River Out Of Eden

CHAPTER 3. DO GOOD BY STEALTH

{59}

Creationism has enduring appeal, and the reason is not far to seek. It is not, at least for most of the people I encounter, because of a commitment to the literal truth of Genesis or some other tribal origin story. Rather it is that people discover for themselves the beauty and complexity of the living world and conclude that it “obviously” must have been designed. Those creationists who recognize that Darwinian evolution provides at least some sort of alternative to their scriptural theory often resort to a slightly more sophisticated objection. They deny the possibility of evolutionary intermediates. “X must have been designed by a Creator,” people say, “because half an X would not work at all. All the parts of X must have been put together simultaneously; they could not have gradually evolved.” For instance, on the day I began writing this chapter I happened to receive a letter. It was from an American minister who had been an atheist but was converted by reading an article in National Geographic. Here is an extract from the letter:

The article was about the amazing adaptations that orchids have made to their environments in order to propagate successfully. As I read I was particularly intrigued by the reproductive strategy of one species, which involved the cooperation of a male {60} wasp. Apparently the flower resembled very closely the female of this species of wasp, including having an opening in the proper place, so that the male wasp might just reach, by copulating with the flower, the pollen produced by the blossom. Flying on to the next flower the process would be repeated, and thus cross-pollination take place. And what made the flower attractive to the wasp in the first place was that it emitted pheromones [specific chemical attractants much used by insects to bring the sexes together] identical to the female of that species of wasp. With some interest I studied the accompanying picture for a minute or so. Then, with a terrific sense of shock, I realized that in order for that reproductive strategy to have worked at all, it had to be perfect the first time. No incremental steps could account for it, for if the orchid did not look like and smell like the female wasp, and have an opening suitable for copulation with the pollen within perfect reach of the male wasp's reproductive organ, the strategy would have been a complete failure.

I will never forget the sinking feeling that overwhelmed me, because it became clear to me in that minute that some kind of God in some kind of fashion must exist, and have an ongoing relationship with the processes by which things come into being. That in short, the creator God was not some antediluvian myth, but something real. And, most reluctantly, I also saw at once that I must search to find out more about that God.

Others, no doubt, come to religion by different routes, but certainly many people have had an experience similar to the one that changed the life of this minister (whose identity I shall withhold out of good manners). They have seen, or read about, some marvel of nature. This has, in a {61} general way, filled them with awe and wonderment, spilling over into reverence. More specifically, like my correspondent, they have decided that this particular natural phenomenon – a spider's web, or an eagle's eye or wing, or whatever it is – cannot have evolved by gradual stages, because the intermediate, half-formed stages could not have been good for anything. The purpose of this chapter is to destroy the argument that complicated contrivances have to be perfect if they are to work at all. Incidentally, orchids were among Charles Darwin's favorite examples, and he devoted a whole book to showing how the principle of gradual evolution by natural selection triumphantly meets the ordeal of explaining “The Various Contrivances by which Orchids are Fertilised by Insects.”

The key to the minister's argument lies in the assertion that “in order for that reproductive strategy to have worked at all, it had to be perfect the first time. No incremental steps could account for it.” The same argument could be made – frequently has been made – for the evolution of the eye, and I'll return to this in the course of the chapter.

What always impresses me whenever I hear this kind of argument is the confidence with which it is asserted. How, I want to ask the minister, can you be so sure that the wasp-mimicking orchid (or the eye, or whatever) wouldn't work unless every part of it was perfect and in place? Have you, in fact, given the matter a split-second's thought? Do you actually know the first thing about orchids, or wasps, or the eyes with which wasps look at females and orchids? What emboldens you to assert that wasps are so hard to fool that the orchid's resemblance would have to be perfect in all dimensions in order to work? {62} Think back to the last time you were fooled by some chance resemblance. Perhaps you raised your hat to a stranger in the street, mistaking her for an acquaintance. Film stars have stand-in stuntmen or stuntwomen to fall off horses or jump off cliffs in their stead. The stuntman's resemblance to the star is usually extremely superficial, but in the fleeting action shot it is enough to fool an audience. Human males are roused to lust by pictures in a magazine. A picture is just printing ink on paper. It is two-dimensional, not three. The image is only a few inches high. It may be a crude caricature consisting of a few lines, rather than a lifelike representation. Yet it can still arouse a man to erection. Perhaps a fleeting view of a female is all a fast-flying wasp can expect to get before attempting to copulate with her. Perhaps male wasps notice only a few key stimuli anyway.

There is every reason to think that wasps might be even easier to fool than humans. Sticklebacks certainly are, and fish have bigger brains and better eyes than wasps. Male sticklebacks have red bellies, and they will threaten not only other males but also crude dummies with red “bellies.” My old maestro, the Nobel Prize-winning ethologist Niko Tinbergen, told a famous story about a red mail van that drove past the window of his laboratory, and how all the male sticklebacks rushed to the window side of their tanks and vigorously threatened it. Female sticklebacks that are ripe with eggs have conspicuously swollen bellies. Tinbergen found that an extremely crude, vaguely elongated, silvery dummy, looking nothing like a stickleback to our eyes but possessed of a well-rounded “belly,” evoked full mating behavior from males. More recent experiments in the school of research founded by Tinbergen have shown {63} that a so-called sex bomb – a pear-shaped object, rounded plumpness personified but not elongated and not fishlike by any stretch of the (human) imagination – was even more effective in arousing the lusts of the male stickleback. The stickleback “sex bomb” is a classic example of a supernormal stimulus – a stimulus even more effective than the real thing. As another example, Tinbergen published a picture of an oystercatcher trying to sit on an egg the size of an ostrich egg. Birds have bigger brains and better eyesight than fish – and a fortiori than wasps – yet oystercatchers apparently “think” that an ostrich-sized egg is a superlative object for incubation.

