chimpanzees and australopithecines and the earliest ape-men fossils, males were one-and-a-half times the size of females, in modern people the ratio is much less. The steady decline of that ratio in the fossil record is one of the most overlooked features of our pre-history. What it means is that the mating system of the species was changing. The promiscuity of the chimp, with its short sexual li-aisons, and the harem polygamy of the gorilla, were being replaced with something much more monogamous: a declining ratio of sexual dimorphism is unambiguous evidence for that. But in a more monogamous system, there would now be pressure on each sex to choose its mate carefully; in polygamy, only the female is choosy. Long pair-bonds shackled each ape-man to its mate for much of its reproductive life: quality rather than quantity was suddenly important. For males it was suddenly vital to choose young mates, because young females had longer reproductive lives ahead of them. A preference for youthful, neotenous characters in either sex meant a preference for the large, domed cranium of youth, so it would have begun the drive towards bigger brains and all that followed therefrom.
Pushing us towards habitual monogamy, or at least pulling us further into it, was the sexual division of labour over food. Like no other species on the planet, we had invented a unique partnership between the sexes. By sharing plant food gathered by women, men had won the freedom to indulge the risky luxury of hunting for meat. By sharing hunted meat gathered by men, women had won access to high-protein, digestible food without having to abandon their young in seeking it. It meant that our species had a way of living on the dry plains of Africa that cut the risk of starvation; when meat was scarce, plant food filled the gap; when nuts and fruits were scarce, meat filled the gap. We had therefore acquired a high-protein diet without developing an intense specialisation for hunting the way the big cats did.
The habit acquired through the sexual division of labour had spread to other aspects of life. We had become compulsively good at sharing things, which had the new benefit of allowing each individual to specialise. It was this division of labour among specialists, S P E C I E S 3 5
unique to our species, that was the key to our ecological success, because it allowed the growth of technology. Today we live in societies that express the division of labour in ever more inventive and global ways.
From the here and now, these trends have a certain coherence.
Big brains needed meat (vegans today avoid protein-deficiency only by eating pulses); food sharing allowed a meaty diet (because it freed the men to risk failure in pursuit of game); food sharing demanded big brains (without detailed calculating memories, you could be easily cheated by a freeloader); the sexual division of labour promoted monogamy (a pair-bond being now an economic unit); monogamy led to neotenous sexual selection (by putting a premium on youthful-ness in mates). And so on, round and round the theories we go in a spiral of comforting justification, proving how we came to be as we are. We have built a scientific house of cards on the flimsiest foundations of evidence, but we have reason to believe that it will one day be testable. The fossil record will tell us only a little about behaviour; the bones are too dry and random to speak. But the genetic record will tell us more. Natural selection is the process by which genes change their sequences. In the process of changing, though, those genes laid down a record of our four-billion year biography as a biological lineage. They are, if we only know how to read them, a more valuable source of information on our past than the manuscripts of the Venerable Bede. In other words, a record of our past is etched into our genes.
Some two per cent of the genome tells the story of our different ecological and social evolution from that of chimpanzees, and theirs from us. When the genome of a typical human being has been fully transcribed into our computers, when the same has been done for the average chimpanzee, when the active genes have been extracted from the noise, and when the differences come to be listed, we will have an extraordinary glimpse of the pressures of the Pleistocene era on two different species derived from a common stock. The genes that will be the same will be the genes for basic biochemistry and body planning. Probably the only differences will be in genes 3 6 G E N O M E
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 S P E C I E S 3 7
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.
C H R O M O S O M E 3
