Certain animals don’t age like the rest of us do. Certain tortoises, e.g., can live several hundred years, but yet maintain their same vitality as if in their younger years. Certain other reptiles, as well as some fish and amphibians also have similar genes. They really don’t die of old age. Eventually some accident or some disease such as cancer eventually strikes them down, but yet they maintained their youthfulness throughout their life.
Here’s a list of some animals that are believed to be negligible senescent from the wiki article:
• Rougheye rockfish (Sebastes aleutianus)—205 years[4][5]
• Aldabra Giant Tortoise—255 years
• Lobsters are believed to live 100 or more years.[6]
• Sea anemones generally only live up to 60–80 years due to environmental dangers[7]
Humans obviously don’t have these genes, our species starts to slow down with the years, we eventually start to look and feel our years, even though some seem to do better than others, but still not quite the fountain of youth. Look at any centenarian and they show their age.
It doesn’t appear that any human has ever been blessed with the fountain of youth genes, or if it ever has it is so rare that none have ever been recorded.
With over 6 billion people on our planet, and with science able to document less than 100 with the Hutchinson-Gilford Progeria (young children having old age symptoms and disease), with evolution bringing us many variations, why hasn’t evolution seemed to have allowed for at least some humans that also have the same negligible aging as some other animals?
It looks as if mammal metabolism, or warm-bloodedness is incompatible with a very long lifespan, as all your examples are cold-blooded. Sorry. Probably a machine that runs at high revs will always wear out eventually.
OR – it has been a very rare condition, and not noticed because until very recently death from “old age” only affected a small percentage of people. Disease, accidents and war likely killed off any Immortals before they got old enough for anyone to notice anything different.
ALSO – Evolution doesn’t give a damn about you once your offspring are old enough to reproduce in their own turn. In fact, evolution depends on the older generation dying out to open space in a niche for new individuals who might be a little better at it than their cousins or uncles…
The basic reason that most species age and die is that there is generally little selective value for reproduction by older individuals, because such individuals make up only a small part of the total population.
Let’s take a population of a hypothetical immortal animal that breeds at the age of one year, and in which the mortality rate due to accident, predation, or disease is 10% per year (pretty low in a lot of natural populations). After 10 years, only a tiny percent of the population will consist of 10-year old individuals, and they will contribute very little to the next generation. There will be little selection against traits that allow bodily processes to break down as the animal becomes older. The result will be that such traits will accumulate in the population, resulting in increasing debility and death of older individuals.
Once we have finished reproducing we have very little evolutionary value left. If we lived just twice as long there wouldn’t be a single lawn in the USA that a young person could walk on.
Although in humans, your usefulness isn’t entirely gone once you stop reproducing, since you can still pass down your wisdom to your descendants and help them survive/thrive/reproduce more. And, accordingly, humans have in fact evolved to live significantly longer than other mammals. Most animals will reach their absolute limit in the close vicinity of a billion heartbeats, but humans can make it to about 4 billion.
And since genes express themselves in a variety of overlapping ways, the same chunk of DNA that makes your body break down with age may even confer a survival advantage when you’re young. A mutation that increases your chances of reaching puberty by 1%, but shortens your expected life span from 70 to 60 may still be a win in evolutionary terms.
Many animals with long lifespans don’t so much live longer than us as they live slower; they have a slower metabolism. In fact we already have an extended lifespan for our metabolic rate; most animals live for about a billion heartbeats, while humans reach 3-4 billion IIRC. Colibri also has a point; animals tend to die of old age about old as they would if they were immortal; subspecies of animals in safer environments like islands apparently age slower than their less-safe relatives in fact. Perhaps our lifespan is as long as it is for that reason; an adult human is fairly hard for nature to kill. And on top of that, natural selection can only work with what genes mutation and recombination gives it; if no “live for 200 years” genes happen to appear, then evolution won’t select for a 200 year lifespan.
I do disagree though that there isn’t anything selecting for a longer lifespan in humans. First, there’s the fact that human males can produce children quite old; unlike a woman who has a reproductive timer, a man who lives for 20 more years can potentially produce another generation’s worth of children. Related to this is the preference of women for older males; a male who remains fit longer can attract women longer. And unlike an animal old men can often provide for their children despite being less physically fit.
There is also the function of older people in pre-literate societies as repositories of knowledge. This can be very important; one example I recall is that of some elderly tribal woman recalling that a particular extremely bad tasting but drought resistant root is edible from her own childhood. With infrequent disasters like “50 year droughts”, if humans only lived to be 20 or 30 by the time the next disaster came around everyone who knew how to survive it would be dead.
And the evidence is that our late pre-human ancestors had a significantly shorter lifespan. The women of the time for example apparently never lived long enough to reach menopause, since their bones lack the changes menopause causes. So our lifespan has increased in recent evolutionary times.
True. In fact, this has been advanced as a possible reason for the evolution of menopause in humans. Removing the possibility of reproduction from older women enhances their survival, since pregnancy and childbirth are stressful and risky. Older women provide more value to survival of their lineage by helping to care for their grandchildren than they do by trying to have another child themselves.
I’m not saying that I buy this idea, just that it’s something that has been proposed.
