Inspired by this thread
If most all of our body cells regularily die off and are replaced by new ones…why doesn’t that process continue indefintely?
-rainy
Someone who is MUCH more knowlegable than me will be along shortly. Different organisms are programmed with a rather specific lifespan (not life expectancy) range. That is why dogs cannot live much beyond 16 years old and humans can’t usually go past the 120 year mark. Modern medicine has been unsuccessful at changing the upper age limit for humans so far. The most popular theory is that DNA is programmed to degrade at a certain rate. Errors get introduced as the strands get more fragile through repeated replication. This DNA degradation seems to cause aging, increase the likliehood of aging related diseases, and cause on upper boundary on lifespan.
Environmental factors play a role too. This part is more complicated. One example is smoking which can directly cause disease and superficial signs of aging.
And actually, most of our cells does not regularly die off and replace themselves (at least if you’re talking about mass and significance of those cells)… muscle and brain cells are so-called post mitotic, i.e. they do not divide after they’ve differentiated. However, they do age. Proteins inside the cell lose their abilities to perform the way they’re supposed to, probably because of the DNA in those cells mutating and not being able to produce the appropriate proteins.
The reason I heard why we age and die is so we won’t compete for resources with the people in the child bearing range, who really need those resources.
The problem with that idea is that it’s perfectly circular. We only become infertile and fall outside child bearing range as a result of aging. So in effect you are saying that we age so that we won’t comepete with people who haven’t yet aged. That makes little sense logically.
It makes absolutley no sense biologically since men suffer relatively little decline in fertile until relatively extreme ages, and yet they still age just as fast as women. Moreover women become infertile around the end of their 5th decade with a notable decline after the mid 30s, and yet the normal maximum lifespan has always been into the 70s. If aging and death were intended to remove unreproductive individauls then it is counterproductive amongst our species where aging produces more competing non-reproductives rather than less.
The most commonly accepted hypothesis is that there’s a tradeoff between eternal youth and cancer prevention. (It’s still being widely debated in the field, though.) Cancer is unregulated cell division; most noncancerous cells can only divide X many times before they automatically die. On the one hand, cells that can keep dividing can refresh organs with new supplies of cells, presumably keeping the organism more youthful, but on the other hand, if cells can keep dividing infinitely, there are far fewer breaks on them turning cancerous.
But we do have cells that can keep dividing indefinitely, they’re called stem cells. It used to be thought that the only stem cells were in the bone marrow to produce new blood cells and in the gonads to produce gametes, but in recent years they’ve found them in just about every tissue of the body. It’s stem cells that do most of the repair work when you are injured. It’s also suspected that stem cells are usually the ones that turn cancerous.
One theory that I’ve heard about aging has to do with mitochodrial DNA. Unlike nuclear DNA, mitochodrial DNA is out there in the cell protoplasm where it can be damaged by free radicals produced during metabolism. Eventually it gets too damaged and its mitochodrion stops working. Lose enough mitochondria and the cell dies. If this happens to too many stem cells, you lose the ability to repair damage to the body. Someone has suggested that if we could move the mDNA into the nucleus, where it’s more protected, we’d live longer.
Well, that’s not the whole story, there are other things that contribute to aging, but it’s probably a big part of it.
Wouldn’t we in fact die within minutes? Mitochondria need rapid gas exchange to function normally and the nucleus is pretty well isolated from those gases.
Well if you look a bit deeper into it, it does make perfect sense. Part of evolution is to change and adapt, to have a broad range of ‘humans’ so an event like a plague won’t wipe out all. If humans didn’t die of old age, and could still bear children, we would have the same genetic stock repopulating over and over.
Not to remove unreproductive individuals, but to remove those who have no benifit to the offspring. It seems apparent to me that grandparents do serve a uesful role for the child, and bio speaking should die off after thier grandchildren are adults (so as not to compete).
This assumes that evolution is striving towards some goal. It’s not. Evolution doesn’t work on the group level or even the individual level, but the genetic level. And a gene doesn’t “care” about genetic diversity one whit.
I think a primary question to ask should be: Why do humans live longer than other animals? The cancer hypothesis doesn’t hold up when you talk about fireflies or mice, who live for radically shorter periods than humans.
If we didn’t die, there’d be no reason to have sex.
There are organisms which do not age, and which only die as a result of unfavorable external conditions - extreme heat or dehydration, being eaten, etc. The vast majority of them are just a single cell, but fundamentally they’re no different from us. They have DNA; they eat, breathe, and reproduce; they compete for resources in their respective environments.
Ageing and death are evolved mechanisms which appeared essentially concurrently with multicellularism. Nearly every multicellular organism dies after reproduction. Asking why this is so is a question based upon a false premise, namely that there is a reason. Evolutionary theory dictates that there is no guiding force or direction for evolution, and while it’s an easy question for someone to ask, there is just one answer, which can be unsatisfying. The descendants of organisms with this trait, characteristic or behavior enjoyed superior reproductive success when compared against organisms which did not display the trait, characteristic or behavior. That’s it, that’s why. It’s not terribly fulfilling, because the actual cause or reason still isn’t clear. If you want me to speculate, then I’ll do so, but my speculation is no better than anyone else’s, and is likely significantly worse than someone else’s.
