Long for This World: The Strange Science of Immortality - Jonathan Weiner (2010)


So teach us to number our days,
That we may apply our hearts unto wisdom.



Mortality is at our core. We are long for this world, compared with life in the microcosm of the paramecium, the bacterium, or the Tokophrya standing on the pillar of its holdfast. We have a greater portion of time than most of the other living things with which we share this planet. And yet how we long for this world, how we wish we had more years to explore and enjoy it! How sharply we feel, at every moment of our lives, that mortality is deeply ingrained within us!

“To be a philosopher is to learn how to die,” said Montaigne. But as a thinker during the Renaissance, he didn’t have much time to learn to do it. He wrote in his tower, in his final essay, “Experience,”

“I have recently passed six years beyond the age of fifty, which some nations, not without cause, have prescribed as such a proper limit of life that they allowed no one to exceed it. Yet I still have flashes of recovery.”

I’m glad we live at a time when a writer who has just turned fifty-six is not all that old. (Flashes of decrepitude here, but still young enough to go forward, I hope.)

“Write as if you were dying,” Annie Dillard advises in her book The Writing Life. “At the same time, assume you write for an audience consisting solely of terminal patients. That is, after all, the case.”

And that has always been the case, although now we live in a moment when, as we philosophize and grope toward wisdom, we can wonder just what and how different the term and the sentence may be, and if, and when, and what then.

From the beginning our philosophers have tried to teach us how to die, and our poets have taught us that to contemplate death is to learn how to live. Seneca wrote, “We must make ready for death before we make ready for life.” “You have noticed that everything an Indian does is in a circle, and that is because the Power of the World always works in circles, and everything tries to be round,” said Black Elk, the Oglala Sioux holy man. “The sky is round, and I have heard that the earth is round like a ball, and so are all the stars. The wind, in its greatest power, whirls; birds make their nests in circles, for theirs is the same religion as ours…. Even the seasons form a great circle in their changing, and always come back again to where they were. The life of a man is a circle from childhood to childhood, and so it is in everything where power moves.” Walt Whitman ends his poem “Out of the Cradle Endlessly Rocking” with a paean to “death, death, death, death.”

I once met Aubrey and Adelaide at the Eagle with my family; he’d offered to take us punting on the Cam. But it rained that day, and we ducked into the Fitzwilliam Museum instead. The glass displays in the museum included one of Isaac Newton’s notebooks and a few of Charles Darwin’s letters. Aubrey strode through the halls of treasures at the same clip and with the same degree of interest with which he’d hurried through Ravenna. He was trying, as always, to recruit my boys to his cause. At one point I stopped at a glass case to read the manuscript of John Keats’s “Ode to a Nightingale,” which the poet wrote one morning on Hampstead Heath, a short walk from the house where we were staying in London.

I have been half in love with easeful death….

Keats had a year and some months to live when he wrote that line, at the age of twenty-three. He had already lost his brother Tom to tuberculosis, and caught the disease himself. While I leaned over the glass museum case, one of my sons squatted on the floor with his back against the wall, and Aubrey settled down right next to him, urgently explaining his own plans for the engineering of thousand-year lives.

From the first age to our own, mortality has been the theme of writers, including the writers who loomed like immortals to my generation, the giants whose very names can still make us feel as small and hopeless as epigones, even though they are all going now or gone, after all that jockeying for immortality. Norman Mailer wrote about WASPS: “They had divorced themselves from odor in order to dominate time, and thereby see if they were able to deliver themselves from death.” Saul Bellow took John Cheever to the Russian baths in Chicago. “Wreathed in vapor he looked more immortal than I,” Cheever reported in a letter to his brother, “but I think he was trying.” “God save us from ever ending, though billions have,” John Updike wrote in his last cycle of poems, Endpoint, when he was dying of cancer at Massachusetts General Hospital.

Mortality, impermanence, ephemerality: this has been the great theme of modern science, too. Galileo’s discovery of sunspots ran counter to traditional astronomy and its view of the sun as immortal. People had always thought the sun was perfect, eternal, and spotless; he argued that the sun could be mortal and decay—like the rest of us. “It proves nothing to say…that it is unbelievable for the dark spots to exist in the sun because the sun is a most lucid body,” Galileo wrote impatiently. “So long as men were in fact obliged to call the sun ‘most pure and most lucid,’ no shadows or impurities whatever had been perceived in it; but now it shows itself to us as partly impure and spotty, why should we not call it ‘spotted and not pure’? For names and attributes must be accommodated to the essence of things, and not the essence to the names, since things come first and names afterwards.”

