This effect on the brain has been observed in a set of experiments into selective breeding. In the 1950s, Russian geneticist Dmitri Belyaev began a programme of research to see if he could domesticate the Siberian Silver Fox.4 Unlike modern dogs that are descendants of a strategy of selective breeding of wolves, most foxes have remained wild. Belyaev thought that domestication depended on temperament. Only those foxes that were less aggressive and less likely to run away when approached by the experimenter were chosen for breeding. These animals were tamer because, coded in their genes, they had slightly different brain chemistry regulating their behaviour. After only about a dozen or so generations of selective breeding, the offspring were markedly more docile. But they experienced significant physical changes too. They developed a white patch on their foreheads, were smaller than wild foxes and, like many dogs, had floppy ears. As Darwin noted in On the Origin of Species, ‘not a single domestic animal can be named which has not in some country drooping ears’. They also had smaller brains.
Breeding for tameness as opposed to aggression means selecting for physiological changes in the systems that govern the body’s hormones and neurochemicals. One possible mechanism to explain smaller brains is that individuals who are more passive may naturally have lower levels of the hormone testosterone. Testosterone is associated with aggression and dominance behaviour in animals, but its anabolic properties also play a role in body size by making muscles and organs larger and stronger. It also increases brain size. Sex-change individuals undergoing hormone treatment to facilitate the change to the opposite sex were found to have either increased or decreased brain volume depending on which hormone they were taking.5
Not only does domestication in animals lead to smaller brains, but it also changes the way they reason. Brian Hare, a leading expert on animal behaviour at Duke University, has shown that domesticated dogs in comparison to wild wolves are much better at reading the social signals of others. We humans can easily read the direction of another person’s gaze to indicate where their focus of attention is. As we shall see in later chapters, it is a social skill that is present in young infants but becomes more sophisticated with the more social interactions we have as we develop. Domesticated dogs can also read human social signals6 such as gaze and even the uniquely human gesture of pointing with the hand, whereas wolves and most other animals are generally baffled or indifferent.
Most fascinating is the change in dependence. Wolves will persist in trying to solve a difficult task through cunning, using different solutions, whereas the dog will typically give up earlier and try to recruit the help of its master. Domestication not only makes the animal more socially skilled but also more dependent on others. Over the years, several of the domesticated foxes from the breeding farms in Russia escaped back into the wild, only to return days later, unable to survive on their own.7 They were dependent on those that had raised them.
Could domestication apply to human evolution as well? As a young researcher at Harvard, Hare went to a dinner where Richard Wrangham, a distinguished primatologist in the anthropology department, described how bonobos, the pygmy chimpanzee species famous for their sexual promiscuity when resolving disputes, were an evolutionary puzzle with a set of unusual traits not found in chimpanzees. Hare realized this was true of silver foxes as well. The more he looked at the similarities between domesticated animals and bonobos, and the way they differed from chimps, the more the evidence seemed to support a hypothesis that this subspecies of primate had become self-domesticated. The way their social groups had evolved placed a greater emphasis on social skills and conciliation rather than aggression. If this was true for bonobos, then why not humans?8 After all, humans are also primates that have evolved the most remarkable capacity for social interaction. Hare would later write, ‘human levels of flexibility in using others’ social cues may have evolved in the human lineage only following the emergence of species-specific social emotions that provide motivation to attend to other individuals’ behaviour and, subsequently, their communicative intent during purely cooperative interactions’.9 In other words, the need to be more sociable by cooperating had altered the operations of the early hominid brain.
This is an old idea that has lately been re-seeded with new research and potential mechanisms. It first appeared under the guise of Social Darwinism in the nineteenth century – the idea that there were selective pressures that emerged from living together that changed the nature of the individuals. At first glance, it seems a bizarre hypothesis that living peacefully together caused the human brain to change, let alone shrink. After all, humans have been civilized for much longer than 20,000 years, with many earlier examples of societies, religions, art and culture. The recent discovery of stone artefacts on the Indonesian island of Flores that date to one million years ago tentatively indicate that an early hominin ancestor, Homo erectus, had inhabited the island.10 If correct, that means that Homo erectus must have had considerable seafaring skills that would have required the cognitive capacity and social cooperation to coordinate such a voyage on early rafts because the land masses were separated by substantial amounts of open sea.11
Clearly our ancestors were cooperating and communicating well before the end of the last Ice Age. But there was a rise in the population around this time that could have increased pressure to adapt to cohabitation in larger groups.12 Analysis of our species’s history reveals that the world’s population rose significantly in three continents well before the Neolithic period began around 12,000 years ago.13 When the ice sheets covering the northern continents began to melt around 20,000 years ago, the demographics of our species changed rapidly, creating social environments that required increased levels of skills to navigate. The process of selection for social traits must have started when our hominid ancestors first began cooperating hundreds of thousands of years ago, when domestication first began to appear, but it could have undergone a sharp acceleration when they settled down to live together after the last great Ice Age.
Strength and aggression were advantageous for hunter-gatherer existence, but in these settled communities cunning, cooperation and trade were necessary. Humans now would have had to keep cool heads and even tempers. Those who prospered in this new selective environment would pass on the temperaments and social abilities that made them skilled at negotiation and diplomacy. Of course, there have been extreme violence and wars in the modern era and we have developed technologies to kill each other in vast numbers, but modern combat is typically orchestrated by groups; brute individual aggression was more prevalent in the smaller hunter-gatherer tribes of our prehistory.
By self-domesticating, we have been changing our species by promoting genes that produce relatively slowly developing brains in comparison to bodies. This would mean longer periods of development and social support that would have incurred more parental investment. It would require mechanisms that modulate temperaments and teach children how to behave in socially appropriate ways. Humans who lived together more peacefully in settled communities reproduced more successfully. They acquired skills that enabled them to cooperate, share information and eventually create our cultures.
Modern civilization arose not because we suddenly became more intelligent as a species, but rather because we learned to improve upon technologies and knowledge that we inherited by sharing information that was a by-product of domestication. Long childhoods were useful for transferring knowledge from one generation to the next, but they originally evolved so that we could learn to get along with everyone in the tribe. It was the drive to learn to live together in harmony that enabled collective intelligence to thrive, not the other way around. By sharing knowledge we became more educated, not necessarily more intelligent.