Eros

and

Evolution

a Natural Philosophy of Sex

by Richard E. Michod

Selections from the book

Forces at work

Before I noticed it, it was gone. Just a momentary streak of light in the night sky. As it came to the end of its journey, the meteor quietly disappeared with a flicker, or so it appeared to me as I witnessed the event from many thousands of miles away. How long had it been up there? How far had it journeyed, free and unencumbered in the vacuum of space? What worlds had it passed before entering mine? The interaction started from afar. Only an imperceptible nudge at first, earth's gravity gently cajoled the unsuspecting rock off its course into its fated mission. Three bodies interacting, a rock, the planet earth, and me. If I had blinked, I would have missed it. Our paths crossed for just a few seconds that lonely summer night, as I looked up into the vast desert sky.

Forces are at work, keeping the world together by attracting one thing to another. In the physical world, the basic forces-gravity, the electromagnetic force, weak and strong nuclear forces-have been identified, although they are not fully understood. But what about the living world, why are living things attracted to one another? A male praying mantis follows his mate dutifully, although after they have sex she may eat him. A robin perches in the shadow of a tree while her mate sings. A green turtle leaves it's wintering ground on the coast of South America and undertakes a 3000 mile journey to tiny Ascension Island in the middle of the Atlantic Ocean to mate. Why do they bother? We humans, why do we love?

Why sex?

For the simplest virus, as well as for the sophisticated human, sex requires a partner. Partners must be attracted to each other. And they must have sex (as we will see). But why?

When we think of sex, we usually think of having sex. This is only natural, given the pleasure we derive from it. When we think of the problem of sex, our attention may turn to our own personal inadequacies or sexual hang-ups. But there is a more universal problem of sex that is much broader and more basic than our feelings. It is the unresolved question of why sex exists at all. What are the consequences of sex that make it so important to us and so widespread in nature? The answer to this question lies not in our own attitudes and feelings about sex, but deep in our evolutionary past.

A simple, if incomplete, answer is that sex is for reproduction. After all, having babies is one thing most of us think about when we have sex, because we either want them or we don't. However, sex is not always associated with having offspring. Single-celled organisms, such as bacteria and viruses have sex without reproducing (I'll explain how they do this in Chapter 6). And they're surely not in it for pleasure, because these creatures have no feelings at all. Such simple organisms, which are most similar to the kinds of things that lived when sex first evolved, must be having sex only for the most fundamental biological reasons. But, what are these reasons?

It may come as a surprise to you, but biologists disagree strongly concerning the purpose of sex. Why sex exists at all is one of the great mysteries in biology. However, there is one thing biologists can agree on-the costs of sex are obvious and plentiful. Consequently, the evolution of sex has proved difficult to explain, since at first glance the costs are large and obvious while the benefits seem unclear.

Immortality

Although every living thing is mortal, born to eventually die, life itself appears to be immortal. It has been around for some 4000 million years now and shows no signs of petering out. Life doesn't begin at some specific moment each generation (as a legal reading of the abortion/choice debate might lead you to believe). Rather, it originated once, and, since, has been passed on from parent to offspring through the eons. It is possible, in principle, for each of us to trace a continuous line of ancestors back to the first living thing. Life's immortality resides in the well-being of the genetic material, DNA. And, as we will see, there is good evidence that sex plays a key role in maintaining the well-being of DNA and hence the immortality of life.

This role is reflected in the way sex appears to rejuvenate life; it allows two aging adults to produce a youthful baby. It was this obvious fact that led nineteenth century scientists to assume that sex exists entirely to rejuvenate life. Their view fell into disfavor, because they could not explain how sexual reproduction worked any better than asexual reproduction in this regard. Weismann had dismissed the idea with the statement "twice nothing cannot make one"-meaning there is no apparent advantage in using two aging cells, from two separate organisms, rather than a single aging cell, from a single asexual organism, to produce a new plant or animal. In recent years, however, armed with an understanding of how DNA operates, Weismann has been proved wrong: twice nothing can make one.

