Editor’s Note: In our latest Magazine Classic from 2010, physicist and author Chet Raymo explores the scientific challenge of probing to “the ineluctable heart of the mystery” that is life.
“We humans cannot see ourselves completely except as part of humanity, humanity as part of life, and life as part of the universe."
So wrote the Jesuit scientist/mystic Teilhard de Chardin. In that Teilhardian spirit, let me begin this celebration of life with two thought experiments that will help put life in a cosmic context.
Imagine removing all visible life from planet Earth. Get rid of the elephants, tigers, apes, dogs and cats. Birds, fish, worms and beetles. Humans too. Remove the plants. Trees, flowers, seaweed, grass. Eliminate the invisible microbes in air, soil and sea. Then, when not an iota of living matter appears to remain, zap the surface of the Earth with a killer blast of 1,000-degree heat to kill off any organisms that managed to escape our attention. Finally, just to be sure, boil away the sea. Scrape off every grain of loose soil down to bedrock, every particle of sediment from the ocean floor, and haul it away. Scrub the bedrock bare.
The planet is now sterile. Life on Earth is finished. Right?
Earth would still harbor an amount of life perhaps as great in mass — if not as diverse — as what we got rid of. Where? Viable bacteria have been found in drill holes two miles below the surface, in microscopic fissures in the rock where they have lived out of touch with surface life since at least the time of the dinosaurs. Pound for pound, there may be just as much life below the surface of the Earth as above.
Surprised? So were scientists. Solid rock may seem an unlikely environment for life. But the rocky crust of the Earth is full of microscopic pores and fissures, through which water percolates. All rocks within a mile or so of the surface are saturated with water. And the water contains bacteria.
How do bacteria survive in total darkness, cut off from the atmosphere and sun? By living off the internal heat of the planet. Subsurface rock is hot; the deeper you go, the hotter it gets. Deep subsurface bacteria take in carbon dioxide and water and use thermal energy to metabolize carbohydrates, releasing methane and hydrogen sulfide waste. These organisms may even live off the rock itself, rock that is “weathered” by the water percolating through it, releasing useful hydrogen.
It’s a grim sort of life, in a kind of permanent hell. Deep subsurface bacteria must survive high temperatures that would kill more familiar forms of life. They may reproduce only once a year, or even once a century, compared to the minutes or hours that are typical of their surface cousins. Deep bacteria are about as close to being dead as something can be and still be alive. But alive they are, living out their lives at a languorous geologic pace.
If life can survive in the hellish conditions that exist deep below the Earth’s surface, with a kind of slowed-down metabolism, then the odds dramatically improve for finding life in such apparently inhospitable places as the Moon, Mars or the frozen seas of Jupiter’s moon Europa. It also becomes less difficult to imagine that the seeds of life may be — as Teilhard would have been thrilled to discover — cosmic, adrift in interstellar space, or carried from place to place by meteor or comet.
Another thought experiment.
Nematodes are threadlike worms that range in length from a millimeter to a meter. A handful of loam might contain a thousand. They live virtually everywhere — soil, water, desert sand, arctic ice, hot springs, and as parasites of plants and animals. Pinworms and hookworms, familiar parasites of humans, are nematodes. Astonishingly, nematodes make up four-fifths of all multicelled animals on Earth.
The American parasitologist N.A. Cobb once imagined this curious scenario: “If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”
Bingo! Everything that isn’t a nematode vanishes. And there, at least for an instant until it disperses, is the shadow world. The giant hollow sphere. The mountains, valleys, oceans, rivers. The plants. The animals. The human habitations. Spookily represented by worms.
I offer these two thought experiments to put human life in perspective. We are part of a teeming sea of living organisms, upon which we depend for the air we breathe and the food we eat. Our bodies consist of tens of trillions of cells that developed from a single fertilized egg, but there are billions of living cells in me and on me that aren’t me. By some estimates, there is a kilogram’s worth of bacteria living in my guts, mostly doing more good than harm. Some biologists suggest that our bodies are themselves elaborate colonies of bacteria that have learned to specialize and work together.
Just what is it?
Life! It’s everywhere, pervasive. We are immersed in a sea of it. We are part of it. But what is it? What is life?
No question in science is more fundamental. No question is more difficult to answer. We recognize life when we see it, but it is devilishly hard to say what it is.
Half a century ago, the great Austrian physicist Erwin Schrodinger proposed a definition in a little book called What Is Life? He was convinced that life would eventually be accounted for by physics and chemistry, and his book helped inspire the biomolecular revolution. The best he could do for a definition, however, was “an elaborate, coherent, meaningful design traced by the great master.”
More recently, the American biologist Lynn Margulis and co-writer Dorion Sagan tackled the question in a book of the same title: What Is Life? They provide a brilliant summary of what we have learned about life, but the sought-for definition remains elusive. To answer the question according to the rules of grammar, we must supply a noun, the name of a thing. But life is more like a verb, they say. They try their hand at several definitions:
“It is a material process, sifting and surfing over matter like a strange, slow wave."
