The Greatest Show on Earth (20 page)

Lang’s photos of infant and adult chimpanzees

Taking the head at face value, even if the shoulders can’t quite bear the burden of authenticity, you can immediately see how a comparison of adult fossil skulls might mislead us. Or, to put it more constructively, the dramatic difference between adult and juvenile heads shows us how easily a characteristic such as muzzle protrusion might change in just the right direction to become more – or indeed less – human. Chimpanzee embryology ‘knows’ how to make a human-like head, because it does it for every chimp as it passes through its infant years. It seems highly plausible that, as Australopithecus evolved through various intermediates to Homo sapiens, shortening the muzzle all along the way, it did so by the obvious route of retaining juvenile characteristics into adulthood (the process called neoteny, mentioned in Chapter 2). In any case, a great deal of evolutionary change consists of changes in the rate at which certain parts grow, relative to other parts. This is called heterochronic (‘differently timed’) growth. I suppose what I want to say is that evolutionary change is a doddle, once you accept the observed facts of embryological change. Embryos are shaped by differential growth – different bits of them grow at different rates. A baby chimpanzee’s skull changes into an adult’s skull via relatively fast growth of the bones of the jaws and muzzle compared to other bones of the skull. To repeat, every animal of every species changes, during its own embryological development, far more dramatically than the typical adult form changes from generation to generation as the geological ages go by. And this is my cue for a chapter on embryology and its relevance to evolution.

* Predictably, the Peking fossil is now sometimes called Beijing Man. Why, since we are talking English rather than Chinese, do we go along with ‘Beijing’ at all, when referring to China’s capital? There’s a rather charming programme on British television called Grumpy Old Men, which is a genially edited collection of grouses and grizzles of just this kind. If I were on it, I would say something like the following. We don’t dab on a splash of Eau de Köln to drown out the smell of Mumbai Duck, or go waltzing to the strains of ‘The Blue Dunaj’ or ‘Tales from the Wien Woods’. We don’t compare Neville Chamberlain, the Man of München, to Napoleon’s retreat from Moskva. Nor yet (though give it time) do we take our snuffling little pet Beij for walkies. What’s wrong with Peking, when it’s the English language we are speaking? I was delighted recently to meet a member of the British diplomatic corps, fluent in Mandarin, who had played a leading role in our embassy in what he insisted on calling Peking.

* Pen name Isak Dinesen, but I like to use her real name because I spent my earliest childhood near Karen, the village ‘at the foot of the Ngong Hills’ which is still named after her.

* Unlike diseases, which are often named after their discoverers, new species are named by their discoverers but never after themselves. It is a nice opportunity for a biologist to honour the name of another, or, as in this case, a benefactor. Not surprisingly, my distinguished colleague the late W. D. Hamilton was several times honoured in this way. Arguably one of Darwin’s greatest successors of the twentieth century, he had a lugubrious manner reminiscent of A. A. Milne’s Eeyore (not the deplorable Walt Disney version, of course). Hamilton was once on a small boat on an expedition up the Amazon when he was stung by a wasp. Knowing what a great entomologist he was, his companion said, ‘Bill, do you know the name of that wasp?’ ‘Yes,’ Bill murmured gloomily in his most Eeyoreish voice. ‘As a matter of fact it’s named after me.’

