Why do we die?

I finally got around to finishing Greta Christina’s Comforting Thoughts About Death That Have Nothing to Do with God. It’s good! This book is the sort of thing atheism needs more of: an acknowledgment that the phenomena most important to human beings can be addressed effectively without imagining fantastic supernatural creatures. Atheists have this reputation of being nerds all wrapped up in abstract concepts and making arguments against the superstitious props that people claim to find useful in day-to-day life, and it’s good that some of us make the effort to show that no, we do deal with real-world concerns, and no, your myth is actually a terribly ineffective way of handling that problem.

So I guess it’s not surprising that my strategy for coping with death isn’t in Greta’s book. I take a developmental and evolutionary view of death.

We (and by ‘we’, I mean all animals) build our bodies in a slightly peculiar way. You might be thinking that we start with one cell, and it divides and divides and divides many times again to produce a mass of cells that then form tissues and organs, and within most of these tissues cells continue to divide throughout your life to maintain and repair yourself. Simple, right?

This isn’t quite true.

What we see in the pattern of cell division is that replication is dangerous: every division is a chance to produce, by accident, monstrous progeny which will endanger the primary evolutionary function of development (more about that in a moment). The worst thing you can do is allow a line of cells to replicate many, many times, because a long series of divisions is an opportunity to accumulate many small mutations, each contributing incrementally to the chance that a cell will go rogue and endanger the whole project.

This is why many biologists were completely unsurprised by that paper that showed that errors in cell division were the major contributor to cancer. Mitosis is a necessary evil. Indulging in it is the path to perdition.

So we animals are in an awkward position: we’re multicellular! We need to build these elaborate bodies that do complex things to shelter and propagate our germ line — the primary evolutionary function of development — and to do that, we need lots of cells derived from our initial zygote! Yet the process of making all those cells is fraught with hazard. How can we cope?

We have a couple of strategies. One is to put a strict time limit on our cells. Many of you have already heard of the Hayflick limit; if you extract human cells and grow them in culture, they’ll divide about 50 times before slowing and stopping. That’s still a lot of cells — 250 is impressive potential — but it’s still a finite number. Even in an environment with little stress and plenty of nutrients, these cells are literally programmed to die.

It’s the Logan’s Run strategy. Once you hit a certain age limit, in this case a certain number of cell divisions, it’s off to Carousel with you. Why is this useful for the organism (or in the case of the story, society)? Because older individuals are more likely to carry a dangerous load of accumulated mutations, so they must be destroyed for the good of the whole.

Another strategy is modularity. We want to limit the number of cell divisions, but we also need to build populations of cells large enough to do the work of forming a 70 kg human being, who will then continue to live and regenerate necessary replacement cells for 70 years or so. So what we do is allocate stem cell founder populations to different tissues. These stem cells divide slowly, minimizing accumulated damage, but produce progeny that divide more rapidly and more routinely…and then die off.

We can call this the Blade Runner strategy: the sheltered, protected founder population spawns workers that do the dangerous jobs, and then wink out before they turn monstrous.

Tyrell: The light that burns twice as bright burns half as long – and you have burned so very, very brightly, Roy. Look at you: you’re the Prodigal Son; you’re quite a prize!

Batty: I’ve done… questionable things.

Tyrell: Also extraordinary things; revel in your time.

That movie is also a good demonstration of why you want your hazard-handling clones to die before they get dangerous ideas.

We see the symptoms of these phenomena of division limits and modular division all the time. It’s called aging. Those sequestered populations of stem cells? They haven’t stopped dividing, they’re just dividing more slowly, and eventually they wear out and die, too, and fewer working cells are produced, and we see the symptoms: the epidermis thins, pigment cells deplete, wound-healing takes longer, the immune system loses resiliency. Take this example of a woman who died at 115 — she had virtually no stem cells left.

Another evolutionary point: just as the modules of your body are produced by a stem cell population, the whole of your body is in service to the primary stem cell population: the germ line. Those cells your gonads produce for the purpose of procreation. The whole of our bodies are an elaborate efflorescence that blooms gloriously for a few years to spread our seed, or to help others spread their seed. If the flower wilts after it has done its job, evolution doesn’t care — if another one blooms next year, it will continue.

