All my life I’ve wondered about life beyond the Earth. On those countless other planets that we think circle other suns, is there also life? Might the beings of other worlds resemble us or would they be astonishingly different? What would they be made of? In the vast Milky Way galaxy how common is what we call life?
The nature of life on Earth and the quest for life elsewhere are the two sides of the same question: the search for who we are. All living things on Earth are made of organic molecules, a complex microscopic architecture built around atoms of carbon. In the great dark between the stars there also are organic molecules in immense clouds of gas and dust. And inside such clouds there are batches of new worlds just forming. Their surfaces are very likely covered with organic molecules. These molecules almost certainly are not made by life although, they are the stuff of life. On suitable worlds, they may lead to life. Organic matter is abundant throughout the cosmos, produced by the same chemistry everywhere.
Perhaps, given enough time, the origin and evolution of life is inevitable on every clement world. There will surely be some planets too hostile for life. On others, it may arise and die out, or never evolve beyond its simplest forms. And on some small fraction of worlds there may develop intelligences and civilizations more advanced than ours.
All life on our planet is closely related. We have a common organic chemistry and a common evolutionary heritage, and so our biologists are profoundly limited. They study a single biology, one lonely theme in the music of life. Is it the only voice for thousands of light years or is there a cosmic fugue, a billion different voices playing the life music of the galaxy?
This blue world is where we grew up. There was once a time before life. Our planet is now burgeoning with life. How did it come about? How were organic molecules originally made? How did life evolve to produce beings as elaborate and complex as we, able to explore the mystery of our own origins? Let me tell you a story about one little phrase in the music of life on Earth.
In the history of humans, in the 12th century, Japan was ruled by a clan of warriors called the Heike. The nominal leader of the Heike, the emperor of Japan, was a seven-year-old boy named Antoku. His guardian was his grandmother, the Lady Nii. The Heike were engaged in a long and bloody war with another Samurai clan, the Genji. Each asserted a superior ancestral claim to the imperial throne. Their decisive encounter occurred at Dannoura in the Japanese Inland Sea on April 24 in the year 1185. The Heike were badly outnumbered and outmaneuvered. With their cause clearly lost, the surviving Heike warriors threw themselves into the sea and drowned. The emperor’s grandmother, the Lady Nii, resolved that they would not be captured by the enemy. What happened next is related in The Tale of the Heike.
The young emperor asked the Lady Nii, “Where are you to take me?” She turned to the youthful sovereign with tears streaming down her cheeks and comforted him. Blinded with tears, the child sovereign put his beautiful small hands together. He turned first to the east to say farewell to the god of Ise, and then to the west to recite the Nambutsu, a prayer to the Amida Buddha. The Lady Nii took him in her arms, and with the words “In the depths of the ocean is our capital” sank with him at last beneath the waves.
The destruction of the Heike battle fleet at Dannoura marked the end of the clan’s thirty-year rule. The Heike all but vanished from history. Only 43 Heike survived; all women. These former ladies-in-waiting of the Imperial Court were reduced to selling flowers and other favors to the fishermen near the scene of the battle. These women and their offspring by the fishermen established a festival to commemorate the battle. To this day, every year, on the 24th of April, their descendants proceed to the Akama shrine which contains the mausoleum of the drowned seven-year-old emperor Antoku. There, they conduct a ceremony of remembrance for the life and death of the Heike warriors.
But there is a strange postscript to this story. The fishermen say that the Heike samurai wander the bottom of the Inland Sea still, in the form of crabs. There are crabs here which have curious markings on their backs; patterns which resemble a human face with the aggressive scowl of a samurai warrior from medieval Japan. These Heike crabs, when caught, are not eaten. They are thrown back into the sea in commemoration of the doleful events of the battle of Dannoura.
