Mutants: On Genetic Variety and the Human Body

OF ALL THE DOCTRINES THAT HAVE BEEN OCCASIONED by human deformity, none is more dismal than the belief that it is due to some moral failing. We can call this idea ‘the fallacy of the mark of Cain’. For killing his brother, so Judeo-Christian tradition has it, God marked Cain and all his descendants. An apocryphal text from Armenia gives Cain a pair of horns; a Middle Irish history gives him lumps on his forehead, cheeks, hands and feet, while the author of Beowulf makes him the ancestor of the monstrous Grendel. None of this can actually be found in Genesis, which is, by comparison, a dull read. There Cain’s punishment is exile, the mark is for his own protection, and its nature is left obscure. But then, the link between moral and physical deformity has never really required biblical authority. It does not even require iniquitous parents. In 1999 the coach of the English national football team opined to an interviewer: ‘You and I have been physically given two hands and two legs and a half-decent brain. Some people have not been born like that for a reason. The karma is working from another life. What you sow, you have to reap.’ He took his cue from a Buddhist faith healer.

The fallacy of the mark of Cain flourished in Britain – football coaches aside – as recently as the seventeenth century. In 1685, in the remote and bleak Galloway village of Wigtown, two religious dissenters, Margaret McLaughlin and Margaret Wilson, were tried and convicted for crimes against the state. The infamy of their case comes from the cruelty of the method by which they were condemned to die. Both women were tied to stakes in the mouth of the River Bladnoch and left to the rising tide. Various accounts, none immediately contemporary, tell how they died. McLaughlin, an elderly widow, was the first to go; Wilson, who was eighteen years old, survived a little longer. A sheriff’s officer, thinking that the widow’s death-throes might concentrate the younger woman’s mind, urged her to recant: ‘Will you not say: God bless King Charlie and get this rope from off your neck?’

He underestimated the girl. Some accounts give her reply as a long and pious speech; others say she sang the 25th Psalm and recited Chapter 8 of Romans; all agree that her last words were pure defiance: ‘God bless King Charlie, if He will.’ The officer’s response was to give vent to his talent for vernacular wit. ‘Clep down among the partens and be drowned!’ he cried. And then he grasped his halberd and drowned her.

The executioner’s words are interesting. In the old Scottish dialect to ‘clep’ is to call; ‘partens’ are crabs. Thus: ‘Call down among the crabs and be drowned.’ In another version of the story, the officer was asked (by someone who had evidently missed the fun) how the women had behaved as the waters rose around them. ‘Oo,’ he replied in high humour, ‘they just clepped roun’ the stobs like partens, and prayed.’ Either way, it is here that the story slides from martyrology into myth. For it seems that shortly after the officer – a man named Bell – had done his cruel work, his wife gave birth to a child who bore the ineradicable mark of its father’s guilt: instead of fingers, its hands bore claws like those of a crab. ‘The bairn is clepped!’ cried the midwife. The mark of Bell’s judicial crime would be visited on his descendants, many of whom would bear the deformity; they would be known as the ‘Cleppie Bells’.

The spot at which the women are supposed to have died was marked by a stone monument in the form of a stake; today it stands in a reed-bed far from the water’s edge, the Bladnoch having shifted course in the intervening three centuries. Another, far more imposing, monument to the martyrs stands on a hill above the town, and their graves, with carefully kept headstones, may be found in the local churchyard. Here, as elsewhere, the Scots nurse the wounds of history with relish.

There are are other modern echoes of the event as well. As recently as 1900, a family bearing the names Bell or Agnew, and possessing hands moulded from birth into a claw-like deformity, lived in the south-east of Scotland and were said to be descendants of the Cleppie Bells. We know nothing more about them; they may be there yet. We do know that in 1908 a large, unnamed family, living in London but of Scottish descent, were the subject of one of the first genetic studies of a human disorder of bodily form. Their deformity, known at the time as ‘lobster-claw’ syndrome, is certainly the same malformation that the Cleppie Bells had, though these days clinical geneticists eschew talk of ‘lobster claws’ and speak of ‘split-hand-split-foot syndrome’ or ‘ectrodactyly’, a term rendered palatable only by the obscurity of Greek, in which it reads as ‘monstrous fingers’. This second Scots family may have been related to the Cleppie Bells, but it is quite possible that they were not and that the deformity arose independently in the two families. At one end of this story there is the historical trial and death of Margarets Wilson and McLaughlin, at the other there are the Cleppie Bells and a clinical literature. The mythical element, of course, lies in the causal connection between the two. Nothing that officer Bell ever did could have caused his descendants to be born with only two digits on each hand, widely spaced apart. If the Bells were clepped, it was because some of them carried a dominant mutation that affected the growth of their limb-buds while they were still in the womb: it certainly had nothing to do with the partens.