So that’s how this atheist finds comfort in the thought of death: I wouldn’t be here in this complex corporeal form without it. Immortal cells don’t need to form massive assemblies of specialized cells to support their continued existence — that a few grams worth of germ line cells require tens of kilograms of me to propagate is a consequence of the inevitability of death. We are life’s answer to death, our existence is the way our line fights back.

We are all Roy Batty. All of our moments will be lost, like tears in rain…but weren’t they extraordinary moments?


  1. says

    By the way, I have no expectation that my answer will be at all comforting to most people. Read Greta’s book for answers with broader appeal.

  2. says

    All true I’m sure. As a computer science major I worked on genetic algorithms for my final year project and examined a bit of sexual selection pressure in the models I worked with.
    My solutions had 2 parts, a scored part and a match part. High scoring solutions would use the match part of their solution to find the ideal match for them to make the next generation. Both parts would be combined and mutated randomly.

    I expected the match solution to improve in fitness just as the scored solution improved, and it did for a while but quickly leveled out just below the scorer’s main fitness. I eventually realized that my high-scorers were always kept in the population – in effect they were immortal – but only provided they weren’t replaced with their better offspring.

    Time and again a population would hit a collection of high scorers that kept themselves there by only “mating” with inferior solutions and progress ground to a halt.

    Removing the selfish solutions from the population resolved the issue and we were able to produce high quality solutions again.

    So in effect, we could say we die because if we didn’t we’d be stunted inferior bodies wanting ugly children. And any competing population that did die would wipe us out by out evolving us.

  3. birgerjohansson says

    You might be interested in the role of retrotransposome elements and their role for ageing;
    Se Science Vol 346, issue 6214, pa ge 1187-1188 “Sleeping Dogs of the Genome”.
    Briefly, reverse trancription of L1 RNA is coupled to insertion into DNA at random sites thoroughout the genome.

  4. machintelligence says

    Because older individuals are more likely to carry a dangerous load of accumulated mutations, so they must be destroyed for the good of the whole.

    Which is essentially group selection, so I wouldn’t give that one much weight.

  5. says

    No, it’s not group selection.

    You, as an individual, will not be able to reproduce if your liver cells begin to proliferate riotously. There is selection for individuals whose pattern of development is constrained to improve their chances of successfully reproducing. Note that the part you quoted specifically refers to the organism.

    I suppose you could call multicellularity a group selectionist heresy, if you want. I wouldn’t.

  6. taxesmycredulity says

    Great post! Reading it, my first thought was “Thank god we die!” What a mess things would be if we didn’t. Then I realized I had said reflexively “Thank god.” OY!

    Surprised at only a few postings so far. My guess is you struck a chord that clangs and hits us where we live, and so temporarily at that.

  7. Bob Macias says

    Thanks, PZ… this subject is near and dear to me and is a constant source of inspiration when the crap of our daily lives begins to stink up the place. I wrote an essay about it a few years ago which encapsulates my personal philosophy and would love to know what you think: http://gortnation.blogspot.com/2011/08/meaning-of-life.html.

    So many of my family and friends are ill and are constantly complaining about their lousy luck. I just tell them their machine is either malfunctioning or broken, because their body IS a machine, albeit a biological one. Gotta keep it lubed and tuned-up, and repair it when it can be repaired. However, there comes a point with every biologic machine when no amount of new parts or repairs will suffice, because the machine is just worn out.

    Our evolved selves are pretty darned amazing, but not enough of us are aware of that… we take our conscious lives for granted, then look to the Invisible Sky Wizard for purpose and meaning. It ain’t there… it’s inside each of us, where it’s been all along.

  8. David Marjanović says

    Which is essentially group selection

    No, kin selection. :-) Only Henrietta Lacks has escaped, and not by her own doing.

