Why We Die

[spamonkey8]

I've heard a few interesting theories relating to the fundamental reason why all living cells eventually deteriorate, and a new one occurred to me. First, I'll relate the most notable, which is based upon biochemical reasoning (don't worry, it's not hard to understand).

 

Oxygen, being nearly the most electronegative of the elements, has a tendency to 'steal' electrons from other compunds. One theory states that cell aging is derived from the slow breakdown of individual molecules that eventually lead to cell death. A notable byciclist whose name eludes me won the Tour de France a few times and then was forced to retire due to a very rare illness. It was determined that his cardiopulminary system was extraordinarily able to cycle blood and oxygenate it far beyond his peers. His disease was related to the mitochondrion in his cells (the part of the cell that uses oxygen to make the cell's energy, ATP). His mitochondrion were all failing and deteriorating. According to that theory, the increased flow of oxygen accellerated the decay that would normally have occurred. It is interesting to note that in this theory, our very life force is our inescapable undoing.

 

My theory relates to evolution and a species' lifespan. The now cliche phrase 'survival of the fittest' tells us that the species able to adapt the quickest to a changing environment will survive to procreate. To explain my point, imagine the development of a species that can live for hundreds of years. Assuming that the period of time in which the species is able to reproduce is substantial enough to match that of humans (roughly 35 years or half the average lifespan). This species would be able to pass on its genetic material for many hundreds of years. The problem with this hypothetical species is this:

 

If you extend the idea of employment in a business to the existance of a certain evolution of a species, then this species would have a problem with too little turnover. The older 'employees' would be alive and still reproducing. Many generations could have sprung forth while the ancestor of those generations is still reproducing. It would be like great-great-grandfathers and great-great-grandmothers having babies at the same time as their great-great-grandchildren are. Having the older genetic material still reproducing would limit the species' ability to adapt to changes as a whole. Unable to 'flush out' the previous generations which had not had as many chances to mutate into more fit creatures, the species would die out.

 

The only exception to this, I believe, is us humans. We are the only species on earth able to actively affect our environment and change it to suit us, so as long as we can keep from killing ourselves, we may never be forced to evolve. (In the grand scheme of things) In addition to us, species such as trees and Galapagos turtles live for exceptionally long periods of time, but both plants and reptiles have been noted for having a much longer cycle of adaptation.

 

Thus, if evolution is used to explain a species' lifespan, you must consider that it has never been a viable option for an organism to live for an extraordinary long time. Due to this fact, there would be no evolutionary drive to make a species that did not carry some inherent flaw that inevitably diminished its longevity.

 

I'm eager to hear what you think on the matter, both via replies and the poll.

[Mokele]

Interesting, but flawed, and I'll use one of your examples to show why:

 

In addition to us, species such as trees and Galapagos turtles live for exceptionally long periods of time, but both plants and reptiles have been noted for having a much longer cycle of adaptation.

 

Actually, the fossil record shows a fairly rapid level of adaptation for reptiles, for precisely the same reason your idea doesn't necessarily work.

 

Let's use crocodiles as an example.

 

First, while they *can* live over a century, few do. You are right about the retention of old individuals, and as a result the territorial battles are fearsome, and often lethal. But older crocs (which are bigger and stronger, since they never stop growing) eventually accumulate enough injuries to be killed by a usurper. The point is that potential lifespan doesn't equal actual lifespan.

 

However, as to species with longer lifespans adapting more slowly, I disagree. Again, consider crocs. Sure, they have a long life, even with premature death in the wild, but look at the reproductive output: a single female produces about 50 eggs a year, for about 40 years of reproductive life. That's 2000 baby crocs, and if they all survived, we'd be waist-deep in crocodiles. Instead, the young undergo horrificly intense selection, with only the very best surviving even to the 1st year. This is where selection acts, not on the old. As a result, even this long-lived species can evolve quite rapidly, as I'm sure you can see, if the environment changes to necessitate a change from the current form.

 

Also, old animals can always be displaced by animals which might be younger but also more vigorous (due to a beneficial mutation), especially in species like crocs with so many violent territorial battles. On top of that is sexual selection: if the new generation is just plain sexier, it doesn't matter how long the older ones live, as they'll not get any.

