by Carl Sagan
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.
Occasionally, the dead manage to have sex and generate offspring. When a bacterium dies, its contents are spilled into the surroundings. Its nucleic acids don’t know much about the death of the bacterium and even as they slowly fall to pieces, the fragments remain for a time functional—like the severed leg of an insect. Should such a fragment be ingested by a passing (and intact) bacterium, it may be incorporated into the resident nucleic acids. Perhaps it is used as an independent record of what undamaged instructions should say, helpful in repairing DNA altered by oxygen. Maybe this extremely rudimentary form of sex arose along with the Earth’s oxygen atmosphere.
Bizarre chimerical gene combinations happen more rarely—for example, between bacteria and fish (not only are there bacterial genes in fish today; there are also fish genes in bacteria), or baboons and cats. They seem to have been brought about by a virus attaching itself to the DNA of a host organism, reproducing with and accommodating to the host over the generations, and then shaking loose to infect another species while carrying some of the original host’s genes with it. Cats are known to have acquired a baboon virogene somewhere on the shores of the Mediterranean Sea 5 to 10 million years ago.5 Viruses are looking more and more as if they are peripatetic genes that cause disease only incidentally. But if genetic exchanges can occur today in such widely divergent organisms, it must be far easier for them to occur, by accident, in organisms of the same or very closely related species. Perhaps sex started out as an infection, becoming later institutionalized by the infecting and infected cells.
Two distant relatives, members of the same species, each in the process of replication, find their nucleic acid strands, one from each, laid down, cozily, alongside one another. A short segment of one very long sequence might be, say,
… ATG AAG TCG ATC CTA …
and the corresponding segment of the other
… TAC TTC GGG CGG AAT …
The long nucleic acid molecules both break apart at the same place in the sequence (here, just after AAG in the first molecule and TTC in the second), whereupon they recombine, each picking up a segment of the other:
… ATG AAG GGG CGG AAT …
and
… TAC TTC TCG ATC CTA …
Because of this genetic recombination, there are two new sequences of instructions and therefore two new organisms in the world—not exactly chimeras, since they come from the same species, but nevertheless each constituting a set of instructions that may never before have coexisted in the same being.
A gene, as we’ve said, is a sequence of perhaps thousands of As, Cs, Gs, and Ts which codes for a particular function, usually by synthesizing a particular enzyme. When DNA molecules are severed, just prior to recombination, the cut occurs at the beginning or the end of a gene, and almost never in its middle. One gene may have many functions. Important characteristics of the organism—height, say, aggressiveness, coat color, or intelligence—will generally be the consequence of many different genes acting in concert.
Because of sex, different combinations of genes can now be tried out, to compete with the more conventional varieties. A promising set of natural experiments is being performed. Instead of generations patiently waiting in line for a lucky sequence of mutations to occur—it might take a million generations for the right one, and the species might not be able to wait that long—the organism can now acquire new traits, new characteristics, new adaptations wholesale. Two or more mutations that don’t do much good by themselves, but that confer an enormous benefit when working in tandem, might be acquired from widely separated hereditary lines. The advantages (for the species, at least) seem clear, if only the costs were bearable. Genetic recombination provides a treasure trove of variability on which natural selection can act.6
Another proposed explanation for the persistence of sex, wonderful in its novelty, invites us to consider the age-old arms race between parasitic microbes and their hosts. There are more disease microorganisms in your body at this moment than there are people on Earth. A single bacterium reproducing twice an hour will leave a million successive generations during your lifetime. With so many microbes and so many generations, an immense number of microbial varieties are available for selection to operate on—especially selection to overcome your body’s defenses. Some microbes change the chemistry and form of their surfaces faster than the body can generate new model antibodies; these tiny beings routinely outwit at least some parts of the human immune system. For example, an alarming 2% of the plasmodium parasites that cause malaria significantly change their shapes and styles of stickiness each generation.7 In light of the formidable adaptive powers of disease microorganisms, a real danger would arise if we humans were genetically identical, generation after generation. Very quickly, the blur of evolving pathogens might have our number. A variety that outsmarts our defenses might click into place. But if our DNA is reassorted every generation, we have a much bette
r chance of keeping ahead of the potentially deadly infestation of disease microbes.8 In this highly regarded hypothesis, sex provides essential confusion to our enemies and is the key to health.