This legend raises a lovely problem: how does it come about that the face of a warrior is cut on the carapace of a Japanese crab? How could it be? The answer seems to be that humans made this face. But how? Like many other features, the patterns on the back—or carapace—of this crab are inherited. But among crabs, as among humans, there are different hereditary lines. Now suppose, purely by chance, among the distant ancestors of this crab, there came to be one which looked just a little bit like a human face. Long before the battle, fishermen may have been reluctant to eat a crab with a human face. In throwing it back into the sea they were setting into motion a process of selection. If you’re a crab and your carapace is just ordinary, the humans are gonna eat you. But if it looks a little bit like a face they’ll throw you back, and you can have lots of baby crabs that all look just like you. As many generations passed—of crabs and fisher-folk alike—the crabs with patterns that looked most like a samurai face preferentially survived until, eventually, there was produced not just a human face, not just a Japanese face, but the face of a samurai warrior. All this has nothing to do with what the crabs might want. Selection is imposed from the outside. The more you look like a samurai, the better your chances of survival. Eventually, there are a lot of crabs that look like samurai warriors.
This process is called artificial selection. In the case of the Heike crab, it was effected more or less unconsciously by the fishermen, and certainly without any serious contemplation by the crabs. Humans, for thousands of years, have deliberately selected which plants and animals shall live. We’re surrounded with farm and domestic animals, fruits, vegetables. Where do they come from? Were they once free-living in their present form in the wild and then induced to adopt some less strenuous life on the farm? No. They are, almost all of them, made by us.
The essence of artificial selection for a horse, or a cow, a grain of rice, or a Heike crab is this: many characteristics are inherited. They breed true. Humans encourage the reproduction of some varieties and discourage the reproduction of others. The variety selected for eventually becomes abundant. The variety selected against becomes rare, maybe extinct. But if artificial selection makes such changes in only a few thousand years, what must natural selection—working for billions of years—be capable of? The answer is all the beauty and diversity in the biological world.
That life evolved over the ages is clear from the changes we’ve made in the beasts and vegetables, but also from the record in the rocks. The fossil evidence speaks to us unambiguously of creatures that were once present in enormous numbers and that have now vanished utterly. There are far more species that have become extinct than exist today. They are the terminated experiments in evolution.
These guys, for example—the trilobites—appeared 600 million years ago. They were around for 300 million years. They’re all gone. There’s none left. But in those old rocks there are no fossils of people or cattle. We’ve evolved only recently. Evolution is a fact, not a theory. It really happened.
That the mechanism of evolution is natural selection was the great discovery of Charles Darwin and Alfred Russel Wallace. Here’s how it works: nature is prolific, there are many more creatures that are born than can possibly survive. So those varieties which are, by accident, less well adapted don’t survive, or at least they leave fewer offspring. Now, mutations—sudden changes in heredity—are passed on. They breed true. The environment selects those occasional mutations which enhance survival. And the resulting series of slow changes in the nature of living beings is the origin of new species.
Well, many people were scandalized by the ideas of evolution and natural selection. Our ancestors looked at the intricacy and the beauty of life and saw evidence for a great designer. The simplest organism is a far more complex machine than the finest pocket watch. And yet, pocket watches don’t spontaneously self-assemble or evolve in slow stages on their own from, say, grandfather clocks. A watch implies a watchmaker. So there seemed to be no way in which atoms could spontaneously fall together and create, say, a dandelion. The idea of a designer is an appealing and altogether human explanation of the biological world. But as Darwin and Wallace showed, there’s another way—equally human and far more compelling. Natural selection, which makes the music of life more beautiful as the eons pass.
To understand the passage of the eons we have compressed all of time into a single cosmic year with the big bang on January first. Every month here represents a little over a billion years. The Earth didn’t form until the cosmic year was two-thirds over. Our understanding of the history of life is very recent, occupying only the last few seconds of December 31st; that small white spot at bottom right in the cosmic calendar.
What happened on Earth may be more or less typical of the evolution of life on many worlds. But in its details the story of life on Earth is probably unique in all the Milky Way galaxy. The secrets of evolution are time and death. Time for the slow accumulation of favorable mutations, and death to make room for new species. Life on Earth arose in September of the cosmic calendar, when our world, still heavily battered and cratered from its violent origin, may have looked a little like the moon.
The Earth is about 4.5 billion years old. In the cosmic calendar, it condensed out of interstellar gas and dust around September 14th. We know from the fossil record that the origin of life originated soon afterwards—maybe around September 25th, something like that. Probably in the ponds and oceans of the primitive Earth.
