Darwin's Island Part 6

Sir Robert Moray was a spy for Cardinal Richelieu, a Freemason, a member of the Scottish army that took Newcastle from the English in 1640 and, in his spare time, the first President of the Royal Society. He wrote at length on the natural history of his native land and made a remarkable discovery, published in the Society's Philosophical Transactions Philosophical Transactions in 1677. On a log on the sh.o.r.es of the island of Uist, he saw 'mult.i.tudes of little sh.e.l.ls; having within them little birds perfectly shaped, supposed to be barnacles . . . This bird . . . I found so curiously and completely formed, that there appears nothing wanting, as to the external parts, for making up a perfect Sea-Fowl; . . . the little bill like that of a goose, the eyes marked, the head, neck, breast, wings, tail and feet formed like those of other water fowl, to my best remembrance. ' Sir Robert had the honesty to admit that he had never observed any of the adult animals but a.s.sured his readers that 'some credible persons have a.s.sured me that they have seen some as big as a fist'. in 1677. On a log on the sh.o.r.es of the island of Uist, he saw 'mult.i.tudes of little sh.e.l.ls; having within them little birds perfectly shaped, supposed to be barnacles . . . This bird . . . I found so curiously and completely formed, that there appears nothing wanting, as to the external parts, for making up a perfect Sea-Fowl; . . . the little bill like that of a goose, the eyes marked, the head, neck, breast, wings, tail and feet formed like those of other water fowl, to my best remembrance. ' Sir Robert had the honesty to admit that he had never observed any of the adult animals but a.s.sured his readers that 'some credible persons have a.s.sured me that they have seen some as big as a fist'.

The myth of the sh.e.l.l-born birds, barnacle geese as we call them today, the sh.e.l.ls themselves supposed to be the seeds of a certain tree, was already widespread. So embedded was the notion that for a time the barnacle goose was counted as a fish and could be eaten by Catholics on Fridays (Thomas Henry Huxley suggested that the mistake came about because such birds were common in Hibernia, or Ireland, and that the shift from Hiberniculae Hiberniculae to to Barnaculae Barnaculae, the term then used for barnacles, was easy enough).

The idea of a bird-bearing tree is foolish, but it arises from an ancient and accurate observation - that the adult form of many creatures is quite distinct from that of their eggs or embryos. The untrained eye finds it hard to tell juveniles apart. A month-old human foetus is almost identical to that of a chimpanzee, the inside of a goose egg looks much like that of an ostrich and a barnacle larva is not very different from those of its relatives among the lobsters and crabs. Even the founder of modern embryology, Karl von Baer, found it difficult. In 1828, he wrote that 'I have two embryos preserved in alcohol that I forgot to label. At present I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals.'

The word 'evolution' (which does not appear in The Origin of Species The Origin of Species) was first applied to the unfolding of the body as egg is transformed into adult. Development is the imposition of pattern upon a formless ma.s.s. Most animals, from barnacles to geese, share the same basic types of cells. As the embryo grows they are organised to make a crab, a goose or an ostrich, a man or a bat. That grand reshuffling builds new and complicated body shapes from the same raw material. As it does it hides the logic upon which bodies are built. Adult anatomy makes much more sense when seen through the eyes of the embryo and The Origin The Origin itself used the similarity of the juvenile stages of apparently unrelated beings to argue that 'community of embryonic structure reveals community of descent'. itself used the similarity of the juvenile stages of apparently unrelated beings to argue that 'community of embryonic structure reveals community of descent'.

Its author saw that many creatures showed 'unity of type', a deep similarity manifest in the young but largely hidden by the complexity of the adult form. Many embryos - those of barnacles included - consisted of repeated segments that are multiplied, reduced or rearranged to produce an adult. An increase or decrease in number or a shift in pattern of growth can generate a vast diversity of size and shape. Evolution, Darwin realised, works as much by the manipulation of repeated units as by tinkering with the details of individual organs as they grow.

The idea finds new life in modern biology, which reveals affinities among the embryos of even distant creatures. DNA, like the bodies it builds, is itself based on a series of variations on a structural theme. As egg becomes adult, complex organs - eyes, ears, hands and brains - are pieced together from elements that can clearly be distinguished only in the embryo.

Nowhere is the contrast between young and old more remarkable than among the barnacles. Once, such creatures were said to be snails because of their solid sh.e.l.ls (and a well-known professor of zoology - or of biochemistry masquerading under that t.i.tle - once tried to convince me that they do belong to that family). In fact, they are jointed-limb animals not unlike crabs, spiders or flies. Their ancestors lived free in the oceans but now many spend most of their lives in a prison cell. Barnacles are close kin not to limpets as once imagined, but to shrimps and lobsters. That affinity was discovered in the 1820s by an army surgeon based in Ireland but for many years the group - the cirripedes or 'curly-footed' to give them their technical name - seemed no more than obscure. Few biologists could be bothered with such tedious creatures.

Until, that is, Charles Darwin spent a sixth of his scientific career on them. His eight years of research, in the interval between the Beagle Beagle voyage and voyage and The Origin of Species The Origin of Species, showed that animals, dull as they might appear, had lessons not just for naturalists but for biology as a whole. As he came slowly to the idea that life was not fixed but might change, he was warned that 'no one has the right to examine the question of species who has not minutely described many'. Perfectionist as ever, he agreed: he realised that to understand the logic of life he needed to become an expert on a single group. Today's biologists are obsessed with 'model organisms' - fruit flies, a certain worm, mice, mustard plants and even humans - that might, when their secrets are unveiled, be exemplars of evolution on a wider stage. Cash pours in, and optimists hope that to understand their favourites in detail will illuminate the science of life. Some of the supposed archetypes turn out, alas, to be quite untypical even of the group to which they belong (and the fruit fly itself falls into that category). The first model organisms of all were Darwin's barnacles. He made - by luck or judgement - an excellent choice.

He wrote four books - well over a thousand pages - on their taxonomy, their embryos and their fossils. Some species were bizarre; so distinct from the familiar rock-dwellers of Welsh or Scottish sh.o.r.es that any kinship appeared almost as improbable as did an affinity to geese. The young naturalist did his job so well that, at the age of forty-four, he was given the Royal Society's Gold Medal for his work. His solid volumes remain a standard reference work today. More important, they laid the foundation of a central theme of evolution: that the embryo is the key to the adult.

