What seems to happen in these cases is that young animals do not yet show either fear or aggression, these developing last during the forward growth of the limbic system at the base of the brain. So the most likely way for evolution to produce a friendly or tame animal is to stop brain development prematurely. The effect is a smaller brain and especially a smaller ‘area 13’, a late-developing part of the limbic system that seems to have the job of disinhibiting adult emotional reactions such as fear and aggression. Intriguingly, such a taming process seems to have happened naturally in bonobos since their separation from the chimpanzee more than two million years ago. For its size the bonobo not only has a small head, but also reduced aggression and several juvenile features retained into adulthood including a white anal tail tuft, high-pitched calls and unusual female genitals. Bonobos have unusually small area 13s.40
So do human beings. Surprisingly, the fossil record suggests that there has been a rather steep decline in human brain size during the past 15,000 years, partly but not wholly reflecting a shrinking body size that seems to have accompanied the arrival of dense and ‘civilised’ human settlement. This followed several million years of more or less steady increases in brain size. In the Mesolithic (around 50,000 years ago) human brains averaged 1,468 cc (in females) and 1,567 cc (in males). Today the numbers have fallen to 1,210 cc and 1,248 cc, and even allowing for some reduction in body weight, this seems to be a steep decline. Perhaps there has been some recent taming of the species. If so, how? Richard Wrangham believes that once human beings became sedentary, living in permanent settlements, they could no longer tolerate anti-social behaviour and they began to banish, imprison or execute especially difficult individuals. In the past in highland New Guinea, more than one in ten of all adult deaths were by the execution of ‘witches’ (mostly men). This might have meant killing the more aggressive and impulsive – hence more developmentally mature and bigger-brained – people.41
Such self-taming, however, seems to be a recent phenomenon in our species and is not able to explain the selective pressures that led to the divergence of human beings from chimp-like ancestors more than five million years ago. But it does support the idea of evolution happening through the adjustment of gene promoters rather than genes themselves: hence the alteration of several irrelevant features caught in the slipstream of a reduction in impulsive aggression.42 Meanwhile, it is suddenly looking possible to understand how the human brain achieved its enlarged size in the first place, thanks to a newly discovered gene on chromosome 1. Following the completion of a dam in Mirpur, in Pakistani-controlled Kashmir in 1967, a large number of local people, displaced from their homes, migrated to Bradford in England. They included some who had married cousins, and among the offspring of these cousin marriages were a few people born with abnormally small, though otherwise normal brains – so-called microcephalics. The family pedigrees allowed scientists to pin down the cause as four different mutations in different families, but all affecting the same gene: the ASPM gene on chromosome 1.
On further investigation, a team of scientists led by Geoffrey Woods in Leeds discovered something rather extraordinary about the gene. It is a large gene, 10,434 letters long and split into 28 paragraphs (called exons). The 16th to 25th paragraphs contain a characteristic motif repeated over and over again. The phrase, usually 75 letters long, begins with the code for the amino acids isoleucine and glutamine, the significance of which I will reveal in a moment. In the human version of the gene, there are 74 such motifs, in the mouse 61, in the fruit fly 24 and in the nematode worm just 2 repetitions. Remarkably, these numbers seem to be in proportion to the number of neurons in the adult brain of the animal.43 Even more remarkably, the standard abbreviation for isoleucine is ‘I’ and the abbreviation for glutamine is ‘Q’. Therefore, the number of IQ repeats may determine the relative IQ of the species, which, according to Woods, ‘is a proof of God’s existence since only someone with a sense of humour could have arranged for the correlation’.44
ASPM seems to work by regulating the number of times neuronal stem cells divide inside the vesicles of the young brain about two weeks after conception. This in turn decides how many neurons the adult brain will have. To have stumbled on a gene with the power to decide brain size in such a simple manner seems almost too good to be true, and complications will undoubtedly crowd in upon this simple story as more comes to be known. But the ASPM gene vindicates that young man who was so startled by the Fuegians: evolution is a difference of degree, not kind.
The startling new truth that has emerged from the human genome – that animals evolve by adjusting the thermostats on the fronts of genes, enabling them to grow different parts of their bodies for longer – has profound implications for the nature–nurture debate. Imagine the possibilities in a system of this kind. You can turn up the expression of one gene, the product of which turns up the expression of another, which suppresses the expression of a third, and so on. And right in the middle of this little network, you can throw in the effects of experience. Something external – education, food, a fight, or requited love, say – can influence one of the thermostats. Suddenly nurture can start to express itself through nature.
CHAPTER TWO A plethora of instincts
When, as by a miracle, the lovely butterfly bursts from the chrysalis full-winged and perfect…it has, for the most part, nothing to learn, because its little life flows from its organization like melody from a music box.
Douglas Alexander Spalding, 18731
Like Charles Darwin, William James was a man of independent means. He inherited a private income from his father Henry, whose father William had amassed $10,000 a year from the Erie Canal. The one-legged Henry used his self-sufficiency to become an intellectual, and spent much of his life shuttling between New York, Geneva, London and Paris with his children in tow. He was articulate, religious and self-assured. His two youngest sons went off to fight in the Civil War, then failed in business and turned to drink or depression. His two eldest sons, William and Henry, were trained almost from birth to be intellectuals. The result was (in Rebecca West’s phrase) that ‘one of them grew up to write fiction as though it were philosophy and the other to write philosophy as though it were fiction’.2
Both brothers were influenced by Darwin. Henry’s novel The Portrait of a Lady was written in thrall to Darwin’s idea of female choice as a force in evolution.3 William’s Principles of Psychology, much of which was first published as a series of articles in the 1880s, contained a manifesto for nativism – the idea that the mind cannot learn unless it has the rudiments of innate knowledge – going against the prevailing fashion for empiricism, the theory that behaviour is shaped by experience. William James believed that human beings were equipped with innate tendencies that were not derived from experience but from the Darwinian process of natural selection. ‘He denies experience!’ wrote James, quoting an imaginary reader. ‘Denies science; believes the mind created by miracle; is a regular old partisan of innate ideas! That is enough! We’ll listen to such antediluvian twaddle no