Germ line

In humans, and other organisms with many differentiated cells, sex culminates with the fusion of two cells-an egg and a sperm. ...[These] germ line cells belong to a distinguished line of cells that can be traced backward and forward in time, for all time. Backward in time, through parents, grandparents, great grandparents, through all recent relatives and ancient ancestors. Back one and a half million years, through those big-brained bipedal hominoids that link our lineage, Homo, with the now extinct Australopithecine. Back six million years, through tribes of arm walking apes that connect us with modern chimpanzees and gorillas. Back 50 million years, to a tree-dwelling cat-like prosimian primate-who held our human destiny in its grasping hands. Back 80 million years, to that little shew-like animal that unites all primates. Back 200 million years, to that now extinct mammal-like reptile Theriodontia. Back, to the seas and our fish ancestors who could be found swimming there some 400 million years ago, having just invented a marvelous internal skeleton. Back, far back, to creatures like a small worm-like marine animal that unites our ancestors to sponges and starfishes, or a single celled Amoebae-like protist that unites all plant and animal life. Creatures so strange and distant that to speak of our family ties with them, seems more like a metaphor than concrete genetic kinship.

But kinship it is. Weismann first used, in the latter part of the last century, the words "germ plasm" to describe this line of cells. "Germ" from the Latin germen means sprig, offshoot, sprout, bud, or embryo. "Plasm" from the Greek plassein means a forming substance. Real, physical, informational connections through time: that is what the germ line is about. The germ line, that ever-branching ever-reticulating web of cells, physically and informationally connects all past, present and future living things. Life originated just once on this planet, and since that time, some 4000 to 5000 million years ago, it has been handed down like a family heirloom, from parent DNA strand to daughter strand, from parent cell to daughter cell, from parent organism to offspring, through our bodies, out of our bodies, into other bodies, through the eons.

The egg and sperm, two cells whose job is to link the generations. Full, as they are, with the potential to create a new and unique individual, we wonder: to whom will they eventually lead? To what unknown and not-yet-designed organism will they eventually connect us? Who could have thought, certainly not our distant shew-like ancestor (probably one of the smartest creatures around at the time) that in just 90 million years of evolution there would be a species that included Albert Einstein, Vincent Van Gogh, and Margaret Mead! Nor can we know what inventions of form and function the millennia will bring forth for our genes. Yet, we-which is to say our genes-are in touch with these distant ancestors through the germ line.

Order from chaos

The world is an orderly place, at least for the time being. The laws of physics tell us that the universe is becoming more and more disordered; that all structure must decay and eventually disappear. Nevertheless, the relentless push toward chaos and confusion can be delayed, at least locally, on planets like our own, that are bathed in entropy procrastinating sunshine. Sunshine is the source of all biological order; sunshine fueled the transition from chemical order to biological order some three to four billion years ago and continues to be the energy that maintains life on our plant today.

While sunlight is the fuel supporting life, natural selection is the engine that moves life forward. To channel sun's energy into useful structures requires its careful guidance, for without natural selection, life would soon coast to a stop. Natural selection itself emerged from the chaos, when replicating molecules occurred spontaneously some three to four billion years ago. The useful structures created by natural selection adapt organisms to new environments, enabling them to reproduce and pass on their genes. Organisms are impressive because of their adaptations, and these beautiful designs are one aspect of biological order.

Species are a second aspect of biological order. The living world is not a blend of living things, each slightly different from the next. Rather it is organized into the categories we call species. Why do species exist? This question is the question Darwin really wanted to solve, as he announced to the world in 1859 in the title of his great book "The Origin of Species." For Darwin, the question of the origin of species was the same as "Why are not all organic beings blended together in an inextricable chaos?" There is order in the living world; it is not an inextricable chaos. Species are, for the most part, distinct and recognizable; we give them names. Cultures without the formal study of modern biology name the same species as do trained biologists. True, there is confusion within certain groups of organisms, for example, in some groups of plants, but species are generally distinct and different from one another. They are so distinct that early biologists took them as evidence for the unchanging categories created by God.

For early biologists, like the great 18th century Swedish botanist Carl Linnaeus, as for the fundamental biblical creationist, there is no problem with species discontinuities. The apparently bridgeless gaps that separate species are modern evidence of God's ancient handiwork. The gaps in existence today were created by God and preserved for all time by the fidelity of reproduction, so thought 17th and 18th century biologists. The essence of each species, or edios in Plato's ontology, was thought to somehow be contained in the seed and thereby passed from parent to offspring through the eons. In this view, species have not changed since the Creation. But, even Linnaeus late in his life admitted that species do in fact change; some species had even gone extinct. How can one reconcile the changeability of species with gaps. The apparently bridgeless gaps must have been bridged. But when, and how?

