Lobstrosity

That's a good argument, but it doesn't really get to the heart of the matter. Malookie could easily just ask why individuals get less fit as they age, why evolution hasn't eliminated this trend. Clearly as we develop from adolescence we increase in fitness. Why does there have to be any subsequent decline? Why can't it level off into steady-state? We take aging as a fundamental property of life because most life on Earth does it. We see it as obvious that as you get older you get weaker and less fit, but is that really the way things have to be? Why don't the cells in our body continue to reproduce as efficiently as they do when we're in the prime of our lives? I personally like the hypothesis that aging is the mechanism life evolved to combat overpopulation.

malookiemaloo

I don't think the over population theory is a sufficient explanation.

Bearing in mind about four fifths (I think this is the correct figure) of those who have ever lived are living now and the population is increasing rapidly and there is still enough food for everyone (although not distributed properly) death cannot exist as a sort of population cull. Aside from everything else, this would imply evolution being pro-active when it would appear that evolution can only ever be reactive. Or have I got that bit wrong?


m

Nic Tamzek

Sorry, I think all these "for the good of the species" and "prevent overpopulation" arguments are officially bunk. The key to the matter is not survival but reproducing as many surviving offspring as fast as possible. She who accomplishes this will crowd out all other genotypes in short order due to exponential growth. From the "overpopulating" gene's "perspective", it doesn't even matter if overpopulation occurs, because even if half the population dies for exceeding the carrying capacity, the "overpopulator" will be disproportionally represented among the survivors, so it still wins.

Darwin even had to deal with this one:

When you consider the prehistoric primate/hominid death rate and life expectancy due to disease, starvation, injury, childbirth, etc., hardly anyone ever made it long enough to actually get around to dying of old age. This is mostly a modern-times thing. The name of the game is to have as many successful offspring as possible before your personal body is too beat up to reproduce. Evolution probably *could* modify animals to have super-healing capacity and no degradation with age, but this would suck resources away from baby making. Better for the selfish genes to use disposable gene containers, in other words.

Once language etc. evolved there probably was some selection for increased lifespan due to the information value of elders to the rest of their family, which may account for the relatively longer lifespan of humans compared to chimps (although the the world's oldest chimpanzee is 71, so perhaps even the beneficial-elder-language theory is bunk.

Lifespan in most multicellular species appears to correlate mostly with size (bigger things take longer to grow) and perhaps metabolic rate (in bad conditions everything goes slower so it takes longer to reach maturity). I expect this explains things to a first approximation.

Lobstrosity

Here you're referring to humans whereas I was referring to more basic forms of life. Humans are a rather big exception to the rule because we have been able to so rapidly augment our own physical abilities with technology. This has allowed us to more than double our lifespans over the last millenium or two. Furthermore, this has allowed us access to resources that we could not otherwise reach. Increases in technology beget more resources, which in turn allows for more population growth. Eventually we will saturate this planet's carrying capacity and overpopulation will be a problem humans will have to face (assuming we aren't intelligent enough to proactively head off this problem before it becomes too late). Right now we're just in the middle of using technology to exploit an ecological niche that's basically empty (at least as far as we're concerned since we can just irradicate anything else occupying that niche). If humans keep growing at this rate we will eventually start to run out of water as rivers feeding major residential areas are bled dry. We will not be able to produce enough food and mass famine will ensue. Problems will abound and people will die off in large numbers if programs aren't implemented to curb our reproduction rates. Humans are statistical outliers because we were able to change our lifespans over such a short time period that evolution just couldn't keep up. What you need to do is look at species that are living in near-evolutionary equilibrium, not humans. Death is not an effective population cull for humans, but it is for other animals. If you think about it, you can see that it allows them to come to equilibrium population distributions even in the absence of large-scale predation.

Lobstrosity

First and foremost, I think this is far from obvious, yet you present it as if it's a fact that doesn't even require justification. Why should one assume that cellular reproduction without degradation would have any effect on our ability to make babies? I could make the same argument and say that vision sucks valuable resources away from baby-making, but clearly it doesn't. I think this is a weak argument you're posing here, especially when quite the opposite seems to be true. We are at our reproductive prime when our cells are replicating without much degradation.

Secondly, Nic, it appears evolution has modified animals to actually have degradation with age. I don't think it's fair to say that we have evidence that aging is just the default natural scheme that we would have to evolve away from. It seems that it might be well be a more complex addition to the genome that tells the cells to start reproducing less efficiently. I say this because there are actually organisms out there that do not experience cellular degradation with age. There are plants that do not experience "aging" in the same way animals do. Similarly, there are single-celled organisms that can reproduce without degradation. Furthermore, scientists in the labs have been able to produce human cells that don't degrade as they reproduce (I believe they did this by toying with the telomeres, but I'm not sure).