Gulls, geese and other ground-nesting birds have a stereotyped response to an egg that has rolled out of the nest. They reach over and roll it back in with the underside of their bill. Tinbergen and his students showed that gulls will do this not just to their own eggs but to hens' eggs and even wooden cylinders or cocoa tins discarded by campers. Baby herring gulls get their food by begging from their parents; they peck at the red spot on the parent's bill, stimulating the parent to regurgitate some fish from its bulging crop. Tinbergen and a colleague showed that crude cardboard dummies of a parent's head are very effective in provoking begging behavior from the young. All that is really necessary is a red spot. As far as the baby gull is concerned, its parent is a red spot. It may well see the rest of its parent, but that doesn't seem to be important.

This apparently restricted vision is not confined to baby gulls. Adult black-headed gulls are conspicuous because of their dark face masks. Tinbergen's student Robert Mash investigated the importance of this to other adults by painting {64} wooden dummy gull heads. Each head was stuck on the end of a wooden rod attached to electric motors in a box so that, by remote control, Mash could raise or lower the head and turn it left or right. He would bury the box near a gull nest and leave it with the head safely out of sight beneath the sand. Then, day after day, he would visit a blind near the nest and observe the nesting gulls' reaction to the dummy head when it was raised and turned this way or that. The birds responded to the head and to its turning just as though it were a real gull, yet it was only a mock-up on the end of a wooden rod, without any body, without legs or wings or tail, silent and without movement apart from a pretty unlifelike, robotic rising, rotating and lowering. To a black-headed gull, it seems, a threatening neighbor is little more than a disembodied black face. No body, or wings, or anything else seem to be necessary.

Just to get into the blind to observe the birds, Mash, like generations of ornithologists before him and since, exploited a long-known limitation of the bird nervous system: birds are not natural mathematicians. Two of you go to the blind, and only one of you leaves it. Without this trick, the birds would be wary of the blind, “knowing” that somebody had entered it. But if they see one person leave, they “assume” that both have left. If a bird can't tell the difference between one person and two, is it all that surprising that a male wasp might be fooled by an orchid that bore a less than perfect resemblance to a female?

One more bird story along these lines, and it is a tragedy. Turkey mothers are fierce protectors of their young. They need to protect them against nest marauders {65} like weasels or scavenging rats. The rule of thumb a turkey mother uses to recognize nest robbers is a dismayingly brusque one: In the vicinity of your nest, attack anything that moves, unless it makes a noise like a baby turkey. This was discovered by an Austrian zoologist named Wolfgang Schleidt. Schleidt once had a mother turkey that savagely killed all her own babies. The reason was woefully simple: she was deaf. Predators, as far as the turkey's nervous system is concerned, are defined as moving objects that don't emit a baby's cry. These baby turkeys, though they looked like baby turkeys, moved like baby turkeys, and ran trustingly to their mother like baby turkeys, fell victim to the mother's restricted definition of a “predator.” She was protecting her own children against themselves, and she massacred them all.

In an insect echo of the tragic story of the turkey, certain of the sensory cells in honeybee antennae are sensitive to only one chemical, oleic acid. (They have other cells sensitive to other chemicals.) Oleic acid is given off by decaying bee corpses, and it triggers the bees' “undertaker behavior,” the removal of dead bodies from the hive. If an experimenter paints a drop of oleic acid on a live bee, the wretched creature is dragged off, kicking and struggling and obviously very much alive, to be thrown out with the dead.

Insect brains are much smaller than turkey brains or human brains. Insect eyes, even the big compound eyes of dragonflies, possess a fraction of the acuity of our eyes or bird eyes. Quite apart from this, it is known that insect eyes see the world in a completely different way from our eyes. The {66} great Austrian zoologist Karl von Frisch discovered as a young man that they are blind to red light but they can see – and see as its own distinct hue – ultraviolet light, to which we are blind. Insect eyes are much preoccupied with something called “flicker,” which seems – at least to a fast-moving insect – to substitute partially for what we would call “shape.” Male butterflies have been seen to “court” dead leaves fluttering down from the trees. We see a female butterfly as a pair of large wings flapping up and down. A flying male butterfly sees her, and courts her, as a concentration of “flicker.” You can fool him with a strobo-scopic lamp, which doesn't move but just flashes on and off. If you get the flickering rate right, he will treat it as if it were another butterfly flapping its wings at that rate. Stripes, to us, are static patterns. To an insect as it flies past, stripes appear as “flicker” and can be mimicked with a stroboscopic lamp flashing at the right rate. The world as seen through an insect's eyes is so alien to us that to make statements based on our own experience when discussing how “perfectly” an orchid needs to mimic a female wasp's body is human presumption.

Wasps themselves were the subject of a classic experiment, originally done by by the great French naturalist Jean-Henri Fabre and repeated by various other workers, including members of Tinbergen's school. The female digger wasp returns to her burrow carrying her stung and paralyzed prey. She leaves it outside the burrow while she enters, apparently to check that all is well before she reappears to drag the prey in. While she is in the burrow, the experimenter moves the prey a few inches away from {67} where she left it. When the wasp resurfaces, she notices the loss and quickly relocates the prey. She then drags it back to the burrow entrance. Only a few seconds have passed since she inspected the inside of the burrow. We think that there is really no good reason why she should not proceed to the next stage in her routine, drag the prey inside and be done with it. But her program has been reset to an earlier stage. She dutifully leaves the prey outside the burrow again and goes inside for yet another inspection. The experimenter may repeat this charade forty times, until he gets bored. The wasp behaves like a washing machine that has been set back to an early stage in its program and doesn't “know” that it has already washed those clothes forty times without a break. The distinguished computer scientist Douglas Hofstadter has adopted a new adjective, “sphexish,” to label such inflexible, mindless automatism. (Sphex is the name of one representative genus of digger wasp.) At least in some respects, then, wasps are easy to fool. It is a very different kind of fooling from that engineered by the orchid. Nevertheless, we must beware of using human intuition to conclude that “in order for that reproductive strategy to have worked at all, it had to be perfect the first time.”

