The Language of the Genes

by Steve Jones

  Sex itself raises other problems — if sex, why sexes? If to shuffle together the genes of two individuals is such a good thing why has evolution not come up with a scheme which allows everyone to mate with everyone else? As we are limited in our choice of partners to those of a different sex, to have just two of them seems very inefficient. Almost all organisms (with the exception of a few simple creatures that have dozens of genders) exist as just males and females. This means that just half the population is available as a potential marc. If there were three genders, then two thirds of the group might be accessible, and a hundred iliHerent reproductive classes could make ninety-nine per cent of our fellows into possible partners. One answer (and it is only one) involves what seems at first sight the antithesis of sex — conflict.

  Males are best defined as the sex with small sex cells, sperm; and females as that with large, eggs. Body cells contain DNA not just in the nucleus, but in the cytoplasm which surrounds it. Some is associated with mitochondria, which have genes of their own. Many creatures have yet more in the cytoplasm. It comes from what were once independent beings which now hitch a ride within cells. This DNA (like that in the nucleus) has its own agenda, which is to be copied and passed to the next generation. The cytoplasm is its territory and like a blackbird or a tiger it defends its homeland against invaders. If sperm and egg were the same size (and each had its own population of extraneous DNA) there is a danger of war breaking out on fertilisation. Then, two sets of cytoplasmic genes find themselves in the same space in the fertilised egg. Just like tigers, one set might attack the other until it prevails. This is expensive and could even harm the genes in the nucleus.

  The dispute is resolved because one sex — the males — unilaterally gives up. The sex which surrenders passes on none (or very few) of its cytoplasmic genes (which are excluded from the sperm at fertilisation) while the winner, the egg-maker, passes on large numbers. As in most wars, the stable number of opponents is two, and the existence of males and females (rather than dozens of genders) represents a truce in the battle of the sexes.

  Biology now understands why sex is there and why it is limited to the tedious dualism of male and female. The technical revolution in genetics has also shown how simple sex is in — and at — conception and what a complicated tangle it later becomes. Existence is, it seems, in its essence female and masculinity just a modification of the feminine experience. The Y chromosome forces the embryo into manhood. If, for some reason, the Y is absent the foetus develops as a female. Some children arc born with an extra X chromosome. Their chromosome set is XXY. They are male, but sterile. People with half a do/en X chromosomes and a Y have been found and these too are male, 1 reminder of the power of this small chromosome to impose its function on the X.

  The discovery of a few males with two X chromosomes helped in the search for the gene responsible. They break the rule that to be a male needs a Y. In fact, in these men (most of whom are unaware of their condition) a tiny part of the Y chromosome has been broken off and attached to an X. This is then armed with the information needed to inflict maleness. Because the transferred segment is small the augmented X was useful in tracking down the crucial gene. The gene is found in all male mammals and is similar to another that determines what passes for masculinity in yeast.

  The machinery that decides the sex of a fertilised egg may be simple, but the road to adult gender is a complicated one. Sexuality is a flexible thing. In crocodiles, for example, it is determined by the temperature at which the eggs develop, so that females must lay their clutches in a place with a temperature range which allows both males and females to be produced. In certain fish, embarrassment — or social pressure — is important. A shoal of females is guarded by a male. To remove him leads to a period of confusion, until one of the females changes sex and assumes his role.

  Once sexuality gets started, great consequences flow from it. Most of natural history is the scientific study of sex, as the characters which differentiate birds, insects and flowers from each other are, in the main, associated with reproduction. To compare the sex lives of different animals hints at how sex evolved and why animals indulge in one or other reproductive preference. Although humans are in many ways distinct, it might even be possible to learn something about our own habits by looking at those of other species.

  Many people have attempted to draw sweeping conclusions about humankind from studies of the private lives of monkeys and apes. It is always dangerous, and usually futile, to try to explain human behaviour in the simple terms used to study animals. Attempts to do so almost all fall into the 'pathetic fallacy', the literary trap which sees emotions mirrored in the weather or the landscape. Occasionally — very occasionally, as in WutberingHeights- this works, but most of the time it ends in bathos. Anthropology has the same problem. It is fatally easy to read into the animal world what we would like to see in our own, to explain the human condition as an inevitable consequence of our biology. Even Charles Darwin, a veritable Bronte among sociobiologists, was at fault. Hidden in his unpublished notebooks is the damning phrase 'Origin of Man now proved — metaphysics must flourish — he who understands baboons will do more towards metaphysics than Locke.'

  Metaphysics is one thing, sex another. The Nobel Prizewinner Konrad Lorenz saw us as 'killer apes' anxious to pass on our own genes by murdering the opposition (which may explain his own flirtation with the Nazis), and any decent airport has a row of paperbacks that purport to explain human nature as the remnants of a history as primates with one or other social preference. Until a few years ago the study of sexual behaviour was little more than a set of unconnected anecdotes. It has been transformed by the rebirth of one of the oldest techniques in biology. Comparative anatomy is what convinced Darwin that men and women are related to monkeys and apes. The new science of comparative behaviour hints at how and why their sexual conduct evolved.

