Why Do Living Beings Die Despite Cellular Reproduction?

[rhia]

If cells can always reproduce and replace themselves then why do living beings die?

 

[Mk]

It has something to do with the DNA of some of the body's cell's mitochondria. Its like a copy machine running out of toner, the copies get worse and worse until they aren't even functional. I remember a study from Scientific American, in which biologists altered lab mice's mitochondrial DNA in such a way that they lived.

Like this one: http://www.cbsnews.com/stories/2004/05/26/health/main619735.shtml

 

[Janitor]

I would love to see an answer to this question within the context of a creationist vs. evolutionist debate. The creationist could legitimately challenge his/her opponent to explain why--taking the our own species as the example--a human typically lives about three generations before dieing. We can reasonably expect to see our grandkids, but if we see our great-grandkids at all, we will probably miss out on their entire adult lives. Why shouldn't natural selection and the "selfish gene" have led to a typical human living ten generations, or 50 generations?

I have a bit of an idea as to an answer the evolutionist could give, but at least for the moment I will not present it, so as not to bias any answers others may care to give.

 

[aychamo]

I think there is a very simple answer to that. Nothing would be selecting for or against the person living to whatever age, because they have already reproduced. Once a person has had their kids, it doesn't matter if they live another day or a thousand more years.

 

[Noticibly F.A.T]

So, aychamo, are you saying that essentially, the meaning of life is to reproduce?

 

[Jikx]

If you mean "meaning" to mean "reason". The reason for life is reproduction.

And about the mitochondria - I know you inherit them mostly from your mother, so they are "old" already. How does that work in regard to aging?

And a personal opinion about aging - we age, we reproduce, because an undying individuals (on a cellular level) would have no capacity for variation and be vunerable to changes in the environment. So basically we age because living forever would mean our deaths ::confusing::

 

[Mazuz]

Evolution cares about the survival of our genes, not necessarily the individual. An individual that could go on reproducing for ever would be an advantage however. Perhaps its a situation similar to this.. We can build very reliable cars that will go hundreds of thousands of miles without a breakdown. But these cars will not perform as well as top of the line sports cars that need constant maintenance and have a high rate of break down. Evolution is a delicate balance involving genes and the environment and there are millions of reasons for the paths our genes have taken that we can't even venture a guess to with our current knowledge, this in no way is a challenge to the theory.

Perhaps we depend on some of the same mechanisms that cause aging but for other beneficial reasons that are actually key to our survival and success.

Or perhaps evolution has taken off from a design that is inherently flawed and simply hasn't been able to overcome the problem of aging yet or maybe ever.

 

[selfAdjoint]

Death and sex are two definite evolutionary strategies to speed up evolution, perhaps to keep up with the speed of virus evolution. They go back to the dawn of the metazoan lifeforms a billion years ago. Sex mixes and matches genes into new combinations. And death gets old combinations off the stage to allow the new ones to flourish.

 

[aychamo]

So, aychamo, are you saying that essentially, the meaning of life is to reproduce?

Yes. Getting more copies of your genes into the next generation.

 

[Phobos]

I think there is a very simple answer to that. Nothing would be selecting for or against the person living to whatever age, because they have already reproduced. Once a person has had their kids, it doesn't matter if they live another day or a thousand more years.

Although there is the grandparent hypothesis (or whatever it's called) in that in regards to our societal behavior, it is advantageous to have elders help their owns kids raise the next generation (ensuring the survival of their genes).

Why not keep going for more generations? Good question. But there is probably a point of diminishing return. Kids share 1/2 your genes, grandkids share 1/4, great grandkids share 1/8, etc. Eventually, the genetic similarity of your descendents to you is not that much different than the rest of the population. Balance that with the cost of upkeeping your body that long.

Also, although many cells can regenerate, there is still an accumulation of damage over an organism's lifetime (genetic, physical) which makes it harder to upkeep over time.

Also check out telomeres.
http://www.infoaging.org/b-tel-home.html

 

[Phobos]

Of course, a good question to what aychamo and I just said would be "why can't humans remain fertile their entire lives and therefore there would be a selection process favoring longer/eternal life?" I'll have to get back to that question (I recall Dawkins addressing that one) but perhaps it relates to the cost-benefit of self-maintenance vs. reproduction.

 

[marcus]

Death and sex are two definite evolutionary strategies to speed up evolution, perhaps to keep up with the speed of virus evolution. They go back to the dawn of the metazoan lifeforms a billion years ago. Sex mixes and matches genes into new combinations. And death gets old combinations off the stage to allow the new ones to flourish.