I may have done my work too well in persuading you that wasps are likely to be easy to fool. You may be nurturing a suspicion almost opposite to that of my ordained correspondent. If insect eyesight is so poor, and if wasps are so easy to fool, why does the orchid bother to make its flower as wasp-like as it is? Well, wasp eyesight is not always so poor. There are situations in which wasps seem to see quite well: when {68} they are locating their burrow after a long hunting flight, for instance. Tinbergen investigated this with the bee-hunting digger wasp, Philanthus. He would wait until a wasp was down in her burrow. Before she reemerged, Tinbergen would hastily place some “landmarks” around the entrance to the burrow – say, a twig and a pinecone. He would then retreat and wait for the wasp to fly out. After she did so, she flew two or three circles around the burrow, as though taking a mental photograph of the area, then flew off to seek her prey. While she was gone, Tinbergen would move the twig and the pinecone to a location a few feet away. When the wasp returned, she missed her burrow and instead dived into the sand at the appropriate point relative to the new positions of the twig and the pinecone. Again, the wasp has been “fooled,” in a sense, but this time she earns our respect for her eyesight. It looks as though “taking a mental photograph” was indeed what she was doing on her preliminary circling flight. She seems to have recognized the pattern, or “gestalt,” of the twig and the pinecone. Tinbergen repeated the experiment many times, using different kinds of landmarks, such as rings of pinecones, with consistent results.

Now here's an experiment of Tinbergen's student Gerard Baerends that contrasts impressively with Fabre's “washing machine” experiment. Baerends' species of digger wasp, Ammophila campestris (a species also studied by Fabre), is unusual in being a “progressive provisioner.” Most digger wasps provision their burrow and lay an egg, then seal up the burrow and leave the young larva to feed on its own. Ammophila is different. Like a bird, it returns daily to the burrow to check on the larva's welfare, and gives it food as {69} needed. Not particularly remarkable, so far. But an individual female Ammophila will have two or three burrows on the go at any one time. One burrow will have a relatively large, nearly grown larva; one a small, new-laid larva; and one, perhaps, a larva of intermediate age and size. The three naturally have different food requirements, and the mother tends them accordingly. By a painstaking series of experiments involving the swapping of nest contents, Baerends was able to show that mother wasps do indeed take account of the different food requirements of each nest. This seems clever, but Baerends found that it is also not clever, in a very odd, alien way. The mother wasp, first thing each morning, makes a round of inspection of all her active burrows. It is the state of each nest at the time of the dawn inspection that the mother measures and that influences her provisioning behavior for the rest of the day. Baerends could swap nest contents as often as he pleased after the dawn inspection, and it made no difference to the mother wasp's provisioning behavior. It was as though she switched on her nest-assessing apparatus only for the duration of the dawn inspection round and then switched it off, to save electricity for the rest of the day.

On the one hand, this story suggests that there is sophisticated equipment for counting, measuring, and even calculating, in the mother wasp's head. It now becomes easy to believe that the wasp brain would indeed be fooled only by a thoroughly detailed resemblance between orchid and female. But at the same time, Baerends' story suggests a capacity for selective blindness and a foolability that are all of a piece with the washing-machine experiment, and make it believable {70} that a crude resemblance between orchid and female might well be sufficient. The general lesson we should learn is never to use human judgment in assessing such matters. Never say, and never take seriously anybody who says, “I cannot believe that so-and-so could have evolved by gradual selection.” I have dubbed this kind of fallacy “the Argument from Personal Incredulity.” Time and again, it has proved the prelude to an intellectual banana-skin experience.

The argument I am attacking is the one that says: gradual evolution of so-and-so couldn't have happened, because so-and-so “obviously” has to be perfect and complete if it is to work at all. So far, in my reply, I've made much of the fact that wasps and other animals have a very different view of the world from our own, and in any case even we are not difficult to fool. But there are other arguments I want to develop that are even more convincing and more general. Let's use the word “brittle” for a device that must be perfect if it is to work at all – as my correspondent alleged of wasp-mimicking orchids. I find it significant that it is actually quite hard to think of an unequivocally brittle device. An airplane is not brittle, because although we'd all prefer to entrust our lives to a Boeing 747 complete with all its myriad parts in perfect working order, a plane that has lost even major pieces of equipment, like one or two of its engines, can still fly. A microscope is not brittle, because although an inferior one gives a fuzzy and ill-lit image, you can still see small objects better with it than you could with no microscope at all. A radio is not brittle; if it is deficient in some respect, it may lose fidelity and its sounds may be {71} tinny and distorted, but you can still make out what the words mean. I have been staring out of the window for ten minutes trying to think of a single really good example of a brittle man-made device, and I can come up with only one: the arch. An arch has a certain near-brittleness in the sense that, once its two sides have come together, it has great strength and stability. But before the two sides come together, neither side will stand up at all. An arch has to be built with the aid of some sort of scaffolding. The scaffolding provides temporary support until the arch is complete; then it can be removed and the arch remains stable for a very long time.

There is no reason in human technology why a device should not in principle be brittle. Engineers are at liberty to design, on their drawing boards, devices that, if half-complete, would not work at all. Even in the field of engineering, however, we are hard put to find a genuinely brittle device. I believe that this is even more true of living devices. Let's look at some of the allegedly brittle devices from the living world that creationist propaganda has served up. The example of the wasp and the orchid is only one example of the fascinating phenomenon of mimicry. Large numbers of animals and some plants gain an advantage because of their resemblance to other objects, often other animals or plants. Almost every aspect of life has somewhere been enhanced or subverted by mimicry: catching food (tigers and leopards are nearly invisible as they stalk their prey in sun-dappled woodland; angler fish resemble the sea bottom on which they sit, and they lure their prey with a long “fishing rod,” on the end of which is a bait that mimics a worm; femmes fatales fireflies mimic {72} the courtship flash patterns of another species, thereby luring males, which they then eat; sabre-toothed blennies mimic other species of fish that specialize in cleaning large fish, and then take bites out of their clients' fins once they have been granted privileged access); avoiding being eaten (prey animals variously resemble tree bark, twigs, fresh green leaves, curled-up dead leaves, flowers, rose thorns, seaweed fronds, stones, bird droppings and other animals known to be poisonous or venomous); decoying predators away from young (avocets and many other ground-nesting birds mimic the attitude and gait of a bird with a broken wing); obtaining care of eggs (cuckoo eggs resemble the eggs of the particular host species parasitized; the females of certain species of mouthbreeder fish have dummy eggs painted on their flanks to attract males to take real eggs into their mouths and brood them).