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Because females and males are physiologically different, they sometimes pursue different strategies, each to propagate its own hereditary line; and these strategies, while of course not wholly incompatible, introduce a certain element of conflict in the relations between the sexes. In many species of reptiles, birds, and mammals, the female produces only a small number of eggs at a time, perhaps only once a year. It then makes evolutionary sense for her to be discriminating in her choice of mates, and devoted to nurturing the fertilized eggs and the young.
The male, on the other hand, with plentiful sperm cells—up to hundreds of millions per ejaculation and the capability of many ejaculations a day in a healthy young primate—can often better continue his hereditary line through numerous and indiscriminate matings, if he can pull it off. He may be much more ardent and eager, and at the same time much more likely to drift from partner to partner—cajoling, displaying, intimidating, and impregnating as many females as possible. Moreover, since there are other males with identical strategies, a male can’t be sure that a particular fertilized egg or hatchling or cub is his; why should he spend time and effort nurturing and raising a youngster that might not even carry his genes? The investment might benefit his rival’s descendants and not his own. Better to be off fertilizing more females.
This is by no means an invariable pattern, though; there are species in which the female is eager to mate with many males, and there are species in which the male plays a major, even a primary, role in raising the young. Over 90% of the known species of birds are “monogamous”; so are 12% of the monkeys and apes, to say nothing of all the wolves, jackals, coyotes, foxes, elephants, shrews, beavers, and miniature antelope.9 However, monogamous doesn’t mean sexually exclusive; in many species in which the male helps raise the children and provides care for their mother, he also is sneaking out for a little sex on the side; and she is often receptive to other males. Biologists call it a “mixed mating strategy,” or “extra-pair copulation.” As much as 40% of the young reared by “monogamous” bird pairs are revealed by DNA fingerprinting to have been sired by extramural encounters, and numbers almost as large may apply to humans. Still, the motif of nurturing females, who are choosy about their sex partners, and males given to sexual adventure and many partners is very widespread, especially among the mammals.
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There’s a good deal of plumbing, odor signaling, and other machinery in higher organisms to get the genes of one organism in contact with those of another, so the molecules can lie down next to one another and recombine. But that’s mere hardware. The central sexual event, from bacteria to humans, is the exchange of DNA sequences. The hardware serves the purpose of the software.
In its beginning, all sex must have been fumbling, confused, haphazard, the microbial equivalent of bedroom farce. But the advantages that sex confers on future generations seem to be so great that, provided the costs were not too high, selection for improved sexual hardware must soon have been up and running, along with whatever new software was required to stiffen a resolve for sexual congress. Passionate organisms, other things being equal, leave more descendants than those of more tepid dispositions. Unenlightened on the selective advantage of new DNA combinations, organisms nevertheless developed an overwhelming compulsion to trade their hereditary instructions. Like hobbyists who exchange comic books, postage stamps, baseball cards, enameled pins, foreign coins, or celebrity autographs, they didn’t think it out; they just couldn’t help themselves. Trade is at least a billion years old.
Two paramecia may conjugate, as it’s called, exchange genetic material, and then drift apart. Recombination does not require gender. There aren’t boy bacteria and girl bacteria, and bacteria do not have sex—do not recombine segments of their DNA—with every act of reproduction. Sexual plants and animals do. However you bring it about, recombination means that every new being has two parents rather than only one It means that members of the same species—and, except during courtship, the members of most species are solitary and asocial—must arrange a centrally important act that can only be performed in pairs. The two genders might have slightly different goals and strategies, but sex calls, as an absolutely minimum requirement, for cooperation.
