Mutants: On Genetic Variety and the Human Body

X ANTHROPOMETAMORPHOSIS ^{[AN EPILOGUE]}

The authors of books about genetics – at least books written for the general reader – disagree about many things. What they agree on is the need either to predict the future course of humanity or to moralise about it, or, better yet, to do both. The predictions invariably concern the role that the ‘new genetic technologies’ – mass genetic screening, embryo selection, cloning, germ-line modification and the like – will have in the lives of individuals and societies. Some writers are sanguine, and assure us that these technologies are as nothing compared to the great demographic forces such as birth rates and migrations that shape the gene pool of our species; others contemplate, with surprising equanimity, the transformation of the human species into something resembling a highly intelligent plant. Some – media scientists usually – would like to clone themselves; others would like to restructure the tax system so that people with inherited disorders have a disincentive to reproduce. Others again – soi-disant ethicists, dialectical biologists and bishops – speak portentously of ‘human dignity’, rail against ‘genetic determinism’ or else mutter darkly about the ‘ethical dilemmas that face us all’ – though rarely condescend to explain exactly what these might be.

I propose to resist the temptation to do any of this. I do not know the future, and my views on the morality of cloning humans, or germ-line engineering, or any other topic, are neither so deeply considered nor so unique as to warrant public exposure – a reticence born not of cowardice, but rather of courtesy, or so I like to think. Instead I will end this book, which has been all about the many things that we know about the construction of the human body, with some thoughts on what we do not know and what we – or at least I – would like to.

I would like to know about variety. Most of this book has been about the rare mutations that damage the body. If I have mentioned variety, I have done so only in passing. By variety I mean the normal variation in human appearance and attributes that we see in healthy people around us. I mean the variety that can be found within the smallest Scottish hamlet, with its brown-, green- and blue-eyed inhabitants. But I also mean the differences in form between populations of people who live near to each other, but are somehow distinct: short pygmies versus taller Bantu farmers, for example. And I also mean the differences in skin colour, hair curliness and eye shape that distinguish – more or less – people who originate from different continents. One of the things, then, that I want to know about is race.

Race has long been under siege. Among scientists, geneticists have led the assault. Their attack has been predicated on two empirical results that have emerged from the study of patterns of genetic variation across the globe. The first was the discovery that most of the variety so abundantly visible in our genomes does not divide humanity along lines that correspond to the races of traditional and folk anthropology. All genes come in different variants, even if most of those variants are ‘silent’ and do not affect the structure of the proteins they encode. Inevitably, some variants are more common in some parts of the world than others. But the ubiquity and rarity of most variant genes across the globe do not correspond to traditional racial boundaries. Racial boundaries are usually held to be sharp; gene variant frequency changes are generally smooth. Changes in variant frequencies are also inconsistent between one gene and another. If there are lines to be drawn through humanity, most genes simply don’t show where they should go.

The second discovery that caused, and causes, geneticists to doubt the existence of races is the ubiquity of genetic variation within even the smallest populations. About ?5 per cent of the global stock of genetic variation can be found within any country or population – Cambodians or Nigerians, say. About another 8 per cent distinguish nations from each other – the Dutch from the Spanish – which leaves only a paltry 7 per cent or so to account for differences between continents or, in the most generous interpretation of the term, ‘races’. To be sure, there are genetic differences between a Dutchman and a Dinka, but not many more than between any two natives of Delft.

These facts about human genetic variation have been known since the 1960s. In each decade since then they have been confirmed, with ever more lavish quantities of data, using ever more sophisticated means of finding and analysing human genetic variation. In the 1960s genetic variation was studied by examining the migration of variant proteins on gels; today it is studied by sequencing entire genomes. Generations of scientists have expounded these results much as I have here – and asserted that, as far as genetics is concerned, races do not exist. They are reifications, social constructs, or else they are the remnants of discredited ideologies.