Galileo saw ruin not only in the sun but also in the moon, when he pointed his telescope there. And he much preferred a cosmos in motion and even in decay to a cosmos that, once created, never changed:

I cannot hear it to be attributed to natural bodies, for a great honour and perfection that they are impassible, immutable, inalterable, &c…. It is my opinion that the Earth is very noble and admirable, by reason of so many and so different alterations, mutations, generations, &c. which are incessantly made therein; and if without being subject to any alteration, it had been all one vast heap of sand, or a masse of Jasper…wherein nothing had ever grown, altered, or changed, I should have esteemed it a lump of no benefit to the World, full of idlenesse, and in a word superfluous, and as if it had never been in nature; and should make the same difference in it, as between a living and a dead creature: The like I say of the Moon, Jupiter, and all other Globes of the World.

And this has been the drift of science ever since, in the discovery of the deep geological layers of the earth, and the vast numbers of species that have gone extinct, to be preserved only within those layers; and in the lives and deaths of the stars; and in the cycle of the life and death of the universe itself—all of which those sunspots prefigured.

If anything, the cosmos of science is as ephemeral as the cosmos of Buddha, who founded a religion on evanescence, as on a rock. Siddhimagertha Gautama, who became the Buddha, wearying when very young of the sights and dread of mortality, shocked by his first sight of an old man by the side of the road, left Lumbini on a pilgrimage into the mountains:

“Grieve not for me,” he said, “but mourn for those who stay behind, bound by longings to which the fruit is sorrow…for what confidence have we in life when death is ever at hand?…Even were I to return to my kindred by reason of affection, yet we should be divided in the end by death. The meeting and parting of living things is as when clouds having come together drift apart again, or as when the leaves are parted from the trees. There is nothing we may call our own in a union that is but a dream.”

Mortality is the central fact of our lives. Contrary to rumor, we do know it even when we are young. We are adept at pushing the thought away, but it is there with us almost from the beginning. There are times in every life when we find it hard to think about and impossible to withdraw from. We try to number our days, so that we may apply our hearts unto wisdom, as we are advised in the Psalms. And it is essential to us at any age to know or to guess roughly where we are in our time—because that knowledge does teach us how to live. Laura L. Carstensen, a psychologist at Stanford, has presented an interesting paper in Science, “The Influence of a Sense of Time on Human Development.” When we have reason to believe that we have decades ahead of us—our whole life ahead of us, as we say—we focus our energies on adventures, new experiences, learning new things: the advancement of learning. When we believe that we have very little time left, we focus more on experiences that have emotional meaning for us; the meaning we have found and made.

For Carstensen and her colleagues, this helps make sense of what psychologists have sometimes called the “paradox of aging.” Older people tend to want to spend their time within a small social circle of a few close friends and loved ones. They want to focus their time and energy where they have already found their greatest satisfaction. And though their world is smaller, they often say they are as happy as young people, if not happier.

Ask old people how they want to spend time, and almost always that is what they say: they want to spend it with their loved ones. Young people asked the same questions will choose to spend time on new experiences. In one test, Carstensen showed people a travel poster with the usual spread of photographs: a cheetah, a parrot, a family picnicking on a trip, the Sphinx. One poster carried the message, “Capture those special moments.” The other poster read, “Capture the unexplored world.” The old people in the study chose to capture those special moments; the young people were more attracted to the unexplored world. “Young or old, when people perceive time as finite,” Carstensen writes, “they attach greater importance to finding emotional meaning and satisfaction from life and invest fewer resources into gathering information and expanding horizons.” And when we see time as virtually infinite, our priorities reverse.

In one experiment, Carstensen and her colleagues asked their subjects to imagine that their doctor had just called to say that science had made a medical breakthrough, which would give them many more years to live. Now they were willing and eager to spend time with new people and broaden their horizons. But if they were asked to imagine that they would soon leave their homes and move somewhere very far away, most of them said that they would spend their remaining time with a few of the people they were closest to. Young and old had the same reaction. What mattered here was not how old they were, how much time they’d lived, but how much time they thought they had ahead. Carstensen writes, “Preferences long thought to reflect intractable effects of biological or psychological aging appear fluid and malleable.”