Now, the first living things were not anything so complex as a one-celled organism, which already is a highly sophisticated form of life. No, the first stirrings of life were much more humble and happened on the molecular level. In those early days, lightning and ultraviolet light from the sun were breaking apart simple hydrogen-rich molecules in the primitive atmosphere, and the fragments of the molecules were spontaneously recombining into more and more complex molecules.
The products of this early chemistry dissolved in the oceans, forming a kind of organic soup of gradually increasing complexity. Until, one day, quite by accident, a molecule arose that was able to make crude copies of itself, using as building blocks the other molecules in the soup. This was the ancestor of DNA, the master molecule of life on Earth. It’s made of four different molecular parts, called nucleotides, which constitute the four letters of the genetic code—the language of heredity. Each of the four nucleotides, the rungs on the DNA ladder, are a different color in this model. The instructions that are spelled out are different for different organisms—that’s why organisms are different.
Now, a mutation is a change of a nucleotide; a misspelling of the genetic instructions. Most mutations spell genetic nonsense, as you’d expect, because they’re random. They’re harmful for the next generation. But a very few, by accident, make better sense than the original instructions and aid the evolution of life. DNA is about a billion times smaller than we see it here. In fact, each of those things that looks like a piece of fruit is an atom. Without the tools of science, the machinery of life would be invisible.
Four billion years ago, the ancestors of DNA were competing for molecular building blocks and leaving crude copies of themselves. There were no predators; the stuff of life was everywhere. So the oceans and murky pools that filled the craters were, for these molecules, a Garden of Eden. With reproduction, mutation, and natural selection, the evolution of living molecules was well underway. Varieties with specialized functions then joined together, making a collective: the first cell. The organic soup eventually ate itself up. But by this time, plants had evolved, able to use sunlight to make their own building blocks. They turned the waters green. A number of one-celled plants joined together—the first multi-cellular organisms.
Equally important was the invention—not made until early November—of sex. It was stumbled upon by the microbes. By December 1st, green plants had released copious amounts of oxygen and nitrogen into the atmosphere. The sky is made by life. Then, suddenly, on December 15th, there was an enormous proliferation of new life forms, an event called the Cambrian explosion.
We know from the fossil record that life arose shortly after the formation of the Earth, suggesting that the origin of life might be an inevitable chemical process on countless Earth-like planets throughout the cosmos. But on the Earth, in nearly 4 billion years, life advanced no further than algae. So maybe more complex forms of life are harder to evolve—harder, even, than the origin of life itself. If this is right, the planets of the galaxy might be filled with microorganisms, but big beasts and vegetables and thinking beings might be comparatively rare.
By December 18th, there were vast herds of trilobites foraging on the ocean bottom, and squid-like creatures with their multicolored shells were everywhere. We know enough to sketch in a few of the subsequent details. The first fish and the first vertebrates appeared on December 19th. Plants began to colonize the land on December 20th. The first winged insects fluttered by on December 22nd. And on this date, also, there were the first amphibians: creatures, something like the lungfish, able to survive both on land and in water. Our direct ancestors were now leaving the oceans behind.
The first trees and the first reptiles evolved on December 23rd; two amazing evolutionary developments. We are descended from some of those reptiles.
The dinosaurs appeared on Christmas Eve. There were many different kinds of dinosaurs. The Earth was once their planet. Many stood upright and had some fair intelligence. Great lizards crashed and thundered through the steaming jungles. Unnoticed by the dinosaurs, a new creature—whose young were born live and helpless—was making its timid debut. The first mammals emerged on December 26th, the first birds on the following day. But the dinosaurs still dominated the planet. Then, suddenly, without warning, all over the planet at once, the dinosaurs died. The cause is unknown, but the lesson is clear: even 160 million years on a planet is no guarantee of survival. The dinosaurs perished around the time of the first flower.
On December 30th, the first creatures who looked even a little bit human evolved, accompanied by a spectacular increase in the size of their brains. And then, on the evening of the last day of the last month—only a few million years ago—the first true humans took their place on the cosmic calendar. The written record of history occupies only the last 10 seconds of the cosmic year.
