Aggression Out, Agriculture In
It’s hard to pin down the biological basis for the changes that took place in early human ancestors and domesticated animals as they developed lower levels of aggression. No one is sure which genes are involved in domestication, but they may have something to do with controlling the release of cortisol and testosterone, the hormones involved with fear and competitive behavior, into the bloodstream.
“There are maybe 10 genes, or 50, or hundreds involved in domestication syndrome,” says Carlos Driscoll, chair of conservation genetics at the Wildlife Institute of India of the National Institutes of Health. Driscoll was part of a team that analyzed the genomes of domestic and wild cats to understand the biology of domestication.
With the dawn of agriculture more than 10,000 years ago, our ancestors began settling in villages and living in larger groups, making tolerance of other people and cooperative behavior increasingly necessary. That lack of competitive drive may show up in the faces of modern men. Males with generally higher testosterone levels are known to have thicker brow ridges and cranial bones than men with lower testosterone. High testosterone also makes people prone to competitive rather than cooperative behaviors. Churchill and Hare were part of a group that studied a sample of Homo sapiens skulls dating from 200,000 years ago through to the present. While the study couldn’t control for every variable that might have affected the shape of the skulls, the trend the researchers found was toward thinner craniums and less prominent brow ridges starting around 80,000 years ago and possibly earlier. This trend was more pronounced in the most recent samples, from individuals who lived in villages or cities, than it was in hunter-gatherers who tended to live in smaller groups.
Bonobo social tolerance may also be related to the hormones that the animals’ bodies produce when reacting to stressful situations. When they anticipate having to compete for food, bonobos show a spike in the amount of the hormone cortisol, which is associated with fear and anxiety. Chimpanzees, on the other hand, show a spike in testosterone. According to Hare, some of the differences between chimps and bonobos may be rooted in these hormonal differences. The more aggressive chimpanzees are likely to deal with competition through violence while bonobos are more likely to calm themselves through sex.
Despite the millions of years since we shared a common ancestor, humans still retain some tendencies in common with chimpanzees. Churchill believes that human behavior is actually something of a mosaic of traits demonstrated by chimpanzees and bonobos. “In some respects, we are very chimpanzee-like. We have a remarkable propensity for violence and warfare,” he says, “and in some respects, we can be very bonobo-like. We can be very prosocial and sexual.”
By studying the differences of these two equally distant relatives, we can come to a better understanding of our own species. “The more we understand why in some cases we behave like chimps and why in some cases we behave like bonobos, the better off we’ll be,” says Churchill.