When we’re in the first few ages of man, the last few ages seem very far away. But we do know those last ages are there, and death is there. And it is healthy and adaptive to know that; to number our days, that we may try to be wise; even if we are adept a moment later at pushing the thought away.

When we arrive at the late ages we are still consumed with the problem of mortality and still adept at pushing it away. In fact, when we’re old (having arrived at that state as if suddenly), mortality means so much to us that it might crowd out everything else, if we weren’t so good at thinking about it and then trying to ignore it again. An old New Yorker cartoon shows a man of a certain age reading the obituaries and thinking: Twelve years older than me…. Five years older than me…. My God, exactly my age…. People have computed that way since the days of the first newspapers, sometimes with a frisson of fear, but often with a strange feeling of comfort afterward. As Dr. Johnson observes, “The computer refers none of his calculations to his own tenure, but persists, in contempt of probability, to foretell old age to himself, and believes that he is marked out to reach the utmost verge of human existence, and see thousands and ten thousands fall into the grave.”

From beginning to end it is this knowledge of the limit, the endpoint, death, that looms largest in our calculations and our struggles, and touches us most deeply in the stories of the struggles of our heroes. At the Eagle, patrons often wander under the blood-red ceiling of the RAF Room and read the initials of the pilots, the nicknames of their squadrons and commanders. “Donald Jimmie Moore.” “Bert’s Boys.” “The Pressure Boys.” You can make out the form of a woman who floats across the ceiling like a constellation. She is remembered in the pub as Ethel. She may have been the land-lady’s sister. Apparently the young airmen lifted her up to the ceiling one night and drew her outline in lipstick, and apparently she had lost her clothes.

The young airmen wrote up there with their lighters, with that lipstick, with candles, and with charcoal from the fireplace.

“Alis Nocturnes,” a motto: “On the Wings of the Night.”

“58.” The Fifty-eighth squadron was commanded by Sir Arthur Travers Harris, known as “Bomber” Harris to the press and as “Butcher” Harris to his men. The men were as young as seventeen, but they knew.

In what is now the Eagle’s DNA Room, James Watson was oppressed by a sense of his own mortality. Watson was convinced that great scientists achieve breakthroughs by the age of twenty-five, and he barely made it. Soon after his eureka moment with Crick, and their victory lunch, Watson made his way to Paris, where he did not have much luck finding girls, in spite of his bohemian long hair and sneakers. He ends The Double Helix on a melancholy note, staring at the girls near Saint-Germain-des-Prés: “I was twenty-five and too old to be unusual.”

Among the innumerable things it does to us, mortality binds us in mutual piety, when we’re young, like those pilots. The problem of mortality pushes us to choose a path; it goads us to accomplish something, like Watson and Crick. We push it away with all of the missions that fill the first ages of man, but we know the problem is there, and it goads us to ask the largest questions—questions of ultimate meaning; questions we might never think to ask if we had all the time in the world.

This problem of mortality will define our next years and decades. It will involve not only writers, philosophers, and biologists but also sociologists, economists, and politicians. It will weigh on our minds at least as much as at any time in history, with or without the discovery of an elixir of youth. Because of our success on the planet, we face a new era even without the elixir.

“Very long lives are not the distant privilege of remote future generations,” according to an analysis by the Danish gerontologist Kaare Christensen and colleagues; “very long lives are the probable destiny of most people alive now in developed countries.” Life expectancy has been rising on a straight line for more than 165 years. This linear progress “does not suggest a looming limit to human life span,” they argue. “If life expectancy were approaching a limit, some deceleration of progress would probably occur. Continued progress in the longest-living populations suggests that we are not close to a limit, and further rise in life expectancy seems likely.”

Life expectancy has doubled over the past two hundred years, and in the last half century most of that rise came from improvements in the lives of the old, whereas before, it had come from improvements for the young. The number of centenarians on the planet has more or less doubled with every decade since 1960. At the moment, Japan is the country that offers the most years of life to its citizens. In 1950 a woman in Japan who had just reached age 65 could expect to live another thirteen years. Fifty years later, a Japanese woman who reached age 65 could expect another twenty-two years. In 1950, her chance of reaching age 100 was less than one in a thousand. By 2002, her chance was one in twenty.