In all cases, there is a temptation to think that the mimicry won't work unless it is perfect. In the particular case of the wasp orchid, I made much of the perceptual imperfections of wasps and other victims of mimicry. To my eyes, indeed, orchids are not all that uncanny in their resemblances to wasps, bees or flies. The resemblance of a leaf insect to a leaf is far more exact to my eyes, possibly because my eyes are more like the eyes of the predators (presumably birds) against which leaf mimicry is aimed.

But there is a more general sense in which it is wrong to suggest that mimicry has to be perfect if it is to work at all. However good the eyes of, say, a predator may be, the conditions for seeing are not always perfect. Moreover, there will almost inevitably be a continuum of seeing conditions, from {73} very bad to very good. Think about some object you know really well, so well that you could never possibly mistake it for anything else. Or think of a person – say, a close friend, so dear and familiar that you could never mistake her for anybody else. But now imagine that she is walking toward you from a great distance. There must be a distance so great that you don't see her at all. And a distance so close that you see every feature, every eyelash, every pore. At intermediate distances, there is no sudden transformation. There is a gradual fade-in or fade-out of recognizability. Military manuals of riflemanship spell it out: “At two hundred yards, all parts of the body are distinctly seen. At three hundred yards, the outline of the face is blurred. At four hundred yards, no face. At six hundred yards, the head is a dot and the body tapers. Any questions?” In the case of the gradually approaching friend, admittedly you may suddenly recognize her. But in this case distance provides a gradient of probability of sudden recognition.

Distance, in one way or another, provides a gradient of visibility. It is essentially gradual. For any degree of resemblance between a model and a mimic, whether the resemblance is brilliant or almost nonexistent, there must be a distance at which a predator's eyes would be fooled and a slightly shorter distance at which they are less likely to be fooled. As evolution proceeds, resemblances of gradually improving perfection can therefore be favored by natural selection, in that the critical distance for being fooled gradually moves nearer. I use “predator's eyes” to stand for “the eyes of whoever needs to be fooled.” In some cases it will be prey's eyes, foster-parent's eyes, female fish's eyes, and so on. {74}

I have demonstrated this effect in public lectures to audiences of children. My colleague Dr. George McGavin, of the Oxford University Museum, kindly manufactured for me a model “woodland floor” strewn with twigs, dead leaves and moss. On it he artfully positioned dozens of dead insects. Some of these, such as a metallic-blue beetle, were quite conspicuous; others, including stick insects and leaf-mimicking butterflies, were exquisitely camouflaged; yet others, such as a brown cockroach, were intermediate. Children were invited out of the audience and asked to walk slowly toward the tableau, looking for insects and singing out as they spotted each one. When they were sufficiently far away, they couldn't see even the conspicuous insects. As they approached, they saw the conspicuous insects first, then those, like the cockroach, of intermediate visibility, and finally the well-camouflaged ones. The very best-camouflaged insects evaded detection even when the children were staring at them at close range, and the children gasped when I pointed them out.

Distance is not the only gradient about which one can make this kind of argument. Twilight is another. At dead of night, almost nothing can be seen, and even a very crude resemblance of mimic to model will pass muster. At high noon, only a meticulously accurate mimic may escape detection. Between these times, at daybreak and dusk, in the gloaming or just on a dull overcast day, in a fog or in a rainstorm, a smooth and unbroken continuum of visibilities obtains. Once again, resemblances of gradually increasing accuracy will be favored by natural selection, because for any given goodness of resemblance there will be a level of visibility at which that particular goodness of resemblance {75} makes all the difference. As evolution proceeds, progressively improving resemblances confer survival advantage, because the critical light intensity for being fooled becomes gradually brighter.

A similar gradient is provided by angle of vision. An insect mimic, whether good or bad, will sometimes be seen out of the corner of a predator's eye. At other times it will be seen in merciless full-frontal aspect. There must be an angle of view so peripheral that the poorest possible mimic will escape detection. There must be a view so central that even the most brilliant mimic will be in danger. Between the two is a steady gradient of view, a continuum of angles. For any given level of perfection of mimicry, there will be a critical angle at which slight improvement or disimprove-ment makes all the difference. As evolution proceeds, resemblances of smoothly increasing quality are favored, because the critical angle for being fooled becomes gradually more central.

Quality of enemies' eyes and brains can be regarded as yet another gradient, and I have already hinted at it in earlier parts of this chapter. For any degree of resemblance between a model and a mimic, there is likely to be an eye that will be fooled and an eye that will not be fooled. Again, as evolution proceeds, resemblances of smoothly increasing quality are favored, because predator eyes of greater and greater sophistication are being fooled. I don't mean that the predators are evolving better eyes in parallel to the improving mimicry, though they might. I mean that there exist, somewhere out there, predators with good eyes and predators with poor eyes. All these predators constitute a danger. A poor mimic fools only the predators with poor eyes. A good mimic fools {76} almost all the predators. There is a smooth continuum in between.

