The Greatest Show on Earth: The Evidence for Evolution

by Richard Dawkins

  In countless other ways, dolphins and whales could be said to have their ancient history written all over and through them, like vestiges of Roman roads drawn out in dead straight cart tracks and bridleways across the map of England. Whales have no hind legs, but there are tiny bones, buried deep inside them, which are the remnants of the pelvic girdle and hind legs of their long-gone walking ancestors. The same is true of the sirenians or sea cows (I’ve already mentioned them several times: the manatees, dugongs and the 8-yard-long Steller’s sea cow, hunted to extinction by humans).* Sirenians are very different from whales and dolphins, but they are the only other group of wholly marine mammals that never come ashore. Where dolphins are fast, actively intelligent carnivores, manatees and dugongs are slow, dreamy herbivores. At the manatee aquarium that I visited in western Florida, for once I didn’t rage against the loudspeakers playing music. It was sleepy lagoon music and it seemed so languidly appropriate that all was forgiven. Manatees and dugongs float effortlessly in hydrostatic equilibrium, not by means of a swim bladder as fish do (see below), but through being equipped with heavy bones as a counterweight to the natural buoyancy of their blubber. Their specific gravity is therefore very close to that of water, and they can make fine adjustments to it by pulling in or expanding the rib cage. The precision of their buoyancy control is enhanced by the possession of a separate cavity for each lung: they have two independent diaphragms.

  Dolphins and whales, dugongs and manatees give birth to live babies, like all mammals. That habit is not actually peculiar to mammals. Many fish are livebearers, but they do it in a very different way (actually a fascinating variety of very different ways, doubtless independently evolved). The dolphin’s placenta is unmistakably mammalian, and so is its habit of suckling the young with milk. Its brain is also beyond question the brain of a mammal, and a very advanced mammal at that. The cerebral cortex of a mammal is a sheet of grey matter, wrapped around the outside of the brain. Getting brainier partly consists in increasing the area of the sheet. This could be done by increasing the total size of the brain, and of the skull that houses it. But there are downsides to having a big skull. It makes it harder to be born, for one thing. As a result, brainy mammals contrive to increase the area of the sheet while staying within limits set by the skull, and they do it by throwing the whole sheet into deep folds and fissures. This is why the human brain looks like a wrinkled walnut; and the brains of dolphins and whales are the only ones to rival those of us apes for wrinkliness. Fish brains don’t have wrinkles at all. Indeed, they don’t have a cerebral cortex, and the whole brain is tiny compared to a dolphin’s or human’s. The dolphin’s mammalian history is deeply etched into the wrinkled surface of its brain. It’s a part of its mammalness, along with the placenta, milk, a four-chambered heart, a lower jaw having only a single bone, warm-bloodedness, and many other specifically mammalian features.

  Brains of human (top), dolphin (middle), brown trout (bottom) (not to scale)

  Warm-blooded is what we call mammals and birds, but really what they have is the ability to keep their temperature constant, regardless of the outside temperature. This is a good idea, because the chemical reactions in a cell can then all be optimized to work best at a particular temperature. ‘Cold-blooded’ animals are not necessarily cold. A lizard has warmer blood than a mammal if both happen to be out in the midday sun in the Sahara desert. A lizard has colder blood than a mammal if they are out in the snow. The mammal has the same temperature all the time, and it has to work hard to keep it constant, using internal mechanisms. Lizards use external means to regulate their temperature, moving into the sun when they need to warm themselves up, and into the shade when they need to cool down. Mammals regulate their body temperature more accurately, and dolphins are no exception. Once again, their mammal history is written all over them, even though they have reverted to life in the sea, where most creatures don’t maintain a constant temperature.

