The Greatest Show on Earth (29 page)

Pill millipede

Pill woodlouse

Almost any zoologist who was not a specialist would say that the skull on the opposite page belongs to a dog. The specialist would discover that it isn’t actually a dog skull by noting the two prominent holes in the roof of the mouth. These are tell-tale signs of marsupials, the large group of mammals nowadays found mostly in Australia. It is in fact the skull of Thylacinus, the ‘Tasmanian wolf’. Thylacines and true dogs (for example dingos, with which they competed in Australia and Tasmania) have converged on a very similar skull because they have (had, alas, in the case of the unfortunate thylacine) a similar lifestyle.
I have already mentioned the magnificent marsupial mammal fauna of Australia, in the chapter on the geographical distribution of animals. The relevant point for this chapter is the repeated convergences between these marsupials and a great variety of opposite numbers among the ‘placental’ (i.e. non-marsupial) mammals, which dominate the rest of the world. Though far from identical, even in superficial characteristics, each marsupial in the illustration overleaf is sufficiently similar to its placental equivalent – that is, the placental that most closely practises the same ‘trade’ – to impress us, but certainly not sufficiently similar to suggest ‘borrowing’ by a creator.

Thylacine ‘marsupial wolf’ or ‘Tasmanian tiger’ skull

The sexual shuffling of the genes in a gene pool could be regarded as a kind of borrowing or sharing of genetic ‘ideas’, but sexual recombination is confined within one species and is therefore irrelevant to this chapter, which is about comparisons between species: for example, comparisons between marsupial and placental mammals. Interestingly, high-level borrowing of DNA is rife among bacteria. In a process that is sometimes regarded as a kind of precursor to sexual reproduction, bacteria – even quite distantly related strains of bacteria – swap DNA ‘ideas’ with promiscuous abandon. ‘Borrowing ideas’ is indeed one of the main ways by which bacteria pick up useful ‘tricks’ such as resistance to particular antibiotics.
The phenomenon is often called by the rather unhelpful name of ‘transformation’. That’s because, when it was discovered in 1928 by Frederick Griffith, nobody understood about DNA. What Griffith found was that a non-virulent strain of Streptococcus could pick up virulence from a completely different strain, even though that virulent strain was dead. Nowadays we would say that the non-virulent strain incorporated into its genome some DNA from the dead virulent strain (DNA doesn’t care about being ‘dead’, it is just coded information). In the language of this chapter, the non-virulent strain ‘borrowed’ a genetic ‘idea’ from the virulent strain. Of course, bacteria borrowing genes from other bacteria is a very different matter from a designer borrowing his own ideas from one ‘theme’ and re-using them in another theme. Nevertheless, it is interesting because, if it were as common in animals as it is in bacteria, it would make it harder to disprove the ‘designer borrowing’ hypothesis. What if bats and birds behaved like bacteria in this respect? What if chunks of bird genome could be ferried across, perhaps by bacterial or viral infection, and implanted in a bat’s genome? Maybe a single species of bats might suddenly sprout feathers, the feather-coding DNA information having been borrowed in a genetic version of a computer’s ‘Copy and Paste’.

Placental and marsupial opposite numbers

In animals, unlike bacteria, gene transfer seems almost entirely confined to sexual congress within species. Indeed, a species can pretty well be defined as a set of animals that engage in gene transfer among themselves. Once two populations of a species have been separated for long enough that they can no longer exchange genes sexually (usually after an initial period of enforced geographical separation, as we saw in Chapter 9), we now define them as separate species, and they will never again exchange genes, other than by the intervention of human genetic engineers. My colleague Jonathan Hodgkin, Oxford’s Professor of Genetics, knows of only three tentative exceptions to the rule that gene transfer is confined within species: in nematode worms, in fruit flies, and (in a bigger way) in bdelloid rotifers.
This last group is especially interesting because, uniquely among major groupings of eucaryotes, they have no sex. Could it be that they have been able to dispense with sex because they have reverted to the ancient bacterial way of exchanging genes? Crossspecies gene transfer seems to be commoner in plants. The parasitic plant dodder (Cuscuta) donates genes to the host plants around which it is entwined.*