  9. hiddenheart says

    I’m genuinely fascinated at the range of things people find comforting and otherwise useful. PZ, thanks for writing this up – there’s someone reading it who’s going to be genuinely grateful for it, getting precisely the relief they need, and I’m grateful for that!

  10. mistertwo says

    I like it. It’s really just another thing that can be comforting that may not apply to everyone. Essentially what I get from this is that parts of us live on through our offspring. Of course, not everyone produces offspring, but all of us ARE someone’s offspring, so this applies especially when we lose our parents. My father died in August at age 85, and it’s amazing how much I remind myself of him all of a sudden! And then there’s my 9 month old granddaughter who has inherited the family nose, so we can actually see a part of my father living on in another generation.

  11. Dhorvath, OM says

    I am too selfish to derive comfort from my genes propagating. Also too selfish to want to live forever, it’s not like it would be me who is alive, just someone who was me once. Memory and identity are fickle, and much like our genes they grow errors as we age, divorcing us from who we were.

  12. toska says

    Because older individuals are more likely to carry a dangerous load of accumulated mutations, so they must be destroyed for the good of the whole.

    Oh dear, I can see this one being taken out of context. PZ says old people should be killed off!

    Great post! I’ve always thought dying would be preferable to living forever. I would think existence would get tiring after the first few hundred years. And that’s before considering the biological implications of accumulating so many mutations. It doesn’t take away the pain of losing a loved one, but for what it’s worth, it does add to this atheist’s comfort with her own mortality.

  13. ragdish says


    Have you seen attack ships on fire of the shoulders of Orion? Have you seen c-beams glitter in the dark near Tannhauser Gate?
    Personally, I’ve seen a friend cut a loud fart at an IHOP. Now that moment truly needs to be lost in time. I recall it was a rainy day and we all of had tears.

  14. Azuma Hazuki says


    Incidentally this is also a problem for our Abrahamic friends, in a slightly different way: infinitely-extended duration is torture for a finite mind like ours, yet, if we are somehow changed not to mind it (and I think this would be a huge set of changes…) we wouldn’t be “us” any longer, so what was the point of heaven exactly? Heaven is, ironically, a larger problem for Christianity and Judaism than hell is!

  15. david73 says

    I found Einstein’s letter to Besso’s family on the death of his friend comforting- to paraphrase —
    He has preceded me by a little time but “for us physicists believe the separation between past, present, and future is only an illusion, although a convincing one.”

  16. Azuma Hazuki says

    *Islam, pardon me, not Judaism…though the ones that believe you spend eternity with Yahweh in olam-ha-ba have the same problem, now I think of it.

  17. ragdish says

    Thoughts from Master Oogway on death:
    Yesterday is history. Tomorrow is a mystery. But today is a gift. That is why we call it the present.

    And The Grass Roots:
    Sha la la la la la live for today
    Sha la la la la la live for today
    And don’t worry ’bout tomorrow, hey…..

  18. Rich Woods says

    @Kevin Kehres #9:

    Well, I think we’re just vectors for e. coli.

    @PZ #10:

    No, no, no: you’re a vector for spreading another single-celled organism, human gametes.

    Unless there have been drunken nights the details of which I’ve long since forgotten, and the outcome never brought to my attention, I’m pretty sure my lifetime ratio of success in being the former vector rather than the latter would result in a divide-by-zero error. So because evolution has no agency and no intent, how can it be said that I’m predominantly a vector for the latter?

  19. Rich Woods says

    @idiot me #26:

    I’d rather have said ‘as compared to’ rather than ‘rather than’. I bet there’s no gene for proof-reading.

  20. unclefrogy says

    ” ‘Tis a consummation
    Devoutly to be wished. To die, to sleep,
    To sleep, perchance to Dream; Aye, there’s the rub,”

    we are aware of our coming end but seldom ask what is this thing called now and how are we experiencing it. all wrapped up in our day to day thoughts.
    We so not look at ourselves as our biology doing what we need to stay living. We create worlds of ideas and concepts and live in them. We live in the biology on top of the chemistry covering the physics.
    uncle frogy

  21. Rich Woods says

    @unclefrogy #28:

    but seldom ask what is this thing called now and how are we experiencing it

    Entirely coincidentally, I am sitting here looking at the front cover of this week’s New Scientist, which bears the title ‘How Long Is Now? Your life plays out three seconds at a time’.