  ONCE PROUD WINGS

  The bodies of whales and sirenians abound in historical relics that we notice because they live in a very different environment from their land-dwelling ancestors. A similar principle applies to birds that have lost the habit and equipment of flight. Not all birds fly, but all birds carry at least relics of the apparatus of flight. Ostriches and emus are fast runners that never fly, but they have stubs of wings as a legacy from remote flying ancestors. Ostrich wing stubs, moreover, have not completely lost their usefulness. Although much too small to fly with, they seem to have some sort of balancing and steering role in running, and they enter into social and sexual displays. Kiwi wings are too small to be seen outside the bird’s fine coat of feathers, but vestiges of wing bones are there. Moas lost their wings entirely. Their home country of New Zealand, by the way, has more than its fair share of flightless birds, probably because the absence of mammals left wide open niches to be filled by any creature that could get there by flying. But those flying pioneers, having arrived on wings, later lost them as they filled the vacant mammal roles on the ground. This probably doesn’t apply to the moas themselves, whose ancestors, as it happened, were already flightless before the great southern continent of Gondwana broke up into fragments, New Zealand among them, each bearing its own cargo of Gondwanan animals. It surely does apply to kakapos, New Zealand’s flightless parrots, whose flying ancestors apparently lived so recently that kakapos still try to fly although they lack the equipment to succeed. In the words of the immortal Douglas Adams, in Last Chance to See,

  It is an extremely fat bird. A good-sized adult will weigh about six or seven pounds, and its wings are just about good for wiggling about a bit if it thinks it’s about to trip over something – but flying is completely out of the question. Sadly, however, it seems that not only has the kakapo forgotten how to fly, but it has also forgotten that it has forgotten how to fly. Apparently a seriously worried kakapo will sometimes run up a tree and jump out of it, whereupon it flies like a brick and lands in a graceless heap on the ground.

  While ostriches, emus and rheas are great runners, penguins and Galapagos flightless cormorants are great swimmers. I was privileged to swim with a flightless cormorant in a large rock pool on the island of Isabela, and I was enchanted to witness the speed and agility with which it sought out one undersea crevice after another, staying under for a breathtakingly long time (I had the advantage of a snorkel). Unlike penguins, who use their short wings to ‘fly underwater’, Galapagos cormorants propel themselves with their powerful legs and huge webbed feet, using their wings only as stabilizers. But all flightless birds, including ostriches and their kind, which lost their wings a very long time ago, are clearly descended from ancestors that used them to fly. No reasonable observer could seriously doubt the truth of that, which means that anybody who thinks about it should find it very hard – why not impossible? – to doubt the fact of evolution.

  Numerous different groups of insects, too, have lost their wings, or greatly reduced them. Unlike primitively wingless insects such as silverfish, fleas and lice have lost the wings their ancestors once had. Female gypsy moths have underdeveloped wing muscles and don’t fly. They don’t need to, for the males fly to them, attracted by a chemical lure which they can detect at astounding dilutions. If the females were to move as well as the males, the system probably wouldn’t work, for by the time the male had flown up the slowly drifting chemical gradient, its source would have moved on!

  Unlike most insects, which have four wings, the flies, as their Latin name Diptera suggests, have only two. The second pair of wings has become reduced to a pair of ‘halteres’. These swing about like very high-speed Indian clubs, which they resemble, functioning as tiny gyroscopes. How do we know that halteres are descended from ancestral wings? Several reasons. They occupy exactly the same place in the third segment of the thorax as the flying wing occupies in the second thoracic segment (and the third too in other insects). They move in the same figure-of-eight pattern as the wings of flies. They have the same embryology a
s wings and, although they are tiny, if you look at them carefully, especially during development, you can see that they are stunted wings, clearly modified – unless you are an evolution-denier – from ancestral wings. Testifying to the same story, there are mutant fruit flies, so-called homeotic mutants, whose embryology is abnormal and who grow not halteres but a second pair of wings, like a bee or any other kind of insect.