Bdelloid rotifer

I am undecided about the politics of GM foods, torn between the potential benefits to agriculture on the one hand and precautionary instincts on the other. But one argument I haven’t heard before is worth a brief mention. Today we curse the way our predecessors introduced species of animals into alien lands just for the fun of it. The American grey squirrel was introduced to Britain by a former Duke of Bedford: a frivolous whim that we now see as disastrously irresponsible. It is interesting to wonder whether taxonomists of the future may regret the way our generation messed around with genomes: transporting, for example, ‘anti-freeze’ genes from Arctic fish into tomatoes to protect them from frost. A gene that gives jellyfish a fluorescent glow has been borrowed from them by scientists and inserted into the genome of potatoes, in the hope of making them light up when they need watering. I have even read of an ‘artist’ who plans an ‘installation’ consisting of luminous dogs, glowing with the aid of jellyfish genes. Such debauchery of science in the name of pretentious ‘art’ offends all my sensibilities. But could the damage go further? Could these frivolous caprices undermine the validity of future studies of evolutionary relationships? Actually I doubt it, but perhaps the point is at least worth raising, in a precautionary spirit. The whole point of the precautionary principle, after all, is to avoid future repercussions of choices and actions that may not be obviously dangerous now.

CRUSTACEANS

I began the chapter with the vertebrate skeleton, which is a lovely example of an invariant pattern linking variable detail. Almost any other major group of animals would show the same kind of thing. I’ll take just one other favourite example: the decapod crustaceans, the group that includes lobsters, prawns, crabs and hermit crabs (which are not crabs, by the way). The body plan of all crustaceans is the same. Whereas our vertebrate skeleton consists of hard bones inside an otherwise soft body, crustaceans have an exoskeleton consisting of hard tubes, inside which the animal keeps and protects its soft bits. The hard tubes are jointed and hinged, in something like the same way as our bones are. Think, for example, of the delicate hinges in the legs of a crab or lobster, and the more robust hinge of the claw. The muscles that power the pinch of a large lobster are inside the tubes that make up the claw. The equivalent muscles when a human hand pinches something attach to the bones that run through the middle of the finger and thumb.
Like vertebrates, but unlike sea urchins or jellyfish, crustaceans are left/right symmetrical, with a train of segments running the length of the body from head to tail. The segments are the same as each other in their underlying plan, but often differ in detail. Each segment consists of a short tube joined, either rigidly or by a hinge, to the two neighbouring segments. As with vertebrates, the organs and organ systems of a crustacean show a repeat pattern as you move from front to rear. For example, the main nerve trunk, which runs the length of the body on the ventral side (not the dorsal side, as the vertebrate spinal cord does), has a pair of ganglia (sort of mini-brains*) in each segment, from which sprout nerves supplying the segment. Most of the segments have a limb on each side, each limb again consisting of a series of tubes joined by hinges. Crustacean limbs usually terminate in a two-way branch, which in many cases you could call a claw. The head is segmented too although, as with the vertebrate head, the segmental pattern is more disguised here than in the rest of the body. There are five pairs of limbs lurking in the head, although it might sound a bit strange to call them limbs since they are modified to become antennae or components of the jaw apparatus. They are therefore usually called appendages rather than limbs. More or less invariably, the five segmental appendages of the head, reading from the front, consist of first antennae (or antennules), second antennae (often just called antennae), mandibles, first maxillae (or maxillules) and second maxillae. The antennules and antennae are mostly engaged in sensing things. The mandibles and maxillae are concerned with chewing, milling or otherwise processing food. As we proceed back along the body, the segmental appendages or limbs are pretty variable, the middle ones often consisting of walking legs, while those sprouting from the rearmost segments are often pressed into service doing other things such as swimming.
In a lobster or a prawn, after the usual five head segment appendages, the first body segment appendages are the claws. The next four pairs are walking legs. The segments bearing claws and walking legs are bunched together as the thorax. The rest of the body is called the abdomen. Its segments, at least until you reach the tip of the tail, are the ‘swimmerets’, feathery appendages that help with swimming, quite importantly so in some delicately graceful prawns. In crabs, the head and thorax have merged into a single large unit, to which all the first ten pairs of limbs are attached. The abdomen is doubled back under the head/thorax so that you can’t see it at all from above. But if you turn a crab over, you can clearly see the abdomen’s segmental pattern. The picture below shows the typical narrow abdomen of a male crab. The female abdomen is wider and resembles an apron, which it is indeed called. Hermit crabs are unusual in that the abdomen is asymmetrical (to fit into the empty mollusc shell which is its house), and soft and unarmoured (because the mollusc shell provides protection).

