Shadows of Forgotten Ancestors (22 page)
Read Shadows of Forgotten Ancestors Online
Authors: Carl Sagan,Ann Druyan
The beings of our planet are imperfectly linked 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, C
2
H
5
OH (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.”
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*
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.
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, Supplements
2
F
ireflies 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, more 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.
Now, instead of slow, random change in the genetic materials, imagine that you could in one step glue onto part of the existing messages a long, complex set of new instructions—not merely a change in one letter of one word of the DNA, but whole volumes of consumer-tested manuals. Imagine the same kind of reshuffling occurring in subsequent generations. This is a dumb idea if you’re ideally adapted to an unchanging or very marginal environment; then any change is for the worse. But if the world you must adapt to is heterogeneous and dynamic, evolutionary progress is better served when reams of new genetic instructions are made available in each generation than when all there is to deal with is an occasional conversion of an A into a C. Also, if you can reshuffle genes, you or your descendants can get out of the trap set by the accumulation, generation after generation, of deleterious mutations.
3
Harmful genes can quickly be replaced by advantageous ones. Sex and natural selection work as a kind of proofreader, replacing the inevitable mutational errors by uncontaminated instructions. This may be why the eukaryotes diversified—into the separate evolutionary lines that would lead to protozoa (like paramecia), plasmodia (like those that cause malaria), algae, fungi, all land plants and all animals—just around the time that eukaryotes hit upon sex.
Some modern organisms—ranging from bacteria to aphids to aspens—sometimes reproduce sexually and sometimes asexually. They can go either way. Others—dandelions, for example, and certain
whiptail lizards—have recently evolved from sexual to asexual forms, as seems clear from their anatomy and behavior: Dandelions produce flowers and nectar that are useless for their current reproductive style; no matter how busy the bees are, they cannot be agents for dandelion fertilization. In the whiptail lizards, everyone is female and the hatchlings have no biological fathers. But reproduction still requires heterosexual foreplay—the formality of copulation with males of other, still sexual, lizard species, even though they cannot impregnate these females, or a ritual pseudocopulation with other females of the same species.
4
Apparently, we are observing these dandelions and lizards so soon after their evolution from sexual to asexual beings that there has been insufficient time for the scripts and props of sex to have withered away. Perhaps there are circumstances when it’s wise to reproduce sexually and others when it isn’t; certain beings may prudently cycle from one state to the other, depending on the external environment. This option is, however, unavailable to us. We are stuck with sex.
Today a reshuffling of genetic instructions, similar to what happens in sex, occurs—oddly—in infection: A microbe enters a larger organism, evades its defenses, and insinuates its nucleic acid onto that of its host. There’s an intricate machinery in the cell, idling and ready to go, which reads and replicates preexisting sequences of A, C, G, and T. The machinery’s not good enough, though, to distinguish foreign nucleic acids from native ones. It’s a printing press for instruction manuals, and it will copy anything when its buttons are pushed. The parasite pushes the buttons, the cell’s enzymes are issued new instructions, and hordes of newly minted parasites are spewed out, itching for more subversion.