Darwin's attention was drawn to cirripedes when, as a medical student in Edinburgh, he spent weeks in the hunt for marine animals in the Firth of Forth. There he fell under the influence of the zoologist Robert Grant, who introduced him to life on the sea sh.o.r.e and encouraged him to publish his first scientific paper (Grant later became Professor of Comparative Anatomy at University College London, but the two fell out over the issue of whether animals showed inevitable progress from low to high and almost never spoke again even when they worked in the same street). Almost a decade after his studies on the chilly sh.o.r.es of the Forth, the Beagle Beagle's naturalist found on the sh.o.r.es of the Chonos archipelago off the coast of Chile an enigmatic soft-bodied creature 2.5 millimetres long drilling into a conch sh.e.l.l. At first he thought it was a worm, but under the lens it became clear that the creature was a great anomaly, for naked as it might be, it looked very like a British barnacle. Could the animal, in spite of its lack of a sh.e.l.l, be related to the denizens of a Scottish sh.o.r.e? If so, how - and why was the creature so different?

Darwin tried to find out. He planned at first just to sort out the Chilean creature's place in nature, but as the work went on, he found more and more distinct and - on the face of it - aberrant kinds. Soon he began to notice what appeared to be intermediate forms between them and series that showed greater or lesser affinity to each other. Oppressed as he was by the tedium of the task ('I may as well do it, as any one else'), barnacles sharpened in his mind the idea - already implanted, as his notebooks show - that one species might change into another. Perhaps, he became convinced, all barnacles - all animals - descended from a common ancestor that could be tracked further and further into the past. Five years after his cirripede opus, that radical notion became the theme of The Origin of Species The Origin of Species.

The juvenile stages revealed unexpected connections between the South American borer and its Scottish kin. That lesson, learned on the sh.o.r.es of Chile, has grown into the science of evolutionary developmental biology, which unites barnacles from across the world with each other, with crabs and lobsters and even with geese. It reveals the common foundations upon which all animals are built.

In the first days of development, many creatures resemble one another more than they do when they become adults for each shares a series of genes that lay down the basic body plan, from head to tail. Such genes are control switches in the journey from fertilisation to the grave. They shepherd the egg towards adulthood. Errors lead to dramatic shifts in form - eyes transformed to legs in fruit flies, lambs with two heads or extra fingers in human babies - together with more persistent changes such as those that made birds from dinosaurs or barnacles from the ancestors of crabs.

Darwin was sent specimens from across the globe. Some would, he realised, stretch the belief of his fellows and he wrote to a colleague about his discoveries that 'You will think me a Baron Munchausen among naturalists.' His first job was to describe what the animals looked like. As ever, he told a simple story in plain prose.

His introductory paragraph is a sober account of what most people imagine such creatures to be: 'Almost every one who has walked over a rocky sh.o.r.e knows that a barnacle or acorn-sh.e.l.l is an irregular cone, formed generally of six compartments, with an orifice at the top, closed by a neatly-fitted, moveable lid, or operculum. Within this sh.e.l.l the animal's body is lodged; and through a slit in the lid, it has the power of protruding six pairs of articulated cirri or legs, and of securing by their means any prey brought by the waters within their reach. The basis is firmly cemented to the surface of attachment.'

That statement introduced the immense variety of cirripede lives. More than twelve hundred different kinds are known and no doubt many more remain to be discovered. All live in salt water. They fall into two main groups, those with a stalk (the goose barnacles, named in homage to the ideas of Sir Robert Moray, and a delicacy in many parts of the world) and those without, many of them, like the familiar acorn barnacle, attached to rocks and other marine structures. All, or almost all, have jointed legs, often tucked away within a sh.e.l.l. Many use them as a net to sweep the seas, while the stalked versions depend more on the movements of the water to bring food. Unlike their relatives the crabs and lobsters, barnacles do not moult their skeletons to grow. Instead their plates increase in size as the animal gets older. Some species sit on rocks, while other kinds burrow through solid stone or into snail sh.e.l.ls or spend most of their time afloat. Yet more are parasites of crabs, jellyfish and starfish. Some among that group are so specialised that, when adult, they look more like a fungus than an animal.

Like insects, barnacles have a head and thorax and, in a few species, what might be the remains of an abdomen. Like them, they have six pairs of jointed legs, fewer than the prawns and lobsters, who have ten. Each leg is covered with hairs and together they lash the sea. The familiar sh.o.r.e versions spend their lives upside down for they stand on their heads and wave their feet in the water.

Those found on rocky sh.o.r.es live in a fortress made of around six tough plates, based, like a snail sh.e.l.l, on a limestone-like substance. Different varieties have more or fewer segments of body armour and many pages of Darwin's four books on the creatures are devoted to the minutiae of how their plates might sort out their patterns of relationship. For the common British form, an additional two plates act as a lid, which opens to let out the legs at high tide and closes to keep in water when the creatures are exposed to the air (which for some individuals means all the time except for a few days each month at spring tide). The mouth has structures that chew and grind and look a little like those of crabs and even of c.o.c.kroaches. Some species excrete through their mouths as their a.n.u.s has faded away. Tucked away in the dark, the adult barnacles lose their eyes. The nervous system, too, is reduced when compared with that of their free-living relatives.

Dull as a cloistered existence within a gloomy fortress might be, all barnacles have a remarkable s.e.x life. Like all good biologists, Darwin spent a lot of time on that topic. He found a wild diversity of reproductive habit. The textbooks of his day said that all known species were hermaphrodites but many, he found, were not. Some have two s.e.xes, some are male when young and female later, and some are true hermaphrodites - while a few among that group secrete small males around their bis.e.xual persons in case they might be useful. Many of those with two s.e.xes spend their adult lives fixed to a single spot. As a result every male must constantly wave his p.e.n.i.s, erected at the cost of lots of hydraulic energy, to reach out and tap his neighbours in the hope that at least one might be a female. Those who find themselves in a spa.r.s.e and scattered group must, if they are to succeed, grow a longer organ than those who live in a crowd. A female, once tapped, may copulate with half a dozen males in series and then pump out most of their seminal fluid as not up to scratch. Her fertilised eggs soon develop into the first of several larval stages.

