by Carl Sagan
All life on Earth is closely related. We have a common organic chemistry and a common evolutionary heritage. As a result, our biologists are profoundly limited. They study only a single kind of biology, one lonely theme in the music of life. Is this faint and reedy tune the only voice for thousands of light-years? Or is there a kind of cosmic fugue, with themes and counterpoints, dissonances and harmonies, a billion different voices playing the life music of the Galaxy?
Let me tell you a story about one little phrase in the music of life on Earth. In the year 1185, the Emperor of Japan was a seven-year-old boy named Antoku. He was the nominal leader of a clan of samurai called the Heike, who 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 naval encounter, with the Emperor on board ship, occurred at Danno-ura in the Japanese Inland Sea on April 24, 1185. The Heike were outnumbered, and outmaneuvered. Many were killed. The survivors, in massive numbers, threw themselves into the sea and drowned. The Lady Nii, grandmother of the Emperor, resolved that she and Antoku would not be captured by the enemy. What happened next is told in The Tale of the Heike:
The Emperor was seven years old that year but looked much older. He was so lovely that he seemed to shed a brilliant radiance and his long, black hair hung loose far down his back. With a look of surprise and anxiety on his face he 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, binding up his long hair in his dove-colored robe. 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 repeat the Nembutsu [a prayer to the Amida Buddha]. The Lady Nii took him tightly in her arms and with the words “In the depths of the ocean is our capitol,” sank with him at last beneath the waves.
The entire Heike battle fleet was destroyed. Only forty-three women survived. These ladies-in-waiting of the imperial court were forced to sell flowers and other favors to the fishermen near the scene of the battle. The Heike almost vanished from history. But a ragtag group of the former ladies-in-waiting and their offspring by the fisherfolk established a festival to commemorate the battle. It takes place on the twenty-fourth of April every year to this day. Fishermen who are the descendants of the Heike dress in hemp and black headgear and proceed to the Akama shrine which contains the mausoleum of the drowned Emperor. There they watch a play portraying the events that followed the Battle of Danno-ura. For centuries after, people imagined that they could discern ghostly samurai armies vainly striving to bail the sea, to cleanse it of blood and defeat and humiliation.
The fishermen say the Heike samurai wander the bottoms of the Inland Sea still—in the form of crabs. There are crabs to be found here with curious markings on their backs, patterns and indentations that disturbingly resemble the face of a samurai. When caught, these crabs are not eaten, but are returned to the sea in commemoration of the doleful events at Danno-ura.
This legend raises a lovely problem. How does it come about that the face of a warrior is incised on the carapace of a crab? The answer seems to be that humans made the face. The patterns on the crab’s shell are inherited. But among crabs, as among people, there are many different hereditary lines. Suppose that, by chance, among the distant ancestors of this crab, one arose with a pattern that resembled, even slightly, a human face. Even before the battle of Danno-ura, fishermen may have been reluctant to eat such a crab. In throwing it back, they set in motion an evolutionary process: If you are a crab and your carapace is ordinary, the humans will eat you. Your line will leave fewer descendants. If your carapace looks a little like a face, they will throw you back. You will leave more descendants. Crabs had a substantial investment in the patterns on their carapaces. As the generations passed, of crabs and fishermen alike, the crabs with patterns that most resembled a samurai face survived preferentially until eventually there was produced not just a human face, not just a Japanese face, but the visage of a fierce and scowling samurai. All this has nothing to do with what the crabs want. Selection is imposed from the outside. The more you look like a samurai, the better are your chances of survival. Eventually, there come to be a great many samurai crabs.
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. But humans have deliberately selected which plants and animals shall live and which shall die for thousands of years. We are surrounded from babyhood by familiar farm and domestic animals, fruits and trees and vegetables. Where do they come from? Were they once free-living in the wild and then induced to adopt a less strenuous life on the farm? No, the truth is quite different. They are, most of them, made by us.
Ten thousand years ago, there were no dairy cows or ferret hounds or large ears of corn. When we domesticated the ancestors of these plants and animals—sometimes creatures who looked quite different—we controlled their breeding. We made sure that certain varieties, having properties we consider desirable, preferentially reproduced. When we wanted a dog to help us care for sheep, we selected breeds that were intelligent, obedient and had some pre-existing talent to herd, which is useful for animals who hunt in packs. The enormous distended udders of dairy cattle are the result of a human interest in milk and cheese. Our corn, or maize, has been bred for ten thousand generations to be more tasty and nutritious than its scrawny ancestors; indeed, it is so changed that it cannot even reproduce without human intervention.
The essence of artificial selection—for a Heike crab, a dog, a cow or an ear of corn—is this: Many physical and behavioral traits of plants and animals are inherited. They breed true. Humans, for whatever reason, encourage the reproduction of some varieties and discourage the reproduction of others. The variety selected for preferentially reproduces; it eventually becomes abundant; the variety selected against becomes rare and perhaps extinct.