According to Christensen, the elderly in Denmark are living longer without spending more years sick, frail, and in pain. A recent study followed more than two thousand elderly Danes. Between ages 92 and 100, the number of those who could live independently, shopping, cooking, and bathing, declined only slightly, from 39 percent to 33 percent. Even at age 100, one in three Danes was still independent. That’s pleasant news for warm, fallible human computers of a certain age (although it’s not quite as comforting as it sounds, because most of those 92-year-olds never made it to 100).

These forecasts could be wrong. The long rise in life expectancy through history has been broken here and there, chipped into jagged and serrated edges like a flint knife, interrupted by the great wars, famines, and epidemics. In the fourteenth century the Great Plague killed nearly half the population of Europe. Baby boomers have to look only one generation back to remember the global cataclysm that brought them into the world. More than 50 million people died in World War Two. Russian men died in such numbers during the war that there was a shortage of able-bodied males in the Soviet Union for a whole generation. Lately the life expectancy of Russian men has been declining again, because of too little work, too little food and medicine, and too much vodka and tobacco.

S. Jay Olshansky, a well-respected demographer at the School of Public Health at the University of Illinois in Chicago, thinks we’re already at the limit of human life expectancy. He believes that we are not likely to extend it beyond about eighty-five years. He warns that in the United States, average life expectancy may soon begin to decline, as in Russia, because of too many burgers and fries.

More optimistic demographers point out that Olshansky has been wrong before. In 1990, he predicted that life expectancy at age fifty would not exceed thirty-five years “unless major breakthroughs occur in controlling the fundamental rate of aging.” As things turned out, Japanese women were already exceeding this life expectancy by 1996.

The implications of these changes for the world’s economies are very mixed. In Italy and Spain today there are now almost twice as many old people as young people. With so many old and so few young, those countries and many others may be in for hard times during the next few decades. As one population expert in Washington has put it, you can’t keep going with the pyramid of civilization standing on its vertex. You can’t run a village, much less a country, if most of your people are in nursing homes. Chekhov wrote a short story about a coffin-maker that begins, “The town was small, worse than a village, and in it lived almost none but old people, who died so rarely it was even annoying. And in the hospital and jail there was very little demand for coffins. In short, business was bad.”

If we are going to turn our population pyramid upside down in the next decades—and that’s what will happen, if it stands at all—then we are looking at a highly unstable situation, socially and politically. “A civilization has the same fragility as a life,” said Paul Valéry. Challenges to civil values, if they are too great, can lead to civil wars. What happens as the baby boomers go gray all over the world and have to be carried on the backs of their small number of adult children? The lengthening of our life span is the crowning achievement of our species, but the crown is heavy and the head that wears the crown is gray.

Global graying will be one of the great challenges of this century. Demographers will argue about the details for the rest of their lives and ours, just as climate scientists will argue about the details of global warming. But about the very broadest features, there are very few skeptics. We are living longer and staying healthy and vigorous later in life, and every man and woman on the street knows it. You can monitor global graying in your own hair. You can time it by the watch on your own wrist. Barring an apocalypse, the generations of humans alive today can expect to live longer (at least a little longer) than any generation before us.

No matter what else happens with the science of aging, more and more of us will follow it as global graying advances.

Since the problem of mortality will be so much on our minds, whatever our age, we will be watching this science from all sides. We’ll argue not only the feasibility of its goals but their desirability.

In France during the summer of 1783, Benjamin Franklin watched the brothers Montgolfier go aloft in a hot-air balloon. “It diminish’d in Apparent Magnitude as it rose,” he reported afterward in a letter to the Royal Society, “till it enter’d the Clouds, when it seemed to me scarce bigger than an Orange, and soon after became invisible, the Clouds concealing it.” A man in the crowd asked, “What’s the use of that?” And Franklin replied, “What is the use of a newborn baby?” He understood that the rise of modern science would mean life itself to us, and although he could not know how far or fast it would travel, he did foresee that the global enterprise would carry us toward ever longer and healthier lives. Later that same year, when the Montgolfiers staged a balloon demonstration before the French court and about 130,000 other onlookers, one Madame d’Houdetot had the same prophetic thought, and found it poignant. Madame d’Houdetot reflected, as she watched the balloon passing over Versailles, “Soon they shall discover how to live forever…and we shall be dead.”