THE USE OF LIMBS

The fragments of myth, folklore and tradition that remain to us from a pre-scientific age are like the marks left in sand by retreating waves: void of power and meaning, yet still possessed of some order. Muddied by time and confused causality, they still bear the imprint of the regularities of the natural world. It is surely significant that in such lore – no matter what its origin – few parts of the human body are as vulnerable to deformity as the limbs. Greek mythology has only one deformed Olympian, crook-foot Hephaestus who, abandoned by Hera (his mother), betrayed by Aphrodite (his wife), and spurned by Athena (his obsession), nevertheless taught humanity the mysteries of working metal and so is the god of craftsmen and smiths. Depicted on black-and-red-ware he is usually given congenital bilateral talipes equinovarus, or two club-feet. Oedipus, perhaps the most famous deformed mortal, wore his swollen foot in his name.

New myths arise even now. In the mid-1960s a Rhodesian Native Affairs administrator claimed that he knew of a tribe of two-toed people in the darker reaches of the Zambezi river valley. In tones reminiscent of Pliny the Elder’s accounts of fabulous races in Aethiopia or the Indies they were, he said, variously called the Wadoma, Vadoma, Doma, Vanyai, Talunda or, most excitingly, the ‘Ostrich-Footed People’ – a primitive and reclusive group of hunter-gatherers who, by virtue of their odd feet, could run as swiftly as gazelles. Veracity was assured by a photograph of a Wadoma displaying his remarkable feet. In 1969 this same photograph appeared in the Thunderbolt, a newsletter published by the American National States Rights Party, illustrating an article which argued that since some Africans had ‘animal feet’ they were obviously a separate species (‘Negro is related to Apes – Not White People’). American academics, rightly outraged, denounced the photograph as a forgery. Wrongly so, for when geneticists investigated the matter, they found that the Wadoma certainly existed, although far from being a whole tribe of ‘ostrich-footed’ people, there was only a single family afflicted with an apparently novel variety of ectrodactyly. But it is impossible to keep a good myth down. In the mid-1980s two South African journalists claimed they had stumbled across a whole tribe of two-toed people in the darker reaches of the Zambezi. Now, websites assert that the Wadoma worship a large metal sphere buried in the jungle and are, in fact, extraterrestrials.

Limbs have an extraordinary knack for going wrong. There are more named congenital disorders that affect our limbs than almost any other part of our bodies. Is it that limbs are particularly delicate, and so prone to register every insult that heredity or the environment imprints upon them? Or is it that they are especially complex? Delicate and complex they are, to be sure, but the more likely reason for the exuberant abundance of their imperfections is simply that they are not needed, at least not for life itself. Children may grow in the womb and be born with extra fingers, a missing tibia, or missing a limb entirely, and yet be otherwise quite healthy. They survive, and we see the damage.

One of the strange things about limbs is how easy it is to compensate for their absence, either partial or entire. As the patriarch of one ectrodactylous family replied to a geneticist: ‘Bless ‘e, sir, the kids don’t mind it. They never had the use o’ fingers and toes, and so they never misses ‘em.’ Indeed, why should they? They could hold their own at school in writing, drawing and even needlework. Among the adults, one was a bootmaker, one drove a cab, and another had a party trick in which he picked up pins from the floor using his two opposable toes.

The neural and physical versatility of limbs is even more striking in people who lack upper limbs altogether. Among the most engagingly feisty of all armless artists was Hermann Unthan, ‘The Armless Fiddler’. Born in 1848 in a small German town, he narrowly escaped smothering by an infanticidal midwife, and was raised by his strict but loving parents on a diet of self-reliance that now seems positively heartless. Within days of his birth, his father ordered that his son was never to be pitied, never to be helped, and was not to be given any shoes or socks. By 1868 the young Hermann was giving violin recitals to delighted Viennese audiences as the younger Johann Strauss conducted. In the course of his long and varied life he travelled widely, finally coming to rest in the United States, which he loved. At the age of eighty he wrote his autobiography, aptly titled The armless fiddler: a pediscript, with his toes and an electric typewriter. This sort of neural flexibility is common in mammals. Among the anatomical wonders of the 1940s was a little Dutch goat that, born without fore-limbs, managed to get about bipedally, rather in the manner of a kangaroo.