Interconnectedness

[A sexual world is] one with interconnectedness, distinctness and immortality. ... A sexual biology gives a pattern of distinctness at each moment in time caste upon a web of interconnectedness from the past. This web continues deep into the past and on into the future, hopefully forever, so long as we humans don't interfere.

As you scan the pool of unfamiliar faces next time you ride the subway or rush through a busy airport, pause for a moment. From the blur, pick out a single face and look into it. Look hard. And remember there was a person somewhere, sometime, perhaps not too far in the distant past, that connects you to that person.

But do not limit your attention just to human ancestors. Take a walk in the woods, or go to the zoo. Pick a creature, any creature. An ape, a tree, a fish, or a worm. For each, there exists an ancestor or relative that the two of you share. A continuous line of ancestors can be traced backward in time to that shared ancestor and then forward in time through its descendants to you. Your most recent relative to the ape was an ape-like creature that lived just 5 million years ago. For the tree, you must go back some 800 million years to find it-a single celled creature similar to a modern day paramecium. For the fish, your most recent relative was a fish that lived in the oceans about 370 years ago, before living things invaded land. For a worm, your most recent relative was an ancient worm-like creature that lived in the oceans some 600 million years ago. There is one ancestor that all past, present and future creatures share-the first living thing. By considering the first living thing, how it arose, and what its problems were, we will discover how sex likely originated

The primordial pool

Imagine sitting cross-legged by the primordial pool on the primitive earth around 4000 million years ago, leaning over the slime and goo and, aided with some magical magnifying glass, watching the very first replication of THE ancestral molecule. Tentative and awkward at first, making many mistakes. Nevertheless, it was a start. Imagine all that this molecule would come to be. Like a human fetus that blossoms from a single cell into a life filled with creativity, achievement and passion, from such modest beginnings, the molecule races forward in our dream, multiplying and diversifying, propelled by nothing but its own replication, guided only by its environment, into some 1,000,000 species and countless individuals, some now living, most long dead. But we have gotten ahead of our story, let us return to the molecule-or to be more exact, the molecular replicator.

The molecular replicator

A molecular replicator is a molecule that makes copies of itself. DNA and RNA are the best known examples of such molecules. They are made up of long strings of nucleotide bases, which, as we know, behave like characters in an alphabet. Information in the First replicator was probably likewise contained in a linear sequence of characters.

But where did the characters come from and how were they strung together? We can only guess. One view is that the characters-the nucleotides-were available in a kind of chemical alphabet soup present in small pools or water droplets, created from physical and chemical reactions that occurred on the primitive earth. (In a famous experiment, scientists showed that by passing lightning-like electrical discharges through a flask containing nothing but methane, ammonia, water and hydrogen (thought to be present on the primitive earth) nucleotides can be produced.) At some point, these characters linked with one another to form short strings.

...I...I...ndividual nucleotides adenine (A), thymine (T), guanine (G) and cytosine (C) are attracted to one another. In the case of DNA, A and T pair with each other, as do G and C. In the primordial pool, likewise, the elements of a string such as AATG (the "parent" strand) could attract the complementary characters available in the alphabet soup. These may then link up to each other to form a complementary string TTAC (the "daughter" strand). Such a pair of strands, held together by hydrogen bonds, is normally stable. However, under certain conditions (high temperature, for example) the bonds would weaken and the strands separate. The two single strands could then drift apart, and when conditions change to again favor attraction, they would be free to attract new complementary strands and replicate as shown in the figure.

Attraction

The forces of attraction and repulsion took the nucleotides, which are just physical chemical entities, and led them across life's doorstep into the world of the living. Attraction and repulsion those two universals found in all interactions. Consider Freud's comments in a famous letter to Albert Einstein in 1932. "We assume that human instincts are of two kinds: those that conserve and unify, which we call "erotic" (in the meaning Plato gives to Eros in his Symposium), or else "sexual" (explicitly extending the popular connotation of "sex"); and, secondly, the instincts to destroy and kill, which we assimilate as the aggressive or destructive instincts. These are, as you perceive, the well-known opposites, Love and Hate. Transformed into theoretical entities, they are, perhaps, another aspect of those eternal polarities, attraction and repulsion, which fall within your province." We will have much more to say about the Eros and Plato's Symposium in later chapters.

