Palaeoanthropology, more than other fields of science, is notoriously plagued - or is it enlivened? - by rivalries. We have to admit that the fossil record connecting the upright-walking ape Australopithecus to the (presumably) quadrupedal ancestor that we share with chimpanzees is still poor. We don't know how our ancestors rose on to their hind legs. We need more fossils. But let's at least rejoice in the good fossil record that we - unlike Darwin - can enjoy, showing us the evolutionary transition from Australopithecus, with its chimpanzee-sized brain, to modern Homo sapiens with our balloon-like skull and big brain.

Throughout this section I have reproduced pictures of skulls and encouraged you to compare them. You perhaps noticed, for example, the protrusion of the muzzle in some fossils, or of the brow ridge. Sometimes the differences are quite subtle, which aids appreciation of the gradual transitions from one fossil to a later one. But now I want to introduce a complication, which will develop into an interesting point in its own right. The changes that take place within an individual's lifetime, as it grows up, are in any case much more dramatic than the changes we see as we compare adults in successive generations.

Chimpanzee shortly before birth

The skull below belongs to a chimpanzee shortly before birth. It is obviously completely different from the adult chimpanzee skull shown on page 187 and far more like a human (adult human as well as baby human). There's a much-reproduced picture (reproduced again overleaf) of an infant and an adult chimpanzee, which is often used to illustrate the interesting idea that in human evolution juvenile characteristics were retained into adulthood (or - which may not necessarily be quite the same thing - we become sexually mature when our bodies are still juvenile). I thought the picture looked too good to be true, and I sent it to my colleague Desmond Morris for his expert opinion. Could it be a fake, I asked him? Had he ever seen a young chimp looking quite so human? Dr Morris is sceptical about the back and shoulders but is happy about the head itself. 'Chimps are characteristically hunched-up in posture and this one has a wonderfully upright human neck. But if you just take the head alone, the picture can be trusted.' Sheila Lee, the publishers' picture researcher for this book, tracked down the original source of this famous photograph, an expedition to the Congo in 1909-15 mounted by the American Museum of Natural History. The animals were dead when photographed, and she points out that the photographer, Herbert Lang, was also a taxidermist. It would be tempting to surmise that the oddly human posture of the baby chimpanzee is due to bad stuffing - were it not for the fact that, according to the museum, Lang photographed his specimens before stuffing them. Nevertheless, the posture of a dead chimp can be adjusted in the way that the posture of a live chimp cannot. Desmond Morris's conclusion seems to stand up. The human-like posture of the baby chimpanzee's shoulders may be suspect, but the head is reliable.

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.

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.

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