THE NEAR TO THE FAR

The ability of animals to survive without their limbs has long proved useful to biologists. Limbs can be counted, dissected and manipulated on a living creature without the need to open the body. They are naked to the biologist’s gaze. This visibility means that, of all the devices that make the body, those that make limbs are now exceptionally well understood. Much is known, for example, about their most salient characteristic: the fact that they stick out from our bodies.

At day 26 after conception, the first signs of a foetus’s arms appear: two small bumps, one on each flank, just behind the neck. By analogy to the precursors of leaves or flowers, these bumps are called limb-buds. A day or so later, another pair of limb-buds forms further down the torso; they will become legs. Like any of the bumps on the surface of an embryo, limb-buds are at first just a bag of ectoderm filled with mesodermal cells. There are as yet no bones, muscles, tendons or blood vessels. The limb-bud remains in this amorphous state for about five weeks, at which time faint outlines of bones – the first signs of structure – begin to form. Even before that, however, the limb-bud has not been quiescent, because from nothing more than a small bump it has grown into an appendage about 2 millimetres long. On day 50 after conception, the embryo crouches and holds its newly formed hands over its heart. On day 56, it touches its nose.

What induces a limb-bud to grow out into space? In 1948 a young American biologist, John Saunders, gave an answer to this question. He had noticed that limb-buds were crowned by a ridge of unusual cells. The cells were clearly ectoderm – the tissue that covers the entire embryo – but at the tips of limbs they resembled tightly packed columns, quite unlike their usual pancake shape. Saunders dubbed this structure the ‘apical ectodermal ridge’ and then, curious to know more, decided to remove it.

As embryonic newts have been used to study the organiser, so chickens have been used to study limbs. Saunders operated on twenty-two foetal chickens, some young, others a little older. In each case he removed the apical ectodermal ridge from one wing-bud, while leaving the one of the other side intact. Having operated, he sealed up the egg and waited until the chicks hatched out. The operated wings all had a characteristic deformity: they were, to varying degrees, amputated. Chickens operated on when the limb-bud had just begun to expand showed severe amputations: they had at best a humerus (the bone closest to the shoulderblade), but below that, the radius, ulna, wrist bones and digits were all gone. Those operated on a little later had a humerus, radius and ulna, but lacked wrists and digits; later yet, only the digits were missing.

This experiment helps to explain why some infants, such as Hermann Unthan, are born without arms or legs. Our limb-buds also have apical ectodermal ridges, and sometimes they must surely fail. The ridges on Hermann’s arm-buds probably malfunctioned soon after they first appeared; perhaps they never appeared at all. Other human deformities resemble the less extreme amputations seen in chicks whose wing ridges are removed only late in their growth. In the Brazilian states of Minas Gerais, São Paulo and Bahia there are families who are afflicted with a disorder called acheiropody – from the Greek: a – absence, cheiros – hand, podos – foot. Instead of hands and feet, the victims of this disorder have limbs that terminate in a tapered stump. They get about by walking on their knees and are called by the locals aleijadinhos, or ‘little cripples’. The disorder is caused by a recessive mutation, probably quite an old one since it appears in more than twenty families, all of Portuguese descent. Because the mutation is recessive, only foetuses who have two copies of the mutant gene fail to develop hands and feet. Having two copies of a mutation is usually a sign of inbreeding: the first family of aleijadinhos ever studied were the children of a Peramá couple who were – local opinion varied – either full siblings, half siblings, or else uncle and niece.

The apical ectodermal ridge is the sculptor of the limb. As the development of the limb-bud draws to a close, the ridge regresses, leaving behind an outline of our fingers and toes. Should it be damaged in any way, the consequences will be visible in the limb’s final form. The ectrodactylous hands of the Wigtown cleppies were the result of a mutation that caused a gap in the middle of the ridge, and so a gap in the middle of the forming limb. Mutations in at least four different genes are known to cause ectrodactyly, but it is quite possible that more will be discovered.

What gives the ridge, which is little more than a clump of cells, such power over the shape of a limb? The most obvious explanation would be that the cells making up the tissues of the limb – bone, sinew, blood vessels and so on – have their origin in the ridge. But this is not the case. All of these tissues are made of the mesoderm that lies beneath the ridge rather than the ridge itself; only the skin is ectoderm. The obvious alternative is that the ridge matters not as a source of cells, but rather as a source of information: it tells mesoderm what to do.