Now, a generation or two after that New Yorker cartoon with the anxious reader of obituaries, we have Web comic strips like XKCD (“A Webcomic of Romance, Sarcasm, Math, and Language”). One strip shows a line of stick figures marching up a hill toward “The Uncomfortable Truths Well.” The figures suggest an endless, eternal line of pilgrims bearing questions, which the well answers one by one. And the first question? We know what it is, of course. Quoth the well: “Science may discover immortality, but it won’t happen in the next eighty years.”

Do we want the science to move faster? Do we want a cure for aging? The question of desirability is going to be hard for us. When we examine it closely our thoughts get tangled in it, much as we are entangled with mortality in our bodies. The spiritual and emotional knots are as tight as the biological. We’re mortals. We’ve wrestled with the problem of mortality for thousands of years in the darkest passages of Scripture and philosophy. Our poets and artists move us profoundly by struggles without answers. No other scientific program raises so many enormous and imponderable questions, and they are so blithely dismissed by the engineers who would build the dam in the valley of the shadow of death.

We can’t know yet if a cure for aging is almost within reach, if it is now low-hanging fruit. But when we turn from feasibility to desirability—when we let ourselves think about science and immortality in the same sentence, and take it seriously, even for a moment—we run into extraordinary turbulence as soon as our thoughts are aloft. Powerful currents run in us, alternating currents of yes and no. We meet internal resistances just as strong as in the body or the cell; and we only half understand them, even though we have been exploring the question “Should we?” for as long as the question “Could we?”

In Paradise Lost, Milton reminds us that we failed to make ourselves immortal when we reached for the low-hanging apple; in fact, we made things infinitely worse.

Of Man’s First Disobedience, and the Fruit

Of that Forbidden Tree, whose mortal taste

Brought Death into the World, and all our woe…

Milton reinforces the point of the lesson by making Satan fall and suffer at least as horribly as Adam and Eve. He does show some sneaking sympathy for the fallen angel, as Blake observes in a famous line in “The Marriage of Heaven and Hell”—“NOTE: The reason Milton wrote in fetters when he wrote of Angels & God, and at liberty when of Devils & Hell, is because he was a true Poet and of the Devil’s party without knowing it.” Even so, as Milton announces at the close of his grave and august first verse, the purpose of this epic, the point of “this great Argument,” is to “assert Eternal Providence, and justify the ways of God to men.”

All of Hebrew and Christian Scripture makes this assertion and insists that the ways of God are just; to be accepted with or without understanding; to be accepted even in the face of horror. Think of the ghastliest story in the book of Genesis, the testing of Abraham: in Jewish tradition, the episode of the Torah that is recited on the first day of the new year, again and again. Abraham and Sara have a child in their old age, a child so long prayed for and despaired of that when at last he is born they name him Isaac, which means “He laughs.” And God comes to Abraham and commands him to take his son, “your only-one, whom you love, Isaac,” up to the mountain and sacrifice him. So early in the morning Abraham saddles his donkey, takes Isaac, splits wood for the sacrifice, and with them goes up to the mountain. They climb the mountain, Isaac carrying the wood and Abraham the torch and the knife.

“Here’s the fire and the wood,” says Isaac, “but where is the sacrifice?”

And Abraham answers, “God will provide.”

When they come to the place, Abraham builds the pyre, binds Isaac on top of it, and stretches out the knife to slay his son—but God stops him. “Abraham lifted up his eyes and saw a ram caught in the thicket by its horns. And he offered up the ram in place of his son.”

Homer tells the same story of another patriarch, King Agamemnon. When Agamemnon wants to sail for Troy, the wind will not come up to fill the sails of the ships. A priest tells him that it is the will of the gods that he sacrifice his firstborn daughter, Iphigeneia. So Agamemnon dispatches a messenger to the girl’s mother, Clytemnestra, and tells her to send their daughter to him. He says Iphigeneia is to be married to his greatest warrior, Achilles. And the girl comes. In Homer, the king sacrifices his daughter; but in Euripides’s play Iphigeneia at Aulis, a goddess spirits the girl away at the very last moment and substitutes a deer, which Agamemnon kills instead.