  Halteres on a cranefly

  What would the intermediate stages between wings and halteres have looked like, and why would natural selection have favoured the intermediates? What is the use of half a haltere? J. W. S. Pringle, my old Oxford professor whose forbidding mien and stiff bearing earned him the nickname ‘Laughing John’, was mainly responsible for working out how halteres work. He pointed out that all insect wings have tiny sense organs in the base, which detect twisting and other forces. The sense organs at the base of halteres are very similar – another piece of evidence that halteres are modified wings. Long before halteres evolved, the information streaming into the nervous system from the sense organs at their base would enable fast buzzing wings, while flying, to act as rudimentary gyroscopes. To the extent that any flying machine is naturally unstable, it needs to compensate with sophisticated instrumentation, for example gyroscopes.

  The whole question of the evolution of stable and unstable fliers is very interesting. Look at these two pterosaurs, extinct flying reptiles, contemporaries of the dinosaurs. Any aero-engineer could tell you that Rhamphorhynchus, the early pterosaur at the top of the picture, must have been a stable flier, because of its long tail with the ping-pong bat on the end. Rhamphorhynchus would not have needed sophisticated gyro-control, such as flies have with their halteres, because its tail made it inherently stable. On the other hand, as the same engineer could tell you, it would not have been very manœuvrable. In any flying machine, there is a trade-off between stability and manœuvrability. The great John Maynard Smith, who worked as an aircraft designer before returning to university to read zoology (on the grounds that aeroplanes were noisy and old-fashioned), pointed out that flying animals can move in evolutionary time, back and forth along the spectrum of this trade-off, sometimes losing inherent stability in the interests of increased manœuvrability, but paying for it in the form of increased instrumentation and computation capability – brain power. At the bottom of the picture on the previous page is Anhanguera, a late pterodactyl from the Cretaceous era, some 60 million years after the Jurassic Rhamphorhynchus. Anhanguera had almost no tail at all, like a modern bat. Like a bat, it would surely have been an unstable aircraft, reliant on instrumentation and computation to exercise subtle, moment-to-moment control over its flight surfaces.

  Rhamphorhynchus (top) and Anhanguera (bottom)

  Anhanguera didn’t have halteres, of course. It would have used other sense organs to provide the equivalent information, probably the semicircular canals of the inner ear. These were indeed very large in those pterosaurs that have been looked at – although, a touch disappointingly for the Maynard Smith hypothesis, they were large in Rhamphorhynchus as well as Anhanguera. But, to return to the flies, Pringle suggests that the four-winged ancestors of flies probably had long abdomens, which would have made them stable. All four wings would have acted as rudimentary gyroscopes. Then, he suggests, the ancestors of flies started to move along the stability continuum, becoming more manœuvrable and less stable as the abdomen got shorter. The hind wings started to shift more towards the gyroscopic function (which they had always performed, in a small way, as wings), becoming smaller, and heavier for their size, while the forewings enlarged to take over more of the flying. There would have been a gradual continuum of change, as the forewings assumed ever more of the burden of aviation, while the hind wings shrank to take over the avionics.

  Worker ants have lost their wings, but not the capacity to grow wings. Their winged history still lurks within them. We know this because queen ants (and males) have wings, and workers are females who could have been queens but who, for environmental, not genetic, reasons failed to become queens.* Presumably worker ants lost their wings in evolution because they are a nuisance and get in the way underground. Poignant testimony to this is provided by queen ants, who use their wings once only, to fly out of the natal nest, find a mate, and then settle down to dig a hole for a new nest. As they begin their new life underground the first thing they do is lose their wings, in some cases by literally biting them off: painful (perhaps; who knows?) evidence that wings are a nuisance underground. No wonder worker ants never grow wings in the first place.

  Parasitic fly from the Phoridae family

  Probably for similar reasons, ants’ nests, and termites’ nests, are home to a horde of wingless hangers-on of many different types, feeding on the rich pickings swept in by the ever-rustling streams of returning foragers. And wings are just as much of a hindrance to them as they are to the ants themselves. Who would ever believe that the monstrosity on the right is a fly? Yet we know from a careful and detailed study of its anatomy that not only is it a fly, this parasite of termite nests belongs to a particular family of flies, the Phoridae. On the next page is a more normal member of the same family, which presumably somewhat resembles the winged ancestors of the weirdly wingless creature above, although it too is a parasite of social insects – bees in this case. You can see the similarity to the sickle-shaped head of the weird monster on the previous page. And the monster’s stunted wings are just visible as the tiny triangles on either side.