Once so powerful an impetus is let out into the world, it might lead, through slow and natural stages, to other kinds of cooperation. Sex brings an entire species together—not just by protecting one another from the cumulative build-up of dangerous mutations, not just by providing new adaptations to a changing environment, but also in the sense of an ongoing, collective enterprise, cross-linking different hereditary lines. This is very different from the asexual practice, where there are many parallel lines of descent, the organisms nearly identical within each line, generation upon generation, and no close relatives between lines.
When sex becomes central to reproduction, the attractiveness of each sex to the other, and the drama of choosing among rivals is moved to center stage. Associated themes include sexual jealousy; real and mock fighting; careful noting of the identities and whereabouts of potential sexual partners and rivals; coercion and rape—all of which in turn lead swiftly, as Darwin pointed out, to the evolution of strange and wonderful appendages, color patterns, and courting behavior that humans often find beautiful, even in members of distantly related species. Darwin thought this sexual selection might be the origin of the human aesthetic sense. Here is a twentieth-century biologist on what sexual selection has brought forth in birds:
crests, wattles, ruffs, collars, tippets, trains, spurs, excrescences on wings and bills, tinted mouths, tails of weird or exquisite form, bladders, highly coloured patches of bare skin, elongated plumes, brightly hued feet and legs … The display is nearly always beautiful10
—especially to the bird of the opposite sex who chooses sexual partners partly on the basis of their good looks. Fashions in beauty then spread rapidly through the population, even if the style isn’t a bit of good in, say, evading predators. Indeed, they spread even if the lifetimes of those who adopt them are thereby considerably shortened, provided the benefit for future generations is sufficiently large. One promising explanation of the showy displays of male birds and fish to the females of their species is that all this is to assure her of his health and prospects.11 Bright plumage and shiny scales demonstrate the absence of an infestation of ticks or mites or fungi, and females—unsurprisingly—prefer to mate with males unburdened by parasites.
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The sockeye salmon exhaust themselves swimming up the mighty Columbia River to spawn, heroically hurdling cataracts, in a single-minded effort that works to propagate their DNA sequences into future generations. The moment their work is done, they fall to pieces. Scales flake off, fins drop, and soon—often within hours of spawning—they are dead and becoming distinctly aromatic. They’ve served their purpose. Nature is unsentimental. Death is built in.
This is very unlike the far less dramatic asexual reproduction of beings like paramecia, where, pretty closely, remote descendants are genetically identical to their distant ancestors. The ancient organisms can with some justice be described as still alive. With all its manifold advantages, sex brought something else: the end of immortality.
Sexual organisms do not generally reproduce by fission, by splitting in two. The big macroscopic sexual organisms reproduce by making special sex cells, often the familiar sperm and egg, that assemble the genes of the next generation. These cells survive just long enough to accomplish their task, and are hardly able to do anything else at all. In sexual beings, the parent does not evenhandedly distribute its body parts and transmute into two offspring; rather, the parent eventually dies, leaving its world to the next generation, which in its time also dies. Individual asexual organisms die by mistake—when they run out of somet
hing, or when they experience a lethal accident. Sexual organisms are designed to die, preprogrammed to do so. Death serves as a poignant reminder of our limitations and frailties—and of the bond with our ancestors who, in a way, died that we might live.
The more active the enzymes devoted to DNA proofreading and repair in big multicellular organisms, the longer the life span tends to be. When these enzymes—themselves of course synthesized under the control of the organism’s DNA—become sparse or inactive, replication errors proliferate and are compounded, and the individual cells increasingly try to implement nonsense instructions. By relaxing the extreme fidelity of its replication, DNA can arrange, at the appropriate moment, for its own death, and that of the organism doing its bidding.
Where sex mandates the death of the individual organism, it provides life to the hereditary line and the species. Still, no matter how many consecutive generations have been recorded of nearly identical asexual beings, eventually the accumulation of deleterious mutations destroys the clone. Eventually, there is a generation where all the individuals are smaller and more feeble, and then you can hear extinction knocking. Sex is the way out. Sex rejuvenates the DNA, revivifies the next generation. There’s a reason we rejoice in it.