The young biologist's studies on cirripede s.e.x brought forth some poetic paragraphs. The male organ of a certain species was 'wonderfully developed . . . it must equal between eight and nine times the entire length of the animal! . . . there [it] lies coiled up, like a great worm . . . there is no mouth, no stomach, no thorax, no abdomen, and no appendages or limbs of any kind'. In another the males were reduced to parasites within the female: 'thus fixed & half embedded in the flesh of their wives they pa.s.s their whole lives & can never move again'. The creatures hold the record for relative p.e.n.i.s size, while a certain beetle comes next, at twice its body length. The organ comes at a price, for individuals from wave-battered sh.o.r.es have shorter and stouter members than do those from calmer places as the male finds it hard to control a lengthy structure in a turbulent world. So expensive are such ma.s.sive genitals that many males lose them at the end of each season and grow a new set the following year.

Barnacles are remarkable for reasons that stretch beyond the p.e.n.i.s. They stick to the rock with a sophisticated cement, a protein that repels water. The stuff is the toughest known natural glue. Like an epoxy adhesive, the material is secreted as a clear fluid with two components. When they mix, cross-links are made between the molecules and its manufacturer becomes almost impossible to dislodge. So powerful is the bond that some of the substances involved may soon be used in surgery.

The creatures cling to ships just as avidly as they do to rocks. Charles Darwin, exhausted by his years of work on them, once wrote that 'I hate a Barnacle as no man ever did before, not even a sailor in a slow-sailing ship', and mariners had good reason to despise the animals. The Beagle Beagle herself had to have her bottom cleaned several times in the cruise around South America. A ship uses 40 per cent more fuel when covered with marine organisms than when its surface is smooth - which is expensive and, in these days of the greenhouse effect, also to be deplored on ecological grounds. Poisonous paints were once used to keep the bottom clean, but as many caused sea snails to change s.e.x most of them have been banned. The best protection is to find a finish to which the animals cannot attach. Given that they can adhere to a non-stick saucepan, the job is not simple, although paints with added carbon nanotubes offer some hope. Some corals and seaweeds manage to stay free of such creatures not with poisons but with chemicals that scare them off. Those unknown substances will make the fortune of the first scientist to extract them. herself had to have her bottom cleaned several times in the cruise around South America. A ship uses 40 per cent more fuel when covered with marine organisms than when its surface is smooth - which is expensive and, in these days of the greenhouse effect, also to be deplored on ecological grounds. Poisonous paints were once used to keep the bottom clean, but as many caused sea snails to change s.e.x most of them have been banned. The best protection is to find a finish to which the animals cannot attach. Given that they can adhere to a non-stick saucepan, the job is not simple, although paints with added carbon nanotubes offer some hope. Some corals and seaweeds manage to stay free of such creatures not with poisons but with chemicals that scare them off. Those unknown substances will make the fortune of the first scientist to extract them.

Barnacles have been pa.s.sengers since long before ships sailed the oceans. Many creatures suffer their attentions. Humpback and grey whales bear large white patches of thousands, some of the individuals several centimetres across. The larvae pick up the scent of their host as they float through the sea and move towards it. Then they dig into the skin - and the huge beasts pay a price in energy as they drag their hangers-on through the sea. The whale retaliates with skin grown at a rate three hundred times that of our own in an attempt to slough off its pa.s.sengers. Some marine mammals secrete enzymes that dissolve the glue and help keep their foes at bay, while grey whales come in to land to try to sc.r.a.pe the hitch-hikers off. Dolphins move fast and are safe from such visitors, who are washed off before they can fix on, but big sharks, who idle through the water, are also free of the pests. Shark skin is covered with tiny ridges - and a film has been developed that mimics its structure. It may find a place in the world of commerce.

Other species of barnacle hitch lifts on the gills of fish, or live around the deep-sea vents that belch out hot rich water that nourishes a thick soup, just right for a filter-feeder. Not all cirripedes have a settled way of life, for some float blithely through the seas and never touch a solid object while yet others bore into coral reefs.

The most aberrant kinds take up a sinister profession. From a whale's or a matelot's point of view, a barnacle is an irritant, but little more. For crabs faced with a marauding cirripede, the situation is far worse. A certain group live as parasites within their living bodies. Their macabre habits give an insight into the spectacular diversity of form that evolution can come up with when it generates variations upon a body plan.

First, a female larva lands on its victim and finds a soft spot in the creature's armour. Then she stabs it with a hollow needle and fires a few of her own cells through. She dies at once. The baleful blob finds its way to the lower part of the crab's body and sends out fine tendrils that run through the host's entire anatomy. They grow to make a mesh that looks more like a mould than a marine animal, and suck in food. The crab stays healthy and continues to eat as fast as it can to feed the visitor. When the time is ripe, the parasite opens up a small hole to the outside and awaits the arrival of a mate - a male larva. Should a male appear it inserts its spiny self through the hole and seals it up to prevent the entry of a rival. Now the crab is in real trouble. The male parasite fertilises his partner and she begins to pump out thousands of larvae. The victim's whole economy is hijacked and it can no longer grow, shed its skin or even replace a damaged part. Instead, sometimes for years, it devotes its energies to its inner barnacle.

Soon the crab, male or female, is spayed by the unwelcome visitor. A castrated male crab starts to look, and behave, just like a female. Both s.e.xes now act as mothers - but mothers who care for another's interests. They develop a pouch on the underside that resembles that made by a healthy female just before she releases her offspring and spreads them through the water with sweeps of her claws. The unfortunate crabs again wave a ma.s.s of newborns on their way, but now they are not their own progeny but barnacle larvae ready for the next target.

The diversity of cirripede lives confused Victorian biologists, who could see no logic in their variety of shape and habit. Large parts of Darwin's work turned, in the traditions of the time, on an attempt to understand how the various species are related to each other and to find where the group as a whole fits into the animal world. That pastime - taxonomy, as the science is called - was once little more than stamp-collecting, but for him it became the raw material for a deep insight into biology. His cla.s.sification was based in part upon the solid plates that surround most settled barnacles and he persuaded himself that he could see a hint of order that reflected their ancient ties (even if he stayed confused by his Chilean burrower and by the parasites). The scheme has been much modified and a system based on the pattern of adult plates remains ambiguous.

