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?