It’s just after daybreak on a plain at the edge of Amboseli National Park in southern Kenya near the border with Tanzania. In an acacia grove, a troop of yellow baboons is getting an easy start to the morning. A few late risers sleep on in the upper branches, but the others have been dropping down to the ground, one by one. More than half of the nearly 70 animals stroll or sit in the scrubby grass.

“They seem really calm this morning,” Susan Alberts says, lowering her binoculars. An evolutionary biologist at Duke University and co-director of the Amboseli Baboon Research Project, Alberts has been observing baboons here for 35 years. She keeps up a field biology play-​by-​play for me.

“There’s a nice grooming, a little lip smacking, some embracing, a little inspection.”

We watch two females greet each other on a tree branch above us.

“[She] was very relaxed when she was approached. No tension, no anxiety, she didn’t cower . . . ​And now there’s a greeting in the opposite direction. Oop!”

For just a second, one female had lunged toward the other.

“The higher-​ranking female just reminded the lower-​ranking female. And the lower-​ranking female said, oh yeah, I submit. We understand each other.”

Our attention turns to a baboon on the ground called Ivy, who is holding an infant. She is approached by Acid, who is pregnant and says a little something as she comes near.

“That’s a hello baby grunt,” Alberts translates, meaning it is a request to handle Ivy’s infant. “Acid is higher ranking than Ivy. You can tell because although Acid was very polite, she wasn’t at all shy about it. And Ivy leaned aside slightly. I don’t know if you saw it. She just went like that.”

Alberts ducks her shoulder ever so slightly then straightens. I hadn’t seen it.

Acid moves in front of Ivy, presenting her shoulder.

“A very clear request for grooming.”

Ivy and Acid are not related, but, Alberts tells me, after consulting a small binder of field notes, they were born in November 2011 and February 2012, respectively. They are agemates that grew up together.

Now Acid returns the favor and begins grooming Ivy, but she also starts playing with Ivy’s baby.

“It’s unpredictable, but Ivy and Acid have known each other their whole lives so there’s a certain amount of trust there.”

You could even say they are friends.

About 15 years ago, Alberts and her colleagues recognized the strong social bonds among the mothers, grandmothers, and great-​grandmothers of these same baboons for what they were—​the nonhuman primate version of friendship. More importantly, they uncovered the value of those bonds in the animals’ lives, friendship’s power to shape outcomes. That discovery was a watershed moment. If humans were the only animal on Earth for whom social relationships affect life span, we would have to search for the roots of friendship in the structure of human society. But since the baboons, along with other species, share our need to be social, that tells us something different. The wish for friendship runs deep.

A potentially game-​changing aspect of the world of early man got very little attention for a long time: his social life.

One of the driving ambitions of anthropologists and evolutionary biologists has been to work out how it was that human beings came to be the dominant species on Earth, able through adaptation and scientific ingenuity to live in any habitat, even Antarctica. We know the process was slow. Seven million years separate humans from the common ancestor we share with our closest relatives: chimpanzees and bonobos. There were several critical developments along the way. When we began to walk upright we could see farther and use our hands to do more than walk or grip tree limbs. Our ability to make tools made building possible and changed the odds when hunting. Language allowed us to communicate as no other species ever has and therefore to cooperate in wildly impressive ways—​ultimately building skyscrapers and sending rockets into outer space. But another potentially game-​changing aspect of the world of early man got very little attention for a long time: his social life. Trends that leave traces in bone or carved stones are more tangible. Just because you cannot touch it, however, doesn’t mean the social world didn’t touch our ancestors in formative ways.

In 1976, Cambridge psychologist Nicholas Humphrey wrote a prescient though purely speculative essay posing a new possibility. He argued that creatures are only as intelligent as their environments demand. Based on the lives of contemporary Bushmen, early humans appeared to have a lot of time for sitting around. What would have challenged their mental capacities? Other individuals. The life of social animals is like a chess game, Humphrey argued, that required players to be “able to calculate the consequences of their own behavior, to calculate the likely behavior of others, to calculate the balance of advantage and loss—​and all this in a context where the evidence on which their calculations are based is ephemeral, ambiguous, and liable to change, not least as a consequence of their own actions.” Social skill, Humphrey concluded, goes hand in hand with intellect and this need for intellect builds on itself like “an evolutionary ratchet, acting like a self-​winding watch to increase the general intellectual standing of the species.” His conclusion: “I propose that the chief role of creative intellect is to hold society together.”