Mention of poor eyes and good eyes brings me to the creationist's favorite conundrum. What is the use of half an eye? How can natural selection favor an eye that is less than perfect? I have treated the question before and have laid out a spectrum of intermediate eyes, drawn from those that actually exist in the various phyla of the animal kingdom. Here I shall incorporate eyes in the rubric I have established of theoretical gradients. There is a gradient, a continuum, of tasks for which an eye might be used. I am at present using my eyes for recognizing letters of the alphabet as they appear on a computer screen. You need good, high-acuity eyes to do that. I have reached an age when I can no longer read without the aid of glasses, at present quite weakly magnifying ones. As I get older still, the strength of my prescription will steadily mount. Without my glasses, I shall find it gradually and steadily harder to see close detail. Here we have yet another continuum – a continuum of age.

Any normal human, however old, has better vision than an insect. There are tasks that can be usefully accomplished by people with relatively poor vision, all the way down to the nearly blind. You can play tennis with quite blurry vision, because a tennis ball is a large object, whose position and movement can be seen even if it is out of focus. Dragonflies' eyes, though poor by our standards, are good by insect standards, and dragonflies can hawk for insects on the wing, a task about as difficult as hitting a tennis ball. Much poorer eyes could be used for the task of avoiding crashing into a wall or walking over the edge of a cliff or into a river. Eyes that are even poorer could tell when a shadow, which might {77} be a cloud but could also portend a predator, looms overhead. And eyes that are still poorer could serve to tell the difference between night and day, which is useful for, among other things, synchronizing breeding seasons and knowing when to go to sleep. There is a continuum of tasks to which an eye might be put, such that for any given quality of eye, from magnificent to terrible, there is a level of task at which a marginal improvement in vision would make all the difference. There is therefore no difficulty in understanding the gradual evolution of the eye, from primitive and crude beginnings, through a smooth continuum of intermediates, to the perfection we see in a hawk or in a young human.

Thus the creationist's question – “What is the use of half an eye?” – is a lightweight question, a doddle to answer. Half an eye is just 1 percent better than 49 percent of an eye, which is already better than 48 percent, and the difference is significant. A more ponderous show of weight seems to lie behind the inevitable supplementary: “Speaking as a physicist, [5] I cannot believe that there has been enough time for an organ as complicated as the eye to have evolved from nothing. Do you really think there has been enough time?” Both questions stem from the Argument from Personal Incredulity. Audiences nevertheless appreciate an answer, and I have usually {78} fallen back on the sheer magnitude of geological time. If one pace represents one century, the whole of Anno Domini time is telescoped into a cricket pitch. To reach the origin of multi-cellular animals on the same scale, you'd have to slog all the way from New York to San Francisco.

It now appears that the shattering enormity of geological time is a steamhammer to crack a peanut. Trudging from coast to coast dramatizes the time available for the evolution of the eye. But a recent study by a pair of Swedish scientists, Dan Nilsson and Susanne Pelger, suggests that a ludicrously small fraction of that time would have been plenty. When one says “the” eye, by the way, one implicitly means the vertebrate eye, but serviceable image-forming eyes have evolved between forty and sixty times, independently from scratch, in many different invertebrate groups. Among these forty-plus independent evolutions, at least nine distinct design principles have been discovered, including pinhole eyes, two kinds of camera-lens eyes, curved-reflector (“satellite dish”) eyes, and several kinds of compound eyes. Nilsson and Pelger have concentrated on camera eyes with lenses, such as are well developed in vertebrates and octopuses.

How do you set about estimating the time required for a given amount of evolutionary change? We have to find a unit to measure the size of each evolutionary step, and it is sensible to express it as a percentage change in what is already there. Nilsson and Pelger used the number of successive changes of 1 percent as their unit for measuring changes of anatomical quantities. This is just a convenient unit – like the calorie, which is defined as the amount of energy needed to do a certain amount of work. It is easiest {79} to use the 1 percent unit when the change is all in one dimension. In the unlikely event, for instance, that natural selection favored bird-of-paradise tails of ever-increasing length, how many steps would it take for the tail to evolve from one meter to one kilometer in length? A 1 percent increase in tail length would not be noticed by the casual bird-watcher. Nevertheless, it takes surprisingly few such steps to elongate the tail to one kilometer – fewer than seven hundred.

Elongating a tail from one meter to one kilometer is all very well (and all very absurd), but how do you place the evolution of an eye on the same scale? The problem is that in the case of the eye, lots of things have to go on in lots of different parts, in parallel. Nilsson and Pelger's task was to set up computer models of evolving eyes to answer two questions. The first is essentially the question we posed again and again in the past several pages, but they asked it more systematically, using a computer: Is there a smooth gradient of change, from flat skin to full camera eye, such that every intermediate is an improvement? (Unlike human designers, natural selection can't go downhill – not even if there is a tempting higher hill on the other side of the valley.) Second – the question with which we began this section – how long would the necessary quantity of evolutionary change take?

In their computer models, Nilsson and Pelger made no attempt to simulate the internal workings of cells. They started their story after the invention of a single light-sensitive cell – it does no harm to call it a photocell. It would be nice, in the future, to do another computer model, this time at the level of the inside of the cell, to show how the first living {80} photocell came into being by step-by-step modification of an earlier, more general-purpose cell. But you have to start somewhere, and Nilsson and Pelger started after the invention of the photocell. They worked at the level of tissues: the level of stuff made of cells rather than the level of individual cells. Skin is a tissue, so is the lining of the intestine, so is muscle and liver. Tissues can change in various ways under the influence of random mutation. Sheets of tissue can become larger or smaller in area. They can become thicker or thinner. In the special case of transparent tissues like lens tissue, they can change the refractive index (the light-bending power) of local parts of the tissue.

The beauty of simulating an eye, as distinct from, say, the leg of a running cheetah, is that its efficiency can be easily measured, using the laws of elementary optics. The eye is represented as a two-dimensional cross section, and the computer can easily calculate its visual acuity, or spatial resolution, as a single real number. It would be much harder to come up with an equivalent numerical expression for the efficacy of a cheetah's leg or backbone. Nilsson and Pelger began with a flat retina atop a flat pigment layer and surmounted by a flat, protective transparent layer. The transparent layer was allowed to undergo localized random mutations of its refractive index. They then let the model deform itself at random, constrained only by the requirement that any change must be small and must be an improvement on what went before.