Action at a distance in the embryo usually implies the work of signals, and so it is in the limb-bud. Apical ectodermal ridges are rich in signalling molecules, especially so in one family of them: the fibroblast growth factors or FGFs. The experiment that identified FGFs as the source of the ridge’s power began with the surgical extirpation, à la Saunders, of the apical ectodermal ridge from the tip of a young wing-bud. The denuded bud was not, however, allowed to grow up into the usual amputee wing. Instead, a silicone bead soaked in FGF was placed on its tip, more or less where the ridge would be. The result was a fully-grown limb – one cured, if you will, by the application of a single protein. Twenty-two genes in the human genome encode FGFs, of which at least four are switched on in the ridge. No one knows why so many are needed there, but collectively they are vital to the workings of the ridge. It would be an exaggeration to say that to grow a leg or an arm one needs only a little FGF, but clearly a little goes a long way.

Ridge FGFs not only keep mesodermal cells proliferating, they also keep them alive. Many cells will, at the slightest provocation, commit suicide. They have a whole molecular machinery to assist them in doing away with themselves. Seen through a microscope, a cell suicide is spectacular. Over the course of an hour or so the doomed cell becomes opaque, then suddenly shrivels and disappears as it is consumed by surrounding cells. In the limb-bud, FGFs block the machinery of death; they give cells a reason to live. Yet while mass cell suicide is clearly a bad thing, at least some cell death is needed to form our fingers and toes, for if the ridge is the sculptor of the limb, cell death is the chisel. At day 37 after conception our extremities are as webbed as the feet of a duck. Over the next few days the cells in the webs die (as they do not in ducks) so that our digits may live free. Should a foetus have too much FGF signalling in its limbs, cells that should die don’t. Such a foetus, or rather the child it becomes, has fingers and toes bound together so that the hand or foot looks as if it is wearing a mitten made of skin.

When Saunders removed the apical ectodermal ridge from a young limb-bud, the result was total amputation. Yet if the bud was older and larger, then only the structures further down – wrists, digits – were lost. Why? Over the last fifty years, various answers have been given to this question. The latest, though surely not the last, turns on two quite new observations. The first of these is that the ridge FGFs only penetrate a short way, about two hundred microns (one fifth of a millimetre) into the mesoderm. In a young limb-bud, two hundred microns-worth of seceding cells cuts very deep as a proportion of total mass; in an older, larger limb-bud, much less so. This difference in proportion matters because limb-buds possess an invisible order. A limb-bud may look like an amorphous sack of cells, but even when newly formed, when it is no more than a bump on the foetal flank, its mesodermal cells have some foreknowledge of their fates. Some are already destined to become a humerus, others digits, yet others the parts between. As the limb-bud grows, each of these populations of cells proliferates and expands in turn. When a young limb-bud is deprived of FGFs, all of these variously fated cell populations suffer; when an older limb-bud is deprived only those closest to the tip do, and with them future hands and feet, toes and fingers.

This account of the making of our limbs contains within it the roots of twentieth-century medicine’s most infamous blunder. In 1961 an Australian physician, William McBride, reported a sudden surge in the numbers of infants born with deformed limbs. Similar findings were reported a few months later by a German named Lenz. Both physicians suggested that the defects were caused by a sedative used to prevent morning sickness that has the chemical name phtalimido-glutarimide, but which swiftly became notorious by its trade-name, thalidomide. More reports rolled in from around the world. By the time it was all over, more than ten thousand infants in forty-six countries with thalidomide-induced teratologies had been found. Only the United States escaped the epidemic because a few sceptical FDA officials had delayed authorisation of a drug that was, at the time, the third best-selling in Europe.

The thalidomide infants had a very particular kind of limb deformity. Unlike acheiropods, their limbs did not suggest amputations in the womb, for most had reasonably formed hands and feet as well as shoulderblades and pelvises; they were simply missing everything else in between. Without long bones, their arms and feet connected almost directly to their torsos. Their limbs had the appearance of flippers – a condition dubbed phocomelia or ‘seal-limb’.

Phocomelic infants have always appeared sporadically. In the sketchbooks of Goya (1746–1828), that compassionate connoisseur of deformity, there is a lovely sepia-wash portrait of a young mother proudly displaying her deformed child to two inquisitive old women. And there are, scattered throughout the early teratological literature, any number of people with the disorder. In his Tabulae (1844–49), Willem Vrolik gave a portrait of a phocomelic, a famous eighteenth-century Parisian juggler, Marc Cazotte, also known as ‘Le Petit Pepin’. Vrolik also shows Cazotte’s skeleton, which still hangs in the Musée Duputryen in Paris, though its legs, by sad irony, are now missing. These cases of phocomelia might have been caused by some chemical or other, but they may also have been due to mutations, several of which cause the disorder. But until the 1960s, phocomelics were rare, little more than anatomical curiosities. Thalidomide turned them into icons of medical hubris.