Most people have remained unconvinced. They have absorbed the message that races are, somehow, not quite what they used to be. Far better, then, to avoid the word and substitute ‘ethnicity’ or some similar term that comfortably conflates cultural and physical variety. For some, the persistence of the idea of race is a sign of racism’s tenacity. I doubt that this is true. Instead I suspect that the reason the lesson of genetics has been so widely ignored is that it seems to contradict the evidence of our eyes. If races don’t exist, then why does a moment’s glance at a stranger’s face serve to identify the continent, perhaps even the country, from which he or his family came?

The answer to this question must lie in that 7 per cent – paltry though it is – of global genetic variation that distinguishes people in different parts of the world. Seven per cent is a small part of global genetic variation, but it is large enough to imply the existence of hundreds, perhaps thousands, of genetic polymorphisms that are common, even ubiquitous, on one continent but rare, or even absent, on another. In recent years some geneticists have begun searching for such variants. The variants are known as AIMs or ‘Ancestry Informative Markers’ – so called because they can indeed tell you roughly where your ancestors came from, and even sort them out if – the case for so many of us – they came from several different places. The search for AIMs, which initially focused on Africans and Europeans but is already being extended, is prompted by the hope of identifying the genetic basis of several diseases, among them type 2 diabetes, the risk of which differs among Africans and Europeans.

Many AIMs have already been found. For some of them, the reasons for their presence on one continent but absence on another is readily apparent. One variant of the FY gene is ubiquitous in Africans but extremely rare among everyone else. The African variant is odd insofar that it prevents the protein that FY encodes from being made. Everyone else makes the protein, though its exact form can vary too. The FY protein is a growth-factor receptor found on blood cells, one that the malaria parasite seems to use as well, and its absence in Africans is almost certainly the result of long-standing natural selection for resistance to the disease. The absence of FY in Africans and its presence everywhere else has been known for decades. Many other differences are now being found – although they are usually not as dramatic as FY’s. No one knows what most AIMS do or why they are there.

Somewhere among all those AIMs, however, will be the genes that give a Han Chinese child the curve in her eyelid and a Solomon Islander his black-, verging on purple-, coloured skin. Among them, too, will be the genes that affect the shape of our skulls. Skull measuring has a long history in anthropology. One of the first really assiduous skull measurers was a Dutchman, Petrus Camper, who in the 1700s invented the ‘facial angle’ – essentially an index of facial flatness. In his most famous diagram, Camper shows a series of heads and skulls – monkey, orangutan, African, European, Greek statue – with ever-declining facial angles. Camper himself was no racist. In his writings he emphasised repeatedly the close relationship between all humans no matter what their origins. ‘Proffer with me,’ he urged in 1764, ‘a fraternal hand to Negroes and recognise them for veritable descendants of the first man, to whom we all look as our common father.’ To which he added that the first man may have been white, brown or black, and that Europeans are really just ‘white Moors’ – and did so at a time when Linnaeus was carving up our species.

Sadly, Camper’s iconography spoke louder than his words, and his diagram with its implicit demonstration of a hierarchy from ape to Apollo (with Africans rather closer to apes than to gods) became a staple of nineteenth-century anthropology. There is no need to recap and critique the craniometric studies carried out in the nineteenth and early twentieth centuries that sought to demonstrate that one subset of humanity was more or less intelligent than another – others have done so with a thoroughness that their scientific influence scarcely merits. But it is worth noting that modern physical anthropologists remain keen on describing skull shape, though nowadays they tend to do so with 3-d laser scanners and multivariate statistics. They find, perhaps unsurprisingly, that for all the variety within populations, people from different parts of the world have different-shaped heads.

Much as Camper claimed, the jaws of sub-Saharan Africans do protrude, on average, further from their foreheads than do the jaws of Europeans – an attribute known as ‘prognathism’. Melanesians and Australian Aborigines are also more prognathic than Europeans. Contra Camper, however, this does not make African (or Aborigine) skulls more like ape skulls than European ones. The facial angle is a rather crude way of capturing an exceedingly complex aspect of skull shape. It does not discriminate between different ways of being prognathic. A chimpanzee has a high facial angle because its whole face and forehead slope; Africans and Aborigines have slightly higher facial angles than Europeans because of a jut in the jaw alone. Besides, Europeans do not even have the flattest faces. That honour – if honour it is – must go to the Inuit of northern Canada.