 

In fact, in species such as crocs, you get this sort of vicious cycle going. Natural selection will favor larger size, which means more eggs and also a longer lifespan. Natural selection will also favor just pure longer lifespan, as it increases reproductive output.

 

However, anything that reduces lifespan, no matter how good for the species, will die out, because "cheaters" who have longer lifespans (with the attendant increases in reproduction) will produce more kids and spread their cheater genes through the gene pool.

 

I would like to say thanks, though. Compared to some of the recent posts, this one is certainly a breath of fresh (and logical, and informed) air.

 

Mokele

[Anjruu]

Actually, I believe, though I cannot back it up, that, besides shrinking in size, crocs are the least changed species in the fossil record.

 

However, if an animal or individual inside a species were to be able to live for a long time, that would imply that the current genetic make-up "works", so evolution would not "want" to cause change, and if a sudden change in enviornment came along, the individuals would die prematurly, so their extra longevity would not come into effect.

 

For example, humans' possible life-span has remained the same, but with advances in medication, more easily accesible nutrition, ect, more of us have been nearing that age (the cut-off seems to be around 120). This has not stopped us from evolving, however, because those unsuited died long before reaching the cutoff. Therefore, a long lifespan, or lack of one, is not a tool of evolution, because those who die do so long before the age limit.

[PhDP]

Having the older genetic material still reproducing would limit the species' ability to adapt to changes as a whole.

 

It's an interesting reasoning, but it's flawed, because it's group selection... You have to ask, why would an individual A have a higher fitness than B if...

 

Therefore, a long lifespan, or lack of one, is not a tool of evolution

 

We age and we die because it maximize our fitness, I already wrote why in another topic;

 

http://www.scienceforums.net/forums/showthread.php?t=8988&page=2

[spamonkey8]

So perhaps the limiting factor of a species' lifespan is a function of how much it must adapt. In the case of crocodiles (great white sharks as well, to an extent), as has already been mentioned, the species as a whole has not needed to adapt greatly. So another point comes to mind: a species that needs to change less would, over time, generally be abe to live longer (with less early deaths due to more fit individuals as any age would be more or less equal), and therefore those individuals able to take advantage of the longer life would be naturally selected, as they would have more time to breed.

 

Bottom line, maybe we die because we have not been given more chance to live. If each generation is easily/likely improved upon by subsequent generations, a longer life's affect on reproduction would be easily offset by the exceeding fitness of adapted young.

 

While the common cockroach has changed little over millenia and yet does not live long (a seeming exception to my theory), I would like to call attention to the general rule that insects and small creatures have shorter lifespans. A cockroach's niche relies on the quick, small, and resilient characteristics only available to a small insect, and this takes precedence over longevity.

 

Thank you all so much for your thoughts, and I'm eager to hear more!

[Mokele]

Actually, I believe, though I cannot back it up, that, besides shrinking in size, crocs are the least changed species in the fossil record.

 

Yes and no. The main line of crocodilian evolution (the form you see now) has been pretty stable since the Triassic (though it's interesting to note that they originally looked like a reptilian greyhound, and the "crocs" of the Permian, phytosaurs, looked almost identical but was unrelated).

 

However, crocs have repeatedly had outbranchings that explore other ecological niches, including becoming fast-running land carnivores (and quite sucessful at that, for a long while), semi-aquatic clam eaters, diminutive vegetarians, and pelican-billed things which seem to have either been filter feeders or sifters like ducks. Most of these divergences have appeared in a geological instant, showing that, while they are perfect for their current niche, change is not only possible but rapid.

 

So perhaps the limiting factor of a species' lifespan is a function of how much it must adapt. In the case of crocodiles (great white sharks as well, to an extent), as has already been mentioned, the species as a whole has not needed to adapt greatly. So another point comes to mind: a species that needs to change less would, over time, generally be abe to live longer (with less early deaths due to more fit individuals as any age would be more or less equal), and therefore those individuals able to take advantage of the longer life would be naturally selected, as they would have more time to breed.

 

The problem with that is what "the species" needs is irrelevant. Cheaters who can live longer will still produce more offspring, spreading the "cheating" gene, even if it's detrimental to the species.