How does thalidomide have its devastating effects? A comprehensive bibliography on the chemical and its consequences would run to about five thousand technical papers, but for all that, thalidomide is still poorly understood. Some things are clear. It is a teratogen and not a mutagen: the children of thalidomide victims are at no greater risk of congenital disorders than any others. Instead thalidomide inhibits cell proliferation. Taken by a pregnant woman during the time when she is most susceptible to morning sickness (thirty-nine to forty-two days after conception), it circulates throughout the bodies of mother and child and stops cells from dividing. This is when the earliest populations of cells that will form each part of the infant’s future limbs are establishing themselves. Depending on the exact duration of the exposure, the precursors of one or more bones will fail to multiply; the result is a limb with missing parts. It is even thought that thalidomide may impede, quite directly, the fibroblast growth factors that are so essential to limb-bud development, but this remains speculation. Whatever its exact modus operandi, thalidomide is clearly a powerful drug and so a perennially attractive one. The taboo that surrounds it is breaking down as proposals for its use against a variety of diseases proliferate. In South America it is used to treat leprosy. Inevitably, infants with limb deformities are appearing once again as it is given to women who do not know that they have conceived.

GOING DIGITAL

Metric, with its base 10 units, exists only because the savants of the Académie Française who devised the system had ten fingers each on which they presumably learned to count. If pigs could do mathematics, they would probably measure their swill using a Système International devised from base 8, for they have only four digits per hoof. Horses have one digit per limb, camels have two, elephants have five, but guinea pigs have four on the fore-limbs and three behind. Cats and dogs have five on the forefeet and five on the hind feet, but one of those is small, and is called a ‘dew-claw’. Apart from some frogs and a kind of dolphin called a vaquita, most vertebrates never have more than five digits per limb.

Why this is so is deeply obscure. It is not as though extra digits are impossible to make. Mammals of all sorts sometimes show extra digits, but they are never common. St Bernards, Great Pyrenees, Newfoundlands and other large dogs are especially prone to having six digits on each foot – the duplication being an extra dew-claw. Ernest Hemingway’s cats were polydactylous, and their many-toed descendants still live in the grounds of his Key West house. Fifteen per cent of the feral cats of Boston are polydactylous (some have up to ten extra toes), but there are no feral polydactylous cats in New York. There are many polydactylous strains of mice: one is called Sasquatch in homage to Big Foot, but most have more prosaic names such as Doublefoot or Extra-Toes. The American geneticist Sewall Wright once produced a baby guinea pig with forty-four fingers and toes in all, but it did not live.

And many people are born with extra digits. About i in 3000 Europeans is born with extra fingers or toes (or both), and about 1 in 300 Africans. Any digit can be duplicated, but in Africans it usually a little finger (pinkie), while in Europeans it tends to be a thumb. Polydactyly is usually genetic, frequently dominant, and can run for many generations in families. Long before Gregor Mendel ever lived, the French mathematician Pierre-Louis Moreau de Maupertuis (1698–1758) described the inheritance of Polydactyly in the ancestors and descendants of a Berlin physician called Jacob Ruhe. Ruhe’s grandmother had six fingers on each hand and six toes on each foot, as did his mother, as did he and three of his seven siblings, and two of his five children. Others have claimed even more impressive polydactylous pedigrees. In 1931 the Russian geneticist E.O. Manoiloff published an account of a polydactylous Georgian, Via?eslav Michailovi? de Camio Scipion, who, he said, was able to document his descent from a lineage of polydactylous forebears reaching back six centuries.

If the apical ectodermal ridge ensures that our limbs grow out into space, another equally unobtrusive piece of limb-bud ensures that we have the right number and kinds of fingers. It was again John Saunders, along with a collaborator, Mary Gasseling, who discovered it. They found that if they transplanted a piece of mesoderm from the tailmost edge of one chicken limb-bud onto the headmost edge of another (so that the bud had two tailmost edges in opposite orientation to each other), the result was a chicken wing with twice the usual number of digits. Most remarkably of all, the experimental wings were like a particularly exotic variety of polydactyly in humans. They resembled people who, far from having just an extra digit or two, have hands and feet that are almost completely duplicated with up to ten digits each. The polydactylous wings had a peculiar mirror-image geometry, one shared by duplicated hands in humans. If each finger is given a code in which the thumb is 1, forefinger 2, index-finger 3, ring-finger 4, and pinkie 5, then a normal, five-fingered, hand has the formula ‘12345’, while a duplicated hand has the formula ‘5432112345’. It is that strangest of things, an anatomical palindrome.