I agree with selfAdjoint.

I think it is in the genes interest for you and me to die after a certain amount of flopping around attempting to reproduce.

that way the genes get to try how it is in some new combinations

they are not only absolutely selfish, they are inveterate gamblers and always impatient to clear the board so they can try the next throw.

watch out for those suckers, they invented death and put the aging clock in you. You have an evolved program to selfdestruct. (our) death is just a way they figured out to give them more chances to recombine. It is no skin off their backs, they like death just fine.

 

[marcus]

Does anybody know when it was that Evolution invented death?
Some species had to accidentally acquire an internal natural death clock and then that species would have experienced an evolutionary advantage over species that hadnt learned the trick of dying

It would only be an advantage if the species had already invented sex.
or some other method of scrambling..
So when did sexual reproduction evolve? Anybody have a guess?

 

[selfAdjoint]

Does anybody know when it was that Evolution invented death?
Some species had to accidentally acquire an internal natural death clock and then that species would have experienced an evolutionary advantage over species that hadnt learned the trick of dying

It would only be an advantage if the species had already invented sex.
or some other method of scrambling..
So when did sexual reproduction evolve? Anybody have a guess?

Finite life span is characteristic of all metazoans (multicelled organisms) except sponges, which diverged early from the main line of the metazoans. According to this theory single celled eucaryotes, metazoan ancestors, may have acquired cell-death (apoptosis) from an apoptopic protozoa in the pre-cambrian. That would be well over a billion years ago.

 

[Janitor]

My own idea is that there are some catastrophes that can overwhelm and kill any individual of a species so easily that evolution just never had the chance or reason to select for genes that would endow an individual with ultra-long life genes. I will give a simple model to show what I am thinking:

A caveman in any given year had these chances of dieing from means other than wearing out due to old age:

Roof of cave collapses: 3%
Sabre-tooth tiger attacks: 2%
Virulent micro-organism runs amock in bloodstream: 1%
Member of the cave tribe on other side of river crushes skull with club: 1%
==========================================================
Sum = 7% chance/year of being killed from one of the above environmental hazards.

Therefore the chance that any individual will live to age 50 is (1-.07)^50, which is so close to zero that the sort of genes that would theoretically allow an individual caveperson to maintain good body and brain function to such an age don't have any reason to establish a foothold in the cavepeople's gene pool.

I will go out on a limb and predict that, given how humans have in the last few thousand years eliminated environmental hazards such sabre-tooth tigers, there in fact is enough advantage to genes endowing a long life and long fertility that in 50,000 years our descendants (even if we don't tamper with ourselves through genetic engineering during all that time--not likely to be the case though!) will typically live substantially longer than three generations, and menopause in women will not occur in most cases until some age substantially older than 40-something.

 

[Chronos]

The Hayflick limit is an appealing explanation
http://www.infoaging.org/feat4.html

 

[Phobos]

Of course, a good question to what aychamo and I just said would be "why can't humans remain fertile their entire lives and therefore there would be a selection process favoring longer/eternal life?" I'll have to get back to that question (I recall Dawkins addressing that one) but perhaps it relates to the cost-benefit of self-maintenance vs. reproduction.

I checked his book (Selfish Gene)...Yep, his argument/speculation was that with older age, women become less efficient at bearing healthy children, so statistically, it eventually becomes more effective for gene survival to care for grandchildren than to risk another birth of your own. So, in that sense, menopause would be an adaptation to promote that behavior. Men do not go through menopause perhaps because their fertility does not decrease that much with age and they invest less (physically) in bearing children than women do.

 

[Phobos]

My own idea is that there are some catastrophes that can overwhelm and kill any individual of a species so easily that evolution just never had the chance or reason to select for genes that would endow an individual with ultra-long life genes. I will give a simple model to show what I am thinking:

Agree. Death by old age is rare for animals (including humans, until recent civilization/technology helped protect elders).

 

[marcus]

... evolution just never had the chance or reason to select for genes that would endow an individual with ultra-long life genes..

I think you may have it backwards.
It takes extra trouble to make a cell that will age and die of its own accord.
This had to evolve.
Early cells apparently did not have this feature.
The original plan for singlecell life did not involve dying. Dying had to be
"invented" by evolution, as an "added wrinkle", because from the selfish gene standpoint it is an improvement.

