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Comment by jostmey | Hacker News Reader

I actually thought this article did a pretty good job explaining why we age. As pointed out, our bodies fall apart because they are the product of an evolutionary process.

Our genetic code is like a computer program that runs into a bunch of fatal errors after it has been left running for too long. These fatal errors do not matter to our genes provided enough time has elapsed since our birth for our DNA to pass on several copies of itself. There is simply an absence of selective pressure for us to live past a certain reproductive age.

(If I wrote in laymen's terms it is because I am a laymen on the issue, but a well informed one at that.)

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Yes, that is one explanation, but it's refuted by other evidence. (And the article covers this.)

The main problem with the argument is that our body does a great job of fixing errors - so why does that process suddenly seem to stop? Why not just keep on fixing errors. If you can fix errors for 40 years why stop there? Why does the process suddenly stop at that age?

It's not like there is a gradual accumulation of errors - it's no errors for 40 years, then suddenly error correction mode is switched off. Why?

It also just raises the question of why there is a reproductive age in the first place? Why does that stop? (For women anyway - it doesn't seem to stop for men, so the evolutionary argument fails there too.)

But the biggest argument against that theory is other animals. Why do they have different lifespans? The genetic machinery for all mammals is more or less the same.

First of all, there are a number of things that go wrong in our bodies as we age. Certainly genetic errors build up over time and are left uncorrected. But this is just one of many things that go wrong. The reason our bodies are allowed to degrade in the first place is because we have usually already survived well into our reproductive age.

The evolutionary theories of aging actually provide a pretty solid explanation as to why separate species have different lifespans. Some species take longer to reach reproductive maturity than others. Some species reproduce more slowly than others, which in turn effects how quickly the animal may age. Individuals in some species don't survive very long due to external pressures such as starvation and predation, which also shapes lifespan.

The best introduction into the evolutionary theories of aging I have found is: http://www.genetics.org/content/156/3/927.full.pdf . It is worth noting this theory explains human mortality curves once external causes of death have been removed.

Would it not make more evolutionary sense to remain reproductively capable for hundreds or thousands of years?

The motive for genes to reproduce does not end after a single set of offspring. In fact, I don't think the motivation is even reduced.

The mathematics of life is exponential growth. Two begets four begets eight and so on. So if you happen to die from old age after the fifth or sixth generation, the effect on how many copies your genes make of themselves is marginal.

You are completely forgetting that if the parent organism has extra copies so do all the children.

Compare 2^100 vs 3^100:

    2^100: 1267650600228229401496703205376
    3^100: 515377520732011331036461129765621272702107522001

No, I am not. I assumed that the offspring will die of old age too.

You neglected the variable of time. That third offspring will be born after the second one, which is born after the first one. You need to use the Euler-Lotka equation. The mathematics was worked out centuries ago. Here is the wikipedia article

http://en.wikipedia.org/wiki/Euler%E2%80%93Lotka_equation

> No, I am not. I assumed that the offspring will die of old age too.

How does that change the conclusion? 3 children vs 2 is still immensely more children, not just a marginal amount as you thought.

When that third child is being born the second child is already beginning to reproduce (if you reproduce like Yeast). Clearly, that second child is more important because it has already given you a grandchild when your third child is still in the process of being born. That second child has already resulted into two offspring, and counts for twice as much.

That only matters at a specific moment in time, but if you take a slightly longer view that 3rd child had an enormous impact.

> That second child has already resulted into two offspring, and counts for twice as much.

Only for one generation, but we care about many many generations. 10 generation down the line the slight difference in timing makes no difference.

But the increase in number made an enormous difference.

You really need to sit and run some numbers and you'll see you are mistaken.

Maybe your intuition will help you if think of it as a steady state - babies in (born), babies out (die), rather than thinking about one moment in time. In a steady state the timing make no difference (because you've already waited out the time) - only the number matters.

> Clearly, that second child is more important because it has already given you a grandchild when your third child is still in the process of being born. That second child has already resulted into two offspring, and counts for twice as much.

Well, to pull the idea from my own other comment... a child+grandchild pair is not twice as good as a child even if all you care about is that specific moment in time). It's 1.5 times as good, because a grandchild is only half as good as a child.

And (as ars says) once all concerned have died, the timing offset is of no significance whatever. However, for the impact on your total representation in the world of the present (no particular present), which does have significance... see my other comment, where you're still quite wrong. The Euler-Lotka equation values all members of the population equally; that has no relationship to reality when you're talking about (in your own exact words) "how many copies your genes make of themselves".

It's the standard problem of tradeoffs in evolution.

Strong is better than not strong. But a stronger model comes at its own fitness costs: maybe it requires more energy upkeep, maybe it sacrifices speed, etc. At some point, fitness benefit from additional strength is less than the fitness cost of the weakness it must accept.

Likewise, an infinitely-living, infinitely fertile organism is better (more fit) than a finite one. (The environmental cost is not a problem for those genes.) But the costs of having it eventually exceed the benefits: perhaps it requires a more robust replication process that's too energy-intensive.

OTOH, if somehow you could reduce this error rate that leads to aging without having to sacrifice in some other way, that would be a mystery.

