From a biologist’s perspective, death isn’t just inevitable; it’s an inherent part of the process of living. We are not static; living things dwell in a domain of constant, dynamic churn. Our cells build proteins that they nearly instantly dismantle. Most of our tissues continuously shed cells and generate new ones. So while our overall appearance seems constant, undergirding it is a near-perpetual cycling. When that cycling stops, we’re dead.
That incessant turnover gives life its flexibility and responsiveness, but there’s a cost: it also involves the slow, inescapable accumulation of error, and, in complex systems like us, some errors can lead to catastrophic collapse. A small error in the construction of an arterial wall leads to a weak spot that balloons outward and ruptures in a deadly aneurysm. A skin cell acquires a tiny genetic error that uncouples the brakes on the control of cell division, and it blossoms into a madly proliferating colony of cancer cells. No process can be perfectly flawless, and when you’re built out of trillions of cells and are losing and replacing hundreds of millions of cells every minute, even a minuscule error rate of one in a million converges on certainty. It’s actually astounding that we human beings can usually hold ourselves together for three score and ten years.
We have evolved mechanisms to minimize error: there are multiple layers of error correction in DNA replication, for instance. But all processes that correct errors will also have rates of failure, so nothing can reduce the buildup of mistakes to zero.
Another way our bodies minimize the likelihood of catastrophic failure is to implicitly acknowledge the inevitability of fatal errors in populations of cells and limit reproduction. A healthy cell is allowed to replicate itself only about fifty times before programmed cell death kills it. Why? Because a cell that old is likely to carry enough defects that it is prone to go rogue or fail to do its job. It will instead be replaced with a relatively young cell (in terms of number of cell divisions) from a slow growing, sheltered population of stem cells.
It’s a smart strategy, honed by a billion years of successful reproduction. In our development, we build tissues and organs in a modular fashion, setting aside groups of cells that will construct the tissue and maintain it. The founding population will contain a small reserve—the stem cells—that will be held back while the other cells divide rapidly to form the tissue and then do the intended work of the organ; they’ll divide and divide until they expire, and then replacements will be doled out from the stem cell reserve to allow prolonged operation. One of the causes of aging is that the stock of stem cells becomes depleted, and there is a dearth of replacements to fill in for normal attrition.
You might be wondering: If error is cumulative and inevitable, and if we multicellular animals have to limit cell divisions to prevent runaway errors, how can we evolve? Shouldn’t the cells of our wormlike ancestor from about a billion years ago have fully exhausted their potential and collapsed into unavoidable corruption by now? And on a shorter scale, doesn’t this mean our children, the products of our dividing cells, have inherited depleted prospects?The answer to that is, again, death.
We produce gametes and offspring prolifically, and the overwhelming majority of them die. Men produce a billion sperm every month; women set aside a few thousand cells as ova. Almost all of them are lost. They are produced and put through a gauntlet, in which the majority is destroyed, failures at the game of fertilization. Even the sperm and eggs that manage to fuse are tested—and many fail, with approximately half of all conceived humans being aborted naturally, as a consequence of errors. The winnowing is intense. The children brought to full term are the products of cells functionally screened for a lack of devastating genetic problems, basically resetting the ticking time bomb of decline to a fresh state. Of course, destructive errors may still appear. And because of the intrinsically imperfect process of replication, offspring cannot be perfect clones of their parents; every human is born with on the order of one hundred novel, generally non-lethal mutations.
Biology and life simply do not work without this pattern of routine rebirth, reuse, renewal, and recycling. As I sit here, I am invisibly shedding a thin cloud of outer skin cells, which are replaced by burgeoning deeper cells; squamous cells lining my throat and gut are pushed out by dividing cells beneath them; red blood cells collapse dead and exhausted, are filtered out by my liver and broken down, and their numbers renewed by budding colonies of progenitor cells in my bone marrow. Even in my stillest, quietest moments, this seething metamorphosis is going on, as bits of me die and new bits are born. You are never at rest; you are always in flux.
As it is with our cells, so it is with us as individuals. We are born, we flourish briefly, we fade and die, and we are replaced with new individuals. Even as culture seems to change only slowly, there is a constant alchemical shift—150,000 people die every day in the world, an immense ongoing tragedy, but over 300,000 are born, so the population goes on, and even grows.
And here’s the personally important point: it’s that resiliency and capacity for change that the death of individuals bestows on a population. Without it, I wouldn’t be here for my brief tenure as a living, breathing, thinking person. Without death, without change, evolution wouldn’t be possible, and Earth’s inhabitants would never have progressed beyond the state of mere fermenting vesicles of simmering chemical broth. It is the transience of individuals that gives the population the potential for growth and change.
Every once in a while, you hear someone talking about science defeating death. It won’t happen. It requires a fundamental misunderstanding of the nature of life to think it can continue without death . . . because you can’t be human without the prospect of change. And I rather like being human.