 

Also, it's contradicted by the availible data. Crocs live for a century, and haven't changed much (aside from the offshoots noted above) in 200 million years. But mayflies have been around and unchanged since at least the Carboniferous (around 300 mya), and their reproductive lifespan ranges from days to mere hours (nymphs may live for years, though).

 

If each generation is easily/likely improved upon by subsequent generations, a longer life's affect on reproduction would be easily offset by the exceeding fitness of adapted young.

 

Again, as Phil pointed out, it's group selection. Whether your kids are "better" than you doesn't matter. If you let them reproduce, you get grandkids that have a 0.25 relationship to you (25% chance of inheriting a particular gene from you), but you say "screw them", be selfish, and reproduce yourself, you make offspring with a 0.5 relationship. As you can see, once again, cheaters prosper.

 

A good counterpoint is whiptail lizards. Sex is important, since it mixes genes and contributes to genetic diversity, which is vital to the species evolution (especially in the face of fast-evoling biotic selective agents like parasites). But in Whiptail lizards, there are 17 species which are all-female, reproducing clonally through parthenogenesis. This is probably bad for the species, and they're probably screwed (pun intended) in an evolutionary time scale. But for the individual, it was a great boon. The first female that could do this could produce just as many eggs, but didn't have to dilute her genes with those of a male, in effect doubling her ability to pass on the genes for this ability (along with the rest of her genes). Before long, these asexual "cheaters" had spread and become the sole form of the species. This has also happened in geckos and snakes.

 

What's good for the species as a whole and helps it adapt is irrelevant, only what's good for the individual. The individual is the fundamental unit of selection, so self-sacrifice only makes sense if there's a genetic payoff for that individual (helping kin and the like).

 

Mokele

[spamonkey8]

But mayflies have been around and unchanged since at least the Carboniferous (around 300 mya), and their reproductive lifespan ranges from days to mere hours (nymphs may live for years, though).

 

Like I mentioned with the cockroaches, I think a mayfly's ecological niche is dependent upon its phisiology to such an extent that deviations that could potentially provide it with a longer life would be impossible to obtain.

 

I know that prehistoric insects were exceedingly large compared to today's specimens, but as far as age goes, the only insects I know of that are capable of living for a substantial length of time would be 18 year cicadas, which is largely spent in incubation. As a general rule, no insect as small as a mayfly could live exorbitant lengths of time.

 

Granted there is evidence opposing the idea, but I think that there are circumstances that take precedence over lifespan. If a mayfly is born that can live for years, it may be so only having sacrificed certain aspects required for its survival. Instead, it simply has a colossal reproductive yield to help make up for it.

 

If each generation is easily/likely improved upon by subsequent generations, a longer life's affect on reproduction would be easily offset by the exceeding fitness of adapted young.

 

Again, as Phil pointed out, it's group selection. Whether your kids are "better" than you doesn't matter. If you let them reproduce, you get grandkids that have a 0.25 relationship to you (25% chance of inheriting a particular gene from you), but you say "screw them", be selfish, and reproduce yourself, you make offspring with a 0.5 relationship. As you can see, once again, cheaters prosper.

 

This is all assuming that the grandfathers don't just look at grandma and say "screw her", going for the sexier/more fit younger generation. (I don't mean inbreeding, just consider how many horny old men get a prescription for Viagra and get a trophy wife in her 20's) This works both ways (grandma-grandpa & vice versa)

[Mokele]

This is all assuming that the grandfathers don't just look at grandma and say "screw her", going for the sexier/more fit younger generation. (I don't mean inbreeding, just consider how many horny old men get a prescription for Viagra and get a trophy wife in her 20's) This works both ways (grandma-grandpa & vice versa)

 

No, that's accounted for. After all, grandpa still has to be alive in order to take a more fit second mate.

 

The main point, however, is that even if short lifespan is good for the species in terms of adaptations, long lifespans will occur (when not simply necessary due to size or complexity of the organism) because if longer lifespan means more reproduction, it'll prosper, evolutionarily. "The good of the species" has only ever really been strived for by one species, and we've never gotten it right either, usually letting our selfish individual desires get in the way, like any other species.

 

Mokele