In the beginning, there was sex

Individual replicating molecules were probably not isolated from one another. Parts of one molecule could easily break off and attach to other molecules. This continual breakage and rejoining of nucleotide strings is directly analogous to the recombination of DNA molecules that goes on during sex. In other words, the living world was born sexual. There was little individuality and much "promiscuity" at this early stage of life. It wasn't that evolution wanted it that way, it just so happened that nothing could be done about it, until, as we will see, the replicators became more advanced and improved their individual integrity.

The first cell

We cannot be certain exactly how the first cell was created. Through a series of chance mutations and recombination events, some replicators acquired the ability to construct little shelters around themselves, enabling them to horde their own proteins for their own exclusive use. Sooner or later this advantage would lead, again by chance mutation or recombination events (probably around 2,000 or 3,000 million years ago) to the creation of the first cell. What a wonderful creation this was: protection and nutrition for the replicator all under the shelter of its own roof. No longer were it's proteins shared by its neighbors. With the advent of the cell, evolution would become a whole new ball game. Individuality was born.

Individuality carried a certain cost, however. The replicator, now encased in a cell, was forced to keep both its bad mutations (improperly sequenced nucleotides) and damages (non-nucleotide intruders on the sequence). Any errors the replicator acquired-by exposure to UV light, for instance-would now be trapped in the cell. The days of free exchange and easy access to spare nucleotides were over. Sex had came naturally and without effort to the naked molecules. Now imprisoned in a cell of their own invention, the replicators had to figure out an alternative way to make repairs and new combinations. A new kind of sex between cells had to be invented.

Further evolution of the cell

With the invention of the first cell, evolution had crossed a threshold of unknowable potential. The First replicator began its journey some 4,000 million years ago, and it took about 1000 million years for evolution to produce the first bacteria-like cell. It would be another 2000 million years before the first eukaryotic cells (cells that make up all higher forms of life in which the DNA is packaged in a nucleus). Most animals and plants evolved rather recently, during the past 1,000 million years. More than 3,000 million years, over three fourths of life's history, was time spent producing the cell, the basic unit of which all higher forms of life are built. Once invented, these cells could combine with one another into ever more elaborate and intricate patterns, creating organisms large enough to be seen by the naked eye.

So it is at this point in the history of life that we must drop our magnifying glass, step back from the primordial pool, and watch as the seas fill with living forms, some strange, from our modern vantage point, and others familiar. Out of this plenitude, a funny-looking fish, with deformed fins inappropriate for swimming but able to support it on land, would emerge. It would crawl out of the sea that nurtured all life for some 3000 million years and, making use of its novelty, strike out alone. This animal would spawn an explosion of new forms of life, some more and some less dependent on the nurturing sea. Eventually, land-dwelling forms would evolve ways of taking the sea with them by first packaging water in cells and later by inventing circulatory systems that bathe each cell in a sea-water-like fluid (blood). These inventions would allow life to colonize new habitats on land and in the air, far away from the sea. Finally, having produced an erect, bipedal ape, too large to avoid its predators yet too small to fend for itself, trapped and exposed in an open savanna, evolution would experiment and, for the first and perhaps last time, produce a thinking conscious being. In time, this being would come to wonder about itself, its origins and nature.

The world might have been otherwise

Life on earth can never be repeated. If life were to begin again on this planet, even given identical initial circumstances, chance events would lead its history down a different path. The evolutionary process opportunistically evaluates random events (such as mutations and new recombinations) and channels them into useful structures. Some broad patterns might be the same-flying animals would still need wing-like structures, swimming animals would need fins and some kind of sex would likely exist-but individual species would certainly be different the second time around. Human beings would probably not exist.