I think people have a tendency to narrow the focus down to humans when they should be asking about all kinds of metazoa (multicell lifeforms). They all seem to age naturally and eventually die of old age (unless some parasite or predator gets them first). that goes for trees in a forest and turtles on Galapagos----even tho they often live a long time and don't seem to have any very sensible reason to die, they don't live forever. If they didnt then they would fill up all the niche room and there would be no place to put the babies. So the genes wouldn't be able to try new combinations and evolution would grind to a halt. Some catastrophic thing like a new disease would probably get 'm

selfAdj. suggests the aging clock may be as old as a billion years.
first invented in some singlecell lifeform.

maybe it reproduced by budding with
the clock getting reset to zero in each infant that swims off on its own
while the clock in the parent keeps on ticking

I would have guessed roughly 600 million years old, for both sex and death.
but don't really have enough detailed knowledge to guess. I would have guessed that both came along roughly at same time as multicell life took over.

Death doesn't make sense for something that is only one cell and reproduces by symmetrical cell-division. It should not age, and i do not think that Amoebas do age. the cell has to go to extra trouble to age.

 

[flippy]

It is beyond physical.

 

[selfAdjoint]

Death doesn't make sense for something that is only one cell and reproduces by symmetrical cell-division. It should not age, and i do not think that Amoebas do age. the cell has to go to extra trouble to age.

I have been trying to think about the life cycle of that protozoan with cell-death in the link i provided. I would suppose it is set to trigger after the cell has divided (duh!) and maybe after it has divided several times, using some Hayflick-like counter. To prevent errors from accumulating. And the eucaryote could use it to provide food for the daughter cell, which would have to expend work to get its new organelles up to speed. Then if the apoptosis failed you would get two cells bound togther, and multicellularity is off and running.

 

[Zantra]

The Hayflick limit is an appealing explanation
http://www.infoaging.org/feat4.html

If the information is correct, then the fountain of youth is right there.. increase production of telmorase, and you will increase telomeres length, and thus, the cellular life.

However that doesn't mean you're going to LIKE your life at 175. Body organs will still decay- including the brain...

 

[aychamo]

Men do not go through menopause perhaps because their fertility does not decrease that much with age and they invest less (physically) in bearing children than women do.

Yes, that is a good note to point out. Biologically, women invest much, much more (parental investment) into their offspring than men do.

 

[aychamo]

My own idea is that there are some catastrophes that can overwhelm and kill any individual of a species so easily that evolution just never had the chance or reason to select for genes that would endow an individual with ultra-long life genes. I will give a simple model to show what I am thinking:

Yup, this seems to be an important concept for ecology. If an organism is likely to experience some adverse change in the environment at some (preditcable) interval, it is likely to evolve a defense against that change. But if it is some event that the organism won't necessarily experience in its lifetime, it won't likley evolve a defense against it.

Extreme example.. A rabbit will not likely evolve a defense against experiencing an asteroid because it is not likely to experience asteroids hitting Earth on any regular basis.

But, for changes that are predictable, like season changes, animals do have adapations evolved for them. Like the changes animals go through for winter.

 

[Phobos]

I think you may have it backwards.
It takes extra trouble to make a cell that will age and die of its own accord.
This had to evolve.
Early cells apparently did not have this feature.
The original plan for singlecell life did not involve dying. Dying had to be
"invented" by evolution, as an "added wrinkle", because from the selfish gene standpoint it is an improvement.

Or could it be an unfortunate consequence of being a multicellular eukaryote? Perhaps because growth & reproduction are more difficult in that case than for bacteria. Do bacteria age (like you mentioned later)? I don't think so either, but I'm not sure. Anyone else? Seems like bacteria have a kind of immortality in that daughter cells are clones of parents, although individual cells certainly die from environmental causes.

If they didnt then they would fill up all the niche room and there would be no place to put the babies. So the genes wouldn't be able to try new combinations and evolution would grind to a halt. Some catastrophic thing like a new disease would probably get 'm

Seems to go against the selfish gene theory (not to harp on that topic, but I'm currently reading that book, so it's on my mind) because the focus is the "good of the species", not the gene.

I'll speculate a moment here...Whether the individual carrying the gene lives or dies may not matter to the gene so long as the individual passes on that gene at least once. Perhaps keeping a successful breeder alive longer would benefit that gene rather than taking risks with new potential breeders. There's no need to self-limit population size because nature will take care of that. New combinations could still be tried with each new breeding partner and the original parent could remain alive for as long as possible. However, such reproduction also presents the risk(?) of changing that gene itself in that next generation. A selfish gene may not want to evolve, but rather stick with high-fidelity duplication.

Not sure where I'm going with this thought, but I suspect I'm about to get off course, so I'll stop there.

