As I'm watching the leaves go from bright to dark green under days of cool rain, there's one thing sure -- there will come a spring I will not see. I can hope next spring will not be that spring, but that spring will get here. Still, if it holds off a decade or so, maybe three, I'd be fine with that. After all, just because something's inevitable doesn't mean we have to look forward to it.
Still, death is clearly a huge evolutionary success for life.
It may not seem obvious that death is a product of evolution, but it is -- and in some ways it's a fairly advanced featured. Understand we're not just talking about generic death. Any organism can end up a snack for a predator, or fall victim to starvation or an accident. We're talking genuine planned obsolescence -- the same thing that does in my vacuum cleaner on a regular basis and guarantees I have plenty of dead electronics on e-cycle day. We, like other advanced animals, don't just fall apart out of some mistake, we're designed to die.
It didn't have to be that way. Far from being programmed for self-destruction, some organisms are incredibly good at living.
Deep under New Mexico, there are caverns filled with salt crystals. That salt came from a sea that dried up more than 250 million years ago during the Permian, before there were such things as birds or mammals. Before there were dinosaurs. In tiny voids within those crystals there are still minuscule droplets of water from that long vanished sea, and in those water drops there are bacteria. When these bacteria are set free from their little salt tombs and given more water and food, they pick up swimming and reproducing just as they were doing more than 100 million years before T. rex made the scene.
That's shouldn't be too surprising. After all, how old is any bacteria? Bacteria, like most single celled organisms, reproduce primarily through fission -- splitting into two copies of the original cell. In a matter of minutes, or days, or years they do it again. They can pull off this nifty trick an unlimited number of times because each bacteria contains a small circular chromosome that can be easily and completely copied by polymerase, the chemical that does the genetic transcription job in both bacteria and humans. So how old is each copy of the bacteria?
Other single celled organisms also have a loose relationship with death. Even simple multicellular organisms may be essentially ageless. Some hydrozoans (predatory animals that range from species of microscopic polyps to big colonial "jellyfish" like the Portuguese Man o' War) appear to age and grow for a few months, then reverse the process and get small and "young" again. They may bounce between the adult and child stage (if those labels make any sense in this situation) an unlimited number of times.
However, most complex organisms have an expiration date built in. Why? Technically it's because these organisms keep their chromosomes arranged in a linear fashion not as neatly handled by good old polymerase. Instead another chemical, telomerase, is added to the process to keep the copies copying. As time passes, telomerase runs short, polymerase starts nipping the end off chromosomes and like a photocopier running low on toner the process starts to unravel. On a less technical level... why? Couldn't we have developed a better system? If 4 billion years of evolution was enough to drive all the diversity of life on Earth, couldn't we find a replacement for our sloppy-copy process? The answer is yes, probably. But the reason we haven't is that death has turned out to be rather a good thing. That copy machine that runs dry after a set number of copies isn't there by accident. It's an advantage.
At first glance, that little "invalid after threescore and ten" stamp may seem like a losing proposition, and it's certainly not pleasant for the individual. But the universality of aging and death among complex organisms gives a hint at its usefulness. There may well have been immortal species of higher animals at some time in the past. Heck, this trait may have popped up more than once. It's just that all the immortals got knocked off by the transients. Apparently, there can be only none.
The reason for this is that immortality comes with some fairly significant disadvantages. A large complex organism requires a good bit of resources and the environment offers only so many available niches in which organisms of a set design can live. If the landscape is already saturated by unaging oldsters doing their timeless thing, there's little room for new and at least possibly improved models to take the stage. For most organisms, the areas where life is most tenuous is at the ends; both predators and disease take the hardest toll on the very young and the very old. If there are no old, then additional stress could be placed on the newcomers, further cutting turnover in a population. That means little chance for newbies, with their occasional mutations and interesting new combinations of genes. For immortals, evolution runs in slow motion.
Immortal species simply don't change quickly enough to keep up. Except, of course, for the immortals that we do know about, those bacteria. Despite their ability to keep on going and going, bacteria change rapidly enough to not only keep up with the changes in death-enabled species, they can outrace all our attempts to fend them off with the latest antibiotics. They can do this for three reasons: they're small, they reproduce quickly, and they have a nifty genetic trick. Though bacteria reproduce by splitting down the middle and making two copies, that doesn't mean they never engage in microscopic sex. Bacteria can, and do, exchange little genetic packages among themselves. They even do this across what we consider to be species boundaries. In essence all bacteria are able to dip into a universal genetic tool box and take out screws or bolts that may be helpful.
When you add this to their sheer numbers (a spoonful of productive soil can have around 1 billion bacteria, more than 1,000 species and hundreds of billions of individuals are at this moment living their immortal lives in your mortal gut), their rapid reproduction (in good conditions, they can do the split about every ten minutes), and their ability to slip each other a little somethin' somethin', bacteria have no trouble keeping up an enormous diversity.
Against that kind of genetic flexibility, what are large organisms that exist in much smaller populations and take much longer to reproduce going to do? Die, that's what. They engineer a system in which the previous generation is constantly forced off stage, freeing up roles for a new mix of genes. Product engineers designing the next great gadget follow a similar path. Yes, eventually we may all drop our iWidget and crack its fragile glass, assuring a new sale, but they can't take that chance. Instead there's always something, like say a battery that craps out after a few years the replacement of which is problematic. Could they design it to make it easier to keep what you have? Yes, but why should they? After all, consumers don't punish them for making their device disposable. Building in a lifespan assures that generations of device will shuffle off the mortal coil pretty much on cue. If there was a gene that could force you to explode immediately after passing along your share of genes to the next generation, a lot of organisms would probably carry it (and mating seasons would be a lot more noisy).
Fortunately for me and my fellow human beings, we don't go up in flames at thirty. We may not get the two hundred years or more that some organisms enjoy, but for a mid-sized mammal, people get a pretty good deal when it comes to time. All of us are familiar with how short a life our companion animals have in comparison. At seventeen, my son was still a kid, but at the same age his poor Golden Retriever was down to her last days. Many larger animals, a cow for example, rarely live to twenty even if no one is eying them for steaks, and plenty of people-sized creatures (kangaroos, for one) don't even get a decade.
Still, despite all those charts of average lifespan over time, our potential for extended life isn't a new invention. Our closest living relatives, chimpanzees, live only 30-40 years out in the forest, but are capable of living 60 years or longer when removed from the stress and resource restrictions of the wild. Humans seem to be right in line with other primates of similar size. The biggest threat to maximizing the potential of many human beings today is not the wear and tear of fending for themselves, but the ease and abundance that causes us to eat, drink, and merry ourselves into an early grave (says the guy who downed a s'mores pop-tart and three cups of coffee while typing this article).
The reason that humans and other primates get that relatively long post-reproductive life is likely because it's proved an advantage. It takes so long to accumulate knowledge of environment, tools, and a complex social hierarchy that there's an advantage in keeping pop, and even grandpop around to teach their hard-won wisdom to the young 'ens. Among many primates, individuals past the rigorous battle for reproductive rights still have positions within the community. Though now that we have Wikipedia, perhaps that explosive gene will start making an appearance.
There will come a spring I will not see, but I've seen this one and I have the poison ivy blisters to prove it. And I think next month I'm going to go out and get a new iPhone. At two years, the one I have is getting really old.