Human skulls are wonderfully diverse. The Inuit are also notable for the largeness of their eye orbits and the massiveness of their cheekbones. Compared to everyone else, the Khoisans of southern Africa have bulging foreheads (frontal bossing); Australian Aborigines have massive brows (supra-orbital ridges); some sub-Saharan Africans have widely set eyes (large inter-orbital distances); Andaman Islanders (negritos) have small, round skulls – the list of differences could be extended indefinitely. Few of these differences are absolute. Just as most of the variance in gene frequency is found within, rather than among, populations (nations, continents), so too is most of the variance in skull shape. And the differences among populations are all subtle. Australian Aborigines and Inuit differ in prognathism by only 6 per cent. Small differences, then, but differences that, given the attention we devote to each other’s faces, strike us immediately.

My claim that we will soon be able to identify the genes responsible for all this diversity in skull shape suggests an important question: namely, do such genes exist? In 1912 the American anthropologist Franz Boas set out to demonstrate that they do not. A humane and tolerant man, he was an implacable opponent of those who sought to make invidious distinctions between humanity based on the shapes of their skulls. The following passage, taken from a serious anthropological article written in 1905 by a German dentist called Rose, gives a flavour of what he was up against: ‘The long heads of German descent represent the bearers of higher spiritual life, the occupants of dominant positions, to which they are destined by nature, the innate defenders of the fatherland and the social order. Their whole character predetermines them to aristocracy.’ And so on, to the detriment of the more democratically minded and un-German ‘round heads’.

The ‘long’ and ‘round’ heads refer to the value of the ‘cephalic index’, the ratio of skull breadth to width (expressed as a percentage, long heads or dolichocephalics have a cephalic index below seventy-five, while round heads or brachycephalics have a cephalic index above eighty; mesocephalics are somewhere in between). Noting that the immigrants who arrived at Ellis Island – Bohemians, Slovakians, Hungarians, Italians, Scots and Eastern European Jews – varied somewhat in their cephalic indexes, Boas asked whether these differences were due to genetic (to use his terminology, ‘racial’) or environmental causes. He reasoned, soundly, that if the skulls of the American-born children of all these various groups were more similar to each other than those of European-born children, then environment rather than ancestry must be the cause of the differences. Boas measured some thirteen thousand heads – a vast undertaking that left him, in the absence of computers, overwhelmed by numbers. Nevertheless, he managed to produce a graph that seemed to show that the cephalic indices of the US-born children of Sicilians and Eastern European Jews (both rather dolichocephalic to begin with) were, indeed, converging. It was a case of new heads for the New World.

Boas’s study dealt a near-fatal blow to craniometry. Over the last ninety years it has been cited innumerable times – not least by the late Stephen Jay Gould – as proof that skull shape is ‘plastic’, that is, caused by non-genetic differences. Boas, however, was wrong. His data have recently been comprehensively re-analysed using modern statistical techniques. The skulls of American-born children do indeed differ from those of their parents, but they do so inconsistently. Indeed, had Boas chosen to compare the children of Scots and Hungarians, rather than those of Sicilians and Eastern European Jews, he could have shown that America causes skulls to diverge rather than converge in shape. But he was wrong in a deeper sense than this. Re-analysis of his data also shows that the changes in skull shape caused by American birth, whatever their direction, are trivial compared to the differences that remain and that are due to ancestry and family – or, to put it more succinctly, to genes. Indeed, looking beyond European immigrants, this is hardly surprising. Forensic anthropologists in the United States and Britain are quite adept at telling whether a given skull, perhaps evidence of some foul deed, once belonged to someone of African or European ancestry. That they can do so after decades, even centuries, of co-existence, not to mention generous amounts of admixture, suggests that our differences are not, as is often said, merely skin deep, but extend to our skulls – if not to what they contain.