Our world, our species, each one of us....might never have been. This may be the most difficult lesson the theory of evolution has to teach us. At first it may seem threatening to consider the idea that we owe our very existence not to the inspiration of a good and omnipotent being, but rather to a blindly mechanistic natural process, evolution. It makes one feel, well..., a bit lonely. Comfort and inspiration can be found in the realization that all living things are one. That, in a real material and informational sense, the living world is one big family. Other cultures, the Native American Indians come to mind, have embodied this attitude about the living world in their myths and teachings. Unfortunately, the Bible got mankind's place in Nature completely wrong. Instead of being a quantum leap above animals as proclaimed in Genesis, our species is found on a tiny twig, on a rather short branch, off to the side of the great tree of life.

Perhaps, this is a major reason why many people do not believe in evolution. It certainly can't be because the theory of evolution is technically difficult or complex, like, say, the theory of relativity. The basic idea of natural selection follows a simple logical progression-requiring just a few sentences to explain. Apparently, for some it is easier to believe in mystical forces and unseen events than to accept Darwin's simple but compelling logic. Others understand Darwin's logic, but are unwilling to embrace it's profound implications. It has been some 130 years since the Origin of Species appeared, yet the Darwinian revolution has barely begun.

Organisms don't evolve

Individual organisms do not evolve. Only populations-groups of individuals that belong to the same species-do. Individuals are born and they die. In between birth and death, they may have some offspring, and some may have more than others depending on the traits they possess. These different rates of reproduction cause the population to change its genetic composition--that is to evolve. Over time, for instance, if the long-necked giraffes reproduce in greater numbers than the short-necked giraffes, the length of giraffe's necks will increase even though each individual giraffe had a fixed neck length during its life (once it had grown to be an adult).

Darwin's genius was in realizing that all the apparent perfection of design in the living world resulted from this simple mechanistic process. Before Darwin, people of the Judaic-Christian or Moslem traditions had assumed that living things must have been created by a Being such as God. Darwin showed that the creator of all living things was, in fact, not a Divine being who stood apart from life, but a process intrinsic to life-natural selection. Ultimately, it is this process that provides the rationale for our existence, and it is in this process that we must seek an explanation of our basic nature, the origin of our needs and desires, and the purpose of sex.

Females and males

We are so caught up in our own biology and psychology that it is difficult to appreciate the original purpose that males and females had. To help us stand back and gain perspective on the confusion let us return to the egg and the sperm cell. Even at this level there are marked differences between males and females in our species. The egg is large in size (over 1000 times the size of the sperm), immobile, and rich in nutrients and other resources needed once it is fertilized and begins developing into a new organism. The sperm, on the other hand, is small, designed for swimming using its tail, and devoid of resources. Even the energy it needs for swimming is provided by the parent cells that produced it (enough for a few hours of frenzied pursuit of the egg)...

Many of the behavioral and morphological differences between males and females stem from the often conflicting needs of nurture and ability to get mates. The female sex typically (but not universally) specializes in nurturing the zygote while the male sex typically specializes at finding females. The egg is far more costly to produce than a sperm. Having invested so much in producing an egg, a female is less inclined to abandon her offspring (if she does, she will have to go through the costly process once again). In contrast, a male having only invested a rather cheap sperm in the zygote is more inclined to abandon it and try to fertilize another egg. Since, the female sex is defined as the one with the large gamete to begin with, she is usually the parent that cares for the offspring. There is much more to gender differences than this; however, the needs of nurture and mate finding are still dominant themes in understanding these other differences.

A bumpy ride

These rather brief excursions may have left you feeling a bit disappointed in the whole enterprise; that evolutionary biology does nothing more than justify certain traditional stereotypes of male and female roles. Keep in mind that we are trying to understand our biological nature and evolutionary history, not how males and females should behave. The way the world happens to be (say as a result of evolution) may not be the way we want it to be; in philosophical jargon: "Is" does not imply "Ought." If nature has endowed us with predispositions to behave in ways we don't like, all the more reason to work hard to change them. Evolution has also endowed us with enough flexibility to try to do that.

While we are on the subject of what comes naturally, evolution is often said to be "selfish" in the sense that organisms who benefit themselves at the expense of others often enjoy an advantage in the struggle to survive. What is commonly not appreciated is that the advantage to selfishness occurs only in a certain kind of population, namely one that is extremely large and well mixed so that organisms do not encounter relatives or meet with non-relatives more than once. However, these conditions are not often met; populations are small or they are structured into groups of relatives or clusters of non-relatives who interact repeatedly. In these situations selfishness doesn't pay; cooperation, even altruism, between organisms can be the favored over selfishness by natural selection. So, selfishness between organisms is not always "natural." The "is" as a result of evolution may often be cooperation. My main point is simply that there is nothing "unnatural" about helping others.