    I haven’t read it yet so I’m afraid I can’t give you a precis, but here’s the online teaser.

  22. Greta Christina says

    Thanks so much for the review, PZ! And I like your approach in this piece: if I ever publish a revised/ expanded version of the book, I may want to cite it.

    I would love to read the book, but I do not have a Kindle and Amazon still does not have a print version.

    Gregory in Seattle @ #18 (and others with this concern): You don’t need a Kindle to read the ebook. If you have a computer, a tablet, or a smart phone, you can read the book. And there’s also an audiobook version. (For those who really want a print edition, it is in the works, but it won’t be out until late summer.)

    I am too selfish to derive comfort from my genes propagating.

    Dhorvath, OM @ #19: I don’t give a damn about my genes propagating, either. Don’t have kids, never wanted ’em, never going to have ’em, more than okay with that. But I am selfish enough to care about my genes having been propagated, so I could be here. If I understand PZ correctly, mortality is literally necessary in order for me to have been born at all.

  23. says

    I want more life…fucker.

    I mean – I say that, but I’m not sure I’d know what to do with it. Not sure what to do with the 40-ish years I’ve got left, frankly. Not even sure what I’m doing next week – or right now, as it happens. Hmm.

  24. sawells says

    Great writing, PZ, as always. You should write a book :)

    Death is the price we pay for being multicellular. Or for being eukaryotic. Or just for being alive. Hang around too long, you’re just going to be outcompeted by a newer, better adapted population- either an external competitor, or an internal neoplasm.

    But I’m not sure “We are all Roy Batty” is really the slogan to march under :)

  25. magistramarla says

    I would like to see PZ’s response to this.
    I have autoimmune issues that usually don’t appear until the 4th or 5th decade, after the age of reproduction.
    I may also have spastic paraplegia, which is a genetic mutation that also is sneaky and doesn’t show up until after the organism is past the age of reproduction.
    Therefore, I had already given birth to five children while I was young and felt immortal. Even more, by the time I began to have symptoms and the doctors began to figure it out, several of my children had also already reproduced. I worry that my children and/or grandchildren will have to deal with these diseases.
    So, what do those of you with biology backgrounds have to say about genetic mutations that stealthily wait until the organism has reproduced before manifesting themselves?

  26. David Wilford says

    This pretty much says it all, IMO. It was a big hit song for Blood, Sweat and Tears, but she wrote it so here’s her singing it.

    And When I Die – Laura Nyro

  27. bluegreenplanet89 . says

    This may be a bit off topic, but I was thinking, isn’t Path to Perdition a great band name? No? Dibs anyway.

  28. procrastinatorordinaire says

    @PZ Myers #10

    No, no, no: you’re a vector for spreading another single-celled organism, human gametes

    No, no, no, no, no …. you’re a vector for spreading grasses. We are just pawns in their quest for global hegemony.

  29. fergl100 says

    Nice post, comforting to me anyway.

    “Because older individuals are more likely to carry a dangerous load of accumulated mutations, so they must be destroyed for the good of the whole.”

    Couldn’t people die purely as a result of the mitotic processes. After we have got beyond the reproductive years there is no evolutionary drive to keep us alive so we die. Nothing to do with for the good of anything?

  30. judykomorita says

    That kind of thinking does fine for we or our loved ones who die of old age. But for those of us who have lost a child, NOTHING helps.

    Not heaven, not nirvana, not valhalla, not so-old-we-become-dangerous, and not “live life well and be happy we had the chance to live it.”


    We just carry around a fiery, bleeding hole in our hearts where our child used to be — every day for the rest of our lives.

    Fuck death and fuck whatever brings it.

  31. says

    I hate the “immortality would be boring” argument. Yes, many dying people accept death as the alternative to never ending sickness, or loneliness, or despair, but how many last words amount to “been there, done that”?