So, genetic differences exist among all sorts of people. Should we try to find out what they are? Many scientists think not. Some find it enough to dismiss such physical diversity as exists among human populations as ‘uninteresting’ – not worthy of study. Others concede that it may be interesting, but that it should not be studied, since even to contemplate doing so is to engender social injustice. They fear a revival of not merely racial, but racist, science.

For my part, I should love to know the genes responsible for human diversity; the genes for the differences – be those differences between men and women who live in the same village or those who have never trodden on each other’s continents. In part this is simply for the pleasure of knowing. This the pleasure that comes from looking at Gabriel Dante Rossetti’s painting La Ghirlandata and knowing that his model, Alexa Wilding, had two loss-of-function MC1R mutations that gave her such glorious red hair. This pleasure of knowing is partly that which all science gives, but to which is added the pleasure that comes from understanding the reason for something that has been hitherto at once familiar but completely mysterious.

The view that human diversity is dull seems to me excessively Olympian. After all, if population geneticists have ignored human variety, they have for decades lavished their (seemingly inexhaustible) energies studying variety in the colours of garden-snail shells and the number of bristles that decorate the backs of fruit flies – problems that are intellectually much like those presented by human variation.

The claim that human genetics is morally dangerous is a more serious one. One can certainly, given the history of racial science, see where such a claim originates. Nevertheless it is misplaced. Reasonable people know that the differences among humans are so slight that they cannot be used to undermine any conceivable commitment to social justice. ‘Human equality,’ to borrow a slogan of Stephen Jay Gould’s, ‘is a contingent fact of human history.’ What is true, however, is that as long as the cause of human variety remains unknown – as long as the 7 per cent of genetic variance that distinguishes people from different parts of the world remains obscure – there will always be those who will use that obscurity to promote theories with socially unjust consequences. Injustice can sometimes be the consequence of new knowledge, but more often – far more often – it slips in through the cracks of our ignorance.

Perhaps the most compelling reason that we should once again turn our attention fully to the study of human physical diversity is that it is disappearing. In South-East Asia the negritos, those enigmatic pygmy-like people, are in decline. They are hunter-gatherers. Overrun by Austronesian-speaking farmers in the Neolithic, they mostly persist on remote islands. Now, modernity threatens. On Lesser Andaman, the remaining Onge live in reservations. On Greater Andaman, a few hundred Jarawas survive by virtue of having fended off the curious with bows and arrows (in the last fifty years they have killed or injured more than a hundred people), but they too have now emerged from the forest, attracted by baubles offered by Indian officials. It is feared that they will soon succumb to tuberculosis, measles and culture shock as their predecessors have.

They are only the latest casualties of Austronesian and European (not to mention Chinese, Bantu and Harappan) expansion. In 1520 Ferdinand Magellan, arriving at the straits that today bear his name, reported the existence of a race of giants that lived in the interior of Tierra del Fuego. He called them the Pataghoni, after a giant in a Spanish tale of chivalry. Subsequent travellers embroidered the account; by 1767 these giants, a wild and brutal people, had grown to about three metres (ten feet) tall. Today, the giants of Tierra del Fuego are as forgotten and fantastical as Pliny’s Arimaspeans. And yet the Pataghoni existed. They called themselves the Selk’nam or Ona, and they had an average adult-male height of 178; centimetres (five feet ten inches) – giant, then, but only to sixteenth-century Spanish sailors. But if their stature was not that remarkable, their skulls certainly are. They have a strength and thickness, a robustness, which other human skulls don’t, and this is true of their skeletons as a whole. Some photographs of the Selk’nam exist. They depict a handsome and physically powerful people who wore cloaks made from the pelts of the guanacos that they hunted on foot using bows as tall as themselves. Argentine sheep ranchers killed the Selk’nam off in a genocidal slaughter, and the last one died some time around 1920.