Now, let’s take a closer look at who our ancestors were. A simple chemical circumstance led to one of the great moments in the history of our planet. There were many kinds of molecules in the primordial soup. Some were attracted to water on one side and repelled by it on the other. This drove them together into a tiny enclosed spherical shell like a soap bubble, which protected the interior. Within the bubble, the ancestors of DNA found a home and the first cell arose. It took hundreds of millions of years for tiny plants to evolve, giving off oxygen. But that branch didn’t lead to us.
Bacteria that could breathe oxygen took over a billion more years to evolve. From a naked nucleus, a cell developed with a nucleus inside. Some of these amoeba-like forms led, eventually, to plants. Others produced colonies with inside and outside cells performing different functions. Becoming a polyp attached to the ocean floor filtering food from the water and evolving little tentacles to direct food into a primitive mouth. This humble ancestor of ours also led to spiny-skinned armored animals with internal organs, including our cousin, the starfish. But we don’t come from starfish.
About 550 million years ago, filter feeders evolved gill slits which were more efficient at straining food particles from the water. One evolutionary branch led to acorn worms. Another led to a creature which swam freely in the larval stage but, as an adult, was still firmly anchored to the ocean floor. Some became living hollow tubes. But others retained the larval forms throughout the life cycle and became free-swimming adults with something like a backbone.
Our ancestors now, 500 million years ago, were jawless filter-feeding fish a little like lampreys. Gradually, those tiny fish evolved eyes and jaws. Fish then began to eat one another. If you could swim fast, you survived. If you had jaws to eat with, you could now use your gills to breathe the oxygen in the water. This is the way modern fish arose.
During the summer, swamps and lakes dried up, so some fish evolved a primitive lung to breathe air until the rains came. Their brains were getting bigger. If the rains didn’t come, it was handy to be able to pull yourself along to the next swamp. That was a very important adaptation.
The first amphibians evolved, still with a fish-like tail. Amphibians, like fish, laid their eggs in water, where they were easily eaten. But then a splendid new invention came along: the hard-shelled egg, laid on the land, where there were—as yet—no predators. Reptiles and turtles go back to those days. Many of the reptiles hatched on land never returned to the waters. Some became the dinosaurs. One line of dinosaurs developed feathers, useful for short flights. Today, the only living descendants of the dinosaurs are the birds. The great dinosaurs evolved along another branch. Some were the largest flesh-eaters ever to walk the land. But 65 million years ago, they all mysteriously perished.
Meanwhile, the forerunners of the dinosaurs were also evolving in a different direction. Small, scurrying creatures with the young growing inside the mother’s body. After the extinction of the dinosaurs, many different forms developed. The young were very immature at birth. In the marsupials—the wombat, for example—and in the mammals, the young had to be taught how to survive. The brain grew larger still. Something like a shrew was the ancestor of all the mammals.
One line took to the trees, developing dexterity, stereo vision, larger brains, and a curiosity about their environment. Some became baboons, but that’s not the line to us. Apes and humans have a recent common ancestor. Bone for bone, muscle for muscle, molecule for molecule. There are almost no important differences between apes and humans. Unlike the chimpanzee, our ancestors walked upright, freeing their hands to poke and fix and experiment. We got smarter. We began to talk. Many collateral branches of the human family became extinct in the last few million years. We, with our brains and our hands, are the survivors. There’s an unbroken thread that stretches from those first cells to us.
Let’s look at it again compressing 4 billion years of evolution into 40 seconds. Those are some of the things that molecules do, given 4 billion years of evolution. We sometimes represent evolution as the ever-branching ramifications of some original trunk, each branch pruned and clipped by natural selection. Every plant and animal alive today has a history as ancient and illustrious as ours. Humans stand on one branch. But now we affect the future of every branch of this 4-billion-year-old tree.
How lovely trees are. The human species grew up in and around them. We have a natural affinity for trees. Trees photosynthesize—they harvest sunlight; they compete for the sun’s favors. Look at those two trees over there: pushing and shoving for sunlight, but with grace and astonishing slowness. There are so many plants on the Earth that there’s a danger of thinking them trivial, of losing sight of the subtlety and efficiency of their design. They are great and beautiful machines, powered by sunlight, taking in water from the ground and carbon dioxide from the air, and converting them into food for their use and ours.
