Anatomy, genes and fossils each place the barnacles in close association 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 passing. 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 shell-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 antennæ’. 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 advertisement 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 penis-length of a female if he is to have a chance to pass 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 analogue of the first birds, which were dinosaurs who shook off their tails.
Goethe - philosopher, scientist and author of
Faust -
had, well before the
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.
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 masses of tissue on either side then begin to break up into a series of evenly spaced 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 repetition 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 mass 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 pass to the brain. The inner ear also gives its owner a sense of physical position and of acceleration or deceleration.