by Steve Jones
Animals - ourselves included - have a similar set of mechanisms, with a variety of genetic identity tests before sex is allowed. Some are obvious, while others are less so.
Simple familiarity can breed contempt. Unrelated Jewish infants brought up together in kibbutzim, or Asian children betrothed and made to live together when they are tiny, may prefer to avoid sexual contact when they grow up, and - in the latter case - are, after the arranged marriage, said to be less fertile and more liable to divorce than average. Brothers and sisters also tend not to fancy each other. Older sibs feel a stronger sense of aversion to their younger fellows than do the young to the old. The degree of kinship is the same, but the older child can be almost certain that the junior members of the household are the products of their own mother for they saw them cared for as babies. A younger sib, on the other hand, knows only that an older individual lives under the same roof - which could happen for other reasons. They are less repelled by the idea of sex with somebody who might not, after all, be a relative. It takes fifteen years of shared residence for a younger brother or sister to build up the erotic revulsion that an older member of the family can generate by watching a few months of childcare.
Social pressures play a large part in our marital patterns, but genes are involved too. Some are obvious - people do, after all, tend to marry someone of the same skin colour as themselves - but others are more subtle.
As a boy, I kept mice in my bedroom, a fad quashed because of the awful stench. At the time that was no more than a nuisance, but in fact the aroma of mouse urine was an introduction to a new world of sexual contact, through the nose. Quite unexpectedly, mice have more genes than we do. Almost all the extras are involved with the sense of smell. The genes that code for smell receptors - most of them decayed in the human race - are in full order. Mice have hundreds, which together can tell apart a vast diversity of scents. The animals choose both food and mates through the nasal passage.
Given the choice, an inbred laboratory female mouse prefers to mate with a male from a different line. So keen is she on a new swain that a pregnant female will resorb her foetuses to render herself available. Bedding soaked with male urine has the same effect. The females assess health as well as kinship. Their acute nostrils sniff out those who carry parasites and avoid them. Perhaps - as in the wormy sheep on the Isle of Soay - the healthiest males, with the most impressive statements of their fine condition, are less inbred.
Mice live in an aggressive sexual universe. Each male dominates a small patch in which he can monopolise the females, but their partners often hop over to a neighbour’s territory for a change. The male marks his boundaries with urine and females base their choice on the same stuff. The more urine there is and the less familiar it smells, the better. The males are forced to engage in liquid battles in which each tries to water down the offerings of his competitors. The females go for the most productive and most aromatic among them. So potent is the identity cue that even the human nose, feeble as it might be, can separate some mouse inbred lines by scent alone.
The perfume is based on a series of proteins, coded for by related genes in four different families, one of which has over a thousand members. Two others are expressed in a special organ with its own set of nerves, at the base of the nose. Not only do the proteins have a strong scent of their own, but they bind other male pheromones to make a cocktail of desire. The genes involved are highly variable. As a result - just as in flowers - females can avoid males with the same odour, and the same family history, as their own. Like them, they steer clear of potential swains with low variability in the smell-related proteins, perhaps because their reduced and inbred state makes them less suitable as fathers.
Primates, too, signal with scent - which is why the aftershave industry does so well. Marmosets and tamarins, small New World monkeys, send out chemical messages with dozens of constituents to mark their territories, to advertise when they are available, and to bond with their partners. A male’s brain lights up in response to female chemicals when she is most fertile. The largest response is in those parts of the marmoset brain associated in humans with emotion.
We smell, as any marathon runner soon finds out. Bloodhounds can sniff out individual identities and are confused by identical twins. Rats, too, can assess human kinship. The animals sniff for longer at an unfamiliar scent than at an odour that they have already experienced. Give them a sweat-soaked shirt and they can tell whether they have smelt it before. When tested with the scent of the brother of a familiar subject, they sniff less than when given a sample from a cousin. To rats, at least, we have an aromatic identity.
But can men and women, like rats, mice or marmosets, themselves identify the sweet smell of the opposite sex? The case is not proved. Many of the human genes for odour reception have rusted away, to leave fewer than half the number at work in mice, and we lack the special organ that is so sensitive to scent in other mammals (although a few of the genes that make it are still at work and will respond to mouse scents). Generations of students have sniffed T-shirts worn by women at different stages of the ovulatory cycle, with inconsistent results. In spite of the undoubted genetic differences that exist in the ability to taste certain chemicals it has been hard to obtain clear results on the role of scent in human mate choice.
Even so, some observations hint that - like dogs around lampposts - men and women do pass on romantic messages through the nose. Many perfumes contain synthetic musks of the kind used by monkeys or mice to choose a mate. One chemical, a relative of testosterone, has long been touted as a chemical messenger. The stuff is sold to farmers as ‘Boar Taint’ to test the sexual receptivity of sows. Some people can smell it while others deny that it has an odour of anything, but after several weeks of exposure even they begin to notice its presence and the number of relevant receptors in the nose goes up and up - and more in women than in men.
