Shadows of Forgotten Ancestors

by Carl Sagan

  On the scale of the Solar System, the extinction of the dinosaurs and the rise of the mammals seem to have been a very near thing. The causality corridor, figuratively speaking, was only inches wide. Had the comet been traveling a little slower or faster or headed in a slightly different direction, no collision would then have occurred. If other comets that in our real history missed the Earth had been on slightly different trajectories, they would have hit the Earth and killed off life in some different epoch. The cosmic collision roulette, the extinction lottery, reaches into our own time.

  At the depth in the fossil record above which there are no more dinosaurs, there is, worldwide, a telltale thin layer of the element iridium, which is abundant in space but not on the Earth’s surface. There also are tiny grains bearing the signs of a collosal impact. This evidence tells us of a high-speed collision of a small world with the Earth which distributed fine particles worldwide. The remains of the impact crater may have been discovered in the Gulf of Mexico near the Yucatan Peninsula. But something else is found in this layer as well: soot. Planet-wide, the time of this great impact was also the time of a global fire. The debris from the impact explosion, spewed out into the high atmosphere and falling back through the air all over the Earth—a continuous meteor shower filling the sky—illuminated the ground far more brightly than the noonday Sun. Land plants everywhere on Earth burst into flames, all at once. Most of them were consumed. There is an odd causal nexus connecting oxygen, plants, giant impacts, and world-immolating fire.

  There are many ways in which such an impact could have extinguished long-established and, if we may call them that, self-confident forms of life. After the initial burst of light and heat, a thick pall of impact dust enveloped the Earth for a year or more. Perhaps even more important than the world fire, the lowered temperatures, and a planet-wide acid rain was the absence for a year or two of enough light for photosynthesis. The primary photosynthesizing organisms in the oceans (then as now covering most of the Earth) are little one-celled plants called phytoplankton. They are especially vulnerable to lowered light levels because they lack major food reserves. Once the lights get turned out their chloroplasts can no longer generate carbohydrates from sunlight, and they die. But these little plants are the principal diet of one-celled animals that are eaten by larger, shrimp-like creatures, that are eaten by small fish, that are in turn eaten by large fish. Turn off the lights, wipe out the phytoplankton, and the entire food chain, this elaborate house of cards, collapses. Something similar is true on land.

  The beings of Earth depend on one another. Life on Earth is an intricately woven tapestry or web. Yank out a few threads here and there, and you can’t be sure whether that’s all the damage you’ve done, or whether the whole fabric will now unravel.

  Insects and other arthropods are the principal agents by which dead plants and animal excrement are cleaned up. Scarabs—the dung beetles identified with the sun god and worshipped by the ancient Egyptians—are specialists in waste management. They collect the nitrogen-rich animal excrement accumulating on the surface of our planet and transport this fertilizer down where the plant roots are. Some sixteen thousand beetles have been counted on a single fresh elephant pat in Africa; two hours later the pat was gone.12 The Earth’s surface would be very different (and very messy) without dung beetles and their like. In addition, the microscopic feces of mites and springtails are major constituents of the soil humus from which the plants grow. Animals then eat the plants. We live off each other’s solid wastes as well.

  Other inhabitants of the soil kill off the young plants. Here is Darwin’s account of a little experiment he did to illustrate the hidden ferocity lurking just beneath the placid surface of a country garden:

  [On] a piece of ground 3 feet long and 2 feet wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of 357 no less than 295 were destroyed, chiefly by slugs and insects If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous though fully grown plants . .13

  Some plants provide food for specific animals, in turn, the animals act as agents for the sexual reproduction of the plants—in effect, couriers taking sperm from male plants and using it for artificial insemination of female plants. This is not quite artificial selection, because the animals are not much in charge. The currency these procurers are paid in is usually food. A bargain has been struck. Maybe the animal is a pollinating insect, or bird, or bat; or a mammal to whose furry coat the reproductive burrs adhere; or maybe the deal is food supplied by the plants in exchange for nitrogenous fertilizer supplied by the animals. Predators have symbionts that clean their coats or scales or pick their teeth in exchange for leavings. A bird eats a sweet fruit; the seeds pass through its digestive tract and are deposited on fertile ground some distance away: another business transaction consummated. Fruit trees and berry-bearing bushes often take care that their offerings to the animals are sweet only when the seeds are ready to be dispersed. Unripe fruit gives bellyaches, the plants’ way of training the animals.

  The cooperation between plants and animals is uneasy. The animals cannot be trusted; given a chance, they’ll eat any plant in sight. So the plants protect themselves from unwelcome attention with thorns, or by producing irritants, or poisons, or chemicals that make the plant indigestible, or agents that interfere with the predator’s DNA. In this endless slow-motion war, the animals then produce substances that disable these adaptations by the plants. And so on.

  The beasts and vegetables and microbes are the interlocking parts, the gear train, of a vast, intricate and very beautiful ecological machine of planetary proportions, a machine plugged into the Sun. Pretty nearly, all flesh is sunlight.