Male crab showing narrow, folded-back abdomen

To get an idea of some of the wonderful ways in which the crustacean body is modified in detail, while the body plan itself is not modified at all, look at the set of drawings opposite by the famous nineteenth-century zoologist Ernst Haeckel, perhaps Darwin’s most devoted disciple in Germany (the devotion was not reciprocated, but even Darwin would surely have admired Haeckel’s draughtsmanship). Just as we did with the vertebrate skeleton, look at each body part of these crabs and crayfish, and see how, without fail, you can find its exact opposite number in all the rest. Every bit of the exoskeleton is joined to the ‘same’ bits, but the shapes of the bits themselves are very different. Once again, the ‘skeleton’ is invariant, while its parts are anything but. And once again the obvious – I would say the only sensible – interpretation is that all these crustaceans have inherited the plan of their skeleton from a common ancestor. They have moulded the individual components into a rich variety of shapes. But the plan itself remains, exactly as inherited from the ancestor.

WHAT WOULD D’ARCY THOMPSON HAVE DONE WITH A COMPUTER?

In 1917 the great Scottish zoologist D’Arcy Thompson wrote a book called On Growth and Form, in the last chapter of which he introduced his famous ‘method of transformations’.* He would draw an animal on graph paper, and then he would distort the graph paper in a mathematically specifiable way and show that the form of the original animal had turned into another, related animal. You could think of the original graph paper as a piece of rubber, on which you draw your first animal. Then the transformed graph paper would be equivalent to the same piece of rubber, stretched or pulled out of shape in some mathematically defined way. For example, he took six species of crab and drew one of them, Geryon, on ordinary graph paper (the undistorted sheet of rubber). He then distorted his mathematical ‘rubber sheet’ in five separate ways, to achieve an approximate representation of the other five species of crab. The details of the mathematics don’t matter, although they are fascinating. What you can clearly see is that it doesn’t take much to transform one crab into another. D’Arcy Thompson himself wasn’t very interested in evolution, but it is easy for us to imagine what the genetic mutations would have to do in order to bring about changes like this. That doesn’t mean we should think of Geryon, or any other one of these six crabs, as being ancestral to the others. None of them was, and in any case that is not the point. The point is that whatever the ancestral crab looked like, transformations of this kind could change any one of these six species (or a putative ancestor) into any other.

Haeckel’s crustaceans. Ernst Haeckel was a distinguished German zoologist and an excellent zoological artist.

Evolution never happened by taking one adult form and coaxing it into the shape of another. Remember that every adult grows as an embryo. The mutations selected would have worked in the developing embryo by changing the rate of growth of parts of the body relative to other parts. In Chapter 7 we interpreted the evolution of the human skull as a series of changes in the rates of growth of some parts relative to other parts, controlled by genes in the developing embryo. We should expect, therefore, that if we draw a human skull on a sheet of ‘mathematical rubber’, it should be possible to distort the rubber in some mathematically tidy way and achieve an approximate likeness to the skull of a close cousin, such as a chimpanzee, or – perhaps with a bigger distortion – a more distant cousin such as a baboon. And this is just what D’Arcy Thompson showed. Note, once again, that it was an arbitrary decision to draw the human skull first, and then transform it into the chimpanzee and the baboon. He could equally well have drawn, say, the chimpanzee first and then worked out the necessary distortions to make the human and the baboon. Or, more interestingly for a book on evolution, which his was not, he might have drawn, say, an Australopithecus skull first on the undistorted rubber, and worked out how to transform it to make a modern human skull. This would surely have worked just as well as the pictures above, and it would have been evolutionarily meaningful in a more direct way.