The history of life has been a bumpy ride, not a trip well planned in advance. It has depended on chance variations and fortuitous events. It is not repeatable and would certainly happen differently a second time. The theory of evolution is the best explanation we have for how it happened, but it is not based on a system of moral beliefs. To use its capricious inventions as substitutes for moral values would be a mistake-a great mistake indeed. Evolution is simply too capricious and undependable for that.

Is sex bad?

The flower of the century plant is so huge, over ten times the size of the basal plant, that it looks more like a tree than a flower as it towers above the sparse Sonoran desert landscape. The century plant waits quietly, ten, twenty, sometimes fifty years, as a rather dull rosette, a clump of spiny shiny leaves, before it shoots its spectacular flowering stalk towards the sky. Soon afterwards it shrivels and dies. Biologists call it "big bang" reproduction. The little Australian marsupial mouse, Antechinus, dies immediately after a single episode of frenzied mating. Or, consider the poor praying mantis father; no sooner has he finished with the pleasures of mating, than he is eaten whole by his mate. Is it our Victorian outlook, or is sex really bad?

...Its hard to do more than one thing at a time and to do it well. One of my first shop tools as a teenager was a "jack of all trades" tool that, by unbolting and turning some levers, could become a drill press, a lathe, a table saw, or a radial arm saw. It didn't perform any of these functions particularly well, but it was all that I could afford (plus, it was small enough to fit in my parent's basement). Each function seemed to work against the other. The long reach needed for the drill press or lathe made for an awkward table saw.

And so it is with organisms. The long and brightly colored tail needed to attract a female peacock makes for awkward goings in the weeds and brush. The well nourished fetus puts mother at all kinds of risks; late in pregnancy it is even difficult for her to walk. Imagine our ancient grandmothers trying to climb a tree to avoid a tiger on the open savanna with a 9 month fetus inside her. For that matter, the modern mother-to-be is not much better off when crossing busy West 53 Street in Manhattan.

Then as now, babies are a burden. Mutant female fruit flies without ovaries live longer. Even sperm, often ridiculed because it is cheap and poorly constructed (and when compared with the egg it is), lowers survival when it is produced by males in the simple parasitic worm C. Elegans. Reproduction, sexual or asexual, involves risks both large and small. A mother whiptail lizard, although able to reproduce without a male, must still allocate a great deal of time and energy to nourishing and guarding her eggs, time and energy that could be spent on herself. But at least she doesn't have to bother with a mate. Asexual reproduction is hard enough and sex only further complicates matters, for it requires coordinating the lives of two entirely different and usually unrelated individuals. Finding, attracting and keeping a mate adds greatly to the already difficult task of reproduction.

Sex is for genes

For individual organisms, sex is not just a bother, it is downright dangerous. If not the harbinger of death-as it is for the century plant, the praying mantis, or Antechinus-sex hastens the journey to the grave for all of us. If the direct costs of energy and time don't get you, the parasites and infectious elements--that, like the AIDS virus, have seized this chance to transfer themselves--will. In spite of the immense pleasure and satisfaction we derive from it, sex is not good for us. Nor is it meant to be-sex is for genes.

Of course, all organisms must die. Being born, dying, and having offspring in-between: this is the biological imperative. Reproducing is, after all, what organisms are about--genes invented organisms as a means for their own perpetuation. We will find in the Chapters that follow that, as far as the perpetuation of genes is concerned, sex is a good thing: cleansing the genome of life stopping damages, purging deleterious mutations, perhaps even providing hopeful variations for new environments. Although organisms come and go, genes are forever.

Plato's theory of repair

In the beginning, according to Aristophanes, speaking in Plato's Symposium, the world was populated by extraordinary human beings, each with four legs, four arms, two faces, two hearts-in short, double the number of each feature possessed by modern man. Zeus, wishing to reduce the power of mankind, had the primeval beings split in two. The god Apollo sliced them down the middle, gathered up the cut skin, and smoothed out all the wrinkles, except a few at the navel. On Zeus's orders, he also turned each of their faces toward the cut side-the navel side-so that they might remember their division. Ever since, humans have lived in constant need to reunite with their other halves, and thus Aristophanes explained, man is motivated by love, "the love which restores us to our ancient state by attempting to weld two beings into one and to heal the wounds which humanity suffered."

