Sunday, May 10, 2009

What causes aging?

In Tolkein’s Lord of the Rings (LOTR) the elves are immortal in the sense that they do not die of old age. They can, of course, be killed by accidents or in battle, be murdered, etc. Man, by contrast, is “doomed to die”, the phrase that appears in the poem about the Nine Rings that opens the novel. Of course, JRRT had a Christian perspective, and viewed an inherent mortality as a planned condition that is part of a scheme to save humankind from a worst fate that would await him if death did not deliver him from the corrupting evils of a “fallen world”. It presumes a sort of “planned obsolescence” that would pave the way for a second life that would, presumably, be free from such spiritually corrupting influences.

Well, me, while I love the mystery and the strange ambiance of LOTR, I don’t know anything for certain about any such second lives. For one thing, I cannot imagine what reality such a second life might be like, and in particular how it might be free from the potential for evil. But this is a complicated issue, to say the least, and it is not what I want to dwell on at the moment. Rather, all this leads me to thinking: what is the scientific reason for the human body ageing and, on a time scale of-- for most of us--- “four score” (80 years), rather rapidly wearing out and dying of “old age”? (if something else doesn’t get us first, of course.)

I always look at such issues from the game theory perspective of evolutionary biology. And it seems to me that the situation of Tolkein’s elves suggests the reason for the body wearing out and the body’s processes grinding to a halt. Namely, there is some expectation value of elapsed time before an accident causes death, and nature does not want to waste any efforts in assuring that the body and mind will last much beyond that average time interval. By the way, by “accidental death” I mean to include all of the causes of death that are not associated with the total body simply wearing out. That is, everything except “dying of old age”.

It is often noted that once the organism has produced offspring, and has ceased being fertile, we can expect it to be on borrowed time for some interval after that. But this begs the question. Why is fertility of finite duration? I suspect that the answer is again, that there is little to be gained by engineering a body that can continue to reproduce for a time that greatly exceeds the temporal expectation value for accidental death.

The wikipedia page on fertility in both human males and females has, in this regard, an unsatisfactory section called “the cause of the decline”. But the title of this section is incorrect because it really just deals with the statistics on when fertility falls off. In other words, it does not address what the real cause of the decline is.

I want to add---perhaps unnecessarily—that when I speak of such things as “nature engineering something” I do not mean that I impute conscious design to any entity outside of nature, or to nature herself. I view this in a Smithsonian sense as being one of those phenomena under the control of an invisible hand, and consider genetic evolution to be a sort of computer that through natural selection reaches an optimal solution.

I look forward to looking into the popular evolutionary literature on what is thought to cause aging and death. The above comments are really intended to simply raise the issue, and I plan to revisit the topic after I have done a bit of research. I noted today a chapter in J. Diamond’s book “The Third Chimpanzee” that speculates about the issue of aging, and I plan to read that first.


Fred said...

Tom, considering aging, there do seem to be limits on how long the human body lasts. We can positively influence the length of life by using good habits such as eating organically, eating high amounts of fruits and vegetables, stimulating the mind at all ages, avoiding high stress, and being physically active throughout life.

The book "The Blue Zones" examines some of the habits of the most long-lived peoples. Aging apparently is fairly complicated as so many things can bring on aging and death, so we can at this time only treat or influence aging in a general way. But I think loss of production of enzymes, beneficial bacteria, and energy-producing compounds, and the general clogging and harming of the body's natural processes through the intake of fast food, sugars, and toxins from water and food supply play a large role.

Consideration of why the time limits exist is interesting. Since some animals and insects live for very short times, say weeks, and other live for perhaps a few hundred years, it might be fruitful to group the short, middle, and long-lived in three or four groups based on average life length, and examine any commonalities such as diet, size, local, stress, reproduction, sun exposure, group relations, etc. Likely this has been done by researchers already, but a fresh look is sometimes rewarding.


Tom said...

Those are good points Fred. Yes, I agree that healthy living and the right nutrition can no doubt lengthen human life spans by some small amount. And the Jared Diamond chapter on aging in "The Third Chimpanzee" (the book referenced in the original blog entry on this) agrees with what you say about how there are many complex factors, not just one or two, in determining the progression of aging.