The new philately - molecular genetics - studies shared descent in DNA itself, rather than in what it makes. The confident a.s.sumption that mutations acc.u.mulate at a regular rate to give steady divergence generates a family tree of the group's evolution. Well-dated fossils can, in principle, be used to measure how fast the changes happen, to give a molecular clock. The tree suggests that stalked forms came first and the others followed on. The double helix also hints that the protective plates emerged after a jointed-legged animal had taken the decision to settle down and wait for food to arrive rather than going out to hunt for it. Several of the cla.s.sical groups, such as the naked barnacles that so confused Darwin, appear to be a mixed bunch with distinct origins, and even the rock-dwellers are an a.s.sorted lot. The deep-sea vent types and the parasites are each, in contrast, a group of true relatives. The genes also confirm his view that the cirripedes as a whole fall into the larger family of crabs and lobsters - the Crustacea - and into the wider clan of insects, spiders and other jointed-legged animals. Some claim, on the basis of their shared molecules, that insects themselves are no more than a specialised group of crustaceans that reached the land. If so, they reveal an unexpected unity between barnacles and b.u.t.terflies.

Whatever the details of their family connections, the diversity of cirripede life began long ago. Two of the great evolutionist's books deal with their fossils. They are not, perhaps, the most riveting of his works but they make, nevertheless, a forceful case that today's kinds descend from forms now long extinct. Darwin referred to modern times as the 'Age of Barnacles', and at least in terms of the number of species known he was right, for their fossils are not abundant and can be hard to identify because the plates fall apart after death. Cirripedes do not appear in the rocks in any numbers until the demise of the dinosaurs, sixty-five million years before the present. A few spots do reveal good evidence of their pa.s.sage. The Red Crag deposits of East Anglia were laid down in the cool Ess.e.x seas of two million years ago. The rust-coloured rocks are still full of their protective plates, mixed in with snail sh.e.l.ls and the teeth of the largest sharks ever to have lived. Further from home, impressive strata in the south of Spain record, in the mix of their cirripede species, the rise and fall of a vanished sea. Their petrified memorials also show that whale barnacles have been around for at least two million years, for a bed in Ecuador is filled with their remains as a hint that the whales once bred there, as they still do, just off the nation's coast. A 164,000-year-old whale barnacle specimen from a human settlement in an African cave shows that our ancestors have long eaten those huge marine mammals. They sc.r.a.ped off the external parasites and may have cooked them.

No more than a few very ancient specimens have been found. A fossil from three hundred million years ago looks rather like a modern barnacle. Another well-preserved remnant, found in Herefordshire, from a hundred million years earlier - about the time of the first land animals - resembles the larva of a modern cirripede and hints that the group was well into its stride by then. The low-growing forms found on rocks, abundant as they now are, emerged far later, perhaps no more than a hundred and forty million years before the present, when Archaeopteryx Archaeopteryx walked the Earth. walked the Earth.

Anatomy, genes and fossils each place the barnacles in close a.s.sociation with crabs and lobsters and in less intimate kinship with insects, spiders and more. That larger group makes its presence obvious early in the record, in the Cambrian, more than half a billion years before the present, the era in which life first left abundant evidence of its pa.s.sing. Some of the mysterious creatures with bizarre body plans found just before that time and once claimed to represent a unique and vanished fauna may in fact have been crustaceans. A molecular clock of the whole group puts the origin of the barnacle lineage well back into the Cambrian, or perhaps earlier, even if no earlier remains have yet been found. If the clock can be trusted, the first cirripedes may have emerged as part of the vast outburst of diversity among jointed-legged animals from lobsters to insects, which began then and is still evident today.

What sparked off the barnacle big bang? Why did they, like their crab and insect brethren, evolve into such diversity of form? And why did vertebrates, the group to which we and the barnacle goose belong, do the same many millions of years later? Backboned animals are less diverse in their body form than are cirripedes, but they include creatures as different as mackerel, toads, pythons and vultures. Why was their evolution, like that of barnacles, so radical while groups such as sponges or flatworms remained, in comparison, tediously conservative? The answer began to emerge from Darwin's labours over the Down House microscope.

Its owner was the first to identify a barnacle larva, from his strange sh.e.l.l-borer from Chile. As he dissected more and more species and examined their juvenile forms a great truth began to dawn: that the creatures were far more distinct from each other as adults than they were in their early stages. From Scottish rock-dweller to naked Chilean and from tasty marine snack to the sinister enemy of crabs, the juvenile forms of the various species were very similar. Even better, they looked quite like the equivalent phases in crabs and lobsters. Darwin's excitement at this discovery is manifest: he writes of a larva 'with six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae'. He knew that he had hit upon a crucial piece of evidence for evolution (although his children laughed because the sentence read like a newspaper advertis.e.m.e.nt by a cirripede manufacturer).

Most barnacles release thousands of tiny fertilised eggs into the sea. Each goes through a series of stages, in most cases as a form that floats free in the plankton. The first has jointed limbs attached to a soft and flattened body. The young animal has an eye spot, sensitive even to dim light, that allows it to choose the level at which it floats. Soon it develops jaws and antennae and starts to feed. It goes through several moults and in time becomes a strong-swimming form with a tough outer coat. Those mature larvae prefer to stay near the surface, do not eat and can be carried far from where they were born. They must find a place to settle down, or - as almost all do - they will die. Some stumble upon a rock, or a whale, or a crab, and glue themselves on with their antennae. The rock- or whale-dwelling species put out a chemical message - a protein hormone - that invites others to join the colony. For them, every visitor is welcome, for a male must land within p.e.n.i.s-length of a female if he is to have a chance to pa.s.s on his genes and the more there are the better.

Much as the first stages of many species might resemble each other as they float through the seas, some - like those that amused the Down House children - do have aberrant juveniles, adapted to their own special way of life. Those of the burrowers cannot swim but scuttle about on the bottom using their antennae as feet. Crab parasites have abandoned the first few stages altogether and hatch as jawed and hungry forms that search for new victims at once. Natural selection is at work on the larval stages, which have to adapt themselves to nature's challenges just as grown-ups do. Even so, the young reveal far more about the group's internal affinities than do the much-modified adults. They show how cirripedes and their relatives are based on a theme with variations.

The same is true of the embryo on a wider stage. That of a barnacle goose is almost identical to the contents of a vulture egg and an embryonic human looks rather like that of a mouse or, indeed, if looked at early enough, of a goose. What emerges into the world is quite distinct from what can be seen as development begins. Now we understand why.