“It is the watery, membrane-bound encapsulation of spacetime.”
“A planetary exuberance.”
All of which are lovely metaphors — and, like Schrodinger’s definition, very Teilhardesque — but none of which get us any closer to the ineluctable heart of the mystery. I like to think of life as a self-sustaining chemical reaction that has animated the surface of the planet for four billion years, and maybe animates the universe.
What we do know for sure is that all life on Earth is related by common descent. We and the subsurface bacteria and the nematodes share a common ancestor. As to where the first self-replicating terrestrial organism came from, no one yet knows.
Let us assume an ancestral living cell, as simple as the simplest bacterium existing today — an unnucleated blob of protoplasm enclosed by a membrane. Microscopically small. Autopoietic: that is, capable of maintaining itself by chemical interaction with the environment.
A typical bacterium reproduces every half hour. One makes two, two make four, four make eight, eight make 16, and so on. Start with a single bacterium on the early Earth, and 20 generations later — 10 hours — you have a million, enough to cover the head of a pin. Two days later — 120 generations — you will have enough bacteria to fill the oceans of the world chock-a-block with life. A few hours later, the entire surface of the Earth will be wrapped in a layer of bacteria 10 miles thick.
Clearly, something must be wrong with my calculation; the globe is not wrapped in a thick layer of living slime. For unrestricted reproduction, organisms need access to chemical nutrients from the environment. As my calculation vividly illustrates, competition for resources can be fierce, even for bacteria.
From the very first days of life on Earth, bacteria were locked in a battle to survive. The corollary of life is death. Culling. Recycling. Portioning out resources. No definition of life is complete that does not include death.
For billions of years, microbes competed “tooth and claw” for the opportunity to reproduce. Far more failed than succeeded. Most branches on the tree of life were nipped in the bud. A few lucky lineages fingered into the future, avoiding the sweeping scythe of extinction, like the few stalks of grain that remain standing in a harvested field. Those lucky lineages were helped by chance, by rare mutations of the genome that honed the chemical machinery of autopoiesis, that gave the organism an edge in the struggle to survive.
There was a random stumble in all of this, but the direction of evolution was anything but random. As the biologist/essayist Lewis Thomas wrote: “The capacity to blunder slightly is the real marvel of DNA. Without this special attribute, we would still be anaerobic bacteria. . . . Viewed individually, one by one, each of the mutations that have brought us along represents a random, totally spontaneous accident, but it is no accident at all that mutations occur; the molecule of DNA was ordained from the beginning to make small mistakes.”
Natural selection — the survival of the best adapted — worked on variation, those “small mistakes” to drive evolution toward complexity. The microbiologist Ursula Goodenough echoes Thomas: “Death is the price paid to have trees and clams and birds and grasshoppers, and death is the price paid to have human consciousness, to be aware of all that shimmering awareness and all that love.”
The complex dance
In biology textbooks, the timeline of the first three billion years of life on Earth is mostly blank. Photosynthesis. Respiration. Nucleated cells. Sexual reproduction. It would appear that not much happened. But in fact everything was happening; life was perfecting the complex chemistry that sustains every living creature on Earth today. Life was evolving ever more efficient ways to take the energy of a star and weave the fabulous tapestry of life that wraps the planet in ever-greater diversification.
By the time the first multicelled organisms appeared about 700 million years ago — and the timeline of Earth history becomes crowded and familiar — most of the real work of evolution was finished. The basic chemical machinery of autopoiesis and reproduction was in place. Everything that followed were variations on a theme. Flowers, forests, grassy meadows, birdsong, butterflies, the prehensile grip of a human infant’s hand, a mind that could conceive and execute the music of a Mozart — all contrived of hydrogen, oxygen, carbon and a smattering of other elements, animated by sunlight (or the internal heat of the Earth) against the degrading tendency of entropy.
I’ve yet to find a popular treatment of the first three billion years of life that paints a sufficiently vivid picture of those delicate lineages of microbes fingering into the future, a branching family tree replete with myriad dead-ends, inching forward under the great overarching shadow of death, always bearing the residue of the past, teasing self-maintenance from the environment, transforming the Earth’s atmosphere and oceans, competing, occasionally turning exploitation to mutual advantage, perfecting metabolic pathways of astonishing complexity.
As we have discovered in recent years, the history of human life is written in the DNA that whirls and dances in each of the trillions of cells in our bodies — an arm’s length of DNA in each cell distributed over 23 pairs of chromosomes. The DNA spins off messenger RNA. The RNA is a template for assembling proteins. Our cells are protein-manufacturing machines, building bodies whose cosmic purpose is to pass on genes.
If you want to get close to the cosmic meaning of life, try to imagine what is going on in each of the trillions of cells of your body even as you read this paragraph. The DNA double-helices unzip in a dervish dance. Protein-based “motors” crawl along the strands of DNA, transcribing the code into single-strand RNA. Other proteins help pack DNA neatly into the nuclei of cells and maintain the tidy chromosome structures. Still other protein-based “motors” are busily at work untying knots that form in DNA as it is unpacked in the nucleus and copied during cell division. Other molecules are in charge of quality control, checking for accuracy in the reproductive process and repairing errors.
