Humans: The Domesticated Primates
The greater peaceability of human societies comes from our nature. We can look each other in the eye. We don’t lose our tempers easily. We normally control our aggressive urges. In primates, one of the most potent stimuli for aggression is the presence of a strange individual. By contrast, Jerome Kagan, a pioneer in developmental psychology, reports that in his hundreds of observations of two-year-olds meeting unfamiliar children, he has never seen one strike out at the other. That willingness to interact peacefully with others, even strangers, is inborn.
What accounts for this human difference? The answer lies in the evolutionary pressures that selected against aggression, particularly in men. The cultural anthropologist Christopher Boehm has found that, in hunter-gatherer societies, a man who threatens others by having too violent a temper is treated in a consistent way. If the bully can’t be contained by the cajoling effects of ridicule or ostracism, the other men reach a consensus, make a plan and execute him. Over the eons, the long-term practice of killing unrepentant aggressors must have favored genes for more peaceful behavior.
No other mammal has the brainpower to organize capital punishment. When language became sufficiently sophisticated, our ancestors’ ability to conspire led not only to a more peaceful species but also to a new kind of hierarchy. No longer would human groups be ruled by the physical force of an individual. The emergence of capital punishment meant that henceforth, anyone aspiring to be an alpha couldn't get away with just being a fighter. He had to be a politician, too.
The result of generations of such selective pressure is that human beings are best understood as an animal species that has been domesticated—like dogs, horses or chickens. Recent archaeological evidence suggests that humans became increasingly docile and less reactively aggressive around the time of becoming Homo sapiens, a process that started about 300,000 years ago.
Dr. Leach listed four characteristics of the bones of domesticated animals: They mainly have smaller bodies than their wild ancestors; their faces tend to be shorter and don’t project as far forward; the differences between males and females are less highly developed; and they tend to have smaller brain cavities (and thus brains). As it turns out, all of these changes appear in human fossils. Even our brain size fits the pattern: While the human brain grew steadily over the last two million years, that trajectory took a sudden turn about 30,000 years ago, when brains started to become smaller.
The differences between modern humans and our earlier ancestors have a clear pattern: They look like the differences between a dog and a wolf. Half a million years ago, our ancestors were heavier-bodied, with relatively bigger males, more masculine faces and bigger teeth. To extrapolate from domesticated animals, these characteristics indicate that our ancestors were less docile than we are today. Pre-sapiens humans would have had a greater propensity for reactive aggression, losing their tempers more easily, quick to threaten and fight one another.