  Another fly from the Phoridae family

  There is an additional reason for winglessness in this riff-raff of lurkers and squatters in ants’ and termites’ nests. Many of them (not the Phorid flies) have over evolutionary time assumed a protective resemblance to ants, either (or both) to fool the ants or to fool would-be predators who might otherwise pick them out from among the less palatable and better-protected ants. Who, on taking only a casual glance, would notice that the insect below, which lives in ants’ nests, is not an ant at all but a beetle? Once again, how do we know? From deep and detailed resemblances to beetles, which hugely outnumber the superficial features in which the insect resembles an ant: exactly the same way as we know that a dolphin is a mammal and not a fish. This creature has its beetle ancestry written all through it, except (again as with dolphins) in those features that define its superficial appearance, such as its winglessness and its ant-like profile.

  Beetle disguised as an ant

  LOST EYES

  Just as ants and their subterranean fellow-travellers lose their wings underground, so numerous different kinds of animals that live in the depths of dark caves where there is no light have reduced or lost their eyes, and are, as Darwin himself noted, more or less completely blind. The word ‘troglobite’* has been coined for an animal that lives only in the darkest part of caves and is so specialized that it can live nowhere else. Troglobites include salamanders, fish, shrimps, crayfish, millipedes, spiders, crickets and many other animals. They are very often white, having lost all pigment, and blind. They usually, however, retain vestiges of eyes, and that is the point of mentioning them here. Vestigial eyes are evidence of evolution. Given that a cave salamander lives in perpetual darkness so has no use for eyes, why would a divine creator nevertheless furnish it with dummy eyes, clearly related to eyes but non-functional?

  Evolutionists, on their side, need to come up with an explanation for the loss of eyes where they are no longer needed. Why not, it might be said, simply hang on to your eyes, even if you never use them? Might they not come in handy at some point in the future? Why ‘bother’ to get rid of them? Notice, by the way, how hard it is to resist the language of intention, purpose and personification. Strictly speaking, I should not have used the word ‘bother’, should I? I should have said something like, ‘How does losing its eyes benefit an individual cave salamander so that it is more likely to survive and reproduce than a rival salamander that keeps a perfect pair of eyes, even though it never uses them?�
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  Well, eyes are almost certainly not cost-free. Setting aside the arguably modest economic costs of making an eye, a moist eye socket, which has to be open to the world to accommodate the swivelling eyeball with its transparent surface, might be vulnerable to infection. So a cave salamander that sealed up its eyes behind tough body skin might survive better than a rival individual that kept its eyes.

  But there is another way to answer the question and, instructively, it doesn’t invoke the language of advantage at all, let alone purpose or personification. When we are talking about natural selection, we think in terms of rare beneficial mutations turning up and being positively favoured by selection. But most mutations are disadvantageous, if only because they are random and there are many more ways of getting worse than there are ways of getting better.* Natural selection promptly penalizes the bad mutations. Individuals possessing them are more likely to die and less likely to reproduce, and this automatically removes the mutations from the gene pool. Every animal and plant genome is subject to a constant bombardment of deleterious mutations: a hailstorm of attrition. It is a bit like the moon’s surface, which becomes increasingly pitted with craters due to the steady bombardment of meteorites. With rare exceptions, every time a gene concerned with an eye, for example, is hit by a marauding mutation, the eye becomes a little less functional, a little less capable of seeing, a little less worthy of the name of eye. In an animal that lives in the light and uses the sense of sight, such deleterious mutations (the majority) are swiftly removed from the gene pool by natural selection.