  Sex is filled with strife, with the very existence of males and females the resolution of a war to pass mi cyioplasmic genes. Further conflict arises as males struck- lor mates and as males and females disagree about the tiuu-aiul effort needed to raise young. The conflicts among nuiles lead to the evolution of spectacular organs of attack such as antlers. Other traits — such as a baboon's gaudy face — are more subtle statements of male talent and may evolve because they are preferred by the opposite sex.

  There is little evidence (in spite of much prurient speculation about beards, breasts and buttocks) that humans have attributes of this kind but, as in most animals, conflict between human males is greater than between females. To be a man is dangerous. At birth there are about 105 males to every roo females, but this drops to 103 to 100 at the age of sixteen and in their seventies women are twice as abundant as men. Men have more accidents, more infectious diseases and kill each other more often than do the opposite sex. As might be expected, eunuchs and monks live for longer than do those condemned to a normal sex life.

  Our close relatives have different life-styles. From a human perspective, chimps are deplorable but gorillas dull. A male chimpanzee copulates hundreds of times with dozens of females each year. The faithful gorilla, on the other hand, has to wait for up to four years for his female to be ready to mate after she has given birth, and even then she is available for just a couple of days each month. That means intense competition among gorilla males foraccess to females. A successful male may accumulate half a dozen or more, which leaves many gorilla wallflowers out in the cold and anxious to fight for their reproductive rights. Often, these fights are savage, since what is at stake is the male's evolutionary future. Humans are unlike any other primate as they live in large groups as {more or less) faithful pairs. In this, people are more similar to seagulls than to any ape. The closest in behaviour to ourselves is the pygmy chimpanzee. This forms long-lasting pairs within a stable but small group of individuals and has other attributes not unlike our own (such as face-to-face copulation). The average Frenchman or Briton has ten sexual partners in his life and,
as in many primates, there is more variation among men in their success than among women. One in a hundred men is responsible for one in six of the females who have sex.

  Monkeys and apes show a good genera! fit between the size difference of the sexes and patterns of mating. In those species with large harems and angry bachelors, males are much bigger than females, because bulk and aggression help in the battle for partners. Gorilla males are twice as large as females, while the chimpanzee's more relaxed society has taken the pressure off sexual hostility and males and females weigh about the same. The argument from anatomy (restricted as it is in a socially complex animal like ourselves) suggests that humans, with men just a little larger than women, have a history of mild polygamy intermediate between that of chimp and gorilla.

  Our own behaviour is flexible and often shifts (as in the recent change towards serial monogamy, constancy within a relationship but more than one relationship in a lifetime). There do seem to be some general rules. Strict monogamy is rare and, in most societies, most men have more than one mate during their lives. Polygamy (one male with several wives at once) is far more common than polyandry, the opposite pattern, although this exists in Tibet. In polygamous societies as a few men have many wives some must have none.

  There are hints of a more salacious past for humankind than that recorded in the modest difference in male and female size. In many mammals, the struggle between males does not stop at copulation. Sperm compere too. Often a female uses the sperm of the male she mated with last, which means that a successful sperm donor must ensure that no other male mates with her until the eggs arc fertilised. Dogs, for example, stay paired after copulation because the male is guarding the female against intruders. A more subtle way to help one's own sperm is to Hood out the contribution of the previous visitor. Different primates show quite a good fit between the size of the testes and the extent of male promiscuity. Chimpanzees, the Lotharios of the primate world, have enormous testes while gorillas, in spite of rumour to the contrary, are far less well endowed. Humans are not too different from chimps in this respect (which may say some startling things about our past). Real enthusiasts for evolutionary explanations point out that men produce more sperm when they return to their partner after a long absence, perhaps to overwhelm any alien sperm that may have intruded. There is also the question — as yet unanswered by science — as to why, in penis size, man stands alone. There are limits to what biology can explain and this may be beyond them.

  James Boswell in his London journal (which reveals him to have been no mean performer in his own right) wrote that 'If venereal delight and the power of propagating the species were permitted only to the virtuous, it would make the world very good.' Darwin, too, noticed that sexual selection (as he called it) might do more than improve a male's ability to defeat his ardent competitors. He was much concerned with the evolution of characters with no obvious biological advantage {such as the peacock's tail or the large human penis). The struggle for sex might, Darwin thought, have subtle consequences. If females prefer, for one reason or another, a particular male attribute (such us a bright tail), then males who have it will reproduce more successfully. The tail or its equivalent will become more common in later generations and the showiest males will once again be preferred. In time there may evolve bizarre structures which are so expensive to the unfortunate males that they can evolve no further. Female choice may, Darwin suggested, be as important a part of the sexual equation as is male aggression.