But if humans can make new varieties of plants and animals, must not nature do so also? This related process is called natural selection. That life has changed fundamentally over the aeons is entirely clear from the alterations we have made in the beasts and vegetables during the short tenure of humans on Earth, and from the fossil evidence. The fossil record speaks to us unambiguously of creatures that once were present in enormous numbers and that have now vanished utterly.* Far more species have become extinct in the history of the Earth than exist today; they are the terminated experiments of evolution.
The genetic changes induced by domestication have occurred very rapidly. The rabbit was not domesticated until early medieval times (it was bred by French monks in the belief that newborn bunnies were fish and therefore exempt from the prohibitions against eating meat on certain days in the Church calendar); coffee in the fifteenth century; the sugar beet in the nineteenth century; and the mink is still in the earliest stages of domestication. In less than ten thousand years, domestication has increased the weight of wool grown by sheep from less than one kilogram of rough hairs to ten or twenty kilograms of uniform, fine down; or the volume of milk given by cattle during a lactation period from a few hundred to a million cubic centimeters. If artificial selection can make such major changes in so short a period of time, what must natural selection, working over billions of years, be capable of? The answer is all the beauty and diversity of the biological world. Evolution is a fact, not a theory.
That the mechanism of evolution is natural selection is the great discovery associated with the names of Charles Darwin and Alfred Russel Wallace. More than a century ago, they stressed that nature is prolific, that many more animals and plants are born than can possibly survive and that therefore the environment selects those varieties which are, by accident, better suited for survival. Mutations—sudden changes in heredity—breed true. They provide the raw material of evolution. The environment selec
ts those few mutations that enhance survival, resulting in a series of slow transformations of one lifeform into another, the origin of new species.*
Darwin’s words in The Origin of Species were:
Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then Nature acts on the organisation, and causes variability. But man can and does select the variations given to him by Nature, and thus accumulate them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.… There is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under Nature.… More individuals are born than can possibly survive.… The slightest advantage in one being, of any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance.
T. H. Huxley, the most effective nineteenth-century defender and popularizer of evolution, wrote that the publications of Darwin and Wallace were a “flash of light, which to a man who has lost himself in a dark night, suddenly reveals a road which, whether it takes him straight home or not, certainly goes his way.… My reflection, when I first made myself master of the central idea of the ‘Origin of Species,’ was, ‘How extremely stupid not to have thought of that!’ I suppose that Columbus’ companions said much the same.… The facts of variability, of the struggle for existence, of adaptation to conditions, were notorious enough; but none of us had suspected that the road to the heart of the species problem lay through them, until Darwin and Wallace dispelled the darkness.”
Many people were scandalized—some still are—at both ideas, evolution and natural selection. Our ancestors looked at the elegance of life on Earth, at how appropriate the structures of organisms are to their functions, and saw evidence for a Great Designer. The simplest one-celled organism is a far more complex machine than the finest pocket watch. And yet pocket watches do not spontaneously self-assemble, or evolve, in slow stages, on their own, from, say, grandfather clocks. A watch implies a watchmaker. There seemed to be no way in which atoms and molecules could somehow spontaneously fall together to create organisms of such awesome complexity and subtle functioning as grace every region of the Earth. That each living thing was specially designed, that one species did not become another, were notions perfectly consistent with what our ancestors with their limited historical records knew about life. The idea that every organism was meticulously constructed by a Great Designer provided a significance and order to nature and an importance to human beings that we crave still. A Designer is a natural, appealing and altogether human explanation of the biological world. But, as Darwin and Wallace showed, there is another way, equally appealing, equally human, and far more compelling: natural selection, which makes the music of life more beautiful as the aeons pass.
The fossil evidence could be consistent with the idea of a Great Designer; perhaps some species are destroyed when the Designer becomes dissatisfied with them, and new experiments are attempted on an improved design. But this notion is a little disconcerting. Each plant and animal is exquisitely made; should not a supremely competent Designer have been able to make the intended variety from the start? The fossil record implies trial and error, an inability to anticipate the future, features inconsistent with an efficient Great Designer (although not with a Designer of a more remote and indirect temperament).
When I was a college undergraduate in the early 1950’s, I was fortunate enough to work in the laboratory of H. J. Muller, a great geneticist and the man who discovered that radiation produces mutations. Muller was the person who first called my attention to the Heike crab as an example of artificial selection. To learn the practical side of genetics, I spent many months working with fruit flies, Drosophila melanogaster (which means the black-bodied dew-lover)—tiny benign beings with two wings and big eyes. We kept them in pint milk bottles. We would cross two varieties to see what new forms emerged from the rearrangement of the parental genes, and from natural and induced mutations. The females would deposit their eggs on a kind of molasses the technicians placed inside the bottles; the bottles were stoppered; and we would wait two weeks for the fertilized eggs to become larvae, the larvae pupae, and the pupae to emerge as new adult fruit flies.
