RE: History And Evolution Of Aging - [Is Aging A Process Of Evolution Or Reverse Evolution?]
@samminator
Thanks for such an interesting article. Really liked arguments you made. Though, I don't think we need to call ageing as reverse evolution. I see ageing simply from the perspective of - molecules have a lifetime, they degrade. If you think about it for most part biolmolecules or organisms with traits that lead to higher reproductive survival time gets selected for.
- DNA as genetic material over RNA to start with.
- All the DNA repair molecules
- Stem cell niche in multi-cellular organisms.
I don't think there is any active purpose of ageing that comes under any selection pressure. It is a process that occurs and different mechanisms exist to calm it down.
Which gets us to your question, why doesn't evolution makes us immortal, or to be more specific why doesn't it lead to eternal youth? I mean what could be the trouble. Even if ever young people over populated, they will starve to death by resource scarcity. While those fit to survive in that environment carry on. Happens quite often in bacterial colonies, I suppose? I think what limits a lifespan of species is time required by it for reproducing and nurturing enough offsprings. And second being how well the germ cell in that species maintain their integrity. I would assume the high cost of germline maintenance could at least in part explain the constraint.
However, in context of humans this gets even more complicated. Why to we out-live far beyond our reproductive age. I mean if you rather consider lifespan rather than life expectancy(which can be slave to environmental factors and infant mortality rate), we may not find much difference between past and present.. I think our social grouping structure rather contributes to max lifespan. For instance the presence of grandmother may reduce infant mortality rate.
I don't know, I think you initiated a very interesting conversation here. I have so many thoughts running in my head right now. Wonderful post.
This wasn't part of my question though. But I think it's nice you raised that too.
Some lines in the post could disagree a little from your question though. Like this line:
Or could it be that the process of aging is also encapsulated in the evolution by natural selection? - Possibly to create space for the future generations to have a fair existence, and to ward off stress from Mother Nature
And this:
No wonder people have been making advances to extend life expectancy; maybe indefinitely (or ultimately achieving immortality), but like you know, that would also be devoid of the natural evolutionary parameters - I mean, the natural evolution does not support immortality, but the fact still remains that; aging has not fit into the definition of evolution
There are more to this topic than is just seen peripherally. I only chose to focus on the analysis that aging hasn't obeyed some terms and theoretical framework of evolution.
And since it doesn't fall into apposition with the normal evolutionary trend; then it wouldn't be wrong to infer that it is rather an evolutionary paradox. Of course, some flaws have been created in evolution; even with the advent of tech (which I wouldn't tilt into now).
Can I ask you this question? Does evolution have to be a forward trend? If yes, then why have there been some fluctuations in average life expectancy?
Thanks for coming around
I would say it is directionless. Neither forward nor reverse. The fluctuations in life expectancy can have many other factors other than ageing itself being under the umbrella of selection.
Moreover, life expectancy itself is not a great readout for ageing.. The ageing itself might have occurred at the same rate as it does now. For most part life expectancy measurements did not result from the fact that people died because of old age at 25 or 35. But infant mortality rate was much higher. Think about it Aristotle died at 62 and Socrates at 70 (due to infection and murder respectively, rather than age associated disease). They were not ageing more or less than us, even though modern medicine increased our life expectancy. Hence, I think using life expectancy fluctuation over different eras of human evolution, doesnt say much about the process of ageing.
As far as ageing itself is concerned, goal would be to maximise survival, but that doesn't come without trade-offs. What I am trying to say is that it is likely that the reduction of ageing process(if observed) is not goal, but result of environment constraints.
For instance you mentioned that ancestors had a stronger immune system. Highly likely. Now as per one hypothesis the stronger inflammatory responses to pathogens might be more a bit towards faster ageing. Now as per some studies (though I still need to see more convincing paper), the African ancestry may age faster. Nonetheless, a sub optimal immune response in Africa might have decreased your life expectancy anyway.
What do you suggest?
What we are looking at is the average life expectancy, not just individual ages.
Moreso, we considered the other hominid ancestors of the extant Homo sapiens - like the Australopithecus; not just the ages of people in this era.
These average ages are; better looked upon; as something that spanned over many millennia.
Like the period of the earlier hominid specied to the Neolithic era isn't just some few years. If it is within a short period of time; then it might not necessarily be termed evolution - though some theories support catastrophism instead of gradualism. But overly; evolution is considered a slow process, which spans millions of years.
I know we are talking about average life expectancy. However, this variable is very different from ageing. Ageing effects average life expectancy of population under measurement. Let's say you are talking about average life expectancy at age 35 or 70. What it will end up telling you is a chance that most of the people in this population will live upto that age. See this data for instance. You can take into account the life expectancy at different ages. McDonald and Ruhe explains this difference very nicely, in their review publication in journal nutrition. They also talk about the evolutionary forces acting on ageing.
In fact we can ask the question that how much of average life expectancy is under the regulation of genes? In fact twin studies show that genetic contribution over here is just some 20-25%. While rest of the variation is explained by environment. I think that 20-25% should be contribution of ageing. But what is under selection? Is slow ageing selected against? Or some other benefit that also accelerates ageing tends to increase fitness? For instance take p53 selected which maintains genomic stability. However p53 achieves so by inducing either cell cycle arrest or apoptosis. Both of these can affect the ageing phenotype..
For the second part, as long as you happen to select a trait, it will fall under umbrella of evolution. And evolution can happen even as a step function. There has been huge bottleneck events in past that have shaped many species. Selection of lactose tolerance trait and fair skin in European populations did not take millions of years either. I think it took something like 8000 years.