Saunders and Gasseling called their potent piece of mesoderm the ‘zone of polarising activity’ or ‘ZPA’. It is thought to be the source of a morphogen. At its source, where it is most concentrated, this morphogen induces naive mesoderm to become the little finger; further away, lower concentrations induce the ring, index, and forefinger in succession, and at the far opposite end of the limb, you get a thumb.

This account of how most of us come by our five fingers brings to mind the organiser. Like the organiser, the ZPA has the uncanny ability to impose order on its surroundings. And, just as the organiser morphogen was so eagerly sought for so long, so too, in recent years, has been the morphogen of the ZPA. It is almost certainly a signalling protein, likely a familiar one, a member of one of the great families of signalling proteins that also work elsewhere in the embryo. But limb-buds contain a plethora of such proteins, and it is hard to know which of them is the morphogen itself. In the past few years, several candidate molecules have been said to fit the bill. One of them is sonic hedgehog.

Sonic hedgehog appears in the limb-bud precisely where one would expect a morphogen to be: only in the mesoderm of the tailmost edge, exactly coincident with Saunders and Gasseling’s ZPA. It also does what one would expect a morphogen to do: shape limbs. Chicken wings can be sculpted into new and improbable forms – including duplicate mirror-image polydactylous ones – simply by manipulating the presence of sonic in the bud. And then there are the mutants. Mutations in at least ten genes cause Polydactyly in humans and all seem to affect, in some way or other, sonic’s role in the limb.

But, as we saw in the previous chapter, sonic hedgehog does not just determine how many fingers and toes we have. It also divides our brains, decides how widely spaced our eyes will be, and regulates much else besides. It is an incorrigibly promiscuous molecule. Could we see the pattern of the sonic hedgehog gene’s activity over time, as in time-lapse photography, we would see it flashing on and off throughout the developing embryo and foetus, now in this incipient organ, now in that one.

The devices responsible for all this have a formidable task, and nowhere, given sonic’s power to direct the destiny of cells, do they have much room for error. These devices are transcription factors or ‘molecular switches’. Some of them keep sonic in check. Should they be disabled by mutation, sonic turns on in parts of the limb-bud that it otherwise would not – and the result is extra fingers and toes. Other mutations do not disable the transcription factors themselves, but rather delete the regulatory elements to which they bind. The result, however, is the same: a confusion of morphogen gradients and an embarrassment of digits.

Polydactyly mutations relax control of sonic hedgehog altering the balance of power in favour of ubiquity. But other mutations have exactly the opposite effect and prevent sonic from appearing in the limb-bud at all. The most blatant example of such a mutation is, of course, one that disables the sonic gene itself. Sonic-less mice have, in addition to their many other defects, no paws. This is strikingly reminiscent of a disorder that we have already come across: acheiropody, the disorder of the aleijadinhos. Indeed, there is some (disputed) evidence that the acheiropody mutation disables a regulatory element essential to sonic’s presence in the limb.

This catalogue of mutations only hints at the complexities of gene regulation in the embryo. Whether or not a gene is turned on in a given cell depends on what transcription factors are found in that cell’s nucleus, and their presence depends on the presence of yet other transcription factors, and so on. At first glance hierarchies of this sort seem to involve us in an infinite regression in which the burden of producing order is merely placed upon a previous set of entities which must, themselves, be ordered. But this dilemma is more apparent than real. The embryo’s order is created iteratively. Sonic’s precise presence in the ZPA is defined in part by the activity of Hox genes in the trunk mesoderm from which limbs grow. But the geometrical order that these genes give to the limb is crude; sonic’s task is to refine it further. Beyond sonic there are, of course, yet further levels of refinement in which order is created on ever smaller scales, and each of them requires subtle and interminable negotiations, the nature of which we scarcely understand.

