by Steve Jones
The most conspicuous response to touch is that of the mimosa, so admired by Shelley. Two centuries after his poem, botanists tried a simple experiment: take a series of chemicals known to act as hormones, dissolve them and water the mimosa to see which bits of DNA respond. For every substance, a previously unknown gene increased in activity by a hundred times. At first it looked as if a crossroads in the hormone labyrinth had been discovered - but sprinkling the plants with pure water had the same effect. Mere contact had done the job. The first of the ‘touch genes’ had been discovered. A drop of rain - or a gust of wind - causes them to leap into action. More than five hundred separate parts of DNA alter their activity when a leaf is prodded, some by ten times and more. A hundred respond in the opposite way, but why, nobody knows.
The touch genes do many things. They cause the cell wall to firm up or loosen in response to stress and growth patterns to alter as a result, which explains the gnarled Highland trees. Some members of the clan respond to night and day, or to the seasons as they pass (which is why leaves fall off in autumn), and others are involved in disease resistance. They might some day be engineered to give fruits that fall from the tree in a breeze as soon as they are ripe or crops that grow tall in windy places. Half the touch genes also respond when placed in darkness, but quite why they do is still not clear. The sensitive plant is, it appears, more sensitive than anyone imagined.
The inner world of plants has emerged as almost as rich as our own. Darwin was cautious in his parallels between the sensory and intellectual lives of the two kingdoms. The most he would say is that ‘It is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals.’ Now we know that the similarities go far further than he thought. One persuasive parallel involves the sense of touch. To everyone’s surprise, some of the signal proteins used by plants to sense a gentle tap resemble certain molecules that do a similar job for us. They control our heartbeats, switch on hormones that determine growth and alter the blood chemicals that change mood from happy to depressed. In a further twist to the tale, young rats caressed by their mothers respond with an increase in the activity of certain genes related to those that react to touch in plants. A shortage of embraces stunts the animals’ physical and emotional growth.
There is something magical in the way that scientific rationalism connects raindrops with heartbeats and battered trees with depressed infants. Shelley himself saw that science told us much that poetry cannot. He filled his Oxford room with electrical gadgetry and saw no contradiction between the worlds of the spirit and of science. He would have been delighted to learn that cooling passions are linked to falling leaves and that the Darwinian universal of shared ancestry shelters beneath its ample branches both the mimosa and its poet.
CHAPTER VII
A PERFECT FOWL
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 in 1677. On a log on the shores of the island of Uist, he saw ‘multitudes of little shells; 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 assured his readers that ‘some credible persons have assured me that they have seen some as big as a fist’.
The myth of the shell-born birds, barnacle geese as we call them today, the shells 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 to 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) was first applied to the unfolding of the body as egg is transformed into adult. Development is the imposition of pattern upon a formless mass. 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 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 shells (and a well-known professor of zoology - or of biochemistry masquerading under that title - 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 voyage and 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 shores 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 shore 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 shores of the Forth, the Beagle’s naturalist found on the shores of the Chonos archipelago off the coast of Chile an enigmatic soft-bodied creature 2.5 millimetres long drilling into a conch shell. 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 shell, be related to the denizens of a Scottish shore? 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 juvenile stages revealed unexpected connections between the South American borer and its Scottish kin. That lesson, learned on the shores 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 Münchausen 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 shore knows that a barnacle or acorn-shell 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 shell 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 shell. 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 shells 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 shore versions spend their lives upside down for they stand on their heads and wave their feet in the water.
Those found on rocky shores live in a fortress made of around six tough plates, based, like a snail shell, 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 cockroaches. Some species excrete through their mouths as their anus 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 sex 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 sexes, some are male when young and female later, and some are true hermaphrodites - while a few among that group secrete small males around their bisexual persons in case they might be useful. Many of those with two sexes spend their adult lives fixed to a single spot. As a result every male must constantly wave his penis, 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 sparse 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 sex 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 pass their whole lives & can never move again’. The creatures hold the record for relative penis size, while a certain beetle comes next, at twice its body length. The organ comes at a price, for individuals from wave-battered shores 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 massive 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 penis. 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 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 sex 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.