ARE humans, by nature, good or evil? The question has split opinions since people began philosophising. Some, like the followers of Jean-Jacques Rousseau, say we are a naturally peaceful species corrupted by society. Others side with Thomas Hobbes and see us as a naturally violent species civilised by society. Both perspectives make sense. To say that we are both 鈥渘aturally peaceful鈥� and 鈥渘aturally violent鈥� seems contradictory, however. This is the paradox at the heart of my new book.
The paradox is resolved if we recognise that human nature is a chimera. The chimera, in classical mythology, was a creature with the body of a goat and the head of a lion. It was neither one thing nor the other: it was both. I argue that, with respect to aggression, a human is both a goat and a lion. We have a low propensity for impulsive aggression, and a high propensity for premeditated aggression. This solution makes both Rousseauians and Hobbesians partially right, but it raises a deeper question: why did such an unusual combination of virtue and violence evolve?
Advertisement
The story of how our species came to possess this unique mixture hasn鈥檛 been told before, and offers a rich and fresh perspective on the evolution of our behavioural and moral tendencies. It also addresses the fascinating but surprisingly neglected question of how and why our species, Homo sapiens, came into existence at all.
Since the 1960s, efforts to understand the biology of aggression have converged on an important idea. Aggression 鈥� meaning behaviour intended to cause physical or mental harm 鈥� falls into two major types, so distinct in their function and biology that from an evolutionary viewpoint they need to be considered separately. I use the terms 鈥減roactive鈥� and 鈥渞eactive鈥� aggression, but many other word pairs describe the same dichotomy, including cold and hot, offensive and defensive, premeditated and impulsive.
To judge from other relevant animals, a high level of proactive aggression is normally associated with high reactive aggression. The common chimpanzee is the primate species that most often uses proactive aggression to kill its own kind, and it also has a high rate of reactive aggression within communities. The wolf鈥檚 proactive aggression against members of its own species is often lethal. As with chimpanzees, although relationships within wolf groups are generally benign and cooperative, they are far more emotionally reactive than dogs are. Lions and hyenas are also wolf-like in these respects.
鈥淭his addresses the fascinating question of why our species came into existence at all鈥�
Something different happened in the human lineage. While proactive aggression stayed high, reactive aggression became suppressed. The evidence, I argue, points to self-domestication being the 鈥渟omething different鈥� that made humans special.
An animal is domesticated if it becomes tame as a result of genetic adaptation, as opposed to being tamed within its lifetime. The idea of self-domestication as a way to understand human docility goes back at least 2000 years to the ancient Greeks and has been repeatedly reinvented. Until recently, the term was used only as a description to emphasise special behavioural qualities that we share with domesticated animals, such as social tolerance and low emotional reactivity to provocation. But now we can add the fact that anatomical changes found in H. sapiens compared with earlier hominins show a strong similarity to the anatomical changes that occur in domestication. 鈥淒omesticates鈥� mostly have smaller bodies than their wild ancestors. Their faces tend to be shorter, projecting relatively less forward. Differences between males and females are less developed. And domesticates usually have smaller brains. The differences between modern humans and our earlier ancestors look like the differences between a dog and a wolf.
So, how did humans become self- domesticated? Evidence from fossils reveals the process started certainly by 200,000 years ago, and possibly with the first glimmerings of H. sapiens a little more than 300,000 years ago. Language-based conspiracy was the key, because it gave whispering subordinates the power to join forces to kill bullies 鈥� presumably, alpha males, since men tend to be more violent than women. As happens in small-scale, traditional societies today, language allowed underdogs to agree on a plan and thereby to make predictably safe murders out of confrontations with intended victims that would otherwise have been dangerous. Genetic selection against the alpha males鈥� propensity for reactive aggression was an unforeseen result of eliminating the would-be despots. The selection against alpha-male behaviour led to an increasingly calm tenor of life within social communities of H. sapiens. Our species is now more Rousseauian than it has ever been.
Conform or die
The same ability to perform capital punishment that led to self-domestication also created the moral senses, as Christopher Boehm at the University of South California has argued. In the past, to be a nonconformist, to offend community standards or to gain a reputation for being mean were dangerous adventures; to some extent this is still true today. Rule breakers threatened the interests of the elders 鈥� the coalition of males holding power 鈥� so they risked being ostracised as outsiders or sorcerers. Nonconformists who refused to change their behaviour were executed. Selection accordingly favoured the evolution of emotional responses that led individuals to feel and display unity with the group. Conformity was vital.