The results were swift and decisive. A trajectory of steadily mounting acuity led unhesitatingly from the flat beginning through a shallow indentation to a steadily deepening cup, as the shape of the model eye deformed itself on the computer screen. The transparent layer thickened to fill {81} the cup and smoothly bulged its outer surface in a curve. And then, almost like a conjuring trick, a portion of this transparent filling condensed into a local, spherical subre-gion of higher refractive index. Not uniformly higher, but a gradient of refractive index such that the spherical region functioned as an excellent graded-index lens. Graded-index lenses are unfamiliar to human lensmakers but they are common in living eyes. Humans make lenses by grinding glass to a particular shape. We make a compound lens, like the expensive violet-tinted lenses of modern cameras, by mounting several lenses together, but each one of those individual lenses is made of uniform glass through its whole thickness. A graded-index lens, by contrast, has a continuously varying refractive index within its own substance. Typically, it has a high refractive index near the center of the lens. Fish eyes have graded-index lenses. Now it has long been known that, for a graded-index lens, the most aberration-free results are obtained when you achieve a particular theoretical optimum value for the ratio between the focal length of the lens and the radius. This ratio is called Mattiessen's ratio. Nilsson and Pelger's computer model homed in unerringly on Mattiessen's ratio.

And so to the question of how long all this evolutionary change might have taken. In order to answer this, Nilsson and Pelger had to make some assumptions about genetics in natural populations. They needed to feed their model plausible values of quantities such as “heritability.” Heritability is a measure of how far variation is governed by heredity. The favored way of measuring it is to see how much monozygotic (that is, “identical”) twins resemble each other compared with ordinary twins. One study found the heritability of leg {82} length in male humans to be 77 percent. A heritability of 100 percent would mean that you could measure one identical twin's leg to obtain perfect knowledge of the other twin's leg length, even if the twins were reared apart. A heritability of 0 percent would mean that the legs of monozygotic twins are no more similar to each other than to the legs of random members of a specified population in a given environment. Some other heritabilities measured for humans are 95 percent for head breadth, 85 percent for sitting height, 80 percent for arm length and 79 percent for stature.

Heritabilities are frequently more than 50 percent, and Nilsson and Pelger therefore felt safe in plugging a heritability of 50 percent into their eye model. This was a conservative, or “pessimistic,” assumption. Compared with a more realistic assumption of, say, 70 percent, a pessimistic assumption tends to increase their final estimate of the time taken for the eye to evolve. They wanted to err on the side of overestima-tion because we are intuitively skeptical of short estimates of the time taken to evolve something as complicated as an eye.

For the same reason, they chose pessimistic values for the coefficient of variation (that is, for how much variation there typically is in the population) and the intensity of selection (the amount of survival advantage improved eyesight confers). They even went so far as to assume that any new generation differed in only one part of the eye at a time: simultaneous changes in different parts of the eye, which would have greatly speeded up evolution, were outlawed. But even with these conservative assumptions, the time taken to evolve a fish eye from flat skin was minuscule: fewer than four hundred thousand generations. For the kinds of small animals we are talking about, we can assume one generation per year, so {83} it seems that it would take less, than half a million years to evolve a good camera eye.

In the light of Nilsson and Pelger's results, it is no wonder “the” eye has evolved at least forty times independently around the animal kingdom. There has been enough time for it to evolve from scratch fifteen hundred times in succession within any one lineage. Assuming typical generation lengths for small animals, the time needed for the evolution of the eye, far from stretching credulity with its vastness, turns out to be too short for geologists to measure! It is a geological blink.

Do good by stealth. A key feature of evolution is its gradu-alness. This is a matter of principle rather than fact. It may or may not be the case that some episodes of evolution take a sudden turn. There may be punctuations of rapid evolution, or even abrupt macromutations – major changes dividing a child from both its parents. There certainly are sudden extinctions – perhaps caused by great natural catastrophes such as comets striking the earth – and these leave vacuums to be filled by rapidly improving understudies, as the mammals replaced the dinosaurs. Evolution is very possibly not, in actual fact, always gradual. But it must be gradual when it is being used to explain the coming into existence of complicated, apparently designed objects, like eyes. For if it is not gradual in these cases, it ceases to have any explanatory power at all. Without gradualness in these cases, we are back to miracle, which is simply a synonym for the total absence of explanation.

The reason eyes and wasp-pollinated orchids impress us so is that they are improbable. The odds against their spontaneously assembling by luck are odds too great to be borne in {84} the real world. Gradual evolution by small steps, each step being lucky but not too lucky, is the solution to the riddle. But if it is not gradual, it is no solution to the riddle: it is just a restatement of the riddle.

There will be times when it is hard to think of what the gradual intermediates may have been. These will be challenges to our ingenuity, but if our ingenuity fails, so much the worse for our ingenuity. It does not constitute evidence that there were no gradual intermediates. One of the most difficult challenges to our ingenuity in thinking of gradual intermediates is provided by the celebrated “dance language” of the bees, discovered in the classic work for which Karl von Frisch is best known. Here the end product of the evolution seems so complicated, so ingenious and far removed from anything we would ordinarily expect an insect to do, that it is hard to imagine the intermediates.

Honeybees tell each other the whereabouts of flowers by means of a carefully coded dance. If the food is very close to the hive, they do the “round dance.” This just excites other bees, and they rush out and search in the vicinity of the hive: Not particularly remarkable. But very remarkable is what happens when the food is farther away from the hive. The forager who has discovered the food performs the so-called “waggle dance,” and its form and timing tell the other bees both the compass direction and the distance from the hive of the food. The waggle dance is performed inside the hive on the vertical surface of the comb. It is dark in the hive, so the waggle dance is not seen by the other bees. It is felt by them, and also heard, for the dancing bee accompanies her performance with little rhythmic piping noises. The dance has the form of a figure eight, with a straight run in the middle. It is the direction of {85} the straight run that, in the form of a cunning code, tells the direction of the food.