One day I was looking through a low-power binocular microscope at a newly arrived batch of adult Drosophila immobilized with a little ether, and was busily separating the different varieties with a camel’s-hair brush. To my astonishment, I came upon something very different: not a small variation such as red eyes instead of white, or neck bristles instead of no neck bristles. This was another, and very well-functioning, kind of creature with much more prominent wings and long feathery antennae. Fate had arranged, I concluded, that an example of a major evolutionary change in a single generation, the very thing Muller had said could never happen, should take place in his own laboratory. It was my unhappy task to explain it to him.
With heavy heart I knocked on his office door. “Come in,” came the muffled cry. I entered to discover the room darkened except for a single small lamp illuminating the stage of the microscope at which he was working. In these gloomy surroundings I stumbled through my explanation. I had found a very different kind of fly. I was sure it had emerged from one of the pupae in the molasses. I didn’t mean to disturb Muller but … “Does it look more like Lepidoptera than Diptera?” he asked, his face illuminated from below. I didn’t know what this meant, so he had to explain: “Does it have big wings? Does it have feathery antennae?” I glumly nodded assent.
Muller switched on the overhead light and smiled benignly. It was an old story. There was a kind of moth that had adapted to Drosophila genetics laboratories. It was nothing like a fruit fly and wanted nothing to do with fruit flies. What it wanted was the fruit flies’ molasses. In the brief time that the laboratory technician took to unstopper and stopper the milk bottle—for example, to add fruit flies—the mother moth made a dive-bombing pass, dropping her eggs on the run into the tasty molasses. I had not discovered a macro-mutation. I had merely stumbled upon another lovely adaptation in nature, itself the product of micromutation and natural selection.
The secrets of evolution are death and time—the deaths of enormous numbers of lifeforms that were imperfectly adapted to the environment; and time for a long succession of small mutations that were by accident adaptive, time for the slow accumulation of patterns of favorable mutations. Part of the resistance to Darwin and Wallace derives from our difficulty in imagining the passage of the millennia, much less the aeons. What does seventy million years mean to beings who live only one-millionth as long? We are like butterflies who flutter for a day and think it is forever.
What happened here on Earth may be more or less typical of the evolution of life on many worlds; but in such details as the chemistry of proteins or the neurology of brains, the story of life on Earth may be unique in all the Milky Way Galaxy. The Earth condensed out of interstellar gas and dust some 4.6 billion years ago. We know from the fossil record that the origin of life happened soon after, perhaps around 4.0 billion years ago, in the ponds and oceans of the primitive Earth. The first living things were not anything so complex as a one-celled organism, already a highly sophisticated form of life. The first stirrings were much more humble. In those early days, lightning and ultraviolent light from the Sun were breaking apart the simple hydrogen-rich molecules of the primitive atmosphere, the fragments spontaneously recombining into more and more complex molecules. The products of this early chemistry were 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 other molecules in the soup. (We will return to this subject la
ter.)
This was the earliest ancestor of deoxyribonucleic acid, DNA, the master molecule of life on Earth. It is shaped like a ladder twisted into a helix, the rungs available in four different molecular parts, which constitute the four letters of the genetic code. These rungs, called nucleotides, spell out the hereditary instructions for making a given organism. Every lifeform on Earth has a different set of instructions, written out in essentially the same language. The reason organisms are different is the differences in their nucleic acid instructions. A mutation is a change in a nucleotide, copied in the next generation, which breeds true. Since mutations are random nucleotide changes, most of them are harmful or lethal, coding into existence nonfunctional enzymes. It is a long wait before a mutation makes an organism work better. And yet it is that improbable event, a small beneficial mutation in a nucleotide a ten-millionth of a centimeter across, that makes evolution go.
Four billion years ago, the Earth was a molecular Garden of Eden. There were as yet no predators. Some molecules reproduced themselves inefficiently, competed for building blocks and left crude copies of themselves. With reproduction, mutation and the selective elimination of the least efficient varieties, evolution was well under way, even at the molecular level. As time went on, they got better at reproducing. Molecules with specialized functions eventually joined together, making a kind of molecular collective—the first cell. Plant cells today have tiny molecular factories, called chloroplasts, which are in charge of photosynthesis—the conversion of sunlight, water and carbon dioxide into carbohydrates and oxygen. The cells in a drop of blood contain a different sort of molecular factory, the mitochondrion, which combines food with oxygen to extract useful energy. These factories exist in plant and animal cells today but may once themselves have been free-living cells.