It’s curious that this same tortured story should reappear at the core of several religions. In Christian tradition, the hill where Abraham bound Isaac and lifted the knife was Golgotha, also known as Calvary. That is the hill up which God sent his own son, carrying the wood of the cross on his back, to be sacrificed for the sake of all of humanity, as symbolized by the lamb of God.

Hindus know the story from the Upanishads. A father, Vajasravasa, pledges to sacrifice all that he has in return for the blessings of heaven. His young son Nachiketas watches as Vajasravasa’s cows are led away. “Dear father,” the son asks, “to whom wilt thou give me?”

His father is silent.

“Dear father,” he asks again, “to whom wilt thou give me?”


“Dear father, to whom wilt thou give me?”

“I shall give thee unto Death!”

So the boy descends to the realms of Yama, who is Death. There he learns all the paradoxes of mortality and immortality, in some of the most celebrated poetry of Hindu scripture, which concludes, “When all the ties of the heart are severed here on earth, then the mortal becomes immortal—here ends the teaching.”

Framed this way, as the sacrifice of the child by the father, the story is even harder to accept than the sacrifice of the self. For anyone who has a father, or a child, it is infinitely more painful and bewildering to contemplate than the failure of Gilgamesh, or the fall of Adam and Eve. The very horror of the story forces us to reflect on the ultimate reason for this sacrifice. Whatever else they do, these stories push us to explore in the strongest possible form the struggle within us between acceptance and defiance, defiance and acceptance, in the flow of the generations. Every father does pass the problem of mortality to his child, because he must. Every child receives the problem of mortality from the father, because he must. And framed as it is, this recurring story makes life’s demands for acceptance and resistance impossible to decide for ourselves, impossible to resolve through reason, too much for mortal minds. This is the way it must be, the story says; we have to take it on faith.

The same story reappears in at least one more tradition: Lucretius retells the story of Agamemnon and Iphigeneia at the beginning of his epic poem The Way Things Are, a heroic effort to replace the epics of Homer and religious faith with the epic of what we now call science. In Book One, at line 101, we find the battle cry of the rationalists: Tantum religio potuit suadere malorum, “See what evils are done in the name of religion.” Lucretius’s furious line became a slogan of the Enlightenment and made him a favorite poet of the birth of science. The Loeb edition notes: “Voltaire, an ardent admirer of Lucretius, believed that line 101 would last as long as the world.”

Lucretius thought he could reason his way out of these terrors. Voltaire thought science could get us past them too. The modern age of the Enlightenment would dispel all darkness. But when we approach these questions now, through secular science, they are deep as ever. The problem of mortality does not go away when we look at it from a scientific point of view. The sacrifices are real and have always been real, our inheritances of loss, borne by each generation; and now we approach them from a new direction.

Some demographers predict, for instance, that we would want fewer children if we lived for hundreds or thousands of years. We see the trend already in the world’s developed countries; the longer we live, the smaller the families we choose. The trend might increase with our life expectancy. Those alive would stay alive. Those unborn would stay unborn. Galileo observed something like this centuries ago when he mocked the folly of people who think they can buy eternity in gems. He had nothing but contempt for the romantic idea that rubies and emeralds are pieces of immortality, that “diamonds are forever.” All of these dreams are ways of escaping for a moment from our mortal bodies, for getting off—in our imagination—from a mortal planet circling a mortal star. Fools, said Galileo, “are reduced to talking this way, I believe, by their great desire to go on living, and by the terror they have of death. They do not reflect that if men were immortal, they themselves would never have come into the world.”

Mortality is sacrifice. And the great argument of Scripture and Paradise Lost has its parallel now in a busy field of research, the study of the origins of aging at the level of single cells. A cell that reproduces by splitting in two will do better in the end if it divides unequally, with one half getting all the new parts, and the other half keeping some of the old parts. Cells began doing this very soon after the origin of life itself, more than three billion years ago.

According to present thinking, it all began with that first sacrifice. That was the moment when life invented aging. Those were the first cells doomed to age and die. From that moment on, mortality was ingrained within us.

Mortality is doubly ingrained in us, because it arose not once but twice. It was discovered first by those single cells, the authors of the first sacrifice, all those millions of years ago. And the invention of aging was so successful that life remained single-celled for two billion years—that is, for two-thirds of the time that there has been life on Earth. Even today, most of the life on the planet is still in the form of single cells.

