PRESIDENT DONALD TRUMP has a favorite epithet, a term of contempt: “like a dog.” Mitt Romney could have been president, but he “choked like a dog.” Broadcaster David Gregory was “fired like a dog.” In a presidential debate, Senator Marco Rubio started to “sweat like a dog.” Brent Bozell of the National Review came “begging for money like a dog.” In their Senate testimony, former Director of National Intelligence James Clapper and former acting Attorney General Sally Yates started “to choke like dogs.” Referring to his former assistant Omarosa Manigault Newman, the president writes, “Good work by General Kelly for quickly firing that dog!”

What does it actually mean, to be “like a dog”? I have a yellow Labrador Retriever named Snow. Her defining characteristic is attunement to individuals. When Snow first met my son, then eight years old, he was afraid of dogs. She was tender and quiet with him. By contrast, my daughter, then five years old, was exuberant, playful, and full of laughter. Snow romped and jumped and nearly laughed with her. While my wife likes dogs, she doesn’t come close to my off-the-charts level of enthusiasm. For weeks, Snow chose to sit gently beside her, do whatever she asked, and focus intently on her. At one point, my wife started saying, aloud and with astonishment, “Why do you like me so much?” Snow touches her deeply.

Where do dogs come from? What is their relationship to wolves? Where does Homo sapiens come from? What is our relationship to Neanderthals, Homo erectus, the Denisovans, and various other human species that went extinct? Researchers are converging on the conclusion that, in all probability, the answers to all of these questions turn on the same phenomenon: domestication. But in important respects, this seemingly mundane word is misleading. It conceals plenty of mysteries, and, perhaps, a miracle or two.

With respect to dogs, it has long been thought that human beings had adopted wolf pups, perhaps the most adorable, and trained them, loved them, and gradually converted them into dogs. Two pioneers in modern thinking about the origins of dogs, Raymond and Lorna Coppinger, call this the “Pinocchio Theory,” after the tale of the woodcarver who turned a block of wood into a real boy. But in view of recent research, it is increasingly difficult to believe that people domesticated dogs. It is far more likely that dogs domesticated themselves. We did not choose them. They chose us.

Some of the defining work, beautifully cataloged by Lee Alan Dugatkin and Lyudmila Trut, and the subject of their two recent books, was spearheaded by Dmitri Belyaev, a visionary Soviet geneticist who had been a hero in World War II. A charismatic scholar who could easily command a massive lecture hall, Belyaev had the great misfortunate of beginning his career in the early 1950s under the reign of Joseph Stalin. Among other things, this meant that his work was severely constrained by the influence of Trofim Denisovich Lysenko, a scientific fraud who abhorred “Western genetics.” He had managed to obtain Stalin’s favor and so he ruled over Soviet science like a tyrant. But Belyaev, while still in his teens, had been inspired by the work of his brother Nikolai, a famous geneticist who was arrested and executed in 1937 for his interest in Western genetics. To some extent, Belyaev was protected from official reprisal by his status as a war hero. Still, he had to watch himself carefully.

Belyaev’s job was to raise silver foxes, whose fur commanded high prices, a great boon to the struggling Soviet economy. At his assigned task, Belyaev was a superstar. But his real interest was in the mysterious evolution of canids. A follower of Darwin, he had a startling hypothesis about the origins of dogs. In brief: All of their characteristics came from docility. Belyaev speculated that at some point in ancient history, relatively docile wolves living near human settlements — wolves that might do especially well interacting with people — mated with one another, and their offspring were more docile still, and the offspring of those offspring were even more docile. Over the course of many generations, dogs emerged. Belyaev surmised that in order for animals to coexist with humans, they could not be fearful; if they were, they would be aggressive. In Belyaev’s view, the physical features of dogs — including floppy ears, multiple colors, two menstrual cycles annually (female wolves have only one) — were all a byproduct of docility.

But how to test this hypothesis? The answer lay in Belyaev’s foxes. He had access to a large population, and he planned a bold experiment. He would identify the tamest among them. He would then arrange for them to breed with each other, and for their offspring to breed with each other, and so on. Because female foxes have a litter every year, it would be possible to have 10 generations in a mere decade. Belyaev was acutely aware that his experiment might fail. After all, foxes are not wolves, and even if the experiment lasted for decades, nothing might happen. But Belyaev had a strong hunch. To conceal what he was doing, he reported to the Soviet authorities, and to everyone but his closest confidantes, that he was actually trying to produce more breeding cycles and also foxes with shinier, richer fur.

In his initial tests, he tried to be rigorous. He had his assistants closely observe young foxes to test for both fear and aggression. His assistants were asked to approach foxes slowly, to open their cages cautiously, and to reach their gloved hands into them with food. