Our bodies are maps of our ancestor's social lives. We evolved, physically and behaviorally, in incredibly complex social groups. Such intense sociability may have favored the very traits we favor in cows, horses, dogs and cats. We may have domesticated each other.
First off, it's worth examining what it means to be domesticated. In the most basic sense, to be domesticated is to be tame. Tame animals are not dangerous. They are easy to manage and hard to provoke. Their adrenal response, the cause of the fight-or-flight reflex, is tamped down. In some sense, this is neoteny in behavior- the lifelong preservation of the trust, playfulness, and sociability of youth. In the case of dogs and the purpose-bred silver fox, domestication seems to have increased social intelligence, at least in regard to interpreting human gestures and emotions.
Not surprisingly, behavioral neoteny goes hand-in-paw with physical neoteny. Domestic animals, on average, have smaller skulls, teeth and brains than those of their wild counterparts. Dogs are particularly pedomorphic, hanging onto the tail-wag, puppyish bark, and floppy ears of a juvenile wolf their whole lives.
Humans certainly preserve a host of juvenile traits into adulthood. Descendants of cattle-herding populations are able to digest lactose into adulthood, a trait normally lost once an infant is weaned. Mature human bodies are less muscular and more fine-limbed than those of Neanderthals, our closest hominid relative. Our jaws and teeth remain small and narrow, in contrast to our broad, babyish foreheads and wide-set eyes.
Like other tame species, modern human brains are noticeably smaller. Over the last 20,000 years, the average male brain has lost a volume equivalent to a tennis ball. There's two possible explanations for this, depending how you feel about Idiocracy. One holds that the connections in our minds have become more efficient, and thus don't require the meaty, energy-hungry brains of our ancestors. The other gloomily insists that our social-safety net means that the same people who might once have starved (or been caught off guard by a human rival, outmaneuvered by a tiger, etc) were increasingly buffered by their more clever peers. Yet the human brain nonetheless retains into maturity much of extraordinary plasticity and adaptability of childhood. Evidence indicates that domestication gives rise to novel behaviors where 'wild' living favors constrained, stereotypic forms of interaction.
The theory that we're all adult-ified children is intriguing, but it remains to be seen if humans are actually more tame than either our ape or hominid relatives. Certainly our willingness to work with strangers, our unusual altruism, and the somewhat orderly functioning of our societies speaks to a more domesticated creature. While humans are astonishingly nasty to each other, unpredictable aggression is not rewarded when manageable group dynamics are key to survival.
In some cases, there are measurable trends towards less violent societies. Murder rates in Europe declined dramatically in the 17th century. Murder was surprisingly common in the Middle Ages, and as form of settling disputes, it elicited at most a shrug from the bystanders. The rise of the state and courtly or 'gentlemanly' manners may have moved the locus of control from the exterior to the interior, labeling group justice as barbaric, and self-control as civilized. The same types of men admired for defending their honor hundreds or years ago are now incarcerated, and perhaps have a lower chance of reproductive success, or go on to have offspring that are not terribly successful themselves. It stands to argue that society-wide changes that devalue aggression also marginalize the most violent and unruly members.
Humans, as in so many other areas, aren't unique in this regard. Compared to chimpanzees, bonobos are more gracile, with reduced skulls and jaws, are more sexual and playful, and are significantly less aggressive. Sound familiar? Bonobos' comparatively happy-go-lucky society has weaned out the hyper-masculine traits that dominate chimpanzee life with rape, infanticide and murder.
How might humans continue to domesticate ourselves in the future? It's clear that the most violent and socially unpredictable humans are sidelined by society. We already medicate, incarcerate or execute the men (and some women) on the extreme ends of antisocial behavior. In the future, these traits may naturally continue to fall out of favor in the gene pool as they have for millennia. Societies might purposefully engineer the most aggressive tendencies out of our genome.
More important for fans of Next Nature, the theory of human-self domestication is another blow to notions of 'old' nature. The highly complex social interactions of ancient humans and near-humans have shaped the basic layout of our bodies and minds. Our nature is, at is core, the result of millions of years of culture.