Then, a billion years ago, for reasons that nobody understands, some of the single cells began to come together to form multicellular bodies. Some of the first colonies that formed were the ancestors of today’s sponges, which are very simple colonies. They are essentially immortal. Their aging is negligible. Other early colonies were the ancestors of today’s cnidarians, another large branch of the tree of life, which includes the hydra. The hydra lives in freshwater, but most cnidarians live in the sea, including sea anemones, corals, sea nettles, sea pens, and sea wasps—which are the world’s most poisonous animals; their sting can kill in less than three minutes. The cnidarians include jellyfish and the Portuguese man-of-war. They have nerves and muscles, and some of them have eyes. But most of these thousands of species hardly age. Like the sponges, they can regenerate from a tiny piece, sometimes even from a few scattered cells. When sponges and cnidarians grow new cells, they just slough off the old ones. Any wastes that have built up in those old cells are gone, and the new cells start afresh. So the cells in those animals age and die, but their bodies live on and on.

These immortalists evolved early; they were some of the first multicellular animals on the planet. And then mortal animals evolved. Why? What advantage did they get from becoming mortal?

A sponge has no nervous system. A hydra has networks of nerves but no brain. Both animals shed their nerves the way they shed the cells that make up their skin and their muscles, and then grow new ones. The forests of delicate synapses, which tie all our long-lived nerves together in the bundles we call the nervous system, were among the most important inventions in the history of life. They allowed animals to store more and more information. Long-lived neurons allowed them to maintain a historical memory, to learn from their experience and carry experience forward. The hydra loses its memories along with its old cells. Its memories go up in the flame and ash of the Phoenix. That is a price it pays for the gift of being born anew. Although the hydra lives much longer than its nerves, it sheds its experiences with them—whereas the nerves in a nervous system can last a lifetime, and with them, we have the memories of a lifetime.

Nerves are cells, and all cells accumulate wastes and damage. They age, but they are so specialized that they can no longer be replaced. The cells in the bone marrow proliferate as long as we live, as do the cells that line our guts. They divide and divide, and any junk that’s built up in them is diluted again and again so that they stay clean. In this way, the bone marrow and the linings of our guts and the cells of our liver can be said to be virtually immortal, like the hydra. But the highly specialized cells of our brains are mortal, and so are the cells of our hearts.

In essence, then, that was the second beginning of old age and mortality, in the evolution of these specialists. Ever since, animals with those kinds of long-lived but mortal cells have accumulated damage, and eventually they have failed. Because key parts of our bodies cannot last, we do not build the rest of our bodies to last. Ultimately, then, the cells that give us our identities are the ones that bring us down to the grave.

Terman and Brunk, the authors of the Garbage Catastrophe hypothesis, are among the gerontologists who have advanced this argument: that the need for the nerves brought death into the world a second time. They argue that our long-lived muscles may also have played a part in this second invention of mortality. What we call muscle memory emerges from a combination of the complex patterns we have laid down with our muscles and the firing of our nerves. It may be that the spectacularly complicated and graceful behavior of the more complex animals owes a great deal to their long-lived muscle fibers and long-lived nerves, lasting as long as the body itself.

This invention may have allowed the amazing diversification of life-forms that we call the Cambrian Explosion. If so, the invention of aging, the feature that ingrains our mortality in our flesh, made us such a success on the planet Earth.

The development of those long-lived cells would also have precluded reproduction by budding, which is the main way that hydra makes another hydra. It would have driven the evolution of the separation of bodies into the disposable soma and the protected germ cells, the sex cells. And so it would have furthered and spurred the evolution of aging. And it would have made possible something else, too. Animals with dangerous lives would have grown up fast and reproduced fast before they died. About half the animals on this planet are short-lived insects. But animals that found their way into protected niches could afford to slow down. Then they could benefit more and more from their long-lived muscles and memories. They could grow more and more intelligent. One animal line that did this more than any other was our own, the species Homo sapiens. We lost the gift of living more or less indefinitely, of aging negligibly. We lost the gift of living more or less negligently, without being aware of our losses. But we gained the gift of memory, of memories that can last all our lives.

We have what the hydra does not have. We have a sense of ourselves that goes back to our beginnings and looks ahead toward the infinitely various possibilities that surround our end. We exist, and we know we exist. But the price we pay is that we age, and that we know we age. The price we pay is that we know we are mortals.

And we must wrestle with these questions of acceptance or defiance.