This vision of successive layers of negotiation and control may seem unimaginably complex. But in truth it is not complex enough, for it fails to capture one of the most pervasive properties of the embryo’s programme: its non-linearities. I argued that the acheiropody mutation causes a failure of sonic to appear in the limb. And yet I began this chapter by arguing that infants with amputations in the womb, of whatever severity, were due to failures of the apical ectodermal ridge and the fibroblast growth factors they produce. This may seem like a contradiction, but it is only one if we think of the various limbs’ signals as being independent of each other, when in fact they are not. For one of the most vital roles of sonic hedgehog is to maintain and shape the apical ectodermal ridge and its fibroblast growth factors; and one of the most vital roles of the apical ectodermal ridge is to maintain and shape the production of sonic hedgehog in the zone of polarising activity. There is a reciprocal flow of information as precarious as the flow of batons between two jugglers standing at opposite ends of a stage. Reciprocity of this sort is ubiquitous in the embryo and it alters the way we think about its growth and development. We begin with notions of linear pathways of command and control and simple geometries – and then watch as they unravel. For when, as in the limb, we actually begin to see the outlines of the embryo’s programme, it invariably turns out to resemble a tangle of circuits that loop vertiginously across time and space. Circuits which, in this case, ensure that when we count our fingers and toes we usually come up with twenty.

HANDS, FEET AND ANCESTORS

Around day 32 after conception, when the human limb-bud is already well grown, its amorphous tissue begins to resolve into patterns. Ghostly precursors of bones appear: conglomerations of cells that have migrated together. The technical word for this process is ‘condensation’. It hints at the way in which bones just quietly appear, rather like dew.

The first condensations to form become the bones closest to the body: the humerus in the arm, the femur in the leg. With time, conglomeration sweeps slowly down the limb-bud. The humerus divides into two new long, thin condensations, each of which will bud off by itself: the radius and the ulna. These condensations, in turn, divide and bud to form an arc of cells from which the twenty-seven bones of the wrist and palm are made. By day 38 after conception, the end of each limb-bud has become flat and broad, rather like a paddle. The paddle then folds into parallel valleys – four on each tip – leaving five islands of condensed cells: the future bones of the fingers and toes.

The shapes of the condensations depend, ultimately, on the reference grid laid down by the signalling systems of the limb. But, as elsewhere in the embryo, this information must be translated into cellular action. Hox genes do this for the head-to-tail axis of the embryo, and they also do it for the limb. As the limb-bud grows, some of the thirty-nine Hox genes appear in intricate overlapping patterns. They seem to be engaged in some combinatorial business analogous to the vertebral Hox code. Infants born with a single defective copy of the Hoxa 13 gene have short big toes and bent little fingers. Another human Hox mutation causes synpolydactyly: extra fingers and toes fused together. A particularly devastating mutation that deletes no fewer than nine Hox genes in one go causes infants to be born with missing bones in the forearm, missing fingers and missing toes.

Limbs are not the only appendages in which Hox genes work. Infants born with Hox mutations that affect limbs tend to have malformed genitalia as well; in the worst cases male infants have just the vestiges of a scrotum and penis. Many of the molecules that make limbs also make genitals, and it should be no surprise that some mutations afflict both. The widely rumoured positive correlation between foot and penis size also, surprisingly, turns out to be at least partly true. No man should be judged by the size of his feet, however, for the correlation, though statistically significant, is weak. And then, such data as there are concern ‘stretched’ rather than erect penis length, surely the variable of interest. Still, when the French refer to the penis as le troisième jambe, pied de roi or petit-doigt; and the English to the best-leg-of-three, down-leg or middle-leg, not forgetting the optimistic yard which elsewhere means three feet, they speak truer than they know.

The Hox genes have also begun to tell us about origins. Where do fingers come from? It may seem that this question has a straightforward answer. Our limbs, flexible in so many dimensions, are the cognates of the structures that propel fish through the sea: their fins. But fish don’t have fingers. One might suppose that the rays, those fine, bony projections that spread a fin like a fan, are their piscine equivalents. But fish rays and tetrapod digits are made of quite different kinds of bone – reason enough, anatomists say, to conclude that they have nothing to do with each other.

Most fish are only distantly related to tetrapods, so perhaps their want of fingers is no surprise. But even our closest piscine relatives are not much help. These are the lobe-finned fishes, among them the Australian lungfish, which spends much of its time buried in desiccated mud-flats, and the coelacanth, which inhabits the deeps of the Indian Ocean. Today’s lobe-fins are often called ‘living fossils’, an allusion to the abundance of their relations four hundred million years ago and their scarcity now. Some fossil lobe-fins have fins that are strikingly like our own limbs; they seem to have cognates of a humerus, radius and ulna. They also have an abundance of smaller bones that look a bit like digits and that are made of the right kind of bone. But the geometry of these little bones is quite different to the stereotyped set of fingers and toes that is the birthright of all tetrapods. One can twist and turn a lung-fish’s fin as much as one pleases, but the rudiments of our hands and feet simply do not appear. The conclusion seems unavoidable: fish don’t have fingers, tetrapods do, and somewhere, around 370 million years ago, something new was made.