Mice, men and flowers have converged in their mutual distaste for sex with a relative, but how did such cues of identity evolve? There are intriguing similarities between the mechanisms of choosing a mate and those that fight off infection. An ancient tie between sex and disease may even be behind some of our own marital preferences.
All mammals, smelly or not, carry inherited identity cards on the surface of every cell. We cannot accept kidney transplants because our immune system compares the donor’s genes with our own, recognises the tissue as foreign and rejects it. The less related the source of the organ, the fewer the genes in common and the lower the chance of success, which is why brothers and sisters are better donors than are pairs of strangers. The identity system is based on a set of genes that sit close together on the DNA. They live in a section that codes for the many functions of the immune system, our prime defence against infectious disease, and, as an incidental, against the novel challenges presented by tissue transplantation. Each comes in many different forms, which means that vast numbers of combinations are possible.
Disease is a potent agent of natural selection. Individuals with the most diverse set of immune-system genes, and those with large numbers of rare variants, tend to fight off infection better than others. Mice and even fish prefer to mate with those least similar to themselves in immune identity, as a hint that the tie between sexual choice and disease resistance is ancient indeed. In the fight against infection, such diversity pays, for the next generation will have, thanks to sex, a new mix of defensive genes, confusing the parasites’ ability to evolve fast enough to evade our immune system. It hence pays to choose someone as different as oneself as possible.
As Darwin discovered, cowslips and other plants are very careful when deciding which pollen is acceptable, with a variety of devices to ensure that their reproductive parts stay free of cells from males physically similar to themselves. The erotic stink of mice does the same job and humans, too, may learn to avoid familiar kin. In truth, the sexual examination goes on well after the male cells arrive. Plants choose what pollen tubes are allowed to grow, and fe
male insects may store the sperm of many males before deciding which should be allowed to travel further. Even after fertilisation, plants and mice are happy to abort a high proportion of their embryos, most of all those that arise from the attentions of a male relative.
The female reproductive system is a difficult and dangerous place for a sperm to find itself. Promiscuous mammals have longer vaginas than do those who stick to a few mates and make the male cells work harder to reach their goal. The vaginal tract is acid, too, and sperm do not much like that. In humans, of the millions implanted by a successful man, no more than a few hundred reach the neighbourhood of the egg, twenty or so make it to the point where they might be able to fertilise it and just a single cell gets in.
The smell of success lingers on after the sex act is over. Human sperm pick up and move towards chemical signals from the egg with the help of a gene that sits right inside the group that codes for smell perception. The complicated pore in the nose or the sperm cell membrane that picks up a single scent molecule, or a signal from the egg, each do more or less the same job and the two look remarkably alike - and, in a nod to their common heritage, some of the genes used by mice as they sniff the air to assess kinship by smell are also active in sperm. In an unexpected link between two sexual worlds, the sperm receptor also responds to the scent of lily of the valley and, given the choice, will swim towards it. Whether human eggs prefer to attract, or to allow entry to, sperm genetically different from themselves, we do not yet know.
Darwin’s work on the sex lives of plants has strayed into fields that would have shocked his contemporaries. His interest in their reproductive habits grew from his concerns about the effects of inbreeding in humans and on his own family in particular. Its influence is real, albeit less severe than he had predicted, and both plants and humans have evolved mechanisms that limit its effects. Faced with the same set of challenges, natural selection has come up with similar solutions in both kingdoms of life, which would not have surprised him (although he would, perhaps, be startled to discover that human sperm are attracted by the scent of a flower).
The great man’s concern about the possible damage done by cousin marriage to his own children was not justified. Of his sons, William became a banker and Leonard an army major. George was elected Professor of Astronomy and Francis Reader in Botany at Cambridge, while Horace set up as a scientific-instrument maker and was for a time mayor of that fair city. The naturalist’s offspring married into several eminent clans including those of Keynes and Huxley and - in spite of their progenitor’s concerns about inherited feebleness - have produced dozens of descendants eminent in science, medicine and the professions. They stand as living proof that intellectual aristocrats, unlike their botanical and blue-blooded equivalents, need not pay the price of keeping their biological heritage in the family.
CHAPTER V
THE DOMESTIC APE
‘Let them eat cake!’ said the Queen, and they did. Two centuries after the demise of Marie Antoinette, the poor are fat and the rich thin. Across the globe death from excess has, for the first time in history, overtaken that from deficiency. Eight hundred million people are hungry while a billion are overweight. The problem comes from evolution, as manipulated by man.
Darwin saw how farmers had bred from the best to produce new forms of life and used that notion to introduce the idea of natural selection. His argument is set out in the first chapter of The Origin of Species. Given time, and with conscious or unconscious selection of the best by breeders, new and modified versions of creatures from pigs to pigeons will soon emerge. Were they to be found in nature rather than in sties or lofts many would be recognised by naturalists as distinct species of their own.