Think about it from the perspective of genetic material temporarily 'trapped' in the body like passengers on a boat. All else being equal, a new boat is better than an old boat. And, a new body is better than an old body (even if all the error correction machinery is there and fully working, a 26 year-old body has an advantage over a 46 year-old one, or an 86 year-old one). After the genetic material has been transferred to a new body, via reproduction, i.e. the passengers have been transferred to the new boat, what purpose does the old boat serve other than to consume resources that could be used by the new one?

I think there is a case to be made that death gives organisms, that is to say it gives genetic material (which is the important bit, not the organism which is a mere vessel for the genes) an evolutionary advantage.

No. Evolution selects based on relative fitness. Killing older members is obviously more efficient for the population as a whole. However individual genes that "defect" and continue reproducing into old age will make more copies and spread themselves more into the next generation than the "aging" genes. Even if it's inefficient and they use more resources (which isn't true, it takes a lot of effort and energy to raise a child, keeping an existing body is way better.)

But, the genes are in the new body as well? So, wouldn't the genes that move to a new host and program the old host to expire be better off?

Genes do not move, they replicate. The old host did not lose the genes when reproducing.

I think the question really is, if the inbuilt mechanisms are working efficiently, why should there be a difference between 46 year old vs 26 year old. I the replenishment of the cells go intact why should it matter how old one is, unless there is something inhering in the genetic material itself that is time bound.

A large life-span would starve the younger generations of resources. And the younger generations are probably more crucial to the survival of the gene pool, than the older generations for various reasons. One reason is that younger generations might be better adapted to the environment because of mutations.

Another reason could be that the brains and bodies of older generations will eventually get saturated / tired. Keeping them alive is inefficient for the species.

"Keeping them alive is inefficient for the species."

A lot of that is based on the idea of a nuclear American 1950s family, mom dad and 2.5 kids in a house and grannie lives in Florida 1000 miles away.

More likely, if I lived for 1000 years I'd spend a lot of time ensuring the economic success of my descendants by teaching programming, electronics, maybe math unless a better mathematician comes along. If this situation existed in the past, I have a GGGGG-uncle (cousin?) who was a Harvard literature prof in the early 1800s and I'm sure he would have taken a decade out of his semi-eternal life to educate me on English lit and related topics. I have some GGGG-Uncles who were in the civil war and fought in Tennessee, surely their personal experiences would have more impact than merely reading a book.

This has certain economic and evolutionary effects, such that a "migrant farm harvesting laborer" not allowed to ever benefit from his own labor, would not be able to help his GGGG-grandkids in any obvious way. So some kids are going to grow up with 16 hours a day of continuous one on one academic tutoring from a team of perhaps a dozen PHD ancestors, while other kids are going to grow up basically playing video games, or at best, watching youtube videos.

Its an obvious sci-fi topic to explore the nature vs nurture question. A permanent overclass would likely develop based solely on ancestry (indirectly "nature" although in practice "nurture")

This is aside from obvious social interaction. Locally I know a lot of the people in my field, and a descendent going into my line of work could be financially (aka reproductively) rewarded by hanging out with me and meeting people. If I do nothing for a descendent other than socially introduce them to local hiring managers, that's still genetically a net win. Everyone else will be doing the same thing so I will have to keep up.

If the Red Queen hypothesis (http://en.wikipedia.org/wiki/Red_Queen_hypothesis) is correct, failing to mutate (e.g. via long lifespans and continued reproduction) would be an evolutionary disadvantage. I don't know enough on the topic to have an opinion about the accuracy of the hypothesis, but it's something to think about when you think about this topic.

No:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjourna...

"Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual, and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model for aging based on assumptions that are compatible with evolutionary theory: i) competition is between individuals; ii) there is some degree of locality, so quite often competition will be between parents and their progeny; iii) optimal conditions are not stationary, and mutation helps each species to keep competitive. When conditions change, a senescent species can drive immortal competitors to extinction. This counter-intuitive result arises from the pruning caused by the death of elder individuals. When there is change and mutation, each generation is slightly better adapted to the new conditions, but some older individuals survive by chance. Senescence can eliminate those from the genetic pool. Even though individual selection forces can sometimes win over group selection ones, it is not exactly the individual that is selected but its lineage. While senescence damages the individuals and has an evolutionary cost, it has a benefit of its own. It allows each lineage to adapt faster to changing conditions. We age because the world changes."

>drive immortal competitors to extinction.

That's something I wouldn't want to be quoted on. :-P

Seriously though, it's making a group selection argument, which stacks the deck against it: any gene that weeds out the host for the benefit of the "generation" will be strongly selected against.

I like how all the replies to your post are appealing to tradition.

“If you lived forever you'd starve people of resources and never evolve, that'd be bad!” If you lived forever, you wouldn't need to evolve, because you wouldn't be dying due to the environment. If you are still alive: you are well suited to be. Reproduction is just a form of genetic immortality if not individual immortality.

All of this presupposes that some immortal people were born and just died out because being immortal wouldn't be rewarded by reproducing.

And ignores the fact that animals that live for a very long time reproduce slower than animals that live for a short time.

> The main problem with the argument is that our body does a great job of fixing errors - so why does that process suddenly seem to stop? Why not just keep on fixing errors. If you can fix errors for 40 years why stop there? Why does the process suddenly stop at that age?

Because then you compete for resources with your offspring and limit the chances of passing on your genetic code.

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