for regulating growth and hormonal development. Somehow in their digital language, these genes will tell the foot of a human foetus to grow into a flat object with a heel and a big toe, whereas the same genes in a chimpanzee tell the foot of a chimp foetus to grow into a more curved object with less of a heel and longer, more prehensile toes.

It is mind-boggling even to try to imagine how that can be done — science still has only the vaguest clues about how growth and form are generated by genes - but that genes are responsible is not in doubt. The differences between human beings and chimpanzees are genetic differences and virtually nothing else. Even those who would stress the cultural side of the human condition and deny or doubt the importance of genetic differences between human individuals or races, accept that the differences between us and other species are primarily genetic. Suppose the nucleus of a chimpanzee cell were injected into an enucleated human egg and that egg were implanted into a human womb, and the resulting baby, if it survived to term, were reared in a human family. What would it look like? You do not even need to do the (highly unethical) experiment to know the answer: a chimpanzee. Although it started with human cytoplasm, used a human placenta and had a human upbringing, it would not look even partly human.

Photography provides a helpful analogy. Imagine you take a photograph of a chimpanzee. To develop it you must put it in a bath of developer for the requisite time, but no matter how hard you try, you cannot develop a picture of a human being on the negative by changing the formula of the developer. The genes are the negative; the womb is the developer. Just as a photograph needs to be immersed in a bath of developer before the picture will appear, so the recipe for a chimpanzee, written in digital form in the genes of its egg, needs the correct milieu to become an adult - the nutrients, the fluids, the food and the care - but it already has the information to make a chimpanzee.

The same is not quite true of behaviour. The typical chimpanzee's hardware can be put together in the womb of a foreign species, but its software would be a little awry. A baby chimpanzee would be as socially confused if reared by human beings as Tarzan would be if reared by chimps. Tarzan, for instance, would not learn to speak, and a human-reared chimp would not learn precisely how to appease dominant animals and intimidate subordinates, to make tree nests or to fish for termites. In the case of behaviour, genes are not sufficient, at least in apes.

But they are necessary. If it is mind-boggling to imagine how small differences in linear digital instructions can direct the two per cent difference between a human body and a chimpanzee body, how much more mind-boggling is it to imagine that a few changes in the same instructions can alter the behaviour of a chimpanzee so precisely. I wrote glibly of the mating system of different apes — the promiscuous chimpanzee, the harem-polygamous gorilla and the long-pair-bond human being. In doing so I assumed, even more glibly, that every species behaves in a characteristic way, which, further, assumes that it is somehow at least partly genetically constrained or influenced. How can a bunch of genes, each one a string of quaternary code, make an animal polygamous or monogamous? Answer: I do not have the foggiest idea, but that it can do so I have no doubt. Genes are recipes for both anatomy and behaviour.

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