelligent descendant from an ancient common ancestor. What the latter paradoxical statement is really implying, is that, if this were to occur, current modern humans would probably only be the common ancestor for that line of evolution.