If a creature only dies due to injury or predation, that gives it the chance to live longer and make more offspring, which, as you point out, would naively seem to be beneficial to those genes. Predators could live for hundreds of years and make thousands of babies, but for some reason they don't. You say it doesn't matter if overpopulation occurs, but I disagree. Overpopulation risks the extinction of the species every time it occurs. If severe enough, the population will not be able to bounce back. It is by no means unreasonable to postulate that evolution would selectively weed out such species over time in favor of ones that found ways of smoothing the amplitude of boom/bust cycles. Ideally one wants steady-state population sizes, not gigantic swings that could destablize everything.

I would argue that average in-population lifespan in most equilibrium multicellular species correlates better with reproduction rate than it does with size. I think your argument might explain things to a zeroth-order approximation, but I would not expect such an approximation to be very good. In some species out-of-population lifespan (i.e. how long they live in the absence of injury and predation) is very important as they rarely die from anything other than old age and its effect on their ability to obtain needed resources (anything that experiences minor to no predation would probably fall under this category). In others predators probably are the dominant factor in lifespan, and for them I would just venture that aging is either a stablizing factor should the predator populations decline or perhaps just simply vestigal (much in the same way as creatures that live in dark caves or in the deep sea retain vestigal eyes).

malookiemaloo

I'm confused.

I always understood evolution to be biological. You appear to be saying that it is (or can be) technological.

Also evolution must surely be more than adapting to conditions? I am told that a horse used to be a small animal that has 'evolved' into what it is today. Clearly, if that is the case, something more than merely adapting to conditions has occurred.

Finally I do not think thast humans lifespan has increased at all. However, more people now live out their 'alloted span' which alters the statistics.


m

Lobstrosity

Evolution is biological, but it's driven by environment (i.e. natural selection). The human environment is dominated by technology and thus technology will have an impact on future human evolution (either indirectly via natural selection or directly in the form of genetic engineering). Anyway, this is beyond the point. I never implied (or at least I never meant to imply, in case I inadvertently did) that evolution was technological; my statement was that technology has had a dramatic impact on how the human animal interacts with its environment.

No, evolution is pretty much just adapting to conditions. Changes in the environment result in changes in survival rates among the population's individuals. These survival rates are in turn partially functions of the individual's genetic makeup. As a result, the genetic makeup of the population changes non-trivially with time, driven by environmental changes. Mathematically one can see the changes driving the population along a highly complex fitness surface towards local maxima. Because the genetic component of evolution is randomly produced (sexual reproduction and mutations result in random probing of the fitness phase space), a species can genetically drift randomly along fitness-surface contours. As a result you get both random and non-random contributions that produce some rather interesting results. But in short, the main effect of evolution is adaptation.

It depends on how we define lifespan. If you took a bunch of humans from 2000 years ago and raised them in today's world, they would almost certainly have the same average lifespan as modern humans. In short, I do not assert that their genetically-encoded lifespans (if aging is in fact genetically encoded) have changed over the last 2000 years. All I'm saying is that if you measure the average lifespan of people today and compare it with the average lifespan of people 2000 years ago, you will find that it has approximately doubled (or something on that order of magnitude). This is due to mainly to developments in medicine. I'm not sure what you mean by "allotted span," however. All people eventually die from something. If they live too long they'll get cancer or suffer a stroke or experience heart failure. I don't really see how you can view this as their "allotted span" but not also consider early deaths due to small pox and influeza "allotted spans." What makes one more allotted than another?

malookiemaloo

Thanks for your responses.

So when Fred Hoyle said that the evolution of the horse was due to better nutrition, he was right.

By 'alloted span' I am referring, of course, to the Biblical 70 years which does not appear to have been improved upon!!

Still don't think science explains WHY death exists though.


m

Lobstrosity

Just out of curiosity, if the allotted span is 70 years, why do people sometimes live longer than that? Is that just supposed to be an average? Another thing to note is that there are populations of people in certain areas who live on average longer than 70 years. For example, in Okinawa the average life expectancy for women is 85.1 years (but sadly only 77.2 years for men).

malookiemaloo

Yes it is an approximation.

I'm paraphrasing but the Bible says something like 'your days will be three score and ten years then you shall die. If by reason of strength it be four score and ten, yet you shall still die'.


m