#36. Survival of the Friendliest

The Self-Domestication Theory of the Origin of Modern Homo sapiens

Credit: The Guardian

Dear friends,

As part of my thinking about the role of play in our biological nature, I’ve been delving into some relatively recent paleontological and archeological discoveries about human origins.

In Letter #35, I presented a theory that human language came about, evolutionarily, sometime in the range of 80,000 to 50,000 years ago. I suggested that this followed upon or coincided with physical changes in Homo sapiens that distinguish modern humans from an earlier version of our species. These changes included a lengthening of childhood and adolescence and thus a prolonged period of play, which, I argued, set the occasion for children to invent language as a facilitator of play over many generations.

The archeological record suggests that for hundreds of thousands of years our species showed relatively little change in behavior. We continued to use the same stone tools, with little advance. Then, beginning roughly 80,000 years ago, the archeological record blooms—slowly at first and then more rapidly--with new, more sophisticated tools, with cave art and sculptures that seem to have symbolic significance, and with evidence of trade over relatively long distances indicating that groups of people interacted cooperatively with other groups. What happened, in the millennia beginning about 80,000 years ago that turned early Homo sapiens behaviorally into modern Homo sapiens?

My goal here is to describe what I see as the most plausible theory about how our ancestors changed at that time. It’s the self-domestication theory, which, as one pair of researchers (Hare & Woods, 2020) put it, involved a shift toward “survival of the friendliest.” I start with a summary of human-induced domestication of animals, then describe a theory about the self-domestication of bonobos, and finally a theory about our own self-domestication.

Human-Induced Domestication of Animals

Most of our farm animals—such as horses, pigs, and sheep—were domesticated by a process of selective breeding for tameness. Even when little was known about heredity, people who were keeping animals tended to keep and breed the tamest ones, because they were the easiest to work with. Over many generations, the breeds became increasingly tame.

The earliest steps in the domestication of wolves to become dogs most likely occurred because wolves that were a bit less fearful of, or less aggressive toward, humans benefited by hanging around human groups and eating scraps. Over time, some joined human groups in ways that benefitted both themselves and the humans, and those that were friendliest were most likely to stay or be allowed to stay, eventually leading to populations far tamer than wolves, the earliest dogs. Of course, in more recent times dogs have been deliberately bred in ways that have produced hugely varying sizes, shapes, and personalities (or more literally “caninalities”).

In the 1950s, Russian researchers began a scientific study of domestication with silver foxes (Trut & Dugatkin, 1999). With each generation in the breeding line, the researchers tested each young fox for its willingness to approach a gloved human hand with neither fear nor aggression. This was their measure of tameness. They then mated the tamest males and females to produce the next generation and continued this for over 30 years. As the foxes became tamer over generations, they also became more playful and showed certain other changes in behavior, anatomy, and physiology like to those seen in other domesticated animals compared to their wild counterparts.

The Self-Domestication Theory of Bonobo Evolution

As I noted in Letter #19, our two closest ape cousins are bonobos and chimpanzees. The evolutionary lines leading to these species diverged about 2 million years ago, when a geological event—probably the creation or widened of the Congo River—separated populations of their common ancestor and they took separate evolutionary pathways.

Behaviorally, bonobos and chimps are quite different from one another. Chimps live in hierarchically organized social groups, headed by an alpha male, with males generally dominant over all females, and with much fighting for status among both males and females. Bonobos live relatively peacefully with more muted dominance hierarchies. Bonobos also play much more in adulthood than do chimpanzees, and this apparently helps them reduce tensions that could otherwise lead to aggression. Also, as I noted in Letter #19, bonobos are unique among apes in that the females, though smaller and weaker than the males, are dominant over males when conflicts arise, because the females join one another to cooperate in subduing an aggressive male, while males do not cooperate in that way.

The self-domestication theory is that some condition of bonobos’ social environment turned the tables from the chimpanzee pattern, such that relatively nonaggressive individuals were more able to survive and reproduce than more aggressive ones. This is called self-domestication because the assumption is that selection for tameness derived from the bonobos’ own behavior toward aggressive and nonaggressive individuals. Perhaps an environmentally induced need for greater cooperation led bonobos to ostracize noncooperative, aggressive individuals. Or perhaps some enterprising females learned that by cooperating they could beat off bullying males, thereby reducing the chance that bullies would produce offspring. Then maybe other females learned to do this by watching. Either of these possibilities would lead to a proliferation of genes promoting cooperation and decreasing aggression.

One regular effect of domestication, whether resulting from selective breeding by humans or alterations in how members of the species treat relatively peaceful versus more aggressive individuals, is an increase in adult playfulness. Domestic farm animals are generally more playful than their wild counterparts. Dogs are more playful than wolves. The silver foxes bred for tameness became more playful with each generation. And bonobos are more playful than chimpanzees.

The Self-Domestication Theory of Human Evolution

When we began to look and act like modern Homo sapiens, about 80,000 to 50,000 years ago, we were not the only human species around. Our closest competitor species were Neanderthals, who had occupied large portions of Europe for 300,000 years prior to the time when anatomically modern humans emerged from Africa and began living close to Neanderthals.

At first the Neanderthals seemed to thrive better than we newcomers did. They were better adapted physically to the cold ice-age weather of Europe and had tools more suited to hunting large game there. Their brains were bigger than those of Homo sapiens, and physically they were stronger. But by about 40,000 years ago the Neanderthals became extinct, while our species survived and proliferated.

