How Did Humans Domesticate Themselves?

Domestication is what happens to creatures you domesticate

In our efforts to understand ourselves and the world that made us, it’s useful to compare ourselves to our cousins. And humans are definitely different from other hominids. We’re hairless. We walk upright. We read blogs.

In many ways, of course, we’re not different at all. We often read about how the behaviors of men and women are hard wired into humans, appearing in infancy. Chimpanzees are the same: young female chimps are fascinated with dolls and babies, and will even mother rocks. Male chimps are have no such interests; they’d rather rip dolls open to see what they’re made of.¹

But other sex differences haven’t been inherited from our primate cousins. In East African chimpanzees, adult females are significantly less social than adult males, spending much of their time with only by their dependent offspring.² This sex difference in chimpanzee social behavior shows up early, as infant male chimpanzees have more friends than infant females.³

Humans aren’t like this. Why not? One answer is to say that random processes have a way of smoothing over irregularities. If we begin with a snowman in a field at the start of winter, after several snowfalls the snowman will become a mound, and finally a lump. If we begin with a carefully tuned adaptation, removal of selective pressure maintaining the adaptation will reduce its functionality, and then remove the feature entirely. Maybe this is what happened to the female preference for solitude—maybe it just disappeared without evolutionary pressures reinforcing it.

Another explanation for the loss of sex-typed behavior is domestication. One of the side effects of domestication turns out to be a reduction in sexual dimorphism, and humans are said to have domesticated themselves.⁴

But if those were a good explanation for the disappearance of most sex differences, we’d have good reason to be confused by some of the differences that persist over millennia. When we look at, for example, the brain size disparity between male and female chimps, we should expect that there is some feature of the external environment, or of chimpanzee society, that maintains that gap:

The adolescent and young adult chimpanzees had the largest brain weights; in these two age groups combined, the mean brain weight (+/- standard deviation) was 368.1 g (+/-37.3) for females (n = 17) and 405.6 g (+/-39.4) for males (n = 17).⁵

This represents a 10% greater brain weight in males than in female chimpanzees. Yet despite over four million years of evolutionary distance between chimpanzees and humans, sexual dimorphism in human brains is not less than that in chimpanzees. Indeed, it may even be greater: the first dataset I found has 1376 g for males, vs 1207 g forfemales, rendering a 14% difference in humans.⁶ What could have caused this?

The Sad Story of Human Domestication

Lately there’s been a lot of discussion around substack on humans being a self-domesticated species which shows low levels of reactive aggression, etc. There are many questions about how this could have come about, and the definitive paper here as of today is Wrangham’s Targeted conspiratorial killing.⁷ Wrangham takes a deep dive into human prehistory. By making comparisons between humans, chimpanzees, Neanderthals, and Denisovans, and looking at the physical changes in humans occurring around 300,000 years ago, he finds some support for the hypothesis that humans domesticated themselves by learning to speak, and then using their newfound linguistic abilities to successfully assassinate aggressive alpha males. The story goes that they eventually became so effective at this tactic that they used it against anyone they found even slightly annoying, making us into the small-jawed, polite creatures we are today.

Although Wrangham’s work is absolutely fascinating and I recommend that you do read it for yourself, there is another, much more straightforward explanation for how humans domesticated themselves.

As with much of evolutionary biology, this explanation has a certain narrative character. This is a long, sad tale, easier to weave together than to substantiate clearly. There’s no way to bottle up ten thousand years of evolution and submit it to chemical analysis in a lab. But we can work like detectives to see if the strands mesh together, and try to form testable hypotheses along the way.

What is domestication?

In its broadest sense, domestication is a form of mutualism between two groups. One takes charge of the other, keeping the other group safe and providing for many of its needs in exchange for something. We often think of domestication in terms of humans keeping livestock for slaughter, but the relationship between ants and aphids can be described in the exact same terms—ants protect aphids, while harvesting the honeydew the aphids produce.

This relationship between ants and aphids is often described as the ants “farming” aphids. The aphids are completely at the mercy of the ants, but they benefit anyway from the ant’s care.

When it comes to mammals, though, something more seems to come into play. Possibly because of the way traits are coded in neighboring regions of the genome, or follow similar biochemical pathways, researchers have noticed that mammals undergo a consistent and predictable series of changes when they’re domesticated. After being domesticated, numerous mammalian species show:⁸

[I]ncreased docility, increased skillfulness in using human cues (gestures and glances), increased fecundity (including non-seasonality of oestrus cycles, hormonal changes, multiple breeding cycles per year and earlier sexual maturity), reduction of tooth size, shortening of the rostrum, reduction of brain size, floppiness of the ears, curliness of the tail and depigmentation of skin and fur.

Many of these traits don’t seem to apply well to humans; I don’t know anybody with floppy ears or a curly tail. But humans do seem to have some of these traits. For instance, the rostrum is the portion of the face including the nose, teeth, and chin, and in hominins, this feature has definitely shrunk.

The problem with Wrangham’s explanation is that, while he might argue that we have some of these traits seen in domesticated animals, and he may have identified a cause for these traits, that cause he identifies is not domestication.

Domestication isn’t the practice of killing the leaders of your own group when they’re mean. Domestication is when members of one group take control over members of another group, often by kidnapping, enslaving, and incorporating the domesticated organisms into their own society.

This kind of thing is extremely common to humans. Even before recorded history, humans were already the most assiduous domesticators around; we brand our cattle, train our dogs, create coops for our chickens and stalls for our horses. But our nearest relatives, the chimpanzees, don’t do any of these things.

Why not? Is it that other apes aren’t intelligent enough, or lack the behavioral repertoire necessary to capture and care for other animals? Probably not—or, not exactly. Many creatures are able to bond with and befriend individuals across species; the problem is that you typically have to restrain the creatures you domesticate. A sheepdog isn’t able to domesticate sheep on its own, because you have to imprison, hobble, tie up, or at the very least, convince the sheep not to run away when you aren’t looking. Ants accomplish this with aphids by biting off their wings, but this doesn’t work for most mammals—if you break their legs, they become a severe liability.

The key ingredient is technology. I’m thinking of something very specific, here: An ancient technology, so primordial that we don’t even know when it first came into being, only that it must have existed for at least 40,000 years, deep in the upper paleolithic. But considering how the past is more than you think, this technology was very likely widespread long before that. Possibly it was even a technology that predated the recent expansion out of Africa.

I am referring, of course, to rope.

Rope

The earliest evidence for ropemaking comes from Hohle Fells, where a mammoth bone has been found with four holes drilled into it. The bone’s purpose in manufacturing rope is suggested by the spiral grooves of the holes which can hold fibers snugly in place. (See images here, including researchers demonstrating its use.)⁹

Image courtesy of Mehmet Turgut Kirkgoz

For domesticating mammals, rope is extremely useful. One of the earliest uses for ropes would have been to tie up other animals, such as dogs—the domestication of the dog occurred over 30,000 years ago, or 30 kya.¹⁰ But the time from 40 kya to 30 kya still leaves quite a while to be sitting around staring at this great new invention. What might people have been doing in the meantime?

Well OK. Maybe they were making shelters, bridges, swings, nets, weapons, or other innocuous sorts of things. But aside from all that, I really strongly suspect they were practicing at domesticating each other. Because if you’re an early human just emerging into behavioral modernity, restraining someone you’ve kidnapped is one of the simplest and most obvious uses for rope.

At first this kind of event would have been pretty rare. If you’re a forager who kidnaps someone, well, you have to go out and forage food for them, and bring back water for them to drink. (Do you even have a waterskin, a bowl, a watertight basket, or something like that? There’s evidence for pottery stretching to 13 kya,¹¹ but I have no evidence for waterskins before about 5 kya.) You can always drag the person around with you, but that makes it hard to run off after game animals, or flee from danger without leaving them behind. You might just, uh, do really mean things to them, and let them go afterwards.

So at first, rope wasn’t such a big deal. But in the long run the story of society is a story of progress. Gradually, haltingly, even with reversals and periods of regression to an earlier way of life, humanity advanced. Slowly, ever so slowly, more technologies were discovered, and society made the slow climb towards larger and more complex organizations, the lifestyle shifted from simple foraging, towards horticulture, pastoralism, and agriculture:

The table above was adapted from Kaplan, Hooper, and Gurven’s “Evolutionary and ecological roots of human social organization.”¹² Look closely at the third-column, where it describes male-female relations. Simple foragers follow predominantly monogamous mating patterns, often characterized by bride service, where husbands agree to work for the sake of a wife. (Think Genesis 29, where Jacob worked for Rachel’s hand.)

But move a short way down the list and “bride capture” shows up. What’s bride capture? Oh, simple: Bride capture is where a man goes out and captures a wife—where men begin to domesticate women. Keep moving down the list from stratified foragers, to horticulturalists, pastoralists, and finally agriculturalists, and watch as this process of domestication becomes the norm.

Human Domestication Explains These 4 Outcomes

Outcome #1: Female sociality

The first thing humans domesticating humans explains is human female sociality. If the female primates who were ancestral to humans and chimpanzees were asocial like modern chimpanzees, human females gradually became more social. But this is exactly what the hypothesis of human domestication would predict.

If you’re a free woman, you should seek alliances with other women and men, and move around in groups, to avoid being kidnapped. Since kidnappers are often going to have a psychopathic streak, and be less desirable men husbands haven’t already found a wife, your reproductive prospects are worse off as a captured bride. A kidnapper isn’t as likely, or even as able, to help you through a rough pregnancy, or provision your children effectively. Social women would have had an advantage over women who went off alone, like chimpanzees do.

What if you’re a prehistoric woman and you are kidnapped? It still helps to be social—to be able to read the expression of your captor, to humanize yourself in his eyes, to communicate unmet needs, avoid beatings, and possibly even to negotiate for your freedom. But sociality by itself is probably not going to be enough in many situations of bride capture.

Outcome #2: Female linguistic ability

Bride capture often occurs between neighbors. But it can also occurs cross-culturally, and in its aftermath, it can result in a woman being sold or exchanged to an even more distant tribe. If you find yourself in an environment where you don’t speak your captor’s language, you will be at a significant disadvantage. There’s only so far body language will take you; learning to communicate effectively helps.

Human beings have a strong bias against people who can’t speak. Our word for dumb is a synonym for stupid. Even people who speak broken English come across as dim-witted and slow, because that’s the way young children and people with disabilities speak. If you seem stupid, it’s much easier for your captor to treat you as an object of scorn and abuse.

And if you’re one of many captured women, as happens more and more frequently as society moves down that list above, it also helps a great deal to navigate the new sorority that emerges. Co-wives under polygynous marriages might be cooperative, or deeply competitive, and verbal ability will be a particular advantage when trying to deal with a hostile woman captured or traded from another society.

Outcome #3: Female sexual response

As everybody knows, there are three responses to being domesticated:

1. Bite him! Kick him! Scratch him!

2. Remain silent, saving your strength in the hopes of escape

3. Whatever, may as well enjoy it

For early humans, being domesticated was probably a straightforwardly traumatic event. If you’ve handled semi-domesticated animals, you’ll understand what I mean when I say they don’t like being put into strange environments with unfamiliar peers. Sometimes animals are so stressed by the experience that they’re unable to breed in captivity.

But rope is unyielding. Usually it’s stronger even than the man who tied it. Women who were severely stressed after being captured were at a disadvantage compared to women whose heartrate didn’t shoot through the roof at the feel of rope around their wrists. So it’s not difficult to realize—though it’s very difficult not to be embarrassed for just about every person on Earth—that human evolution took the third route on this one. Mother Nature isn’t really much of a humanitarian. She looked at the situation, said “When life hands you lemons, make lemonade,” and now we have a female psychology that looks like this Age Restricted Video. (Safe for work, but may generate awkward questions if your kids watch it)

Outcome #4: Domestication Syndrome

Because this hypothesis holds that bride capture is widespread across our evolutionary history, we should all show signs of domestication. The decline in brain size around 10 kya roughly corresponds to the point where this would have begun in earnest. But since it was primarily our maternal lineages that were subjected to domestication, it should be primarily human females who show domestication syndrome.

Thus, this hypothesis predicts that, more than men, women should show signs of

[I]ncreased docility, increased skillfulness in using human cues (gestures and glances), increased fecundity (including non-seasonality of oestrus cycles, hormonal changes, multiple breeding cycles per year and earlier sexual maturity), reduction of tooth size, shortening of the rostrum, reduction of brain size… and depigmentation of skin and [hair]

I already mentioned that the sex difference in brain sizes is, if anything, greater in humans than chimpanzees. And despite the way gestation takes more time for human babies, women menstruate around every 28 days, while chimpanzee females cycle every 34 days. In chimpanzees, males and females both begin puberty between ages 8 to 10;¹³ in humans, girls begin puberty a year earlier than boys.

Evolutionary biologists commonly refer to women as being more neotenous than men. Instead, it might be more correct to say that women show more signs of domestication syndrome.

Conclusion

I mentioned before that I really didn’t like patrilineal societies, writing about how they encourage women to develop in strange ways, without really explaining what I meant. Well, this is what I meant. The chart in the section on rope places patrilineal pastoralists as further along in the exploitation of women than matrilineal horticulturalists, which is striking, given that horticulturalists are usually more advanced than pastoralists.

But any kind of settled society in prehistory probably created an evolutionary environment where social complexity and rope left an imprint on human women in terms of their sociality, superior linguistic ability, a suite of traits related to domestication syndrome, and Every Kidnapping Video From 2021:

Disclaimer

This essay does not endorse any of the things that it claims happened in prehistory as normal or desirable. I had all this in mind for months, but was reluctant to post it because as I’ve said before, it’s not exactly my favorite thing. If you take this post as an argument that women deserve to be kidnapped, abused, or anything else, I’m just going to say its a shame you can’t read very well. The end.

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