I just a read science fiction book, "The Old Man’s War" by John Scalzi where the interesting idea is floated that the way to get around aging is to project the consciousness of an aged individual into a completely new bionically engineered youthful body. I don’t know if that is possible---it raises what are as yet poorly understood aspects of the relationship of a given set of cells and molecules to an individual’s sense of self.

For now I fear we are inextricably bound to being what William Yates poignantly referred to in his poem “Sailing to Byzantium” as “a soul attached to a dying animal”.

Your idea about the various classes of animals with such disparate life spans is intriguing. I think that there are tiny insects that live only a day or so, while there are large (relatively predator free) animals such as certain elephants and tortoises that can live as long or longer than humans. This all seems to comport well with the theory that “mother nature” doesn’t waste any time prolonging a lifespan (through regeneration of cells) beyond a time interval that is reasonably consistent with the mean time between fatal accidents. Small bugs, for example, probably are likely to meet with an “accident” in the form of being gobbled up by a bird or other predator within a few weeks or so. And I doubt whether there are many predators that would try to take down an elephant.

There are numerous web sites that present tables of typical life spans (just a google search on "life span of animals" will bring up dozens of them). A big surprise to me is that certain birds such as parrots and cockatoos can live for a century or so, according to these sites. I am not sure how that fits the “mean time between accidents” theory.

Cindy said...

Regarding your piece on aging, the idea that – “There is some expectation value of elapsed time before an accident causes death, and nature does not want to waste any efforts in assuring that the body and mind will last much beyond that average time interval” is interesting. But based on that idea, wouldn’t we age even faster, since evolutionarily speaking, we have spent most of history dying way younger than 80? But, that aside, I am intrigued by this part of that statement - nature does not want to waste any efforts. I’m not sure what “effort” it would be. Unless what you are really saying is that there would be no point in using extra resources to sustain health when you should already be gone? Is that what is meant by effort? Because, you know, as long as the being (man or animal) is still productive, no reason not to blow a few resources on him! The bigger “waste” is to use resources to keep alive people that are decrepit and, not to be nasty, but really pretty useless, evolutionarily speaking. If no one gets decrepit, they are worth the resources. It seems that nature was playing the odds that no one would outlast accidental death and therefore hang around to chew up resources. So, from a nature engineering viewpoint, why not let people (and animals) stay viable until they die, so the resources (“effort”) is always used on something worthwhile no matter what the age?

The reproduction thing is interesting as well. Do we die of old age because we cannot reproduce and therefore serve no purpose? Or does reproduction stop because our body is going to crap and could not bear healthy children, or have the energy to raise them? Chicken or egg, my dear. Perhaps we are not allowed to reproduce after a certain age because nature wants to limit the number of offspring from each female to ensure genetic diversity. If a female can start reproducing at, say 14, and can have one (multiple births being rare) a year, and we shut down at, say 50, that’s 36 children from one woman. Although the fathers would likely not all be the same, there’s enough potential for inbreeding even in a reasonable sized community right there; even if, evolutionarily speaking one half to three quarters of the offspring didn’t live to maturity. To go past 50 and keep producing indefinitely would cause even more inbreeding risks – perhaps the age is based on the maximum number of offspring from one woman to lessen inbreeding probabilities.

Men stay fertile longer. Why are females more limited? My theory is that men wandered far and wide, scattering their seed where ever they could. Women tended to stay in one community. With the scattered seed approach, for half sister and half brother, or whatever else permutation, to mate was perhaps less of a risk. Whereas with the female, and therefore her offspring, tending to stay in the same mating pool, the odds get way more dangerous.

Seen from this viewpoint, the philandering male mating wherever he gets the chance is a good thing. A necessary thing. Else inbreeding would have destroyed the whole race! See, we give you guys such crap about that. But, you know, it was really probably necessary for genetic mixing of the pot. Men aren’t morale degenerates; they’re drawn to a lifestyle geared towards keeping the species safe. Not horn-dogs, but heroes! Not that I recommend that now, of course, because women have become much better shots these days. You can run, and you can mate, but you can’t hide!

Tom said...

