We are the only humans, but not so long ago we had company. In the roughly 300,000 years of our existence, Homo sapiens has shared the planet with at least four other human species. In hindsight, it seems obvious why we prevailed. We were the best hunters, the smartest, the most technologically savvy.
But that is only the story we tell ourselves. Some of the other human species were more technologically advanced, had been around for much longer—a million years—or had brains as big as or bigger than ours. Going back 100,000 years ago, if you were to guess which human species was going to make it, one of the other humans, perhaps Neandertals, would have been a good bet.
We shared a common ancestor with Neandertals. They were stronger than us, barrel-chested with muscle. They were highly skilled with weapons and hunted every large mammal in the Ice Age. They even shared with us a variant of a gene known as FOXP2, thought to be required for the finely calibrated movements needed for speech. Their culture demonstrated high levels of sophistication: Neandertals buried their dead, cared for the sick and injured, painted themselves with pigment, and adorned themselves with jewelry made of shells, feathers and bone.
The first H. sapiens to arrive in Europe met a relatively large population of Neandertals who were well adapted to a cold climate. Later, as glaciers advanced, modern humans fled, and Neandertals stayed and thrived. Compared with our closest living relatives, bonobos and chimpanzees, our species has little genetic variation, which suggests that at some time, perhaps several times, we experienced a severe population bottleneck, which means we might almost have gone extinct.
If we were not the strongest or the smartest, how did we win?
Compared with other human species, it turns out we were the friendliest. What allowed us to thrive was a kind of cognitive superpower: a particular type of affability called cooperative communication. We are experts at working together with other people, even strangers. We can communicate with someone we have never met about a shared goal and work together to accomplish it. We develop this superpower before we can walk or talk, and it is the gateway to a sophisticated social and cultural world. It allows us to plug our minds into the minds of others and inherit the knowledge of generations. It is the foundation for all forms of culture and learning, including sophisticated language.
This friendliness evolved through self-domestication. Domestication is a process that involves intense selection for friendliness. When an animal is domesticated, in addition to becoming much friendlier, it undergoes many changes that appear completely unrelated to one another. This domestication syndrome shows up in the shape of the face, the size of the teeth and the pigmentation of different body parts or hair; it includes changes to hormones, reproductive cycles and the nervous system. Although we think of domestication as something that we do to animals, it can also occur through natural selection, a process known as self-domestication.
The self-domestication hypothesis was developed over the past 20 or so years from our work with anthropologist Richard Wrangham of Harvard University and psychologist Michael Tomasello of Duke University. What we discovered through our research is that self-domestication also increases the key to our success—the ability to cooperatively communicate with others. The hypothesis predicts that if H. sapiens were self-domesticated, we should find evidence of selection for friendliness in the Pleistocene (2.6 million to 11,700 years ago). Although behavior does not fossilize, the neurohormones that regulate behavior shape our skeletons, and we can trace these changes through paleoanthropological specimens.
For example, the more testosterone you have available during puberty, the thicker your brow ridge and the longer your face becomes. Men tend to have thicker, more overhanging brow ridges and slightly longer faces than women, so we call a face with these traits masculinized. Testosterone does not directly cause human aggression, but its levels and its interactions with other hormones do modulate aggressive behavior.
Anthropologists have frequently remarked on the decreasing brow ridges, shortening faces and shrinking heads of humans throughout the Paleolithic. In our own research, we realized that if we documented those changes, they would point to when physiological changes occurred that shaped our behavior and our bodies at the same time.
Together with researchers Steven Churchill and Robert Cieri, then both at Duke, we found that H. sapiens prior to the 80,000-year mark, the Middle Pleistocene, had longer faces and much larger brow ridges than in the Late Pleistocene. On average, skulls more recent than 80,000 years ago had a 40 percent reduction in how far their brow ridges projected from the face. They were also 10 percent shorter and 5 percent narrower than the older skulls before that dividing point. Although the pattern varied, it continued so that the faces of modern hunter-gatherers and agriculturalists grew more delicate in appearance, indicating a decrease in testosterone. Another neurohormone, serotonin, may have promoted a set of changes that led to smaller brains and less aggression. Increases in serotonin appear early on during the domestication syndrome—and the chemical may also be involved in skull development.
Drugs that increase serotonin availability in the brain, such as selective serotonin reuptake inhibitors (SSRIs), make people more cooperative and less willing to harm others when tested during social science experiments examining moral dilemmas and cooperation. Serotonin does not just change behavior. If exposure occurs early in development, it also appears to alter skull morphology. Pregnant mice given SSRIs have babies with shorter, narrower snouts and skulls described as globular.
Every other human species had a low, flat forehead and a thick skull. Neandertals had heads shaped like footballs. Only we have the balloonlike skulls that anthropologists call globular. This shape indicates a possible increase in the availability of serotonin during our evolutionary development. Based on the fossil record, these changes started after we split from our common ancestor with Neandertals—and they have continued in the relatively recent evolutionary past. In fact, the work of one of us (Hare) with Churchill and Cieri suggests that our skulls—and hence brain size—have been shrinking over the past 20,000 years.
If testosterone and serotonin levels changed in H. sapiens as a result of domestication, another molecule probably did as well. Lower testosterone and higher serotonin enhance the effects of the hormone oxytocin on social bonding. Oxytocin floods through mothers during childbirth. It facilitates milk production and is passed on through breast milk. Eye contact between parents and babies creates an oxytocin interactive loop, making both parent and baby feel loving and loved. When psychologist Carsten de Dreu of Leiden University in the Netherlands and other researchers gave people oxytocin to inhale in an experiment, the subjects tended to be more cooperative, empathetic and trusting in financial and social games.