  Where the ground is covered with plants perhaps 0.1% of the sunlight is converted into organic molecules. A plant-eating animal saunters by and eats one of these plants. Typically the herbivore extracts about a tenth of the energy in the plant, or about one ten-thousandth of the sunlight that could, with 100% efficiency, have been stored in the plant. If the herbivore is now attacked and eaten by a carnivore, about 10% of the available energy in the prey will wind up in the predator. Only one part in a hundred thousand of the original solar energy makes it to the carnivore. There are no perfectly efficient engines, of course, and we expect losses at each stage in the food chain. But the organisms at the top of the food chain seem inefficient to the point of irresponsibility.*

  A vivid image of the interconnection and interdependence of life on Earth was provided by the biologist Clair Folsome, who asks you to imagine what you would see if all the cells of your body, flesh and bones, were magically removed:

  What would remain would be a ghostly image, the skin outlined by a shimmer of bacteria, fungi, round worms, pin worms and various other microbial inhabitants. The gut would appear as a densely packed tube of anaerobic and aerobic bacteria, yeasts, and other microorganisms. Could one look in more detail, viruses of hundreds of kinds would be apparent throughout all tissues.

  And, Folsome stresses, any other plant or animal on Earth, under the same dispensation, would reveal a similar “seething zoo of microbes.” 14

  ——

  A biologist from some other solar system, in an unblinking examination of the teeming lifeforms of Earth, would surely note that they are all made of almost exactly the same organic stuff, the same molecules almost always performing the same functions, with the same genetic codebook in use by almost everybody. The organisms on this planet are not only kin; they live in intimate mutual contact, imbibing each other’s wastes, dependent on one another for life itself, and sharing the same fragile surface layer. This conclusion is not ideology, but reality. It depends not on authority, faith, or special pleading by its proponents, but on repeatable observation and experiment.

  The beings of our planet are imperfectly link
ed and coordinated; and there is certainly nothing like a collective intelligence of all the life on Earth—in the sense that all the cells of a human body are subject, within stringent constraints, to a supervening volition. Still, the alien biologist might be excused for lumping together the whole biosphere—all the retroviruses, mantas, foraminifera, mongongo trees, tetanus bacilli, hydras, diatoms, stromatolite-builders, sea slugs, flatworms, gazelles, lichens, corals, spirochetes, banyans, cave ticks, least bitterns, caracaras, tufted puffins, ragweed pollen, wolf spiders, horseshoe crabs, black mambas, monarch butterflies, whiptail lizards, trypanosomes, birds of paradise, electric eels, wild parsnips, arctic terns, fireflies, titis, chrysanthemums, hammerhead sharks, rotifers, wallabies, malarial plasmodia, tapirs, aphids, water moccasins, morning glories, whooping cranes, komodo dragons, periwinkles, millipede larvae, angler fish, jellyfish, lungfish, yeast, giant redwoods, tardigrades, archaebacteria, sea lilies, lilies of the valley, humans, bonobos, squid and humpback whales—as, simply, Earthlife. The arcane distinctions among these swarming variations on a common theme may be left to specialists or graduate students. The pretensions and conceits of this or that species can readily be ignored. There are, after all, so many worlds about which an extraterrestrial biologist must know. It will be enough if a few salient and generic characteristics of life on yet another obscure planet are noted for the cavernous recesses of the galactic archives.

  * Seawater itself is opaque to ultraviolet light beyond a certain depth, and the early oceans were very likely covered by a slick of ultraviolet-absorbing organic molecules. The seas were safe.

  * A biochemical imperfection exploited by the beer, wine, and liquor industries, which profitably manufacture this addictive and dangerous drug, C2H5OH (where C stands for a carbon atom, O for oxygen, and H for hydrogen). Millions of people worldwide die from imbibing it each year. Or, looked at another way, distillers have been exploited by the fermenting bacteria and yeast, who have gotten us to arrange for their growth and reproduction on a worldwide, industrial scale—because we love to drink ourselves senseless on microbial wastes. If they could speak, perhaps they would boast about how cleverly they’ve domesticated the humans. Yeasts also colonize dark, moist, oxygen-poor parts of the human body, another way in which we serve them.

  † Another example was given by the ancient Greek philosopher Heraclitus: “The sea,” he said, “is most pure and most polluted water: for fish, drinkable and life-preserving; for men, undrinkable and death-dealing.”2

  * The genetic code of the mitochondrion is just a little different from that of the nucleus—as if it had evolved so that the nuclear DNA could not tell the mitochondria what to do, a token of independence. For example, AGA means STOP for mitochondrial nucleic acids, whereas for the nucleic acids that hail from the nucleus of a cell, it codes for a particular amino acid, arginine.3 The mitochondria simply ignore instructions from the capital, which to them are mainly gibberish with occasional lucid passages; they follow the commands of their own feudal leader, the mitochondrial DNA.

  * Ninety-five percent seems awfully close to 100%, and it’s disquieting to be reminded that the great rumbling, internal tectonic engine can inadvertently kill off so many of us up here because of some hiccups down there.

  * In principle the ecological machine could continue as long as the Sun continues to shine, estimated at another 5 billion years. It’s hard not to wonder—we carnivores at the apex of the food chain, the beneficiaries of a process with a thousandth of a percent efficiency—if there might not be some more efficient way for us to harness the Sun.