It’s an unceasing buzz of activity. Unceasing! In a sense, your conscious life is the surface manifestation of an unseen and unfelt frenzy of molecular activity that is the animating fire of life.
Bacteria, nematodes, elephants and hummingbirds: We share that buzz and hum. We share the “four-letter” code of the DNA. We share the strings of amino acids, just 20 kinds in all, that are the proteins. We are made of a Tinkertoy set of sorts, but what a set!
As I write, I look out the window. Fields and woods, reaching away to the horizon. Spiders, butterflies, hawks, crows. Invisible organisms in their uncountable myriads. A panorama of teeming biological activity. All of those DNA programs running continuously, inexhaustibly, creatively. Running ceaselessly here on Earth for four billion years, and for all we know throughout the universe. Running to the beat of a rhythm that has been humming at the heart of creation since the dawn of time.
So what is life? How does one define a flame, falling in love, a Mozart symphony? How do we grasp in our mind’s eye the 50 trillion generations of microbial evolution on the planet Earth? Perhaps, after all, it is not possible to improve upon Schrodinger’s definition: “An elaborate, coherent, meaningful design traced by the great master.”
Teilhard de Chardin implied something similar in “The Mass on the World”: “In the beginning there was not coldness and darkness: there was the fire. . . . The flame has lit up the whole world from within . . . from the inmost core of the tiniest atom to the mighty sweep of the most universal laws of being.”
We do not yet know how life began on Earth, although theories abound. But this, with Teilhard, we surely know: The potential for life was there from the beginning, from the instant of the Big Bang.
The universe began as a speck of superhot energy that exploded outward. In the first tiny fraction of a second, the universe inflated rapidly, like a balloon blowing up from nothing. Three minutes later, there was hydrogen and helium. Within a billion years, stars and galaxies had begun to shine, forging the heavy elements of life. Somewhere, somehow, the first self-replicating molecules came into existence — here on Earth, perhaps elsewhere in the universe, or maybe at many sites independently — and the rest, as they say, is history. Every aspect of the universe we inhabit today — from quarks to quasars, from bacteria to human beings — was, as Teilhard suggested, implicit in the Big Bang beginning.
It’s a wild, beautiful story, but why should we believe it? Who are these physicists and biologists that we should pay attention to what they say?
They are us. They are our sons and daughters, brothers and sisters. They come from every ethnic and religious background. They just happen to be very smart and very well-trained in some specialist branches of science. We believe them for the same reason they believe each other. The story of life is written in the out-rush of the galaxies, in the relative amounts of hydrogen and helium of which the stars are made, and in the radiant microwave energy that fills the universe. It is written here on Earth in the atoms, in the molecules of life, in the genomes of creatures great and small.
Curiosity and discovery
We trust the scientists who tell the story because they are part of a long and glorious history of human curiosity and discovery: Aristarchus, Ptolemy, Copernicus, Galileo, Herschel, Darwin, Curie, Hubble, Einstein, Watson, Crick and Franklin, and countless others. And the story isn’t over yet.
What do we make of it so far? Teilhard de Chardin wrote: “If the Fire has come down into the heart of the world, it is, in the last resort, to lay hold on me and to absorb me.” There is no point in bothering about stories of creation unless they enrich and illuminate our lives. And for that, we can’t rely on the physicists and biologists. For that, we turn to the poets and the mystics.
Simon Bartholomew, translator of Teilhard’s “The Mass on the World,” writes: "The aim of scientific language is to provide exactly defined and unambiguous statements about reality; that of poetic language is to communicate reality itself, as experienced.”
Teilhard was a scientist, but he was first and foremost a poet and mystic. His language is full of poetic imagery, ambiguity, paradox, even, some would say, vaporous jargon. But his great gift was to embrace unhesitatingly the scientific creation story as his starting point. He began with the evolving Fire and drew it down into the heart of his world.
Teilhard’s Christocentric language won’t resonate with everyone, but his lifelong struggle to integrate his deepest spiritual life with the scientific story of life won for him the admiration of men and women of every religious faith and philosophical persuasion. He wrote: “It is a terrifying thing to have been born: I mean, to find oneself, without having willed it, swept irrevocably along on a torrent of fearful energy.” Throughout his life he sought to turn the terror into an overwhelming joy.
We are caught up in an evolving universe we only partly understand, illuminated by a flame of life that reaches into the deep crustal rocks of the Earth and perhaps to distant planets. We may take this astonishing story on the word of the scientists, but if we are attentive to what they say — and imaginative and courageous — we too will feel the terrifying, exhilarating wind of primeval fire blowing through our lives.
Chet Raymo is a professor emeritus of physics at Stonehill College. His latest book is When God Is Gone, Everything Is Holy: The Making of a Religious Naturalist. His blog can be found at sciencemusings.com.
Sunset over water photo by Lorne Resnick.