Humphrey got others thinking. If you leap from intellect to the size of a brain required to contain it, you will notice that monkeys and apes (that is, primates) had much bigger brains relative to body size than most other animals. In 1982 primatologist Frans de Waal published his best-​selling book Chimpanzee Politics, which noted the ­Machiavellian scheming required for success in large social groups. The book recounted de Waal’s work watching a large population of captive chimpanzees at the Burgers’ Zoo in Arnhem, the Netherlands. That research included a pivotal moment that he described in later books. One morning, de Waal witnessed a fight between two chimpanzees. The same afternoon, he spotted the same two animals embracing. It looked very much as if they were kissing and making up. That observation marked the beginning of his interest in the positive side of chimpanzee interactions. Ultimately, he shifted the focus of his studies from aggression to reconciliation, empathy, and morality.

In 1990 two Scottish primatologists, Andrew Whiten and Richard Byrne, took the idea further. They linked primates’ capacity for tactical deception and propensity for forming coalitions with the complexity of their societies and the size of their brains. They called their theory the Machiavellian intelligence hypothesis. It emphasized that unlike bees, for example, whose hive has structural complexity with different individuals in different roles, primates live in social systems that involve tight social bonds between pairs of individuals that adjust their behavior according to the subtleties of what’s happening around them. In time, since deception was only part of the story, the theory was renamed the social brain hypothesis, which has been popularized by evolutionary psychologist Robin Dunbar of Oxford University.

The social brain hypothesis is based on the idea that nonhuman primate societies have something to tell us about early human societies. It was that suspicion that inspired Louis Leakey to send Jane Goodall to Gombe to watch the chimpanzees. The societies of monkeys and apes in Africa are viewed as indicators of how early hominins might have lived—​especially those nonhuman primates that are close relatives or live in habitats similar to those of early humans. Anyone who watches them for any length of time can see pretty quickly that primates have an unusually rich and complex social life. They live in groups with all the drama of those cliques I described from high school.

Living in groups helps animals solve two substantial ecological problems. One is the risk of being attacked by predators. There has always been strength in numbers. A lone individual—​whether baboon or human—​is a much easier target for a lion than a herd of individuals. The other major problem to solve is finding food. Animals must “make a living” off the land, gathering enough nutritional resources to sustain themselves. There has been a long-​running debate within primatology and evolutionary biology as to which problem—​predation or foraging—​might be primary, and therefore, an animal’s first order of business. According to the social brain hypothesis, both problems are solved by first creating a cohesive group whose members can coordinate their actions. The assumption has been that the average size of a group provides a proxy for how hard an animal has to work on its social life, and animals that live in larger groups have been found, on average, to have larger brains.

An awful lot—​if not most—​of what people do could be summed up in one phrase: the need to belong.

For a decade or more, the social brain hypothesis was widely accepted. But debate over it never went away and as recently as 2017, a group of primatologists from New York University published a paper using much larger sample sizes and updated statistical techniques to call the idea into question. Their results found that diet, not sociality, predicted brain size—​a victory for the camp that has long argued for the centrality of foraging. The authors suggested that perhaps development of social skills followed as brains grew. In an accompanying commentary, British biologist Chris Venditti doubts this will be the last word on the matter. Instead, he believes it is likely to “reinvigorate and refocus” research into cognitive complexity. To know exactly where sociality fits in the evolutionary pecking order, we will have to wait for primatologists to fight it out in the pages of evolutionary biology journals. Already, however, the emergence of the social brain hypothesis has rewritten the story of social behavior in a way that cannot be undone. Sociality is no longer invisible. Whether it is the driving force or a secondary development, it has a starring role in evolutionary theory. “Frankly it’s a mistake to search for one and only one cause of brain growth in primates when it seems so clear that there are multiple plausible explanations,” says primatologist Robert Seyfarth.

A second important idea, this one from social psychology, was hatched late on a summer night in 1993. Roy Baumeister and Mark Leary sat up talking in the living room of the beach house where they were staying in Nags Head, North Carolina. Everyone else had gone to bed. But Baumeister and Leary were bothered by something.

Even though they were on the beach, they were not on vacation. They were attending an intimate conference of psychologists. There weren’t more than 25 of them. Mornings were spent listening to one another’s presentations and afternoons were devoted to talking inside or out on the beach. The group that week was particularly interested in questions of self and identity. Some of the others were arguing for a new theory that held that humans embrace cultural values that provide life with meaning because of a fundamental fear of dying. (It’s known as terror management theory.)