True, but the point is that in earlier human societies, due to a much lower life expectancy, older males made up only a small percentage of all adult males. In hunter-gather societies, even older males won’t be able to support more than a few wives. The percentage of children produced by males over the age of fifty will be very small compared to the total number of children produced in the population.
The percentage of children produced by older males is no doubt greater today, but still not enough to have much effect on life span.
Addressing the OP more directly, it seems unlikely that there is a single or even handful of genes that could make a human perpetually youthful. As you have pointed out, such a mutation would be likely to happen and would be selected for.
It is probable that our metabolic pathways are sufficiently different from rockfishes’ that asking why we don’t have their longevity gene is like asking why my car doesn’t have the property of flight. And why of all the cars in the world, and all the dents and scrapes that occur, none of these “mutations” has resulted in a flying machine.
Not to say it’s impossible for us to be forever youthful: there’s no physical law forcing us to age. Just that there is no “Press this button and live forever” button.
There are a number of genes that may increase lifespan OTOH (but still ageing within that lifespan).
That’s still relevant in the first world today, not just in pre-literate societies. Every college professor you’ve ever had was past the equivalent age where any other mammal would be dead. Yeah, everything a person learns can be written down, but there’s still no substitute for teachers.
While I agree with the basic sentiment, I don’t think your example exactly true. I was a college professor at age 26, and I know there are others who did so at a younger age. I became a father at age 28. I’m not sure how you’re computing equivalent age. Certainly other mammals live to twice the age at which they are sexually mature (and I’m not sure that’s 13 for humans in any case.) Certainly other mammals live to an age 50% older than when they reach full growth. And certainly there are other mammals that live to age 26.
If by professor you mean tenured, then that happened for me at 33, which is younger than twice the age at which I reached my full height. Certainly other mammals live more than twice the age at which they reach full growth.
There are few reasons that we know of. Not sure how many I remember though - just 2 right now.
Telomeres: Every time a cell divides, there is a repeating sequence on the end of each chromosome that gets slightly shorter. This region is known as the telomere. There is some error in transcription that occurs during mitosis (replication) such that part of the sequence is not copied. Therefore every eukaryotic cell (ones with a nucleus, as compared to bacteria) has what is know as a Hayflick limit. The limit is reached when the telomere sequence is gone and the cell self destructs (apoptosis).
It is possible to re-engineer cells to produce telomerase. This would replenish the telomeres, but the other consequences of doing this are still unknown AFAIK. Some cancer cells produce telomerase as do fetal cells.
There is genetic variation in how long your telomeres are to start with but I don’t know if there is conclusive proof of a correlation between telomere length and longevity.
Oxidative damage. During normal metabolism, your cells produce a variety of highly reactive molecules that can damage the machinery of the cell, the DNA, cell membranes, etc. Superoxide is one example but there are many others. We do produce an enzyme (SOD, or superoxide dismutase) that is used to quell the activity of SO, but you still get some damage.
edit: transcription errors: Mitosis is not foolproof and with every cell division, it is possible to have errors. Obviously these errors accumulate over time.
I was referring to heartbeat count, which IIRC would put a mid-20s human at the same point as an extreme geriatric mammal of any other species. And of course, 26 is extremely young for a professor.
Slight threadjack, is there any way of knowing if the premise of Heinlein’s “Methusthelah’s Children” series could work in reality? For those that aren’t familiar, a foundation in the 1890’s starts giving grants to people who marry who have all four grandparents still alive and in good health. By the 22nd century the resulting offspring have natural livespans approaching 160.
We misunderstand the basic situation. We are not evolving. Our genes are evolving. We are carriers that serve the evolutionary purpose of propagating our genes. And our genes have evolved the ability to regularly shed carriers (us), so they can increase the priority of optimizing their propagation by compromising on the priority of keeping us going.
You know why raising a family is such hard work? It’s because you’re getting consumed by the process.
The only reason we don’t have a “Fountain of Youth” gene is that none of our ancestor had it. And unless there’s a mutation or scientific manipulation, it will remain that way. And as others have pointed out, even if some people remain relatively youthful into old age, their reproductive work is already done, so there’s no way to retroactively benefit from it.
Given that the longest confirmed human lifespan, out of many billions, was less than 123 years, it is very unlikely that the maximum could be increased by than much within only about 12 generations (if ever).
Although the average life expectancy has gone up tremendously over the past several centuries, the maximum life span has not gone up by much, if at all. Even centuries ago some people reached 110 years or more.
That would be successful only if those grants caused the recipients to have more children than average (thereby increasing the incidence of good genetics for old age in the human race at large) or if the recipients showed a strong bias for endogamy, or marriage within the group, eventually creating a subgroup of humans with particularly good genetics for old age. Neither approach would be likely to show those kind of results by 2100 C.E.; evolution doesn’t work that quickly. Even if we assume twenty year average generations, that’s only ten reproductive cycles.
That said, this could conceivably increase human lifespan in the very long run. It creates an evolutionary pressure toward longer total lifespans, something that (as pointed out upthread) tends not to be selected for in the natural world.
I like the way DrFidelius puts it; “Probably a machine that runs at high revs will always wear out eventually.” Immortality is probably not an achievable goal through genetics.