When organisms compete for an ecological niche, they compete both against individuals of the same population and against other species for the same limited set of resources. When compared to other members of the population, ageing seems to be a disadvantage – some individuals become old, feeble, less capable of finding sources of food and less likely to produce healthy offspring. But ideally, they have already produced healthy offspring, whom they are now competing against for the limited pool of resources. These progeny collectively contain the genetic legacy of the previous generation, and in addition random genetic change which will make some small percentage (on the order of 0.1%, maybe) of them more fit than their parents were. It is ultimately to the advantage of the entire population to give the new generation a large enough portion of the total resources to guarantee that the more successful organisms have ample opportunity to pass on their own genetic material. Evolution acts not only on individuals, but on populations, and in different and sometimes seemingly opposing ways. If pruning of the previous generation provides an advantage to a population, then species which have members that age and die will be more successful than other species.
If you’re talking about protists and bacteria, you’re talking about life that is in some ways similar to transformed cells in a multicellular organisms. Such cells don’t die, it’s true, but they persist by fissioning, and, inevitably, evolving.
In a person, such a cell and its progeny are cancer. We aren’t built to have parts of us divide forever and evolve while we’re alive, really, because they tend to lose all specialization and localization, and thus disturb normal function of more well-behaved portions of the organism, and suck up too many needed resources.
Complex multicellular organisms like ourselves die because DNA isn’t copied with 100% fidelity, and the only way to maintain systemic integrity in the face of this inevitable consequence of entropy is through forced culling of damaged cells. Ultimately this compensatory mechanism is itself lethal to the organism. It’s either that or become a tumorous mass, which is itself, due to its disorganized structure, unable to effectively seek nourishment and dispose of waste, and will eventually perish as well.
There’s plenty of immortal human life growing and thriving in culure dishes and flasks all over the world, but you wouldn’t want to take a chunk of it out on a date, much less be able to have a conversation with it if you tried. And even those cell lines can senesce or evolve beyond all utility with too much passaging. Death is simply a natural consequence of multicellular life that lacks the ability to perfectly repair all DNA damage, and that happens to describe all multicellular life above even a rudimentary level of complexity.
Is that why teenagers are dying to have sex?
Carry on.
We live as long as is needed to survive and procreate, essentially, and different organisms have different parameters to deal with in that regard. Complex social structures might for the basis for an explanation of an evolutionary advantage conferred by having individuals within a species live beyond their optimal reproductive years, for instance. The tradeoff might be rate of reproduction (big, highly plastic brains take a long time to grow up, and are difficult to birth, etc.); and neither strategy is necessarily “better” or “worse” but merely a different pathway that life has explored, so to speak. Ultimately, no matter what, relatively complex multicellular life does smack into the “cancer” hypothesis, from which there is no escaping, presently.
Well, perhaps, but then why do we see mice die of old age at around 2 years of age, and humans at 72+? Unless mouse cells overturn 36 or more times faster than human cells. The cancer theory may hold up when we look at the extreme upper limit for ANY multicellular animal, but not when we look at short-lived versus long-live organisms.
Then, we have the problem of turtles, which appear to be immortal:
Theory as to the mechanism by which they resist senesence:
I think I addressed some parts of that issue in my previous post. At any rate, programmed cell death occurs in senescing organisms primarily to cope with DNA damage (if not because of infection or other toxic insults), and the cumulative effect of this adaptation is felt sooner or later by virtually all complex life. It’s not clear to me why turtles, or even a handful of such examples, provides a sufficient body of evidence to challenge the basic facts of the ultimate limits of longevity. Has anyone kept these turtles around in protected conditions long enough to determine they will not eventually succumb to senescence, just as all other vertibrate life appears to do? Do they simply mature more slowly, and do their cells turn over as rapidly as in other organisms? That they have come up with a mechanism to live a really long time doesn’t mean they’re “immortal”, so that characteristic remaind to be demonstrated, in my mind. I’d be cautious about making too much out of a few anomalies, in the absence of thorough investigation.
There were immortal humans… but they didn’t adapt… they had the same old ideas and ways of doing things and eventually died out. 
Imagine having a 250 year old american... someone who still boasts about killing Limeys in the Independence War. He still thinks flintlocks and oil lamps are great... blacks should be slaves.. and a miriad of other stuff. If we were immortal we probably would have died out as a species. Constant "renovation" means new ideas and technology gets to be tried out. New generations have to seek new areas to live off or new stuff to make a living from.The problem with this is twofold.
The first is, as others have pointed out, that evolution doesn’t have such goals. It operates on the level of the good of the individual today, it can never operate on the good of the population tomorrow as you suggest it does. Having the same old genetic stock ( read genes) repopulating over and over is the ideal situation from an evolutionary POV and a guaranteed method of winning.
The second problem is that if a plague wipes out a population then it will wipe out reproductive young and reproductive old alike. As such an immortal population will have no more or less protection than an aging population. When the first plague hits there will be exactly the same distribution of genes in both populations since there has never been a cause to select for genetic resistance. After the plague hits both populations will still have exactly the same genetic spread because both will have been subject to the same selective pressures.
The idea makes no sense.
But this argument still has the same obvious flaw. If grandparents never aged they would never need to reach a point where all their children and grandchildren are adults. Women in their 70s only have adult children because they were rendered infertile due to aging 20 years earlier, it wasn’t a choice in most cases. If they never aged they would continue to be of benefit to their children and grandchildren and thus never “need” to be removed.
Once again this has become an argument that people age and die to get rid of people suffering the ill effects of aging and dieing. Of course if nobody ever aged then nobody would ever need to be removed.