We don’t know just why our species proliferated while Neanderthals disappeared. Perhaps confrontations led to cross-species warfare, and we were somehow better at that. Or perhaps we were somehow better able to adapt to environmental changes that required new ways of living. Regardless of the mechanism, most archeologists agree that our advantage lay in our recently evolved ability to network, cooperate, and help one another in the struggle for survival, whether that struggle was with the forces of nature or a competing human species or both. How did that ability come about?

The most plausible theory I have seen so far is that it came about by self-domestication. The anatomical changes in our skeleton, especially in our cranium, as we became modern humans were very similar in direction to the changes resulting from the domestication of farm animals and dogs and the self-domestication of bonobos. The selective process that leads to reduced aggression and increased cooperation in mammals generally also leads to certain changes in body and bone structure, perhaps as a side effect resulting from the way genes are arranged on chromosomes.

The anatomical differences between modern and earlier Homo sapiens, and between modern Homo sapiens and Neanderthals, are remarkably like the differences between bonobos and chimpanzees. Among these changes are somewhat slenderer bones, a rounder (less oblong) cranium, and a reduction in size of brow ridges (the cranial protrusions above the eyes). There is also evidence from DNA analyses that changes in certain hormones, including reduction in testosterone and increased oxytocin (a neurohormone that promotes affection), may have accompanied both the shift from early to modern humans and that from chimp-like apes to bonobos (Summers & Summers, 2023).

It seems quite reasonable to me that an increased preference to mate with playful rather than domineering partners was part of the mechanism of self-domestication, for bonobos and humans.

Playfulness as Sexual Adornment

Sexual selection—that is, a systematic preference to mate with individuals who have a certain characteristic—may well have been a major contributor to the self-domestication of our species. There is good reason to believe that women in hunter-gatherer bands preferred men who were good hunters but were also humble about their hunting. As discussed in Letter #21, hunter-gatherers strongly discouraged boasting and encouraged humility. Play requires a degree of vulnerability, which is part and parcel of humility. So, playfulness might well have been an attractive feature in potential mates.

Among most of our primate relatives (bonobos excluded), alpha males, which achieve their positions largely through aggressive threats and fights, do most of the mating. Females have relatively little choice about that. But somewhere along our evolutionary line, our species became relatively monogamous, such that men and women formed long-term pair bonds, primarily to enable two-parent care of the offspring. In a system of long-term pairing, it is advantageous to choose a mate who is not only healthy and physically fit, but also manifests behavioral characteristics indicative of willingness to cooperate over a long term.

This choice is especially advantageous to females, because females are the ones left to care for the young if the male leaves or fails to cooperate. It is also especially important to females because, throughout much of the primate world, male aggression often causes the death of females and offspring. Even today, despite our relative within-group peacefulness compared to other primates, domestic violence is the leading cause of murder of women and young children everywhere. Women, especially, would do well to choose a mate who manifests playfulness, humility, willingness to cooperate.

Indeed, at least one study suggests that playfulness is in fact one of the most attractive features to potential mating partners. In that study, college students were asked to rate the value of each of 16 characteristics in a hypothetical romantic partner, with the result that playful, fun loving, and sense of humor were all near the top for both men and women (Chick et al, 2012). The only trait that ranked higher than these was kind and understanding. These four characteristics all ranked considerably higher even than such valued characteristics as intelligence, physical attractiveness, and exciting personality.

So, sexual selection for playfulness may still be occurring in our species. Yay!

Further Thoughts

So, here’s a thought that I have presented before, in many different ways. We are what we are because of our capacity for play. Play Makes Us Human. It underlies, also, what we like to think of as our “humanity” when we use that term to refer to what we most like about our species, our capacity to care for one another. And yet, we live at a time when people who run schools—who should be most concerned about developing the humanity of our children—are taking play time away from children to ever greater degrees. And for what? For more time to study for tests that have little to do with real life. Why do we let them do that? What is wrong with us?

As always, I invite you to share your thoughts and questions in the comments section below. These increase the value of the letters for me and other readers.

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With respect and best wishes,

Peter

References

Chick, G. et al (2012). Play and mate preference: Testing the signal theory of adult playfulness. American Journal of Play, 4, 407-440.

Hare, B. (2017). Survival of the friendliest: Homo sapiens evolved via selection for prosociality. Annual Reviews of Psychology, 68, 155-186.

Hare, B., & Woods, V. (2020). Survival of the friendliest. Scientific American, August, 2020, 58-63.

Nowell, A. (2016). Childhood, Play and the Evolution of Cultural Capacity in Neanderthals and Modern Humans. pp 87-97 in Haidle, M.N. et al (eds), The nature of culture.

Sanchez-Villagra, M.R., & van Schaik, C. P. (2019). Evaluating the self-domestication hypothesis of human evolution. Evolutionary Anthropology, 28, 133-143.

Summers, K., & Summers, V. (2023). Concordant evidence for positive selection on genes related to self-domestication in bonobos and early humans.  Evolutionary Behavioral Sciences, 17, 322-332.

Trut, L. & Dugatkin, L.A. (1999). Early canid domestication: The farm-fox experiment. American Scientist, 87, 160.