Adult cirripedes apart from the crab parasites are - like lobsters and insects - arranged in obvious sections, with a head and a thorax divided into six segments, but they lack an abdomen, found in almost all their relatives. We do not often think of ourselves as segmented creatures, but the vertebrate body is, like that of a barnacle or a lobster, also based on a series of distinct units, arranged from front to back. The human head, thorax and abdomen are obvious enough but our muscles, or our brain-case, show little sign of order. A glance at the embryo, however, reveals that men and women, like their submarine relatives, are constructed from a series of modules, neatly arranged in early life but shuffled around and modified as growth proceeds.

The remains of our watery past as primitive fish, together with the juvenile forms of our relatives among fish, snakes and birds say more. They show how the building blocks have multiplied and rearranged themselves to make the complicated creatures of today.

Just three of the thirty or so major divisions of the animal world are organised in obvious segments; they include the worms, the jointed-legged creatures such as insects, spiders, lobsters and barnacles, and the animals with backbones. For all of them a subdivided way of life has been an evolutionary triumph.

Segmented beings make their first appearance at - or even before - the first signs of the fossil record. They played a large part in the Cambrian explosion of diversity. Fossils from that time show how the addition of new pieces to a simple body, like beads on a string, can spark off a burst of change. Many of its strange animals were worm-like beasts, or had jointed legs and external skeletons. In time they added more and more sections. As they did, they evolved into a wild diversity of form. One ancient marine group, the trilobites (now extinct), started off with around eight segments. In time, some kinds ended up with a hundred and others with three. That process then, for some reason, reversed itself and at the peak of their success most trilobites had at most thirty-five separate elements.

As Darwin noticed, barnacles and their relatives have been through the same process of increase, decrease and divergence. He persuaded himself that the archetypal crustacean, the ancestor of both cirripedes and lobsters, was based on twenty-one parts, divided among head, middle and abdomen. Many modern species have six elements in the head, six in the thorax (the middle part of the body) and five in the last, abdominal, section. Some have multiplied and modified particular elements while others have done the opposite. Lobsters, for example, have many more paired and jointed appendages - legs and swimmerets plus others used to mate or to help brood the young - than do crabs, while the barnacles themselves lack the whole rear segment of the body. They are the Manx cats of the crustacean world and, for that matter, are an excellent a.n.a.logue of the first birds, which were dinosaurs who shook off their tails.

Goethe - philosopher, scientist and author of Faust - Faust - had, well before the had, well before the Beagle Beagle voyage, noticed hints of pattern within the bodies of fish, birds and mammals. He came up with a universal theory of anatomy, based on the notion that vertebrae - the individual sections of the backbone - were units from which many of our various parts were derived. The leaf, he imagined, had the same role in plants. Goethe saw life as emerging from a sort of biological Proteus; a simple component that could be multiplied and modified into a diversity of structures, the skull most of all. He was wrong in the details, but his idea contains an element of truth. voyage, noticed hints of pattern within the bodies of fish, birds and mammals. He came up with a universal theory of anatomy, based on the notion that vertebrae - the individual sections of the backbone - were units from which many of our various parts were derived. The leaf, he imagined, had the same role in plants. Goethe saw life as emerging from a sort of biological Proteus; a simple component that could be multiplied and modified into a diversity of structures, the skull most of all. He was wrong in the details, but his idea contains an element of truth.

Although the simplistic claim, never made by Darwin, that animals relive their ancient history as they develop from the egg is wrong, the embryo is a reminder of where we came from. The shift from fertilised egg - a formless ball of protoplasm - to man or woman looks complex but is in its basics simple. As in origami, a limited set of instructions persuades pattern to emerge from simplicity. As the embryo folds itself into being, its past unfolds before our eyes.

Hints of order soon appear. A fertilised egg divides to form a ball of cells, which in time turns itself inside out and becomes attached to the wall of the uterus. It lengthens, and a ridge - which soon becomes a tube, the precursor of the spinal cord and brain - forms along the upper surface. The ma.s.ses of tissue on either side then begin to break up into a series of evenly s.p.a.ced blocks called somites. Those near the front appear first, and tissue stains show that ordered structures arranged from front to back are present long before the somites themselves become visible.

The somites in their rows look simple, but they give rise to complex structures, some of which have no obvious hint of regularity; to vertebrae (which would have pleased Goethe), to ribs, to muscles of the back and the limbs, to skin and tendons and even to certain blood vessels. The organised nature of vertebrae is obvious enough, but to the untutored eye the muscles of the leg or the skin on the back give no hint of segmentation. Even so, they - like many other organs - began as blocks of tissue.

As development goes on, the front half of one somite fuses with the back of the somite ahead of it to form the precursors of vertebrae - the repeated units of the spine, the structure shared by fish, frogs, snakes, birds and humans. They surround the spinal cord with a protective and flexible sheath that solidifies as bone is formed. The process is controlled by special growth factors, which sometimes go wrong. That has an echo of Goethe, for after a failed attempt by the East Germans in the 1960s to conserve his corpse his body was stripped of flesh - and it was revealed that the great poet suffered from a debilitating fusion of several spinal bones.

How can a uniform embryonic tissue break up into segments and then into distinct organs? In 1891, William Bateson - later the rediscoverer of Gregor Mendel's work - came up with a 'vibratory theory of the repet.i.tion of parts': the notion that a flow of chemicals did the job. Just as waves on the sea create ripples on the sand, their equivalents in the body stamp order on to disorder. A century and more later, he was proved right.

As the embryo develops, chemical signals that promote growth diffuse from its rear end towards the front. They are matched by a second molecular message that travels in the opposite direction and tells the tissue to mature and stop dividing. Each potential somite has an internal timer that instructs genes to work for the appropriate time and then to switch off. When the signal arrives, the clock starts. The somites each contain a hundred or more genes that cycle in and out of phase with each other, many with opposed effects on cell division, growth and movement. Together they build the block of tissue - and the genes that do the job are similar in mice, chickens and barnacles, proof that the basic rules of segmentation began before they last shared an ancestor, long ago.

Vertebrae still retain strong hints of their segmented history. Their numbers vary from species to species. Most people have thirty-three of the bones (with several fused together), geese have more (particularly in their necks), but snakes may have over five hundred. The vast increase among the serpents arises because the clock within each of their somites ticks several times faster than does our own. As a result, the ma.s.s of tissue is converted into many more segments in the time available - and the animal gains its long and flexible backbone. Perhaps the same is true in the goose's neck.

