by Steve Jones
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.
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 passes 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 passed 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 pass 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 ashore 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 fir
st 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 scraping 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 shore 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 passage 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 clusters 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 clusters 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 analogue 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 repetition; 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.