The straight run of the dance does not point directly toward the food. It cannot, since the dance is performed on the vertical surface of the comb and the alignment of the comb itself is fixed regardless of where the food might be. The food has to be located in horizontal geography. The vertical comb is more like a map pinned to the wall. A line drawn on a wall map doesn't point directly toward a particular destination, but you can read the direction by means of an arbitrary convention.

To understand the convention the bees use, you must first know that bees, like many insects, navigate using the sun as a compass. We do this too, in an approximate way. The method has two drawbacks. First, the sun is often hidden behind clouds. Bees solve this problem by means of a sense we don't have. Again, it was von Frisch who discovered that they can see the direction of polarization of light and this tells them where the sun is even if the sun itself is invisible. The second problem with a sun compass is that the sun “moves” across the sky as the hours progress. Bees cope with this by using an internal clock. Von Frisch found, almost unbelievably, that dancing bees trapped in the hive for hours after their foraging expedition would slowly rotate the direction of the straight run of the dance, as if this run were the hour hand of a twenty-four-hour clock. They could not see the sun inside the hive, but they were slowly angling the direction of their dance in order to keep pace with the movement of the sun, which, their internal clocks told them, must be going on outside. Fascinatingly, bee races hailing from the Southern Hemisphere do the same thing in reverse, just as they should. {86}

Now, to the dance code itself. A dance run pointing straight up the comb signals that food is in the same direction as the sun. Straight down signals food in the exact opposite direction. All intermediate angles signal what you would expect. Fifty degrees to the left of the vertical signifies 50° to the left of the sun's direction in the horizontal plane. The accuracy of the dance is not to the nearest degree, however. Why should it be, for it is our arbitrary convention to divide the compass into 360°? Bees divide the compass into about 8 bee degrees. Actually, this is approximately what we do when we are not professional navigators. We divide our informal compass into eight quadrants: N, NE, E, SE, S, SW, W, NW.

The bee dance codes the distance of food, too. Or rather, various aspects of the dance – the rate of turning, the rate of waggling, the rate of peeping – are correlated with the distance of the food, and any one of them or any combination of them could therefore be used by the other bees to read the distance. The nearer the food, the faster the dance. You can remember this by reflecting that a bee that has found food near the hive might be expected to be more excited, and less tired, than a bee that has found food a long distance away. This is more than just an aide-memoire; it gives a clue to how the dance evolved, as we shall see.

To summarize, a foraging bee finds a good source of food. She returns to the hive, laden with nectar and pollen, and delivers her cargo to receiving workers. Then she begins her dance. Somewhere on a vertical comb, it doesn't matter where, she rushes round and round in a tight figure eight. Other worker bees cluster around her, feeling and listening. They count the rate of peeping and perhaps the rate of turning too. They measure, relative to the vertical, the angle of the {87} straight run of the dance while the dancer is waggling her abdomen. They then move to the hive's door and burst out of the darkness into the sunlight. They observe the position of the sun – not its vertical height but its compass bearing in the horizontal plane. And they fly off in a straight line, whose angle relative to the sun matches the angle of the original forager's dance relative to the vertical on the comb. They keep flying on this bearing, not for an indefinite distance but for a distance (inversely) proportional to (the logarithm of) the rate of peeping of the original dancer. Intriguingly, if the original bee had flown in a detour in order to find the food, she points her dance not in the direction of her detour but in the reconstructed compass direction of the food.

The story of the dancing bees is hard to believe, and some have disbelieved it. I'll return to the skeptics, and to the recent experiments that finally clinched the evidence, in the next chapter. In this chapter, I want to discuss the gradual evolution of the bee dance. What might the intermediate stages in its evolution have looked like, and how did they work when the dance was still incomplete?

The way the question is phrased is not quite right, by the way. No creature makes a living at being an “incomplete,” “intermediate stage.” The ancient, long-dead bees whose dances can be interpreted, with hindsight, as intermediates on the way to the modern honeybee dance made a good living. They lived full bee lives and had no thought of being “on the way” to something “better.” Moreover, our “modern” bee dance may not be the last word but may evolve into something even more spectacular when we and our bees are gone. Nevertheless, we do have the puzzle of how the current bee dance could have evolved by gradual degrees. How might {88} those graduated intermediates have looked, and how did they work?

Von Frisch himself has attended to the question, and he tackled it by looking around the family tree, at modern distant cousins of the honeybee. These are not ancestors of the honeybee, for they are its contemporaries. But they may retain features of the ancestors. The honeybee itself is a temperate-zone insect that nests for shelter in hollow trees or caves. Its closest relatives are tropical bees that can nest in the open, hanging their combs from tree boughs or rocky outcrops. They are therefore able to see the sun while dancing, and they don't have to resort to the convention of letting the vertical “stand for” the direction of the sun. The sun can stand for itself.

One of these tropical relations, the dwarf bee Apis florea, dances on the horizontal surface on top of the comb. The straight run of the dance points directly toward the food. There is no need for a map convention; direct pointing will do. A plausible transitional stage on the road to the honeybee, certainly, but we still have to think about the other intermediates that preceded and followed this stage. What could have been the forerunners of the dwarf bee's dance? Why should a bee that has recently found food rush round and round in a figure eight whose straight run points toward the food? The suggestion is that it is a ritualized form of the take-off run. Before the dance evolved, von Frisch suggests, a forager that has just unloaded food would simply take off in the same direction, to fly back to the food source. Preparatory to launching itself into the air, it would turn its face in the right direction and might walk a few steps. Natural selection would have favored any tendency to exaggerate or prolong the takeoff run if it encouraged other bees to follow. Perhaps the {89} dance is a kind of ritually repeated take-off run. This is plausible because, whether or not they use a dance, bees frequently use the more direct tactic of simply following each other to food sources. Another fact that gives the idea plausibility is that dancing bees hold their wings out slightly, as though preparing to fly, and they vibrate the wing muscles, not vigorously enough to take off but enough to make the noise that is an important part of the dance signal.