There is one more thing I should like to know about. And it is a phenomenon more general and nearly as contentious as race. It is beauty. Beauty is that which we see (or hear or touch or smell) that gives us pleasure, and as such its forms are, or at least seem to be, infinitely various. Here I am concerned with physical beauty alone.

‘Beauty,’ says the philosopher Elaine Scarry, ‘prompts the begetting of children: when the eye sees something beautiful, the whole body wants to reproduce it.’ Plato, she points out, had the same idea. In The symposium, Socrates tells how he was instructed in the arts of love by Diotima, a woman of Mantinea, and how they spoke of the nature of love and beauty. ‘I will put it more plainly,’ says Diotima. ‘The object of love, Socrates, is not what you think, beauty.’ ‘What is it then?’ ‘Its object is to procreate and bring forth beauty.’ ‘Really?’ ‘It is so, I assure you.’

Darwin could not have put it better himself. Much of his The descent of man and selection in relation to sex is devoted to investigating the presence, perception and purpose of beauty. ‘The most refined beauty,’ he wrote, ‘may serve as a charm for the female, and for no other purpose.’ He was thinking of the tail feathers of the male Argus pheasant with its geometrical arrays of ocelli. But the psychologies of pheasants and Fijians are really much the same. For Darwin, the love of beauty is a very general evolutionary force, second only to natural selection itself in power. Creatures choosing beauty for generation upon generation have given the natural world much of its exuberance. Sexual selection has given the Madagascar chameleon its horns; it has given the swordtail fish its sword and Birds of Paradise and Argus pheasants their tails; it has given the human species its variety.

One of the fascinating things about Darwin’s account of beauty is that without reference to philosophers or artists he stakes out a position on the great issues of aesthetics. He wants to know whether or not beauty is universal or particular, whether it is common or rare, and whether or not it has meaning. To all of these questions Darwin has an unequivocal answer. Physical beauty, he asserts, is not universal, but rather particular. Different people in different parts of the world each have their own standard of beauty. And it is rare. To be beautiful is to be a little different from everyone else around us. It is also meaningless. Our brains, for whatever reason, perceive some things as beautiful, and do so regardless of the other qualities that those things may have. Beauty does not signify anything. It exists for its own sake.

Darwin’s views on beauty are characteristically, effortlessly, original. The descent of man contains nothing, for example, about the classical ideal of beauty – the ideal that, from Archaic kouros to Antinous’ scowl, was replicated across the Mediterranean for centuries as if there were a formula for it; which there was, one that by the Renaissance had become a theory of human beauty in which proportions were divine, a theory that in the eighteenth century turned into the standard by which all humanity was judged. It was this ideal that caused Winckelmann to assert that the ancient Greeks were the most beautiful of all people (though he thought modern Neapolitans comely as well); that caused Camper to place the head of a Greek statue at one end of his continuum of facial angles; Buffon to identify a ‘beauty zone’ between 20 and 35 degrees north that stretched from the Ganges to Morocco and took in the Persians, Turks, Circassians, Greeks and Europeans; and Bougainville, when he arrived in Tahiti in 1768, to eulogise its inhabitants in terms of a classical idyll painted by Watteau. Darwin avoids all this. He does not tell us what he thinks is beautiful; instead he attempts to find out what other people think. He collects travellers’ reports. American Indians, he is told, believe that female beauty consists of a broad, flat face, small eyes, high cheekbones, a low forehead, a broad chin, a hook nose and breasts hanging down to the belt. Manchu Chinese prefer women with enormous ears. In Cochin China, beauties have round heads; in Siam, they have divergent nostrils; Hottentots like their women so immensely steatopygous that, having sat down, they cannot stand up again.