But how? Fish fin-buds are a lot like tetrapod limb-buds. They have apical ectodermal ridges, fibroblast growth factors, zones of polarising activity, sonic hedgehog, and panoplies of Hox genes that switch on and off in complicated ways as the bud pushes out into space. This tells us (what we already knew) that fins, legs and wings, so various in form and function, evolved from some Ür-appendage that stuck out from the side of some long-extinct Ür-fish.

We, however, are interested in the differences. One such difference lies in the details of the Hox genes. Early in the development of either a fin or a limb, Hoxd13 is switched on in the tailmost half, just around the zone of polarising activity. But as fins and limbs grow, differences begin to appear. In fish, the reign of Hoxd13 is brief; as the fin-buds grow it just gradually fades away. In mice, however, Hoxd13 stays on in an arc that stretches right across the outermost part of the limb. It seems to be doing something new, something that is not, and never has been, done in fish: Hoxd13 is specifying digits.

Such differences (which are true of other Hox genes as well) give Hox gene mutations their deeper meaning. If, in its last flourish of activity, Hoxd13 is specifying digits, one would expect that a mouse in which Hoxd13 has been deleted would be a mouse with no digits. It would be a mouse in which just one of the many layers of change that have accreted over the course of five-hundred-odd million years of evolution has been stripped away. Its paws would be atavistic: incrementally less tetrapod-like and incrementally more fish-like. As it turns out, however, Hoxd13-mutant mice, far from having a lack of digits, have a surplus of them. Their digits are small and crippled, but instead of the usual five, they also have a sixth.

This result is rather puzzling. It seems to suggest that something, somewhere, in our evolutionary history not only had fingers and toes, but had more of them than we, and nearly all living tetrapods, do. The idea that Polydactyly (be it in mice, guinea pigs, dogs, cats or humans) is an atavism is an old one. Darwin claimed as much in the first edition of his The variation of animals and plants under domestication (1868), a work in which he attempted to develop the theory of inheritance that evolution by natural selection so badly needed. ‘When the child resembles either grandparent more closely than its immediate parent,’ he wrote, ‘our attention is not much arrested, though in truth the fact is highly remarkable; but when the child resembles some remote ancestor or some distant member of a collateral line, – and in the last case we must attribute this to the descent of all members from a common progenitor, – we feel a just degree of astonishment.’

This is certainly true, but Darwin’s reasons for thinking that Polydactyly in humans is an atavism (or ‘reversion’ to use his terminology) are, to say the least, obscure. Salamanders, he noted, could regrow digits following amputation, and he had read somewhere that supernumerary fingers in humans could do the same thing even if normal ones could not. Extra digits were somehow, then, the product of a primitive regenerative ability, and hence atavisms.

It was a woolly argument, and it did not go unchallenged. The German anatomist Carl Gegenbauer pointed out that human fingers, supernumerary or otherwise, could not regenerate if amputated, and even if they could, so what? Polydactyly could not be an atavism without a polydactylous ancestor, and all known tetrapods, living or dead, had five fingers. In the next edition of The variation seven years later, Darwin, ever reasonable, admitted that he’d been wrong: polydactylous fingers weren’t atavisms; they were just monstrous.

But Darwin may have been right after all – albeit for the wrong reasons. In the last ten years or so, the ancestry of the tetrapods has undergone a radical revision. New fossils have come out of the rocks, and strange things are being seen. Contrary to all expectations, humans – and all living tetrapods – do have polydactylous ancestors. The earliest unambiguous tetrapods in the fossil record are a trio of Devonian swamp-beasts that lived about 360 million years ago: Acanthostega, Turlepreton and Ichthyostega. All of them are, by modern tetrapod standards, weirdly polydactylous: Acanthostega has eight digits on each paw, Turlepreton and Ichthyostega have either six or seven. Suddenly it seems quite possible that Hoxd13-mutant mice, and mutant polydactylous mammals of all sorts, are indeed remembrances of times past – only the memory is of an early amphibian and not a fish.

Perhaps more genetic fiddling is required to get back to a fish fin; more layers have to be removed. This seems to be so. Mice that are mutant for Hoxd13 may be polydactylous, but mice that are mutant for Hoxd13 as well as other Hox genes – that is, are doubly or even trebly mutant – have no digits at all. It may be that as developmental geneticists strip successive Hox genes from the genomes of their mice, they are reversing history in the laboratory; they are plumbing a five-hundred-million-year odyssey that reaches from fish with no fingers to Devonian amphibians with a surplus of them, and that ends, finally, with our familiar five.