  Chapter 8

  SEX AND DEATH

  [S]ex endows the individual with a dumb and

  powerful instinct, which carries his body and

  soul continually towards another; makes it one

  of the dearest employments of his life to select

  and pursue a companion, and joins to

  possession the keenest pleasure, to rivalry the

  fiercest rage, and to solitude an eternal

  melancholy. What more could be needed to

  suffuse the world with the deepest meaning

  and beauty?

  GEORGE SANTAYANA,

  The Sense of Beauty (1896)1

  Death is the great reprimand which the will to

  live, or more especially the egoism which is

  essential to this, receives through the course of

  nature; and it may be conceived of as a

  punishment for our existence. It is the painful

  loosening of the knot which the act of

  generation had tied …

  ARTHUR SCHOPENHAUER,

  The World as Will and Idea, Supplements2

  Fireflies out on a warm summer’s night, seeing the urgent, flashing, yellow-white phosphorescence below them, go crazy with desire; moths cast to the winds an enchantment potion that draws the opposite sex, wings beating hurriedly, from kilometers away; peacocks display a devastating corona of blue and green and the peahens are all aflutter; competing pollen grains extrude tiny tubes that race each other down the flower’s female orifice to the waiting egg below; luminescent squid present rhapsodic light shows, altering the pattern, brightness, and color radiated from their heads, tentacles, and eyeballs; a tapeworm diligently lays a hundred thousand fertilized eggs in a single day; a great whale rumbles through the ocean depths uttering plaintive cries that are understood hundreds or thousands of kilometers away, where another lonely behemoth is attentively listening; bacteria sidle up to one another and merge; cicadas chorus in a collective serenade of love; honeybee couples soar on matrimonial flights from which only one partner returns; male fish spray their spunk over a slimy clutch of eggs laid by God-knows-who; dogs, out cruising, sniff each other’s nether parts, seeking erotic stimuli; flowers exude sultry perfumes and decorate their petals with garish ultraviolet advertisements for passing insects, birds, and bats; and men and women sing, dance, dress, adorn, paint, posture, self-mutilate, demand, coerce, dissemble, plead, succumb, and risk their lives. To say that love makes the world go around is to go too far. The Earth spins because it did so as it was formed and there has been nothing to stop it since But the nearly maniacal devotion to sex and love by most of the plants, animals, and microbes with which we are familiar is a pervasive and striking aspect of life on Earth. It cries out for explanation.

  What is all this in aid of? What is this torrent of passion and obsession about? Why will organisms go without sleep, without food, gladly put themselves in mortal danger for sex? Some beings, among them good-sized plants and animals such as dandelions, salamanders, some lizards and fish, can reproduce without sex. For more than half the history of life on Earth organisms seem to have done perfectly well without it. What good is sex?

  What’s more, sex is expensive. It takes formidable genetic programming to wire in seductive songs and dances; to manufacture sexual pheromones; to grow heroic antlers used only in defeating rivals; to establish interlocking parts, rhythmic motions, and mutual zest for sex. All this represents a drain on energy resources that could just as well be used for something of more obvious short-term benefit to the organism. Also, some of what the beings of Earth do or endure for sex endangers them directly: The displaying peacock is much more vulnerable to predators than if he were inconspicuous, fearful, and dun-colored. Sex provides a convenient and potentially deadly channel for the transmission of disease. All these costs must be more than offset by the benefits of sex. What are those benefits?

  ——

  Embarrassingly, biologists don’t fully understand what sex is for. In this respect the situation has hardly changed since 1862 when Darwin wrote

  We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements … The whole subject is as yet hidden in darkness.

  Through 4 billion years of natural selection, instructions have been honed and fine-tuned—more elaborate, more redundant, m
ore foolproof, more multiply capable instructions—sequences of As, Cs, Gs, and Ts, manuals written out in the alphabet of life in competition with other similar manuals published by other firms. The organisms become the means through which the instructions flow and copy themselves, by which new instructions are tried out, on which selection operates. “The hen,” said Samuel Butler, “is the egg’s way of making another egg.” It is on this level that we must understand what sex is for.

  We do understand much of the molecular machinery of sex. To begin with, let’s consider some of those microbial beings that routinely do what many people would consider impossible—reproducing without sex*: Once every generation their nucleic acids faithfully copy themselves out of the A, C, G, and T molecular building blocks they manufacture for the purpose. The two functionally identical DNAs then each take half the cell and run—a little like a property settlement in a divorce. Sometime later, the process repeats itself. Each generation is a dreary repetition of the one before, and every organism is the spitting image—nearly identical, down to the last mitochondrion and flagellar propulsion system—of its single parent. If the organism is well-adapted and the environment repetitive and static, this arrangement might work well. The monotony is broken, rarely, by mutation. But mutation, as we’ve stressed, is random and much more likely to do harm than good. All subsequent generations will be afflicted unless, improbably, there’s a compensating mutation down the line. The pace of evolution under such circumstances must be slow, as indeed seems to be reflected in the fossil record between 3.5 and about 1 billion years ago—until the invention of sex.