  In his book on the subject, The Descent of Man and Selection in Relation to Sex, he went further. He suggested that sexual preferences explained why human races looked so different. It was not that they had evolved to fit the place in which they live, but as a consequence of arbitrary choice of a partner. In different places, those looking for a mate may have made different and quite capricious choices. In time, the people of the world diverged: for example, Darwin speculated, those with darker skins might have been seen as more attractive in Africa and those with lighter in Europe. People do tend to marry others who are similar to themselves in intelligence, colour and body build, but there is no evidence that such choices are important in evolution. On average, men — of whatever racial group — do tend to prefer relatively light-coloured females. If sexual selection was important then the blondes will prevail. As they have not, perhaps Darwin was wrong; or, perhaps, the whole issue of sexual choice is so open to social convention that the argument can never be tested.

  Men do tend to agree in their estimation of how attractive a particular female might be. Galton made composite photographs in which the pictures of a number of society beauties were printed one on top of the other in the hope of some vision of the ideal woman. His Ms Averages look rather insipid to the modern eye. The job can now be done by computer. For both male and female faces most people find an image made up of several individuals more attractive than one based on a single person, and the more faces used the more appealing it seems. Why there should be this triumph of the typical is not certain (although some suggest that those with extreme faces might also have aberrant — and less desirable — genes). Faces mixed together even out the differences between left and right. A simple experiment with a photocopier shows how two-tanxl most of us are. Two left cheeks or two rights often look alarmingly different one from the other. As each side of the face is made by the same set of genes, perhaps the greater the asymmetry the feebler the genes. Models (noted for their sexual attractiveness) tend to have symmetrical features and often reveal as much with a full-face smirk into the camera. Again, sexual choice may be involved, although the evidence is weak.

  Any discussion of the evolution of sex seems doomed to stray onto such untamed shores of speculation. Males carry eccentric and expensive ornaments, some say, to demonstrate to potential spouses that their genes are good enough to bear the cost. The idea has been used to explain bizarre patterns of human behaviour. Perhaps men take alcohol, tobacco or stronger drugs to demonstrate to women how tough they are, how their constitutions can cope with mistreatment and how they might make excellent fathers as a result. The small tubes found in the tombs of Maya Indians might have been used to give ritual enemas of toxic drugs to the most powerful men, as a guarantee of instant intoxication and a statement of sexual prowess. The habit did not spread.

  Conflict between males for the attention of females is obvious, but there are also plenty of chances for disagreement between the sexes. In some animals, the reluctance of females to accept a new mate, persistent though he might be, arises because males invest less in bringing up offspring. It pays them to mate and run; to try and father as many children with as many females as possible. Females need to be more cautious. As it costs so much to produce a child they should choose the male who will be the best father and reject the rest.

  The divergence of interest is sometimes obvious. Some males kill a mother's brood by another male with the aim of making her available to themselves. Among the langur monkeys, most of the young die for this reason. Some species even have a form of prenatal cannibalism. Pregnant female horses exposed to a new male reabsorb their foetuses, a behaviour which may have evolved because of the near certainty that if born they will be killed.

  Humans reveal the intersexual struggle in less blatant ways. Their battle is an economic rather than a mortal one. If tribal peoples are any guide, societies with private property are more polygamous, as women prefer the better-endowed as mates. When wealth is concentrated into few hands, society becomes more like that of a gorilla, with the richest males monopolising the females. The philoprogenitive (and opulent) Moulay Ismail the Bloodthirsty of Morocco admitted to 888 children. We in the West now seem to be moving towards the chimpanzees, as most men have at least a fair chance of a Ms Right, but in most societies success is still related to wealth. Among the Kip-sigis people of south-west Kenya a wealthy man may have as many as a dozen wives and eighty children. The more land a man has the more wives he obtains and the poorest males leave the comm
unity as teenagers and have no children at all. All women, in contrast, tend to have families of about the same size. In Britain, too, men from higher social groups have more partners than do those less well off. An economic conflict between the sexes means that men provide the capital and women choose where to invest.

  The battle of the sexes may explain another unusual attribute of human reproduction. Women are the only female primates who do not make it obvious when they are most fertile. Most female primates advertise the two or three days in each cycle when they are able to conceive*. Often, this is accompanied by a frenzy of copulation with a series of males. Before modern medicine, most women (and all men) were unaware of when the fertile period was. Women's reproductive coyness might reflect the change in the economic relation of the sexes which came with the origin of society. It could, some suggest, be an attempt to resolve the conflict between male promiscuity and the female's need to ensure the care of her children. By concealing when she is fertile she ensures constant attention from her mate. If he is not sure when she can conceive then he dare not leave her for a new woman in case another male takes advantage of his absence. This is historical speculation with no evidence for or against it — and, as so often with theories of history, several other interpretations are possible.

  Males do, needless to say, contribute to the care of their children, but in most societies the sexes differ in their commitment to the next generation. The mother is usually left holding the baby when a relationship breaks up. The difference can be subtle. Many genetic tests can tell parents whether they carry a harmful gene and whether it is wise for them to plan to have children. In a few cases, the test also tells the parents themselves that they are at risk of illness themselves. Huntington's disease is of this kind. Twice as many women as men volunteer for a test, perhaps because their concern for their potential child's future is greater than that for their own peace of mind.