Each human vertebra has a personality of its own. Some are reduced to form a vestigial tail and others fuse to make a solid block at the lower end. Those in the upper back grow large spines to which muscles are attached while the seven vertebrae in the neck are specialised to allow the head to move from side to side or up and down. Whatever its task, every vertebra has, as a reminder of its shared embryonic experience, a strong resemblance to its neighbours.

The skull, or so it seems, is different. Its twenty-two bones show no obvious signs of segmentation and, apart from the lower jaw, all are fused together. The cranium is a round case with many openings and a variety of special structures such as the eye-socket, the teeth, the jaws and the ear. It appears at first sight to have little in common with the backbone upon which it perches. Now, science has shown that - as Goethe had hoped - it does.

Once again, the embryo is the key. The skull is in part built from somites (with most of the rest formed from bone laid down by precursors of other tissues). The genes prove that parts of at least the first two somites contribute to the skull. As further evidence, mouse mutations that damage the somite signalling machinery also affect the cranium. The skull, complicated as it might be, began as just another block in the body's support axis.

Its anatomy, its fossils and its genes say a lot about the way in which segmentation can make complex structures from simple precursors. The organs of sense and of thought that live within the skull have long been used by anti-evolutionists to cast doubt on Darwinism. In fact, every part of the skull puts paid to the 'argument from design', the ancient and threadbare claim that complex organs must need a designer. Darwin himself quoted the eye as evidence against that notion. The ear makes the case even better and has the additional advantage that fossils can join the embryos to show how evolution has cobbled together solutions from whatever is available. If a designer did the same, he would lose his job.

The human ear has an outer, middle and inner section. Together they pick up vibrations from the outside world. The outer ear receives the sound waves, the middle amplifies them with the help of physical movements of a set of bony levers while the inner ear transforms that mechanical energy into pulses of liquid and, in the final stage, into electrical and chemical impulses that pa.s.s to the brain. The inner ear also gives its owner a sense of physical position and of acceleration or deceleration.

The organ in its intricacy is witness to the power of variation on a theme and to the joys of improvisation. Genes, embryos and fossils combine to show that it evolved from the skeletons of ancient fish - and that the human ear shares some of its components even with the sense organs of barnacles.

All land vertebrates have some form of ear. The outer part of the organ, the pinna - that elegant appendage on each side of the head - is rather new, for frogs, reptiles and birds do not bother with it. It is made from the same cartilage and skin as much of the rest of the body surface. Darwin noted that in humans and apes, unlike dogs, it could not move, perhaps because as large animals able to climb trees they no longer needed eternal vigilance. He was told by 'the celebrated sculptor, Mr Woolner' that, while working on the figure of Puck, he had noted that some people had a small point folded in from the outer margin - perhaps, Darwin suggested, a vestige of a pointed ear. The structure is now known as Darwin's point.

The membrane of the eardrum is the gateway to the middle ear. It vibrates when sound strikes and pa.s.ses the energy to three tiny bones - the hammer, the anvil and the stirrup, each named after their shape - that act as levers. Each fits into the next and together they amplify the movements of the drum into larger movements that are pa.s.sed down the chain to a small, membrane-covered window on the surface of the liquid-filled inner ear. Because the eardrum itself is larger than the tiny window, the system increases the pressure upon it by twenty times or more. As a safety measure, tiny muscles attached to two of the bones damp down the harmful effects of loud noise. The inner ear transforms the energy of sound waves into messages about intensity, pitch and direction that pa.s.s to the brain.

The three-part middle ear is as specific to the mammals as a whole as is hair or milk, for all other land vertebrates have just a single bone in that organ. The structure is a wonderful example of how repeated elements can be used for a diversity of ends. Fossils, embryos and the animals of today paint a remarkable picture of how a body built on modules adapts itself when faced with a new challenge.

The ability to pick up waves in air or water began long ago. Most marine creatures can do it, with the help of simple sensory cells. Fish, too, are quite modest in their talents. Living as they do in water, an excellent conductor of wave energy, they manage with just a series of pressure sensors on either side of the head and body. Land animals face a more demanding task, for they need to amplify the feeble power of waves in air. They use the middle ear to do so. Fossils show that the structure appeared around two hundred and fifty million years ago, a hundred million years after the direct ancestors of mammals split off from their reptile ancestors, and seventy-five million after the bird and lizard lineages diverged from the same source. All three lineages developed a bony lever independently and with its help each of them improved its own ability to detect high-pitched sounds. Reptile and bird eardrums still connect to the inner ear via just a single bone, the stirrup. A set of triple levers, unique as it is to mammals, does a lot to improve our own hearing. Each of the bones can be traced to simpler structures in primitive animals with an easier way of life.

Anatomists came up with the first evidence of the history of the ear at about the time that Darwin began his barnacle work. They saw that in the first phases of development the embryos of fish, reptiles and mammals generate, just after the somites appear, a series of looped arches on either side of the front end. Those six repeated structures grow into matched pouches on each side of the developing head.

Four hundred million years ago, the only animals with backbones were flat-headed fish that swam in an immemorial sea. Their bodies were covered in bony plates and those primeval vertebrates ate without the benefit of jaws. In time they were succeeded by fish-like creatures with necks and simple limbs. They clambered ash.o.r.e around 365 million years ago and evolved into frogs, lizards, birds and people. The fossils of those antecedents of all the vertebrates tell the story of the middle ear. It confirms that told by the embryo and by the genes.

In antediluvian fish, the arches were supports for the gills, the structures that extract oxygen from the water. They did a simple job that lasted for millions of years. As their descendants grew bolder and moved on to land, natural selection spotted the opportunity offered by a repeated structure. In time the arch nearest the front was hijacked to become modified into the first jaws of all. The lower and the upper jaw of all vertebrates, one hinged into the other, hence trace their origin to an ancient aid to fish respiration. The second arch was then picked up to make a bone that connects the upper jaw to the brain-case. As their descendants crawled on to land, that structure evolved into a lever able to amplify sound.