This is a museum of living plants. The Royal Botanic Gardens at Kew in London. Every plant uses the carbohydrates it makes as an energy source to go about its planty business. And we animals—who are ultimately parasites on the plants—we steal the carbohydrates so we can go about our business. In eating the plants and their fruits, we combine the carbohydrates with oxygen which—as a result of breathing—we’ve dissolved in our blood. From this chemical reaction, we extract the energy which makes us go. In the process, we exhale carbon dioxide into the atmosphere, which the plants then use to make more carbohydrates. What a marvelous cooperative arrangement! Plants and animals each using the other’s waste gases; the whole cycle powered by abundant sunlight. But there would be carbon dioxide in the air even if there were no animals. We need the plants much more than they need us.
There are many family resemblances among the organisms of the Earth. Some are very apparent, such as the use of the number five. Humans have five major bodily projections: one head, two arms, two legs. So do ducks—although the functions of their bodily projections are not quite the same. An octopus or a centipede has a different plan. And a being from another planet might be much stranger still.
These family resemblances continue, and on a much deeper level, when we go to the molecular basis of life. There are tens of billions of different kinds of organic molecules. Yet, only about 50 of them are used in the essential machinery of life. The same 50 employed over and over again, ingenious, for different functions in every living thing. And when we go to the very kernel of life on Earth—to the proteins that control cell chemistry,to the spiral or helix of nucleic acids which carry the hereditary information—we find these molecules to be absolutely identical in all plants and animals of our planet.
This oak tree and me, we’re made of the same stuff. If you go back, you’ll find that we have a common ancestor. That’s why our chemistry is so alike. Let’s take a trip to examine this common basis of life; a voyage to investigate the molecular machinery at the heart of life on Earth: a journey to the nucleus of the cell. First we need a cell. I have trillions, I can afford to donate a few. The casual act of pricking a finger is an event of some magnitude on the scale of the very small. Millions of red blood cells are detoured from their usual routes. But most continue to cruise about the body carrying their cargoes of oxygen to the remotest freckle. We’re about to enter the living cell—a realm, in its own way, as complex and beautiful as the realm of galaxies and stars.
Among the red blood cells, we encounter a white blood cell—a lymphocyte—whose job it is to protect me against invading microbes. It makes antibodies on its furrowed surface. But its interior is like that of many cells. Plunging through the membrane, we find ourselves inside the cell. Here, every structure has its function. Those dark green blobs are factories where messenger molecules are busy building the enzymes which control the chemistry of the cell. The messengers were instructed and dispatched from within the nucleus, the heart and brain of the cell. All the instructions on how to get a cell to work and how to make another are hidden away in there. We find a tunnel, a nuclear pore, an approach to the biological holy of holies. These necklaces, these intricately looped and coiled strands are nucleic acids, DNA. Everything you need to know on how to make a human being is encoded in the language of life, in the DNA molecule.
This is the DNA double helix: a machine with about 100 billion moving parts, called atoms. There are as many atoms in one molecule of DNA as there are stars in a typical galaxy. The sequence of nucleotides, here brightly colored, is all that’s passed on from generation to generation. Change the order of the nucleotides and you change the genetic instructions. DNA must replicate itself with extreme fidelity. The reproduction of a DNA molecule begins by separating the two helices. This is accomplished by an unwinding enzyme. Like some precision tool, this enzyme, shown in blue, breaks the chemical bonds that bind the two helices of DNA together. The enzyme works its way down the molecule, unzipping DNA as it goes. Each helix copies the other, supervised by special enzymes. The organic soup inside the nucleus contains many free nucleotides. The enzyme recognizes an approaching nucleotide and clicks it into place, reproducing another rung in the double helix.
When the DNA is replicating in one of your cells, a few dozen nucleotides are added every second. Thousands of these enzymes may be working on a given DNA molecule. When an arriving nucleotide doesn’t fit, the enzyme throws it away. We call this proofreading. On the rare occasions of a proofreading error, the wrong nucleotide is attached and a small random change has been made in the genetic instructions—a mutation has occurred. This enzyme is a pretty small molecule, but it catches nucleotides, assembles them in the right order,it knows how to proofread, it’s responsible in the most fundamental way for the reproduction of every cell and every being on Earth.