Later speakers in the Symposium add a layer of complexity to Aristophanes' notion. Yes, love constitutes a desire for wholeness, the prophet Diotima tells Socrates, but it can exist only if the whole is itself something good. "Its object," she says, "is to procreate and bring forth in beauty...Because procreation is the nearest thing to perpetuity and immortality that a mortal being can attain." Humans do not remain forever the same, as do gods, Diotima explains, but through procreation, "the losses caused by age are repaired."

The discussion in the Symposium ultimately is directed toward love's importance and complexity as a human emotion. The speakers draw a continuum from the physical love of two individuals to man's inherent love of wisdom. But in so doing, the ancient philosophers unintentionally touch upon the solution to one of the most basic, unsolved mysteries in biology: Why is there sex?

Sex makes babies

Sex between aging adults produces babies that are youthful. The contrast between a baby's youthfulness and its parents maturity is, perhaps, the most obvious consequence of sex. The regenerative and rejuvenating powers of reproduction have appeared time and time again in psychology, philosophy, literature and art. Plato's The Symposium is but one example. Freud recognized the sexual instinct, Eros, as the preserver of all things. In Germinal, Le Docteur Pascal and Fécondité , the 19th century French author Émile Zola develops the principle of fecundity into a myth of regenerative optimism, the guarantee of hope and continuity. Many other examples exist in science, literature and art.

By the late 19th century, the idea that sex rejuvenates life was being discussed among scientists. However, there were problems with this view-the most serious of which was that scientists didn't know how sex could rejuvenate life. As a consequence, the hypothesis that sex rejuvenates life seemed a bit mystical. In 1886, August Weismann simply dismissed the rejuvenation view by stating categorically that "twice nothing cannot make one."

From our more modern vantage point, armed with our understanding of the workings of DNA, we can see a way around this apparent contradiction, that Weismann saw as fatal to the rejuvenation view. The immortality of life must in some way be connected to DNA. DNA, like any physical thing, gets damaged and the resulting errors would destroy life unless there was some efficient means to get rid of them. DNA damages can be repaired by recombination, if spare parts, in the form of genetic redundancy, are available. [Among molecular replicators], random exchange of segments between two damaged molecules can result in a repaired molecule. As life continued to evolve recombination got better at repairing DNA. Master molecular mechanics (so called recombinational repair enzymes) would evolve to make the process more precise and effective. As a consequence of recombinational repair, the DNA passed on by parents becomes undamaged or whole again. Each damaged DNA molecule is, in Aristophanes' words, a "broken tally...in search of his corresponding tally," so as to become whole again. Two damaged complements can make one!

Organisms don't evolve

It is easy to be confused by organisms and their role in evolution. Endowed and ornamented as they are with such wondrous designs, it is easy to see them as the product of evolution. We are organisms and we seek to understand organisms to understand ourselves. But, organisms are of only fleeting existence. Each is born genetically unique (because of sex) and each lives but a moment in evolutionary time. Organisms don't evolve. When viewed from the perspective of the evolutionary process, organisms appear as vehicles for the perpetuation of genes. Even "fitness" is best understood as a process of genetic change rather than as a property of organisms. In this sense, evolution is all process, it has no ultimate products.

Sexual biology

Interconnectedness and unity among distinctly different (yet still interconnected) organisms. That is sexual biology. A sexual biology contains unique organisms who exist but fleetingly as temporary repositories for gene combinations; each combination existing for just a blink of evolutionary time until they are destroyed by the same sexual process that created them. Organisms are born and they die, in-between being born and dying they have offspring, some more than others according to the traits they possess. Their wondrous adaptations are impressive. However, because of sex, organisms as individuals play no enduring role in the evolutionary process.

Genes, however, go on forever. This is what sex is about. The interwoveness and timelessness of gene lineages. The immortality of life resides in the well-being of genes, which stems in large part from damage repair and the cleansing of mutations. Sex began as a necessary but mechanical process of gene repair, but has ended up forever changing the landscape of the living world.