The moral senses of individuals thus evolved to be self-protective to a degree not shown by other primates. The strongly conformist behaviours produced by the new tendencies provided a safe passage through life, and they had a second effect as well. By reducing competition and selfishness, they promoted behaviour that benefited the group as a whole. The idea that the moral senses evolved to protect individuals from the socially powerful suggests that group selection may be unnecessary for explaining why we are such a group-oriented species.
So what about proactive aggression? A predisposition for premeditated violence was in place in our Homo ancestors by at least 300,000 years ago, and perhaps as much as 2 million years ago. How much earlier it was present isn鈥檛 marked by anything so concrete as domestication, which comes with a syndrome of behavioural and physical characteristics. Based on inferring the behaviour of our ancestors, however, a high propensity for coalitionary proactive aggression probably operated at least through the 2 million or more years of the Pleistocene.
The reason for this claim is the antiquity of hunting. Homo erectus, the first ancestor of H. sapiens that was committed, like us, to living on the ground, evolved around 2 million years ago. Cut marks that H. erectus left on meat-bearing bones show that they butchered animals the size of large antelope. By 1 million years ago, ambush hunting is suggested (humans repeatedly reused a site of that age at Olorgesailie, Kenya, a place where animal prey were limited to narrow travel routes and could therefore be killed easily); this implies cooperation and planning too. However, only with H. sapiens and Neanderthals, in the past few hundred thousand years, have we found sufficient evidence that hunting had clearly become premeditated: using spears, catching small animals apparently by setting snares, and hunting from elevated positions. So a conservative interpretation might limit proactive hunting to the Mid-Pleistocene.
After our ancestors became good hunters, they could have killed strangers; hunting is a transferable skill. Hunting and simple war both require searching and safe dispatching, and both benefit from long-distance travel and well-honed coordination. Wolves, lions and spotted hyenas use coalitionary proactive aggression not only to get food but also to kill rivals in other groups. Chimpanzees are also social hunters and killers of their own species. Bonobos, by contrast, aren鈥檛 known to be social hunters (despite their liking of meat) and, to date, haven鈥檛 shown clear evidence of planned aggression. Among humans still living in small-scale societies, there is a similar association: societies relying more on hunting tend to have more frequent war.
鈥淭he moral senses of individuals evolved to be self-protective鈥�
For all these reasons, human hunting of prey seems likely to have been associated with the ability to kill rivals in neighbouring groups 2 million years ago. The origins of proactive violence may be far deeper, however. In our 1996 book, Demonic Males, Dale Peterson and I argued that the killing of strangers probably went back 7 million years or more to our common lineage with chimpanzees and bonobos, when our Central African ape ancestor was probably a chimpanzee-like hunter and killer. Much as chimpanzees and wolves attack strangers, once our ancestors had achieved the ability to kill safely, a motivation to kill strangers would probably have been present too. There seems no reason to excuse our ancestors from the links between hunting and violence found in other animals.
Regardless of when coalitionary proactive aggression began against strangers, killing within groups was limited until humans developed a superior language ability. Much changed after individuals became able to share complex ideas with one another. People could then form alliances based on shared interests that they could articulate. With the arrival of planned and communally approved executions, the bullying by an alpha male was exchanged for the subtler tyranny of the previous underdogs. The newly powerful coalitions of males became the set of elders who would rule society 鈥� a system that largely continues today, albeit more with laws, threats and imprisonment than with execution.
Both our 鈥渁ngelic鈥� and 鈥渄emonic鈥� tendencies, therefore, depended for their evolution on the sophisticated forms of shared intentionality made possible by high-level language 鈥� an ability that undoubtedly also contributed to much pro-social behaviour. A chimpanzee-style form of shared intentionality launched the process at least 7 million years ago. It took the mysterious dawning of an improved language facility, sometime between 500,000 and 300,000 years ago, to shake us into a new world. Language created the chimeric aspects of our personality, high killing power lying alongside reduced emotional reactivity. A unique cognitive ability gave us a uniquely contradictory psychology 鈥� a predisposition for both virtue and violence.