That enzyme, and DNA itself, are molecular machines with awesome powers. Within every living thing, the molecular machines are busy making sure that nucleic acids will continue to reproduce. A minor cut in my skin sounds a local alarm, and the blood spins a complex net of strong fibers to form a clot and staunch the flow of blood. There’s a very delicate balance here: too much clotting and your blood stream will solidify. Too little clotting and you’ll bleed to death from the merest scratch. The balance is controlled by enzymes instructed by DNA. Down here, there’s also a kind of sanitation squad comprised of white blood cells that swings into action, surrounds invading bacteria, and ravenously consumes them. This mopping-up operation is another part of the healing process—again controlled by DNA. These cells are parts of us, but how alien they seem. Within each of them, within every cell, there are exquisitely evolved molecular machines. Nucleic acids, enzymes, the cell architecture—every cell is a triumph of natural selection. And we’re made of trillions of cells. We are, each of us, a multitude. Within us is a little universe.
Human DNA is a coiled ladder a billion nucleotides long. Many possible combinations of nucleotides are nonsense—that is, they translate into proteins which serve no useful function whatever. Only a comparatively few nucleic acid molecules are any good for life forms as complicated as we are. But even so, the number of useful ways of assembling nucleic acids is stupefyingly large. It’s probably larger than the total number of atoms in the universe. This means that the number of possible kinds of human beings is vastly greater than the number of human beings that has ever lived. This untapped potential of the human species is immense. There must be ways of putting nucleic acids together which will function far better by any criterion you wish to choose than the hereditary instructions of any human being who has ever lived. Fortunately, we do not know—or, at least, do not yet know—how to assemble alternative sequences of nucleotides to make alternative kinds of human beings. But in the future we might well be able to put nucleotides together in any desired sequence to produce whatever human characteristics we think desirable. A disquieting and awesome prospect.
We human beings don’t look very much like a tree. We certainly view the world differently than a tree does. But down deep, at the molecular heart of life, we’re essentially identical to trees. We both use nucleic acids as the hereditary material, we both use proteins as enzymes to control the chemistry of the cell, and—most significantly—we both use the identical codebook to translate nucleic acid information into protein information. Any tree could read my genetic code. How did such astonishing similarities come about? Why are we cousins to the trees? Would life on some other planet use proteins? The same proteins? The same nucleic acids? The same genetic code? The usual explanation is that we are—all of us: trees and people, anglerfish, slime molds, bacteria—all descended from a single and common instance of the origin of life four billion years ago in the early days of our planet. Now, how did the molecules of life arise?
In a laboratory at Cornell University we mix together the gases and waters of the primitive Earth, supply some energy, and see if we can make the stuff of life. But what was the early atmosphere made of? Ordinary air? If we start with our present atmosphere, the experiment is a dismal failure. Instead of making proteins and nucleic acids, all we make is smog—a backwards step. Why doesn’t such an experiment work? Because the air of today contains molecular oxygen. But oxygen is made by plants. It’s pretty obvious that there were no plants before the origin of life. We mustn’t use oxygen in our experiments because there wasn’t any in the early atmosphere. This is perfectly reasonable because the cosmos is made mostly of hydrogen, which gobbles oxygen up. The Earth’s low gravity has allowed most of our original hydrogen gas to trickle away to space. There’s almost none left. But 4 billion years ago, our atmosphere was full of hydrogen-rich gases: methane, ammonia, water vapor. These are the gases we should use.
Taking great care to ensure the purity of these gases, my colleague, Bishun Khare, pumps them from their holding flasks. An experiment like this was first performed by Stanley Miller and Harold Urey in the 1950s. The starting gases are now introduced into a large reaction vessel. We could shine ultraviolet light on this mixture, simulating the early sun. But in this experiment the gases will be sparked, as the primitive atmosphere was, by early lightning. After only a few hours, the interior of the reaction vessel becomes streaked with a strange brown pigment—a rich collection of complex organic molecules including the building blocks of the proteins and the nucleic acids. Under the right conditions, these building blocks assemble themselves into molecules resembling little proteins and little nucleic acids. These nucleic acids can even make identical copies of themselves. In this vessel are the notes of the music of life, although not yet the music itself.
