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Scientific American

The Domesticated Savage

published September 2003 | Comments Off
Science reveals a way to rise above our natures
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Evolutionary biologist Jared Diamond of the University of California at Los Angeles once classified humans as the “third chimpanzee” (the second being the bonobo). Genetically, we are very similar, and when it comes to high levels of aggression between members of two different groups, as I noted in last month’s column on “The Ignoble Savage,” we also resemble chimpanzees. Although humans have a brutal history, there’s hope that the pessimists who forecast our eventual demise are wrong: recent evidence indicates that, like bonobos, we may be evolving in a more peaceful direction.

One of the most striking features in artificially selecting for docility among wild animals is that, along with far less aggression, you also get a suite of other changes, including a reduction in skull, jaw and tooth size. In genetics, this is called pleiotropy. Selecting for one trait may generate additional, unintended changes.

The most famous study on selective breeding for passivity began in 1959 by Russian geneticist Dmitri Belyaev of the Institute of Cytology and Genetics in Siberia. It continues today under the direction of Lyudmila N. Trut. Silver foxes were bred for friendliness toward humans, defined by a graduating series of criteria, from the animal allowing itself to be approached, to being hand fed, to being petted, to proactively seeking human contact. In only 35 generations the researchers produced tail-wagging, hand-licking, peaceful foxes. What they also created were foxes with smaller skulls, jaws and teeth than their wild ancestors.

The Russian scientists believe that in selecting for docility, they inadvertently selected for paedomorphism — the retention of juvenile features into adulthood — such as curly tails and floppy ears found in wild pups but not in wild adults, a delayed onset of the fear response to unknown stimuli, and lower levels of aggression. The selection process led to a significant decrease in levels of stress-related hormones such as corticosteroids, which
are produced by the adrenal glands during the fight-or-flight response, as well as a significant increase in levels of serotonin thought to play a leading role in the inhibition of aggression. The Russian scientists were also able to accomplish what no breeder had ever achieved before — a lengthened breeding season.

Like the foxes, humans have become more agreeable as we’ve become more domesticated. Whereas humans are like chimpanzees when it comes to between-group aggression, when it comes to levels of aggression among members of the same social group, we are much more like peaceful, highly sexual bonobos. Harvard University anthropologist Richard W. Wrangham proffers a plausible theory: as a result of selection pressures for greater within-group peacefulness and sexuality, humans and bonobos have gone down a different behavioral evolutionary path than chimps have.

Wrangham suggests that over the past 20,000 years, as humans became more sedentary and their populations grew, selection pressures acted to reduce within-group aggression. This effect can be seen in such features as smaller jaws and teeth than our immediate hominid ancestors, as well as our year-round breeding season and prodigious sexuality; bonobos were once called the “pygmy chimpanzee” because of their paedomorphic features. (Emory University psychologist Frans B. M. de Waal has documented how bonobos in particular use sexual contact as an important form of conflict resolution and social bonding.) Wrangham also shows how Area 13 in the human limbic frontal cortex, believed to mediate aggression, more closely resembles in size the equivalent area in bonobo brains than it does that same area in chimpanzees.

A plausible evolutionary hypothesis suggests itself: limited resources led to the selection for within-group cooperation and between-group competition in humans, resulting in within-group amity and between-group enmity. This evolutionary scenario bodes well for our species — if we can continue to expand the circle of whom we consider to be members of our in-group. Recent conflicts are not encouraging, but in the long run there is a trend toward including more people (such as women and minorities) within the in-group deserving of human rights.

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