Baumeister and Leary weren’t convinced. “Yeah, human behavior is affected by our concerns with dying,” says Leary, who is now a professor at Duke. “There’s no question about that. But we felt like a lot of the things they were trying to explain with their theory, you didn’t need to bring death into the equation. It’s much more mundane than that.”

If you disagree with a theory in science, it’s a good idea to have something else to offer in its place. “We just started talking,” Leary says. “If we were going to have one theory in social psychology that was that big, that we thought identified the number one motive that controlled more human behaviors than any other, what would it be?”

They started speculating and, in the wee hours, came up with a new, far more life-​affirming idea. An awful lot—​if not most—​of what people do could be summed up in one phrase: the need to belong. “Human beings,” they would go on to write, “have a pervasive drive to form and maintain at least a minimum quantity of lasting, positive, and significant relationships.”

At first the idea seemed so obvious that Leary and Baumeister felt sure someone else must have thought of it. No one had, not exactly anyway. Belonging had popped up in psychological literature in various ways over many years, but no one had proposed it as what Leary calls “a master motive.”

“It wasn’t that we thought the need to belong explained everything on the face of the Earth,” Leary says. People need other things. They seek power, achievement, and intimacy. They seek and need material goods. What the “need to belong” theory did was recast behavior that other social psychologists were attributing to concern with death as something that felt more immediate to people’s everyday concerns.

Though they were on to something, there were a lot of details to work out. At the time, Leary was a professor at Wake Forest University in North Carolina and Baumeister was at Case Western Reserve in Ohio. Conveniently, though, Baumeister had accepted a visiting position at the University of Virginia for the 1993–​1994 school year, and his sister happened to live around the corner from Leary in Winston-​Salem. It was natural that Baumeister would make the relatively easy drive from Charlottesville a few times that fall. Every time he visited his sister, he also spent a day sitting in Leary’s living room, continuing the conversation they had started in Nags Head.

They covered the walls with giant sheets of paper. Whenever inspiration struck, one or the other would grab a Magic Marker and add to the notes they were creating as they tried to plot out what belonging would have to look like if it could be said to account for a big chunk of human behavior. One side of the room was devoted to what they called criteria. To be a master motive, the need to belong would have to be universal. A lack of it would have to constitute deprivation. It should show up under nearly any circumstance. It should make people feel good (or bad). It should affect how they thought about the world. It should be something people strive for, and it should affect a wide variety of behaviors.

On another wall they assembled evidence in order to bolster their theory with some proof. They cited evolutionary thinking about the need to live in groups in order to defend against predators and improve chances at resources. They included a raft of laboratory studies showing how quickly and easily people formed in-​group bonds even when the group they were “in” had been made up by the researchers on trivial grounds (think red shirts versus green shirts). They showed that social attachments form under adverse circumstances, such as military service, where the strength of bonds increases with the intensity of action soldiers see together. They cited proof that people are reluctant to break bonds. And they found early evidence—​this was before the explosion in neuroscientific knowledge—​of the extent to which cognitive resources were devoted to social relationships.

Finally, on a third wall, they listed implications. What were all the things they could possibly explain about human behavior in light of the need to belong? “We could have written an encyclopedia,” Leary once said. They thought their theory explained patterns of group behavior and close relationships, and that group conformity, excuse-making, and patterns of self-​presentation made sense in the context of enhancing one’s chances of inclusion. They mentioned the use of social inclusion as a form of reward and punishment. They saw a role for belongingness in religion and thought they could account for the pursuit of power as arising from the need to belong, too. And they noted “it remains plausible (but unproven) that the need to belong is part of the human biological inheritance.” If such a thing could be proven, they said, it would have considerable ramifications for health.

Their paper was published in 1995. Baumeister and Leary hoped their work would make people sit up and say, “We missed this.” That is exactly what happened. The theory has been cited (as I write) by nearly 18,000 others—​an enormous number in the academic realm where 100 citations is a lot. It wasn’t that people had denied the need to belong, but they had certainly failed to appreciate its importance.

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Reprinted from Friendship by Lydia Denworth. Copyright © 2020 by Lydia Denworth. Used with permission of the publisher, W.W. Norton & Company, Inc. All rights reserved.