In The Variation of Animals and Plants under Domestication, published in 1868, Darwin went further in exploring the tame as the key to the wild. The book speaks of ancient times, when ‘a wild and unusually good variety of a native plant might attract the attention of some wise old savage; and he would transplant it, or sow its seed’. That interesting event - the choice of favoured parents to form the next generation - was a microcosm of what had moulded life since it began. The variety of breeds seen on the farm was, he wrote, ‘an experiment on a gigantic scale’, both a test of his theory and a proof of its power.
Savages have been replaced by scientists. Their work has produced many new varieties of plants and animals and, on the way, has revealed the eccentric history of the food on our plates. Modern biology has transformed farming. Planned breeding - directed evolution - has led to an enormous drop in the effort needed to feed ourselves. The British spend a sixth of their income on breakfast, lunch and dinner, a proportion down by half in the past five decades and by far more in the past five centuries. For most people, shortage has given way to glut and for many citizens of the developed world food is in effect free.
The blessings so brought are equivocal. The real price of sugar, starch and fat - high energy but low-quality comestibles - has plummeted. Famine disguised as feast has spread across the globe. Evolution on the farm transformed society ten millennia ago and is doing the same today. Farmers have been powerful agents of selection on wheat, maize, cows, pigs, chickens and more, but the influence of those domestic creatures on the biology of the farmers themselves has been almost as great. Diet began to act as an agent of natural selection as soon as the wild was domesticated ten thousand years ago and caused people to evolve the ability to deal with new kinds of food. Today’s shift in what we eat will have equally powerful effects on the genes of our descendants.
A new global power - and a new agent of natural selection - is on the move. The empire of obesity began to flex its stomach in the 1980s and shows no sign of retreat. Twenty years before that dubious decade there was, in spite of a collapse in the real price of food, little sign of the coming wave of lard. Then, thanks to technology, came the industrialisation of diet; the last step in the scientific exploitation of the Darwinian machine. Now, a tsunami of fat has struck the world and its inhabitants are paying the price.
It does not take much to alter a nation’s waistline. The rise in American obesity over the past thirty years can be blamed on an increase in calories equivalent to no more than an extra bottle of fizzy drink for each person each day. At the present rate two-thirds of Americans and half of all Britons will be overweight by 2025 and Britain will be the fattest nation in Europe. Among industrial powers, only China and its neighbours are insulated from the scourge.
The twenty-first-century plague is a side-effect of the triumph of scientific agriculture. Many of those worst afflicted suffer because they bear genes that make it hard for them to deal with the new diet. Many of the obese will die young or fail to find a mate. As a result obesity will soon be - as farming itself was when it began - a potent cause of evolutionary change.
The people who laid out the first fields lived above the rivers that snaked across a green and leafy Levant. For millennia they hunted game and gathered seeds as man had done for the whole of history. Just after the peak of the last ice age the Middle Eastern weather became wetter and warmer and the grasses flourished. The gatherers prospered. Thirteen thousand years ago came a nasty shock, for the climate turned cold and dry for several centuries. The chill persuaded people to plant grains, rather than just to collect them. Soon the thermometer went up once more, the crops flourished and agriculture made its presence felt. Within a few centuries, the Fertile Crescent was filled with tillers of the soil.
A similar way of life, based on maize and rice rather than on wheat and chickpeas, soon got under way in South America and China and, in time, even in Papua New Guinea, where banana and sugarcane cultivation emerged six and a half thousand years ago. The habit spread fast. Farming reached Britain some four thousand years ago. The shift to the new economy was quite rapid, and the pursuit of wild game was more or less replaced by agriculture within just a couple of centuries, although people still ate plenty of seafood (and that remnant of the chase persists today).
As new crops emerged the locals began to husband animals that could feed on them. Soon a hundred people could live on the space that had previously supported but one.
The new economic system led to a grand simplification of diet. Homo sapiens has eaten some eighty thousand kinds of food since he first appeared on Earth. A dig in Syria of the homes of hunters who lived just before the new economy emerged revealed a hundred and fifty varieties of edible fruit, grain and leaf in that single society. Even in the nineteenth century, Queensland aborigines feasted on two hundred and forty different kinds of plant. As the new way of life spread, the cuisine became simpler. Within a few years, the Middle East had just eight crops: emmer and einkorn (antecedents of wheat), barley, peas, lentils, bitter vetch and chickpeas. Quite soon the people of the whole world considered together ate no more than half the number of plants once used by a single hunter-gatherer band. In most places just a couple of crops - rice, maize or wheat included - became the staple food. They kept that status for ten millennia.
Now, things have changed once more. Some lucky citizens have taken a great leap backwards. The middle classes have returned to the hunter-gatherer diet. They forage in pricey supermarkets for an eclectic range of edibles, from avocado to zucchini, imported from across the globe. The revolution of the rich began soon after Columbus, when exotic delicacies such as potatoes, peanuts and tomatoes were brought from the New World. Other delicacies went the other way, albeit sometimes after a long delay; broccoli, for example, was almost unknown in the United States until the 1920s. On both sides of the Atlantic, those who can afford it have put ten thousand years of dietary history into reverse.