All these changes had lasting impacts on our social relationships. In fact, we think these changes produced a new social category: the intragroup stranger. Our evolutionary cousins bonobos and chimpanzees recognize strangers based only on familiarity. Someone who lives with them inside their territory is a group member. Everyone else is a stranger. Recognition is clear-cut. An individual is either familiar or an outsider.
Chimpanzees may hear or see their neighbors, but the interaction is almost always brief and hostile; in contrast, bonobos are friendlier with outsiders. We, too, respond to individuals who are unfamiliar in different ways, but unlike any other animal, we also have the ability to instantly recognize whether a stranger belongs to our group. Only humans can define our groups based on appearance, language or a set of beliefs. Our ever changing conception of group status allows us to recognize those like us—even if we have never met them. It also lets us expand our social network far beyond the size of that of any other human species.
Every day, without thinking about it, we adorn ourselves in ways that make us identifiable to one another—donning sports jerseys, political pins or religious symbols on necklaces. This capacity dominates our modern lives. It encourages us to perform acts of kindness both great and small—donating an organ to a stranger or helping someone cross the street. It also helps us share and improve our best ideas.
The Lights Stay On
Even though our Neandertal cousins seemed to have an edge on us early on, around 80,000 years ago, signs that H. sapiens might not just prevail but flourish began to appear.
Glimpses of social sophistication and advanced technology can be found in archaeological remains from when we first emerged as a species in Africa as long as 300,000 years ago. But these sites were like lights blinking on and off. Technology and other signs of progress appeared, then disappeared. After 80,000 years ago these lights seemed to stay on and grow stronger. We think the new category of intragroup stranger appeared in our species around this time, when the fossil record suggests complex cultural traditions and technologies started to spread across long distances. Expanded social networks meant more cultural innovations could be shared at greater speed. Cultural and technological progress exploded.
From 50,000 years onward we began to leave evidence of our expanding social networks and cultural prowess wherever humans lived around the world. Jewelry made from shells has been found hundreds of miles inland, implying that an object with no practical value either was worth carrying some distance or was obtained from someone else who had traveled on one of our first trade routes. We painted animals on rocks so skillfully that the contours of the stone rippled beneath their bodies and seemingly gave them a third dimension.
The idea that friendliness led to our success is not new. Neither is the idea that as a species, we became more intelligent. Our discovery lies in the relationship between the two ideas: it was an increase in social tolerance that led to cognitive changes, especially those related to cooperative communication.
The arrival of human self-domestication would have led to both the increase in population and the revolution in technology we see in the fossil record. Friendliness drove these changes by linking groups of innovators together in a way other human species never could. Self-domestication gave us a superpower, and in the blink of an evolutionary eye, we took over the world. One by one, every other human species went extinct.
This optimistic view of our species is immediately at odds with the misery and suffering we still inflict on one another. If human self-domestication explains the best in us, does it also explain the worst? How do we reconcile our kindness with our cruelty?
Some of the same neurohormonal changes underlying friendliness also support horrific violence. Oxytocin seems crucial to parental behavior and has been called the hug hormone. But a better name would be the momma bear hormone. The same oxytocin that floods through a mother with the arrival of her newborn feeds the rage she feels when someone threatens that baby. For example, hamster mothers given extra oxytocin are more likely to attack and bite a threatening male. Oxytocin is also implicated in related forms of male aggression. Available oxytocin increases when a male rat bonds with his mate. He is more caring toward her but also more likely to attack a stranger threatening her. This link connecting social bonding, oxytocin and aggression is seen widely among mammals.
As our species was shaped by self-domestication, our increased friendliness also brought a new form of aggression. A higher availability of serotonin during human brain growth increased the impact of oxytocin on our behavior. Group members had the ability to connect with one another, and the bonds among them were so strong, they felt like family. New concern for others came with a willingness to violently defend unrelated group members. Humans became more violent when those we evolved to love more intensely were threatened.
Love Is a Contact Sport
Despite the evolutionary paradoxes of human nature, the perception of who belongs in our group is malleable. H. sapiens as a species has already demonstrated its capacity to expand the concept of group membership into the thousands and millions.
It can be extended further. The best way to diffuse conflict among groups is to diminish the perceived sense of threat through social interaction. If feeling threatened makes us want to protect others in our group, nonthreatening contact between groups allows us to expand the definition of who our group is.
White children who went to school with Black children in the 1960s were more likely, as they grew up, to support interracial marriage, have Black friends, and be willing to welcome Black people into their neighborhoods.
That formula still works in education. Pairs of roommates at the University of California, Los Angeles, who each were from a different race reported more comfort in mixed-race interactions and approval of mixed-race dating. One study found that imagining positive contact with one of the most dehumanized groups of people—the homeless—helps others to empathize with them. The friendships of individuals from different groups can also generalize beyond their friendship to other group members.
Most policies are enacted with the assumption that a change in attitude will lead to a change in behavior, but in the case of intergroup conflict, it is the altered behavior—in the form of human contact—that will most likely change minds. The self-domestication hypothesis explains why we as a species evolved to relate to others. Making contact between people of different ideologies, cultures or races is a universally effective reminder that we all belong to a single group called H. sapiens.
This gave us the edge we needed to outlast other members in the hominin line. In evolutionary terms, the definition of friendliness relates to positive behaviors, either intentional or unintentional, toward others. It involves not only close physical proximity while group size expanded but also an ability to rapidly read people’s intentions. The benefits of social interactions on our species’ success—the ability to solve problems better than individuals can on their own—proved so beneficial that it influenced the way selection shaped our bodies and minds. The resulting ability to share knowledge across generations produced the technology and culture that allowed us to populate every corner of the planet.