Lizards and their descendants the birds had but a single such bone. Then, as the immediate ancestors of modern mammals appeared, the ear began to commandeer other parts of its ancestors' anatomy. First, the position of the hinge between the upper and lower jaw shifted compared with that found in reptiles. As it did, it freed a bone within the upper jaw, and another one within the lower. Those redundant structures were seized by evolution to make the hammer and anvil bones of the middle ear - which means that we hear, in part, with what our ancestors chewed with. Fossils of the first mammals as they began to evolve from their reptilian ancestors three hundred million years ago reveal the whole process, in all its steps, in a series of creatures with more and more complete middle ears. The shift from food processor to hearing aid happened several times in different mammal lineages, most of which are now extinct. Those small creatures of our first days ate insects and moved around at night - and any improvement in the ability to hear would have been useful indeed. Anatomy agrees about the ear, for the nerves which serve the stirrup bone branch from that to the face, while those to the other two are offshoots of a different nerve (a fact otherwise inexplicable).

Each of the three bones of the middle ear hence comes via a different route from two of the fish gill arches. Those ancient structures have also been taken up for other ends. In mammals remnants of the first arch help make some of the chewing muscles. The second evolved into some of the muscles of the face and into the bone in the neck that supports the tongue and is important in speech.

The embryo tells the same story, for as it develops the famous arches can be seen to reinvent themselves to become parts of the middle ear. The genes that build them, too, resemble others still active in the gill-slits of modern fish. The case for the middle ear as a pastiche based on an ancient marine structure is watertight.

The inner ear, deep within the skull, is another legacy of an extinct fish - and even of an early barnacle. It, too, reveals its history in fossils, embryos and DNA. The sea is a noisy place, for water is almost transparent to sound. Whales sing, fish grunt and crustaceans join in; the pistol shrimp gains its name from the loud clicks it makes with its claws, while its relative the mantis shrimp, whose claw can break a fisherman's finger, emits a deep rumble that frightens off predators. Lobsters, in the same way, make alarm signals by sc.r.a.ping their antennae across a ridged section of carapace. The larvae of lobsters and crabs - with their close resemblance to those of barnacles - pick up the roar made by waves upon a reef, and make their way towards the sound from kilometres away. Fish are even more responsive to such stimuli.

All three groups use the same fundamental mechanism for those jobs: a set of specialised pressure-sensitive cells filled with jelly, into which is affixed a hair-like structure that extends to the outside. A wave - caused by a current, the echoes of a surf-battered sh.o.r.e or the movements of a nearby enemy or friend - causes the hair to flex and the cell to pick up that movement, to transform it into chemical and electrical activity and transmit the information to the brain. Our own inner ear has just the same arrangement, for the physical movements of the middle ear bones make waves that disturb a set of sensitive hairs, which in turn generate a nervous impulse. Damage to a certain gene causes deafness and a search through fish DNA finds the same gene active in the pressure-sensitive cells. So similar are the two systems that fish are used to test drugs that might damage hearing if used on ourselves.

As Wagnerians can attest, human ears do rather more than just notice changes in pressure. Our ability to tell notes apart, impressive as it might be, emerges - once more - from expansion and diversification, in this case of the simple fish system into a series of sensory cells with different sensitivities to particular tones, multiplied and arranged in order within a long coiled structure. The reptile version is short, which means that snakes and their allies can hear only low sounds, that of birds intermediate, and the mammal inner ear sensor the longest of all. The story of the ear is of make do and mend, and of multiplied structures modified by natural selection for a new and different end. Perfect pitch, for those who have it, has been reached by most imperfect means.

The double helix shows that the modular plan upon which life as a whole is built goes back to long before the evolution of barnacles, geese or men. Whole sections of the molecule have been multiplied or lost as evolution made crustacean, birds or mammals.

Certain fruit-fly mutations that double up the number of wings or antennae are due to changes in the genes that control the pa.s.sage from embryo to adult. Such homeobox genes, as they are called after a short repeated DNA-binding sequence (or 'box') found in all of them, alter the timing and rate of growth of various segments of the genetic material and change the shape of what they build. They are a molecular mirror of Darwin's discoveries among the barnacles: of duplication, reshuffling and deletions of parts. As they multiply, such sections diverge to take up new tasks and on the way remove another plank of the creationist cause: that evolution can only remove information and cannot create it.

One surprise in modern genetics was to find how small the molecular divergence among animals actually is. A goose and a chicken are almost identical at the DNA level and neither is particularly distinct from a human. The barnacles, in turn, are close to the crabs and not very different from flies. Many of their genes have changed not at all in the millions of years since they diverged. Geese and barnacles may look quite unalike - just as cars and aeroplanes are distinct even if each is built from the same basic elements. Genes make the nuts and bolts of the body. What they make is put together in different combinations and instructed when and where to do their job. Some act as switches that activate or suppress the activity of particular genes in the embryo. The evolution of segmented animals depends in large part on their gain or loss. In some creatures they are arranged in the same order as the body parts, with head first, then the middle section and then the abdomen, but that neat arrangement is often disrupted as the homeoboxes are broken up into separate cl.u.s.ters or scrambled altogether. Different creatures have from around four homeoboxes to four dozen or so. Their presence in barnacles and buzzards, sea-urchins and squirrels, or spiders and snails, suggests that the universal ancestor of all those animals was a segmented worm-like creature in an ancient sea, with around eight of the famous genes.

Our own homeobox system is arranged in four cl.u.s.ters with about ten members in each. Many are arranged in order to give strings of such structures each specialised to its task. Some help build the ear and are - as the fossils predicted - related to those responsible for gill slits, jaws and fish sensors. The vertebral column, too, is the product of such genes.

The simplest extant member of the greater vertebrate clan (fish, fowl and people included) is a small marine creature called the lancelet that spends most of its time buried in sand in shallow seas, where it filters food through its jawless mouth. It has a segmented body and, instead of a proper backbone, a simple stiff rod along its back. The animal's homeoboxes are arranged in the same order as its body parts. Many of its relatives (ourselves included) have four times or more as many copies of such structures. Their multiplication, followed by the divergence of the various copies, promoted the wild diversity of animals with backbones. The bony fish, which have doubled the number again, are the most variable of all in size, shape and way of life.