Darwin does wonder about the quality of his data – and rightly so. But in general he is quite convinced that different people perceive beauty in different ways. His vision is an appealing one. Per molto variare la natur è bella – nature’s beauty is its variety; it could be Darwin’s slogan and it could be Benetton’s (though it was Elizabeth I’s). Indeed, when we consider the whirlwind of fashion it is impossible to doubt that the love of beauty is frequently the particular love of rare and meaningless things. Among scientists who study beauty, however – and the study of beauty is itself increasingly fashionable – Darwin’s views are seen as rather quaint. These days, most research on the subject begins with the notions that the standard of beauty is universal, that the presence of beauty is rather common, and that far from being meaningless, it has a great deal to say.

The universality of beauty’s standard is as self-evident as its particularity. The apparent contradiction is resolved if we simply recognise that there are some things about which tastes differ, and some about which they do not. Tastes in hairiness (cranial, facial, bodily), pigmentation (eye, hair and skin colour) and perhaps even body shape (hip–waist ratio) all seem to differ quite a lot from person to person, place to place, time to time. But the taste for relative youth – at least when men judge women – does not. Nor, it seems, does the taste for certain kinds of faces. Average faces seem to be universally more attractive than most, but not all, variant faces. Symmetry is preferred over asymmetry. These are some of the results of a large literature devoted to finding out who finds what beautiful when. Much of it demonstrates the obvious. After all, were a Papua New Guinea tribesman brought to London’s National Gallery and offered the choice of Botticelli’s Venus (she of Mars and Venus) and Massys’s Grotesque Old Woman as a mate, he might well be unimpressed by either, but we can be sure which one he’d choose.

Beauty’s meaning is more controversial. Here I wish to pursue just one idea: that it has something to do with physiological condition; that it is, indeed, a certificate of health. In its simplest form the truth of this idea is also quite self-evident. Clear skin, bright eyes and white teeth are manifestly signs of beauty and health. It is no accident that Brazilian men, glimpsing a beautiful carioca, sigh ‘Que saúde’ – what health. Whether particular facial proportions and symmetry signify health is, however, less obvious. Studies using computer-generated faces show that we perceive beautiful faces as being healthy ones. But searches for a correlation between the beauty and health of real people have found only weak and inconsistent effects.

Perhaps this is because beauty is no longer what it was. For all of human history, poor health has mostly been about nutrition and pathogens – a lack of the first and an excess of the second. Beauty was an indicator of the salubriousness of the environment or else the ability to resist its vicissitudes. To the degree that this is true, then the variance in beauty must be declining in the most developed nations at least, even as its mean increases. Goitres and cretinism may still afflict large parts of the world, but they no longer afflict the Swiss. The scars of smallpox have disappeared everywhere. Even in England most people now keep their teeth until they die. One wonders whether the diseases – filariasis, malaria, sleeping sickness, not to mention nutritional deprivation in its many forms – that afflict so many of the world’s children can be read in the symmetry and proportions of their faces if, as adults, they should have survived them. There is no doubt that prosperity exacts a cost to beauty in the form of obesity, dental cavities and stress. But if the balance of its effects is favourable, and it must be, then any classroom of American or European undergraduates contains an abundance of beauty that has never existed in human history before.

That may seem implausible, but only because we have little grasp of beauty’s advance. Beauty is like wealth. It increases over time, yet its distribution remains unequal. However much of it we have, it always seems that someone else has more. In part this is because beauty, as the consequence of health, is also the consequence of wealth. But suppose there existed a society so wealthy and egalitarian that, as far as pathogens and nutrition are concerned, all were equally healthy. A society of the sort approached by the Netherlands (but from which Great Britain and the United States remain woefully distant), in which the socioeconomic background of a child cannot be judged from his or her physical appearance alone. Would all be equally beautiful in such a society? Would beauty’s difference have disappeared? I doubt it. However beautiful the average Dutchman may believe himself to be, some of his compatriots will be more beautiful yet. I suspect that there is a residual variance in beauty that even the most controlled upbringing cannot eradicate. A residuum that lies in our genes.