Now, no one so far has mixed together the gases and waters of the primitive Earth and, at the end of the experiment, had something crawl out of the flask. There’s still a great deal to be understood about the origin of life, including the origin of the genetic code. But we’ve only been at such experiments for 30 years. Nature’s had a 4-billion-year head start. Incidentally, there’s nothing in such experiments that’s unique to the Earth. The gases we start with, the energy sources we use are entirely common through the cosmos. So, chemical reactions something like these must be responsible for the organic matter in interstellar space and the amino acids in the meteorites. Similar chemical reactions must have occurred on a billion other worlds in the Milky Way galaxy. Look how easy it is to make great globs of this stuff! The molecules of life fill the cosmos.
Now, what would life elsewhere look like? Even if it had an identical molecular chemistry to life on Earth—which I very much doubt—it could not be very similar in form to familiar organisms on the Earth. The random character of the evolutionary process must create, elsewhere, creatures very different from any that we know.
Think of a world something like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water, and ammonia, in which organic molecules might be falling from the skies like manna from heaven—like the products of the Miller-Urey experiment. Could there be life on such a world? Well, there’s a special problem. The atmosphere is turbulent, and down deep, before we ever come to a surface, it’s very hot. If you’re not careful, you’ll be carried down and fried. So one way to make a living is to reproduce before you’re fried. Turbulence will carry your offspring into the higher and cooler layers. Such organisms could be very little. We call them sinkers. The physicist E. E. Salpeter and I at Cornell have calculated something about the other kinds of life that might exist on such a world. Vast living balloons could stay buoyant by pumping heavy gases from their interiors or by keeping their insides warm. They might eat the organic molecules in the air or make their own with sunlight. We call these creatures floaters. We imagine floaters kilometers across, enormously larger than the greatest whale that ever was—beings the size of cities. We conceive of them arrayed in great, lazy herds as far as the eye can see, concentrated in the updrafts in the enormous sea of clouds. But there can be other creatures in this alien environment: hunters. Hunters are fast and maneuverable. They eat the floaters, both for their organic molecules and for their store of pure hydrogen. But there can’t be many hunters, because if they destroy all the floaters, they themselves will perish. Physics and chemistry permit such life forms. Art presents them with a certain reality, but nature is not obliged to follow our speculations. However, if there are billions of inhabited worlds in the Milky Way galaxy, then I think it’s likely there are a few places which might have hunters and floaters and sinkers.
Biology is more like history than it is like physics. You have to know the past to understand the present. There is no predictive theory of biology, just as there is no predictive theory of history. The reason is the same: both subjects are still too complicated for us. But we can understand ourselves much better by understanding other cases. The study of a single instance of extraterrestrial life, no matter how humble—a microbe would be just fine—will de-provincialize biology. It will show us what else is possible. We’ve heard so far the voice of life on only a single world. But for the first time, as we shall see, we’ve begun a serious scientific search for the cosmic fugue.
In recent years, we’ve learned much more about the origin of life. Do you remember RNA—that nucleic acid that our cells use as messengers carrying the genetic information out of the cell nucleus? Well, it’s been found that RNA, like protein, can control chemical reactions as well as reproduce itself, which proteins can’t do. Many scientists now wonder if the first life on Earth was an RNA molecule.
And it now seems feasible that key molecular building blocks for the origin of life fell out of the skies 4 billion years ago. Comets have now been found to have a lot of organic molecules in them, and they fell in huge numbers on the primitive Earth.
We also mention the extinction of the dinosaurs, and most of the other species of life on Earth about 65 million years ago. We now know that a large comet hit the Earth at just that time. The dust pall from that collision must’ve cooled and darkened the Earth, perhaps killing all the dinosaurs but sparing the small, furry mammals who were our ancestors. Other cometary mass extinctions in other epochs seem likely. If true, this would mean that comets have been the bringers both of life and death.