On the evolution of immortality

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.

Brontosaurus Dolly: Feasibility of dinosaur cloning?

In response to an off topic question, posed during a research methodology lecture, I was humbled by the response of one logically minded undergraduate.

Can we clone a dinosaur? Not as far as I know, but apparently the internet knows more. An Australian billionaire by the name of Clive palmer, has hinted at the possibility of funding research for the purpose of cloning a dinosaur. First of all, we would have to look at a long list of factors governing its feasibility. Initial impressions would suggest that eccentric billionaire, Clive Palmer, is posing a challenge, laced with more humor than sincere ambition. The reality of live dinosaurs should in my mind, present a slightly larger public fascination than even the sensational athletic prowess on display at the recently concluded London Olympics.

Then a student responded, that they have indeed succeeded in cloning a dinosaur, which according to a bit of research, is postulated by some individual, to have occurred at the University of Florida. At first glance, it seemed to be too good to be true, and this opinion has since prevailed.

This publication seems at odds with the research publications and news from the University of Florida’s homepage. I find it strange, that they would have failed to be the first ones to make public this discovery, especially considering the funding (and inadvertent criticism) they would be privy to. But the apparently online-presence-deprived Dr. Norman Trudell, Biology Professor at UF, has neglected several important aspects considering the potential cloning of dinosaurs, and as it may happen, even more recently extinct species, of which he have near complete DNA databases.

But let us steer away from the breaking news of cloned dinosaurs and get into the challenges of feasibility surrounding the cloning of an extinct species.

Is Clive Palmer wasting his time and money? More importantly, is he potentially monopolizing resources that can best be allocated to disciplines related to more pressing public concerns, such as those of cancer and HIV?

The cloning of previous organisms such as the immortally famous (now deceased), Dolly, required some key elements, that is currently in short supply, with respect to dinosaurs. Dolly was cloned using a technique called nuclear transfer, where the nucleus of a somatic cell from primary in vitro cell culture, is introduced to an enucleated oocyte. The oocyte containing a full diploid complement, complete with all associated structural and regulatory protein, is transferred to a surrogate, until it reaches terminal gestational development.

From what I could gather, modern research has not yet produced a complete DNA sequence from any species that has retired from existence during ancient times, which includes many who have gone extinct far more recently than did the dinosaurs. With extensive biological and physiochemical DNA damage that far exceeds 60 million years, I have very little doubt that the scientific community might be expecting the recovery of an intact dinosaur genome any time soon and since we are only just beginning to understand the cellular physiology of extinct animals, the synthesis of a viable genome is as yet, probably even less likely.

According an article published by Nature (Ancient Biomolecules in Quaternary peleocology), we are seeing significant technological advances that increases the rate at which ancient DNA and other biomolecules can be analyzed, yet the National Center for Biotechnology Information (NCBI), does not mention any entry of DNA sequences that is related to the Apatosaurus (Dinosaur postulated to have been cloned).

Even if – with the advances made hitherto – we are capable of extracting sufficient quantities of DNA from paleontological specimens to obtain a complete DNA sequence of some prehistoric species, we would still be very far away from understanding the organization and nuclear morphology of ancient chromatin. I would think that this would make any attempts at cloning redundant. Even if, by some means, assembly of a complete and viable sequence (comparative to that taken from somatic cells in Dolly’s case) could be theoretically feasible (perhaps in cell culture of cells from a species with a close phylogenetic association), there would exist but still, a vast number of challenges in ‘interspecies’ nuclear transfer. There is no telling what difficulties such imperfect chromatin organization could produce, let alone the compatibility, or probable incompatibility of genes and gene products of donor DNA, with cellular components of the oocyte, that has had a hundred million odd years to evolve.

The question remains though, if there should be a public denouncement of this sort of frivolous allocation of resources, to the likes of unrealistic ambition, even if privately funded? This researcher remains a skeptic as to it feasibility, yet I firmly encourage any private funding that will, no doubt, result in the advancement of technologies that could in the future be applied for numerous ambitions, other than frivolity.

In reality, we are unlikely to see this research endeavor materialize to anything more than debated fantasy. Even if Clive Palmer is pursuing this avenue of research merely to satisfy personal curiosity, it may still inadvertently lead to the development of a technology, or even technique, that could justify such expenditure many times over. In any case though, he will be contributing to the advancement of science, or at least propagating some humor in a scientific community, that at times, seems much deprived thereof.