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 prehistory. What it means is that the mating system of the species was changing. The promiscuity of the chimp, with its short sexual liaisons, 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 pairbonds 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, 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
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