Ah, let me clarify what I meant by the phrase, "nature does not want to waste any effort...". I was making an analogy here, and viewing evolution as a type of "computational process" with the gene pool. Sort of along the lines of Adam Smith's "Invisible Hand" in free market economics.

I agree with your point about how the end of fertility and the age of death are closely related, or intertwined. I was struck by the point that Richard Dawkins makes in, I think, his book The Selfish Gene. He says that at some point the best gene strategy for females, being generally more nurturing than males, is to stop reproducing themselves and instead devoting their efforts to grand children, where they have a 1/4 genetic interest. Males, being less inclined to nurture grand children, continue to attempt to spread their genes by impregnation of females, where they have a gene interest of 1/2 with any offspring that result. This would seem to explain why males do not go through menopause where they stop being fertile, and also why perhaps males do not live quite as long as females. Hmmm….I wonder if there is any anthropological data that would confirm that that has always been the case?

But I do think that Tolkein’s elves, who were imagined to live forever if they were not killed by accident or murdered, gives insight into why organisms live a finite number of years. That is, at some point, the probability of being killed by accident (P) approaches unity. I was suggesting that the evolutionary process in effect cuts off the life span of a given type of organism at some P less than unity.

32oH2O said...

I just watched the movie, "In Time". Though it has some major shortfalls in how modern economic systems work, it does show a new way of looking at the world, one in which the natural lifespan is no longer a constraint within our system. I find it worth watching just to see the different paradigm.

Regarding your aging posts: Evolution follows many design strategies. (Although I use verbs indicating evolution acting, evolution is a passive finding algorithm rather than an active searching algorithm.) One strategy is to invest a lot of knowledge and social skills into turning babies into functional adults (examples include elephants, dolphins, apes, people, and parrots). Another strategy is to create a multitude of spawn, let them all go into the world and try to create more spawn (examples include rabbits, doves, trout, mayflies).

Keep in mind that successful evolving species/individuals do exactly one thing--reproduce. Though sometimes at odds, they must reproduce the species, the individual, and the sex.

Let’s look at a population of apelike creatures on some planet. The females effectively breed at 15 years, peak at 20, and wane linearly to 0% fertility at 45. The males effectively breed at 16 years, peak at 27, and wane linearly to 0% fertility at 70. This apelike species has attained steady state population in its environment. Now, let’s say that a major mutation hits half of the population instantly increasing the aging dynamic by a factor of ten. The methuselah still have the same growth and development potential. But the 0% fertility ages will be 450 years and 700 years respectively between the sexes. Let’s make the tiny assumption that the mutation came with a conspicuous marker such as a blue nose, or rainbow hair so that the methuselah and apelikes never reproduce together—they are different species. Let’s make the huge assumption that the methuselah and apelikes can go through the social transition and several lifespans without anything happening and reach a steady state population. Let’s make the steady state population be 50% of each species. So we now have set up the two independent species which are identical except for life span…

Things start to happen in the environment. A baseball sized meteor blasts into the side of a mountain causing a rockslide. Some radon-rich rock slides down the hill next to a stand of aspen trees. A benign plant bacteria which carries small amounts of host dna gets hit in the solar plexus by a gamma decay and BAMMM! The amount of xylan in the plant world doubles as the xylan producing gene gets spread among much of the flora. Now, which population is going to be the first to exploit this new energy source. The apelikes go through generations on the order of 10 times as fast as the methuselah. Mutations won’t be 10 times as fast since some are time induced, but the sex-based mutations will be. The methuselah clearly have a disadvantage in this game. What happens if we slightly relax the rule against reproducing with a Methuselah (more like in the real world)? Then it doesn’t matter which species gets the xylase gene. The apelikes are going to outcompete the methuselah.

Aging makes room for the young. The young are more likely to carry and transfer mutations. Mutations give the species a chance to thrive in an environment of changing constraints. Getting old is only a positive factor for the species if it helps the species reproduce or adapt. The plain fact is that even if you can reproduce, your offspring can do that, and they can adapt better than you.

So, "What causes aging?" The answer may well be that aging is an evolutionary advance making the species more apt to survive in its environment than the species preceding it.