human self-domesti- cation events

the idea that humans have self-domesticated themselves (ourselves!) is definitely in vogue: here’s a recent irl+online symposium held on the topic.

domestication in mammals results in a specific suite of traits known as the “domestication syndrome”:

“[I]ncreased docility and tameness, coat color changes, reductions in tooth size, changes in craniofacial morphology, alterations in ear and tail form (e.g., floppy ears), more frequent and nonseasonal estrus cycles, alterations in adrenocorticotropic hormone levels, changed concentrations of several neurotransmitters, prolongations in juvenile behavior, and reductions in both total brain size and of particular brain regions.”

humans — in some cases some human populations — appear to exhibit many, if not all, of these traits: increased docility, skin color changes, reduction in tooth size, craniofacial feminization, more frequent/nonseasonal estrus cycle, neotenous gene expression in the developing human brain, prolongation in juvenile behavior, and reduction in total brain size.

much of the current thinking seems to be centered on the idea that humans self-domesticated “in the more distant past,” but the fact that humans have been able to dwell together at all in ridiculously large numbers beginning around the time of the agricultural revolution suggests that human self-domestication did not stop “in the more distant past” and is probably even ongoing. this is 10,000 Year Explosion territory, and cochran and harpending have been here already [pgs. 110-113 — my emphases]:

“If your ancestors were farmers for a long time, you’re descended from people who decided it was better to live on their knees than to die on their feet.

“Farming led to elites, and there was no avoiding their power. Foragers could walk away from trouble, but farms were too valuable (too important to the farmers’ fitness) to abandon. So farmers had to submit to authority: The old-style, independent-minded personalities that had worked well among egalitarian hunter-gatherers (‘A Man’s a Man for a’ That’) were obsolete. Even when some group had a chance to refound society on a more egalitarian basis, as in the case of the medieval Iceland republic, elites tended to reappear.

“Aggressive, combative people may also have experienced lowered fitness once ruling elites began to appear. With strong states, the personal payoff for aggression may have become smaller, while law and order made combativeness for self-defense less necessary. Sheer crowding must also have disfavored some personality traits that had worked in the past. Intuitively, it seems that a high level of aggressiveness would be less favored when encounters with strangers were frequent. Fight too often and you’re sure to lose. Moreover, although the winner of a deadly struggle between two peasants might conceivably gain something, his owners, the elites who taxed both of those peasants, would not, any more than a farmer benefits when one bull kills another.

“Farmers don’t benefit from competition between their domesticated animals or plants. In fact, reduced competition between individual members of domesticated species is the secret of some big gains in farm productivity, such as the dwarf strains of wheat and rice that made up the ‘Green Revolution.’ Since the elites were in a very real sense raising peasants, just as peasants raised cows, there must have been a tendency for them to cull individuals who were more aggressive than average, which over time would have changed the frequencies of those alleles that induced such aggressiveness. This would have been particularly likely in strong, long-lived states, because situations in which rebels often won might well have favored aggressive personalities. This meant some people were taming others, but with reasonable amounts of gene flow between classes, populations as a whole should have become tamer.

“We know of a gene that may play a part in this story: the 7R (for 7-repeat) allele of the DRD4 (dopamine receptor D4) gene. It is associated with Attention-Deficit/Hyperactivity Disorder (ADHD), a behavioral syndrome best characterized by actions that annoy elementary school teachers: restless-impulsive behavior, inattention, distractibility, and the like.

“The polymorphism is found at varying but significant levels in many parts of the world, but is almost totally absent from East Asia. Interestingly, alleles derived from the 7R allele are fairly common in China, even though the 7R alleles themselves are extremely rare there. It is possible that individuals bearing these alleles were selected against because of cultural patterns in China. The Japanese say that the nail that sticks out is hammered down, but in China it may have been pulled out and thrown away.

“Selection for submission to authority sounds unnervingly like domestication. In fact, there are parallels between the process of domestication in animals and the changes that have occurred in humans during the Holocene period. In both humans and domesticated animals, we see a reduction in brain size, broader skulls, changes in hair color or coat color, and smaller teeth. As Dmitri Belyaev’s experiment with foxes shows, some of the changes that are characteristic of domesticated animals may be side effects of selection for tameness. As for humans, we know of a number of recent changes in genes involving serotonin metabolism in Europeans that may well influence personality, but we don’t know what effect those changes have had — since we don’t yet know whether they increase or decrease serotonin levels. Floppy ears are not seen in any human population (as far as we know), but then, changes in the external ear might interfere with recognition of speech sounds. Since speech is of great importance to fitness in humans, it may be that the negative effects of floppy ears have kept them from arising.

“Some of these favored changes could be viewed as examples of neoteny — retention of childlike characteristics. Children routinely submit to their parents — at least in comparison to teenagers — and it’s possible that natural selection modified mechanisms active in children in ways that resulted in tamer human adults, just as the behaviors of adult dogs often seem relatively juvenile in comparison with adult wolf behavior.

“If the strong governments made possible by agriculture essentially ‘tamed’ people, one might expect members of groups with shallow or nonexistent agricultural experience to be less submissive, on average, than members of longtime agricultural cultures. One possible indicator of tameness is the ease with which people can be enslaved, and our reading of history suggests that some peoples with little or no evolutionary exposure to agriculture ‘would not endure the yoke,’ as was said of Indians captured by the Puritans in the Pequot War of 1636. In the same vein, the typical Bushman, a classic hunter-gatherer, has been described as ‘the anarchist of South Africa.'”

what i’d like to draw attention to is the idea that there have been multiple (probably multiple multiples of) human self-domestication events which occurred at different places and at different times — all sorta within the broader human self-domestication project which began back in some stone age or, perhaps, even before. one of these, i propose, was the manorialism/outbreeding/execution-of-violent-criminals combo of medieval europe which left “core” europeans with a very specific set of behavioral traits. another might very well be whatever domestication package went along with rice farming in southern china as peter frost has discussed. others undoubtedly include the sorts of civilizations described by cochran & harpending in the passage quoted above — those “strong, long-lived states” — like those found in ancient egypt, ancient china, and ancient india.

just like how we (prolly together with dogs themselves) domesticated dogs thousands of years ago, but then continued and honed the process by selecting for specific traits in specific breeds, i think we should consider that, not only did humans start self-domesticating themselves a very long time ago so that there are common domestication traits in (nearly?) all humans populations, but also that there have been more localized self-domestication events which selected for somewhat different behavioral traits depending on what sort of selection pressures were present in these various events.

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