YOU DID IT YOURSELF IN NINE MONTHS

THAT irascible genius J. B. S. Haldane, who did so much else besides being one of the three leading architects of neo-Darwinism, was once challenged by a lady after a public lecture. It’s a word-of-mouth anecdote, and John Maynard Smith is sadly not available to confirm the exact words, but this is approximately how the exchange went:Evolution sceptic: Professor Haldane, even given the billions of years that you say were available for evolution, I simply cannot believe it is possible to go from a single cell to a complicated human body, with its trillions of cells organized into bones and muscles and nerves, a heart that pumps without ceasing for decades, miles and miles of blood vessels and kidney tubules, and a brain capable of thinking and talking and feeling.
JBS: But madam, you did it yourself. And it only took you nine months.
The questioner was perhaps momentarily thrown off balance by the veering unexpectedness of Haldane’s reply. Wind taken out of sails would have seemed an understatement. But maybe in one respect Haldane’s retort left her unsatisfied. I don’t know whether she asked a supplementary but, if so, it might have gone along these lines:
Evolution sceptic: Ah yes, but the developing embryo follows genetic instructions. It is the instructions for how to build a complicated body that you, Professor Haldane, claim evolved by natural selection. And I still find it hard to believe, even given a billion years for that evolution.
Perhaps she had a point. And even if a divine intelligence did prove to be ultimately responsible for designing living complexity, it is definitely not true that he fashions living bodies in anything like the way that clay modellers, for example, or carpenters, potters, tailors or car manufacturers go about their tasks. We may be ‘wonderfully developed’ but we are not ‘wonderfully made’. When children sing, ‘He made their glowing colours / He made their tiny wings’,* they are uttering a childishly obvious falsehood. Whatever else God does, he certainly doesn’t make glowing colours and tiny wings. If he did anything at all, it would be to supervise the embryonic development of things, for example by splicing together sequences of genes that direct a process of automated development. Wings are not made, they grow – progressively – from limb buds inside an egg.
God, to repeat this important point, which ought to be obvious but isn’t, never made a tiny wing in his eternal life. If he made anything (he didn’t in my view, but let it pass, that’s not what I’m about here), what he made was an embryological recipe, or something like a computer program for controlling the embryonic development of a tiny wing (plus lots of other things too). Of course, God might claim that it is just as clever, just as breathtaking a feat of skill, to design a recipe or a program for a wing, as to make a wing. But for the moment, I just want to develop the distinction between making something like a wing, and what really happens in embryology.