The effects of poor childhood nutrition and exposure to pathogens upon the face may be uncertain, but the effects of mutations are not. When clinical geneticists attempt to classify the symptoms that their patients present, it is to the face that they first look. They are expert in recognising the subtleties that are often the only outward sign of deeper disturbances in the genetic order: shallow philtrum, low-set ears, upturned nose, narrow or wide-set eyes. Many, perhaps most, of the disorders that I have discussed in this book – from achondroplasia to pycnodysostosis – can be read in the face.

It seems that our faces are very vulnerable to mutation. Or perhaps we are just very good at reading mutation’s effects in them. Either way, it seems likely that mutation’s effects are written on all of our faces – not simply the faces of people with identified clinical disorders. I began this book by observing that every newly conceived embryo has, at an educated guess, an average of three hundred mutations that affect its health for the worse. It may seem impossible that we could, as a species, be so poorly. But a certain number of mutations are eliminated by selection in the womb. A woman who knows that she is pregnant has a 15 per cent chance of miscarriage; many more embryos must be lost to women who are unaware that they have conceived. More than 70 per cent of spontaneously aborted foetuses bear severe chromosomal abnormalities, and it is likely that many also bear mutations in particular genes. It is now widely supposed that miscarriage is an evolved device that enables mothers to screen for, and rid themselves of, genetically impaired progeny.

Mutation is a game of chance, one we must all play, and at which we all lose. But some of us lose more heavily than others. Some calculations hint at the distribution of our losses. If we suppose that, of the three hundred mutations that burden the average newly conceived embryo, five are lost from the population each generation by death (miscarriage, infant and childhood mortality), then the average adult carries 295 deleterious mutations. The least burdened 1 per cent of the population will have about 250 mutations, and the most burdened 342. Somewhere in the world there is a person who has the fewest mutations of all, about 191 of them.

These calculations confirm the intuition that no one leaves the genetic casino unscathed. But they are just educated guesses. They also take no account of the relative cost of each mutation. They are the equivalent of estimating gambling losses by counting the number of chips surrendered to the house without noting their value. It seems likely that the cost of most mutations is quite small. They give us minor ailments such as bad backs and weak eyes. I suspect that they also give us misaligned teeth, graceless noses and asymmetrical ears. If this is so, then the true meaning of beauty is the relative absence of genetic error.

There is, admittedly, very little evidence for this idea, at least in humans. Evolutionary biologists have long suspected that the peacock’s tail and the red deer’s roar are signals of genetic quality, and have amassed much evidence in the support of this theory, most of it weak. The mutational-load explanation of beauty is however consistent with our intuitions – or prejudices – about the distribution of beauty. If deleterious mutations rob us of beauty, they should do so with particular efficacy if we marry our relatives. Most novel mutations are at least partly recessive, and inbreeding should accentuate their negative effects as they become homozygous. There is no doubt that consanguinous marriages have a cost: the children of cousins have a 2 to 4 per cent higher incidence of birth defects than those of unrelated individuals. One wonders if such children would be judged less beautiful than their outbred peers as well. Pakistan, where around 60 per cent of marriages are between first cousins, would be a good place to look. Conversely, people of mixed ancestry, such as Brazilians, should show the aesthetic benefits of concealing their recessive mutations – Que saúde.

What makes physical beauty so wonderful? What enables it to take us by surprise, to prevent us from treating it with indifference no matter how saturated we are by the worlds of advertising and celebrity that have appropriated it, indeed made us suspicious of its power? If the answer that I have sketched contains any truth, then each image of a beautiful face or perfectly turned limb is not really about the subject that it appears to be, but rather what it is not. It is about the imperfections that are absent: the machine errors that arise from the vicissitudes of the womb, childhood, maturity and old age, that are written all over our bodies and that are so ubiquitous that when we see someone who appears to have evaded them, however fleetingly, we pause to look with amazed delight. Beauty, Stendhal says, is only the promise of happiness. Perhaps. But it is equally the recollection of sorrow.