The vast variety of the crustaceans and their relatives - from spider crabs to fungus-like parasites to wasps - also emerges from their group's flexibility in development and they too have homeobox genes quite similar to our own. The variation upon a common theme reaches a peak among the cirripedes. Compared with their close relatives the lobsters they seem simple, for they have no obvious abdomen, and no more than a few jointed legs. Like snakes, lizards and whales the barnacles have lost limbs and, like birds compared with dinosaurs, have abandoned their back ends. The parasitic forms are even simpler. All this can be tracked to changes in their homeobox genes. The ancestral barnacle had ten, each of which has an a.n.a.logue in geese and humans. The number of legs varies from species to species - and that variation is matched by the activity of two of the famous genes. The absence of an abdomen is due to a deletion of a group of homeobox genes similar to those that code for our own posteriors.

Darwin's 'unity of type' hence stretches from cirripedes to men and to the intimate details of the DNA itself. Homeobox genes draw together animals that at first sight show almost no resemblance to each other. Sir Robert Moray was, in a way, almost right about the barnacle's relationship to its eponymous goose: for perhaps those who saw a similarity between the two noticed their shared pattern of repet.i.tion; of vertebrae in the goose and body segments in the goose barnacle. If so, they were ahead of their time, for what might appear to be an accidental resemblance is proof of an ancient unity of form. Bird and barnacle each show how multiplication and divergence rule the world of life. They put paid to the absurd idea that complexity demands design or that evolution cannot generate information. The anatomy of those two sea-loving creatures, the pressure sensors of fish, the ear of an opera fan and large parts of the human genome are each a messy and expedient solution to a set of immediate problems. As Darwin noticed on the coast of Chile and as modern genetics can affirm, inelegant, redundant and wasteful as biology might be, it works well, but only as well as it must.

CHAPTER VIII.

WHERE THE BEE SNIFFS.

A gift of orchids is a statement of a gentleman's intentions towards a potential partner. A man willing to spend so much on his mate must be devoted indeed - or rich enough not to care, which comes to more or less the same thing. An orchid, with its extravagant flowers and a price tag to match, is a real test of his readiness to invest in a relationship.

The plants feel the same. Their Latin name, Orchidaceae, means 't.e.s.t.i.c.l.e' after the unexpected shape of their roots. Orchids advertise their prowess with expensive and often bizarre blooms. So impressive are their carnal powers that the English herbalist Nicholas Culpeper called for caution when they were used as aphrodisiacs. In The Descent of Man and Selection in Relation to s.e.x The Descent of Man and Selection in Relation to s.e.x Charles Darwin had shown how, in the animal kingdom, the battle to find a mate was as formidable an agent of selection as was the struggle to stay alive. Males, in general, have the potential to have far more offspring than do females - if, that is, they can fight off their rivals and persuade enough members of the opposite s.e.x to play along with their carnal desires. Losers in the conflict reach the end of their evolutionary road for their genes go nowhere. Evolution as played out in the universe of s.e.x is as pitiless as is that in the battle for survival. s.e.xual selection, as Darwin called it, can lead to rapid change: to the evolution of gigantic antlers, a vivid posterior or - for species interested in such things - gold watches and flashy clothes. Charles Darwin had shown how, in the animal kingdom, the battle to find a mate was as formidable an agent of selection as was the struggle to stay alive. Males, in general, have the potential to have far more offspring than do females - if, that is, they can fight off their rivals and persuade enough members of the opposite s.e.x to play along with their carnal desires. Losers in the conflict reach the end of their evolutionary road for their genes go nowhere. Evolution as played out in the universe of s.e.x is as pitiless as is that in the battle for survival. s.e.xual selection, as Darwin called it, can lead to rapid change: to the evolution of gigantic antlers, a vivid posterior or - for species interested in such things - gold watches and flashy clothes.

Later in his career, Charles Darwin examined the s.e.xual struggles within the second great realm of life, the plants. He showed how the search for a partner can be as much of a challenge for them as it is for stags or peac.o.c.ks. Plant reproductive habits were obscure and their mere existence often denied until the seventeenth century, but within a hundred years or so the basic machinery had been worked out. Flowers were both the home of the reproductive organs and an eloquent statement of erotic need. Darwin found that they evolved in rather the same way as an animal's s.e.xual displays and were subject to the same forces of selection, which often achieved ends equally - or more - bizarre than those found in animals. In addition he discovered (although he found it hard to believe) that for orchids s.e.x was full of dishonesty and discord, with all those involved ready to cheat whenever necessary.

Any botanical marriage is - by definition - more crowded than its animal equivalent, for a third party is needed to consummate it by moving male s.e.x cells to the female. For some species, wind or water step in to help, but most flowers need a flying p.e.n.i.s - a pollinator - to carry their DNA to the next individual (Ruskin, with his pa.s.sion for the beauties of nature, strongly advised his young female readers not to enquire 'how far flowers invite, or require, flies to interfere in their family affairs'). Darwin himself saw how antagonism between the plant and animal partners is as powerful an agent of selection as is the process of female choice and male compet.i.tion that gives rise to the peac.o.c.k's tail. Flower and pollinator each become trapped into the embrace of the other and enter an evolutionary race that may end with the emergence of structures as unexpected, and tactics as devious, as anything in the animal world.

The interests of those who manufacture the crucial DNA and those who deliver it are quite different. From a female flower's point of view, or that of the female part of a hermaphrodite plant, one or a few visits by a winged phallus is enough to do the job (although the more callers she gets, the more choice she has of which s.e.x cell to use). To beat its rivals, however, a male is forced to attract the distribution service again and again - and that can be expensive.

In his 1862 volume On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing, Darwin studied the divergence of interests between the two parties. He used the showiest and most diverse of all flowers as an exemplar. He found that 'the contrivances by which Orchids are fertilised, are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom'. As well as an exhaustive account of the structure of the orchids themselves ('I fear, however, that the necessary details will be too minute and complex for any one who has not a strong taste for Natural History'), his work introduced the idea - much developed nine years later, in The Descent of Man, and Selection in Relation to s.e.x - The Descent of Man, and Selection in Relation to s.e.x - that large parts of evolution depend on an ancient and endless s.e.xual conflict that crafts the future of all those who are drawn in. that large parts of evolution depend on an ancient and endless s.e.xual conflict that crafts the future of all those who are drawn in.

The war between flowers and insects became an overture to a wider world of biological discord. It has led to spectacular bonds between improbable partners, As it does, it reveals many of the details of the mechanism of natural selection, including its uncanny ability to subvert the tactics of any opponent. The orchids and their pollinators were, for Charles Darwin, an introduction to the dishonesty that pervades the world of life.