  32. brett says

    I hate it, too. For one thing, your perspective would change – you’d feel a lot more comfortable letting things ride out, laying out gradual plans over years, and then just enjoying life and being comfortable amidst the process. If you decided to spend 50 years working an easy job while taking night classes on all kinds of stuff, you could do it.

    Just in general, the value of immortality isn’t that you don’t die. It’s that you get to choose when you want to die, barring accidents, illness, and foul play. If you wanted to live a couple of centuries and then decided you were tired of it all and ready to die for good, you could.

  33. Dhorvath, OM says

    Greta C,

    If I understand PZ correctly, mortality is literally necessary in order for me to have been born at all.

    I had less coherence earlier than my comment required to make sense. I didn’t quote the relevant text

    Essentially what I get from this is that parts of us live on through our offspring.

    from mistertwo @ 17 that my response was constructed towards.
    I do have a child, and we get along just fine, at least so far, and with a little work on both our ends should continue that through the rest of the lifetime that we share. But he isn’t me, and when I die, (which with any manner of luck will be the earlier of us because I would prefer that he has as full a life as he can,) I expect to cease while he will continue. This doesn’t feel anything like comfort for the fact that I will die, anymore than his being successful would cause me to embrace failure. I want all I can wring out of life.

  34. Tigger_the_Wing, asking "Where's the justice?" says

    Like magistramarla, I had already produced five offspring, and had grandchildren, by the time I was diagnosed with genetic disorders.

    How do we explain these aberrations, especially the ones that don’t shorten our lifespan? They don’t prevent the affected person from reproducing, they don’t even take us out of the population any earlier, they just make for a shitty old age.

    I’m so sorry, judykomorita.

    Looking forward to a ‘good’ death after living long enough is a luxury that few people get. And no other death is ‘good’. Far too many lives are cut short by a death which isn’t necessary in the least, and causes enormous harm to the people left behind. You are right – death isn’t something for our species blithely to accept, but to rail against. We should examine and eliminate all the causes of premature death and death that is only welcome because of disease etc. making life unbearable, until every one of us can live just as long as each of us wants to.

    P.S. So, if my ancestors hadn’t died, then I almost certainly wouldn’t exist. Of course, I wouldn’t be around to regret my non-existence, would I? I’m perfectly resigned, even a tad contented, at this stage to leave the world to future generations. If those generations find a way to conquer disease, stabilise the population and only die when they want to, why shouldn’t they do that?

  35. SpaceGhoti says

    A thought has been percolating in my layman’s brain for a couple of days, particularly about stem cells. What happens to a geriatric zebrafish who gets a fresh infusion of zebrafish stem cells?

  36. says

    I don’t know. It ought to reduce the symptoms of aging.

    The unfortunate side-effect you might expect, though, is that if the stem cells were produced by multiple generations in culture, they’d have a greater likelihood of going cancerous.

  37. SpaceGhoti says

    Thank you very much. I think it sounds like it might be an interesting experiment for the minions students. Would it ease the impact of aging or extend the life of the organism? What effect from multiple culture generations? What effect from non-cultured cells taken from eggs?

  38. says

    @ procrastinatorordinaire #41

    No, no, no, no, no …. you’re a vector for spreading grasses. We are just pawns in their quest for global hegemony.

    From John Green’s admittedly terrible zombie apocalypse novella:

    Pre-zombification, my father was already obsessed with corn. He told me almost every day that corn was in control of us. Corn wants the world to contain more corn, so corn evolves us to agree with it: Corn tells us that we could make sugar out of corn, or fuel out of corn, or plastic out of corn, etc. The flu makes us cough, which spreads the flu. Corn makes us corn-hungry, which spreads corn. He got this idea from a book, and he never ceased to be amazed by it. For years, he would talk about it. We’d be eating mashed potatoes or something, and he’d say, “You know, potatoes are impossible without corn. That corn, it’s everywhere.” (He meant this metaphorically, although it is now nearly true literally.)