NO CHOREOGRAPHER

The early history of embryology was riven between two opposing doctrines called preformationism and epigenesis. The distinction between them is not always clearly understood, so I shall spend a little time explaining these two terms. The preformationists believed that the egg (or sperm, for the preformationists were subdivided into ‘ovists’ versus ‘spermists’) contained a tiny miniature baby or ‘homunculus’. All the parts of the baby were intricately in place, correctly disposed to each other, waiting only to be inflated like a compartmentalized balloon. This raises obvious problems. First, at least in its early naïve form, it requires what everybody knows to be false: that we inherit only from one parent – the mother for the ovists, the father for the spermists. Second, preformationists of this kind had to face a Russian-doll-style infinite regress of homunculi within homunculi – or if not infinite, at least long enough to take us back to Eve (Adam for the spermists). The only escape from the regress would be to construct the homunculus afresh in every generation by elaborately scanning the adult body of the previous generation. This ‘inheritance of acquired characteristics’ doesn’t happen – otherwise Jewish boys would be born without foreskins, and gym-frequenting body-builders (but not their couch-potato twins) would conceive babies with rippling six-packs, pecs and glutes.
To be fair to the preformationists, they did face up, fairly and squarely, to the logical necessity of the regress, however absurd it seemed. At least some of them really did believe that the first woman (or man) contained miniaturized embryos of all her descendants, nested inside each other like Russian dolls. And there is a sense in which they had to believe that: a sense that is worth mentioning because it prefigures the nub of this chapter. If you believe Adam was ‘made’ rather than being born, you imply that Adam didn’t have genes – or at least didn’t need them in order to develop. Adam had no embryology but just sprang into existence. A related inference led the Victorian writer Philip Gosse (the father in Edmund Gosse’s Father and Son) to write a book called Omphalos (Greek for ‘navel’) arguing that Adam must have had a navel, even though he was never born. A more sophisticated consequence of omphalogical reasoning would be that stars whose distance from us is more than a few thousand light years must have been created with ready-made light beams stretching almost all the way to us – otherwise we wouldn’t be able to see them until the distant future! Making fun of omphalogy sounds frivolous, but there is a serious point here about embryology, which is the subject of this chapter. It is quite a difficult point to grasp – indeed, I am only in the process of grasping it myself – and I am approaching it from various directions.
For the reasons given, preformationism, at least in its original ‘Russian doll’ version, was always a non-starter. Is there a version of preformationism that could be sensibly revived in the DNA age? Well, perhaps, but I doubt it. Textbooks of biology repeat time and again that DNA is a ‘blueprint’ for building a body. It isn’t. A true blueprint of, say, a car or a house embodies a one-to-one mapping from paper to finished product. It follows from this that a blueprint is reversible. It is as easy to go from house to blueprint as the other way around, precisely because it is a one-to-one mapping. Actually, it’s easier, because you have to build the house, but you only have to take some measurements and then draw the blueprint. If you take an animal’s body, no matter how many detailed measurements you take, you can’t reconstruct its DNA. That’s what makes it false to say that DNA is a blueprint.
It is theoretically possible to imagine – maybe that’s the way things work on some alien planet – that DNA might have been a coded description of a body: a kind of three-dimensional map rendered into the linear code of DNA ‘letters’. That really would be reversible. Scanning the body to make a genetic blueprint is not a totally ridiculous idea. If that is how DNA worked, we could represent it as a kind of neo-preformationism. It wouldn’t raise the spectre of the Russian dolls. It isn’t clear to me whether it would raise the spectre of inheritance from only one parent. DNA has a breathtakingly precise way of intersplicing half the paternal information with exactly half the maternal information, but how would it go about intersplicing half a scan of the mother’s body with half a scan of the father’s body? Let it pass: this is all so far from reality.
DNA, then, is emphatically not a blueprint. Unlike Adam, who was fashioned directly into his adult form, all real bodies develop and grow from a single cell through the intermediate stages of embryo, foetus, baby, child and adolescent. Maybe in some alien world living creatures assemble themselves from tip to toe as an ordered set of three-dimensional bio-pixels read out from a coded scan line. But that is not the way things work on our planet, and actually I think there are reasons – which I have dealt with elsewhere and so won’t go into here – why it could never be so on any planet.*
The historical alternative to preformationism is epigenesis. If preformationism is all about blueprints, epigenesis is about something more like a recipe or a computer program. The Shorter Oxford English Dictionary’s definition is rather modern, and I’m not sure that Aristotle, who coined the word, would recognize it:
epigenesis: A theory of the development of an organism by progressive differentiation of an initially undifferentiated whole.*
Principles of Development, by Lewis Wolpert and colleagues, describes epigenesis as the idea that new structures arise progressively. There is a sense in which epigenesis is self-evidently true, but details matter and the devil is in the cliché. How does the organism develop progressively? How does an initially undifferentiated whole ‘know’ how to differentiate progressively, if not by following a blueprint? The distinction I want to make in this chapter, which largely corresponds to the distinction between preformationism and epigenesis, is the distinction between planned architecture and self-assembly. The meaning of planned architecture is clear to us because we see it all around us in our buildings and other artefacts. Self-assembly is less familiar, and will need some attention from me. In the field of development, self-assembly occupies a position analogous to natural selection in evolution, although it is definitely not the same process. Both achieve, by automatic, non-deliberate, unplanned means, results that look, to a superficial gaze, as though they were meticulously planned.
J. B. S. Haldane spoke simple truth to his sceptical questioner, but he would not have denied that there is mystery, verging on the miraculous (but never quite getting there) in the very fact that a single cell gives rise to a human body in all its complexity. And the mystery is only somewhat mitigated by the feat’s being achieved with the aid of DNA instructions. The reason the mystery remains is that we find it hard to imagine, even in principle, how we might set about writing the instructions for building a body in the way that the body is in fact built, namely by what I have just called ‘self-assembly’, which is related to what computer programmers sometimes call a ‘bottom-up’, as opposed to ‘top-down’, procedure.