An obvious way to prolong and exaggerate the take-off run is to repeat it. Repeating it means going back to the start and again taking a few steps in the direction of the food. There are two ways of going back to the start: you can turn right or turn left at the end of the runway. If you consistently turn left or consistently turn right, it will be ambiguous which direction is the true take-off direction and which the return journey to the start of the runway. The best way to remove ambiguity is to turn alternately left and right. Hence the natural selection of the figure-eight pattern.

But how did the relationship between distance of food and rate of dancing evolve? If the rate of dancing were positively related to the distance of the food, it would be hard to explain. But, you'll remember, it is actually the other way around: the closer the food, the faster the dance. This immediately suggests a plausible pathway of gradual evolution. Before the dance proper evolved, foragers might have performed their ritualized repetition of the take-off run but at no particular speed. The rate of dancing would have been whatever they happened to feel like. Now, if you had just flown home from several miles away, laden to the gunwales with nectar and pollen, would you feel like charging at high speed around the comb? No, you would probably be exhausted. On {90} the other hand, if you had just discovered a rich source of food rather close to the hive, your short homeward journey would have left you fresh and energetic. It is not difficult to imagine how an original accidental relationship between distance of food and slowness of dance could have become ritualized into a formal, reliable code.

Now for the most challenging intermediate of all. How did an ancient dance in which the straight run pointed directly toward the food get transformed into a dance in which the angle relative to the vertical becomes a code for the angle of the food relative to the sun? Such a transformation was necessary partly because the interior of the honeybee hive is dark and you can't see the sun, and partly because when dancing on a vertical comb you can't point directly toward the food unless the surface itself happens to be pointing toward the food. But it isn't enough to show that some such transformation was necessary. We also have to explain how this difficult transition was achieved via a plausible series of step-by-step intermediates.

It seems baffling, but a singular fact about the insect nervous system comes to our rescue. The following remarkable experiment has been done on a variety of insects, from beetles to ants. Start with a beetle walking along a horizontal wooden board in the presence of an electric light. The first thing to show is that the insect is using a light-compass. Shift the position of the lightbulb, and the insect will change its direction accordingly. If it was holding a bearing of, say, 30° to the light, it will change its path so as to maintain a bearing of 30° to the new position of the light. In fact you can steer the beetle wherever you like, using the light beam as a tiller. This fact about insects has long been known: they use the sun (or moon {91} or stars) as a compass, and you can easily fool them with a lightbulb. So far so good. Now for the interesting experiment. Switch the light off and at the same moment tilt the board into the vertical. Undaunted, the beetle continues walking. And, mirabile dictu, it shifts its direction of walking so that its angle relative to the vertical is the same as the previous angle relative to the light: 30° in our example. Nobody knows why this happens, but it does. It seems to betray an accidental quirk of the insect nervous system – a confusion of senses, a crossing of wires between the gravity sense and the sense of sight, perhaps a bit like our seeing a flash of light when hit on the head. At all events, it probably provided the necessary bridge for the evolution of the “vertical stands for sun” code of the honeybee's dance.

Revealingly, if you switch on a light inside a hive, honeybees abandon their gravity sense and use the direction of the light to stand, directly, for the sun in their code. This fact, long known, has been exploited in one of the most ingenious experiments ever performed, the experiment that finally clinched the evidence that the honeybee dance really works. I'll return to this in the next chapter. Meanwhile, we have found a plausible series of graded intermediates by which the modern bee dance could have evolved from simpler beginnings. The story as I have told it, based on von Frisch's ideas, may not actually be the right one. But something a bit like it surely did happen. I told the story as an answer to the natural skepticism – the Argument from Personal Incredulity – that arises in people when they are faced with a really ingenious or complicated natural phenomenon. The skeptic says, “I cannot imagine a plausible series of intermediates, therefore there were none, and the phenomenon arose by a spontaneous {92} miracle.” Von Frisch has provided a plausible series of intermediates. Even if it is not quite the right series, the fact that it is plausible is enough to confound the Argument from Personal Incredulity. The same is true of all the other examples we have looked at, from wasp-mimicking orchids to camera eyes.

Any number of curious and intriguing facts of nature could be mustered by people skeptical of gradualistic Darwinism. I have been asked to explain, for example, the gradual evolution of those creatures that live in the deep trenches of the Pacific Ocean, where there is no light and where water pressures may exceed 1000 atmospheres. A whole community of animals has grown up around hot, volcanic vents deep in the Pacific trenches. A whole alternative biochemistry is run by bacteria, using the heat from the vents and metabolizing sulfur instead of oxygen. The community of larger animals is ultimately dependent on these sulfur bacteria, just as ordinary life is dependent on green plants capturing energy from the sun.

The animals in the sulfur community are all relatives of more conventional animals found elsewhere. How did they evolve and through what intermediate stages? Well, the form of the argument will be exactly the same. All we need for our explanation is at least one natural gradient, and gradients abound as we descend in the sea. A thousand atmospheres js a horrendous pressure, but it is only quantitatively greater, than 999 atmospheres, which is only quantitatively greater than 998 and so on. The sea bottom offers gradients of depth from 0 feet through all intermediates to 33,000 feet. Pressures vary smoothly from 1 atmosphere to 1000 atmospheres. Light intensities vary smoothly from bright daylight near the surface {93} to total darkness in the deeps, relieved only by rare clusters of luminescent bacteria in the luminous organs of fishes. There are no sharp cut-offs. For every level of pressure and darkness already adapted to, there will be a design of animal, only slightly different from existing animals, that can survive one fathom deeper, one lumen darker. For every… but this chapter is more than long enough. You know my methods, Watson. Apply them. {94}