 

[antiflag403]

The subject of why cells die and why we age is constantly debated. One interesting fact I do know is that heart cells are "programmed" to complete a set amount of cell divisions in a lifetime. If a heart cell is removed after 20 divisions, and frozen, when it is "thawed" it will complete 30 more divisions before dying. If it were removed after 30 divisions, it would complete 20 more before dying. The magic number is 50 (i think that's what it was). I found that very interesting.

NOTE: OK i just read that article that I previously skipped. I realize that the article says just about the same thing as what i have said above. Excuse my stupidity.

 

[Monique]

Caloric restriction has been shown to increase the life-span of an organism. During metabolism free radicals are formed, that attack the proteins and the DNA in a cell and induce mutations. Such continuous oxidative stress might induce a state of cellular senescence and thus a reduced tissue renewability.

Also, I think the loss of stem cells must play a major role in aging although I haven't seen any reports on it. Stem cells are involved in tissue regeneration and production of immune cells. Middle aged children have a very strong ability to heal wounds and have a strong immune system that diminishes over life. It would be hard to study since stem cells are tucked away in niches, but it would be interesting to find out whether centenarians have a higher count of stem cells. It (should) also possible to induce the proliferation of stem cells, I wonder whether that would increase life span (with a cautious eye out for tumor formation).

 

[ryokan]

If cells can always reproduce and replace themselves then why do living beings die?

I believe that the answers in this thread were more related with lifespan, aging, than with the fact of death. With independence of apoptosis at cellular level, death of complex organism seems to be due in most cases to mistakes (somatic mutations) or to lesions induced by environment (infectious agents, chemical, physical traumas...)
Most of people, most of animals, don't die by aging, but by disease or accident.
So the question on death in metazoans is, at the moment, enough clear without need of an evolutive selection of mortal versus immortal beings.
Another question is lifespan and its relationship with reproduction.
Another question is programmed death in cells.
I think that an excessive appeal to evolution in this case (with the selfish gene argument) could be here an unnecessary teleological argument.

 

[sciart]

The Second Law of Thermodynamics

http://china77.yeah.net/

 

[physicskid]

The Second Law of Thermodynamics

I agree with Sciart. The accumulation of entropy - like physical damages, little gene mutations, and etc. - leads to death being a neccesary occurence. Although these body cells with defects may be replaced by new cells, this would become unnecessary as the defects accumulates more and more as the individual grows old, and would be better for the gene to be passed through reproduction.

From the Evolution theory aspect, having longing age does not mean this 'advantageous trait' would be more widespread through natural selection. A person with longer age may not necessarily reproduce more or get to reproduce earlier than those who don't. He or she would still reproduce at the same rate as others would do, and longer life span is NOT a factor in sexual selection.


One the other hand, single-sex organisms get to live longer life - the most notable example are trees. There is no age limit to trees' reproduction process, and therefore tree with longer life reproduce more. Thus tree genes with longer life get to replicate more than tree genes with shorter life. Through natural selection, those trees with short lifes will be eliminated through nat. selection and therefore explains the long life span of trees.

 

[transit442003]

i am a christian and for obvious reasons this wil affect my response to this sort of question but plaese bare with me. i believe that the human body was made to live but not to die but because of the genesis downfall of our sin our punishment is death but that is not it for me, i believe that there is a life beyond death, heaven and hell.

 

[z4955]

if the reason for life is to reproduce and to die after reproducing, why do people who never have kids die? If that were the case, you would live until you reproduce, or forfeit reproducing in order to live longer/forever? It doesn't work that way, as we all know.

 

[ravisastry]

genes

survival of genes is more important than the individual. i read this at the begininig of this thread. how true it seems to be :) what is the information that our genes carry.. I am sure it is much more than just defining the physical characteristics of the individual.

 

[ravisastry]

im not sure about the evolution theory. but i feel, we pass on something.. some kinda information to our future generation. this is just a speculation of mine, but its worth giving a thought. what do I care the most.. continuation of my dogs family or my friends family or mine ? everything boils down to the individuals interest. 'I' come before my species, ie humans. why is it so ? I'm confused and I am moving towards philosophy :)

 

[scott_alexsk]

There was an interesting thread I read before in which someone asked for the mechanism for restraining the growth of a cell population. I responded that the current mechanism for restraining the growth of cell populations is in the cell multiplying only when there is an excess of the resources, it needs to survive.

When life first started out, it did not have a mechanism for restraining such growth, therefore from growing so much, a cell population might wipe out its own resources, therefore killing itself. The mechanism with expanding only in excess resources may have also evolved with killing off parts of the population as to control resource consumption, which I believe Phobos mentioned earlier.
-scott