An architect designs a great cathedral. Then, through a hierarchical chain of command, the building operation is broken down into separate departments, which break it down further into sub-departments, and so on until instructions are finally handed out to individual masons, carpenters and glaziers, who go to work until the cathedral is built, looking pretty much like the architect’s original drawing. That’s top-down design.
Bottom-up design works completely differently. I never believed this, but there used to be a myth that some of the finest medieval cathedrals in Europe had no architect. Nobody designed the cathedral. Each mason and carpenter would busy himself, in his own skilled way, with his own little corner of the building, paying scant attention to what the others were doing and no attention to any overall plan. Somehow, out of such anarchy, a cathedral would emerge. If that really happened, it would be bottom-up architecture. Notwithstanding the myth, it surely didn’t happen like that for cathedrals.* But that pretty much is what happens in the building of a termite mound or an ant’s nest – and in the development of an embryo. It is what makes embryology so remarkably different from anything we humans are familiar with, in the way of construction or manufacture.
The same principle works for certain types of computer program, for certain types of animal behaviour, and – bringing the two together – for computer programs that are designed to simulate animal behaviour. Suppose we wanted to understand the flocking behaviour of starlings. There are some stunning films available on YouTube, from which the stills on colour page 16 are taken. These balletic manœuvres were photographed over Otmoor, near Oxford, by Dylan Winter. What is remarkable about the starlings’ behaviour is that, despite all appearances, there is no choreographer and, as far as we know, no leader. Each individual bird is just following local rules.
The numbers of individual birds in these flocks can run into thousands, yet they almost literally never collide. That is just as well for, given the speed at which they fly, any such impact would severely injure them. Often the whole flock seems to behave as a single individual, wheeling and turning as one. It can look as though the separate flocks are moving through each other in opposite directions, maintaining their coherence intact as separate flocks. This makes it seem almost miraculous, but actually the flocks are at different distances from the camera and do not literally move through each other. It adds to the aesthetic pleasure that the edges of the flocks are so sharply defined. They don’t peter off gradually, but come to an abrupt boundary. The density of the birds just inside the boundary is no less than in the middle of the flock, while it is zero outside the boundary. As soon as you think about it in that way, isn’t it wondrously surprising?
The whole performance would make a more than usually elegant screensaver on a computer. You wouldn’t want a real film of starlings because your screensaver would repeat the same identical balletic moves over and over, and therefore wouldn’t exercise all the pixels equally. What you would want is a computer simulation of starling flocks; and, as any programmer will tell you, there’s a right way and a wrong way to do it. Don’t try to choreograph the whole ballet – that would be terribly bad programming style for this kind of task. I need to talk about the better way to do it because something like it is almost certainly how the birds themselves are programmed, in their brains. More to the point, it is a great analogy for how embryology works.
Here’s how to program flocking behaviour in starlings. Devote almost all your effort to programming the behaviour of a single individual bird. Build into your robo-starling detailed rules for how to fly, and how to react to the presence of neighbouring starlings, depending on their distance and relative position. Build in rules for how much weight to give to the behaviour of neighbours, and how much weight to give to individual initiative in changing direction. These model rules would be informed by careful measurements of real birds in action. Endow your cyberbird with a certain tendency to vary its rules at random. Having written a complicated program to specify the behavioural rules of a single starling, now comes the definitive step that I am emphasizing in this chapter. Don’t try to program the behaviour of a whole flock, as an earlier generation of computer programmers might have done. Instead, clone the single computer starling you have programmed. Make a thousand copies of your robo-bird, maybe all the same as each other, or maybe with some slight random variation among them in their rules. And now ‘release’ thousands of model starlings in your computer, so they are free to interact with each other, all obeying the same rules.
If you’ve got the behavioural rules right for a single starling, a thousand computer starlings, each one a dot on the screen, will behave like real starlings flocking in winter. If the flocking behaviour isn’t quite right, you can go back and adjust the behaviour of the individual starling, perhaps in the light of further measurements of the behaviour of real starlings. Now clone up the new version a thousand times, in place of the thousand that didn’t quite work. Keep iterating your reprogramming of the cloned-up single starling, until the flocking behaviour of thousands of them on the screen is a satisfyingly realistic screensaver. Calling it ‘Boids’, Craig Reynolds wrote a program along these lines (not specifically for starlings) in 1986.
The key point is that there is no choreographer and no leader. Order, organization, structure – these all emerge as by-products of rules which are obeyed locally and many times over, not globally. And that is how embryology works. It is all done by local rules, at various levels but especially the level of the single cell. No choreographer. No conductor of the orchestra. No central planning. No architect. In the field of development, or manufacture, the equivalent of this kind of programming is self-assembly.

The body of a human, an eagle, a mole, a dolphin, a cheetah, a leopard frog, a swallow: these are so beautifully put together, it seems impossible to believe that the genes that program their development don’t function as a blueprint, a design, a master plan. But no: as with the computer starlings, it is all done by individual cells obeying local rules. The beautifully ‘designed’ body emerges as a consequence of rules being locally obeyed by individual cells, with no reference to anything that could be called an overall global plan. The cells of a developing embryo wheel and dance around each other like starlings in gigantic flocks. There are differences, and they are important. Unlike starlings, cells are physically attached to each other in sheets and blocks: their ‘flocks’ are called ‘tissues’. When they wheel and dance like miniature starlings, the consequence is that three-dimensional shapes are formed, as tissues invaginate in response to the movements of cells;* or swell or shrink due to local patterns of growth and cell death. The analogy I like for this is the paper-folding art of origami, suggested by the distinguished embryologist Lewis Wolpert in his book The Triumph of the Embryo; but before coming to that I need to clear out of the way some alternative analogies that might come to mind – analogies from among human crafts and manufacturing processes.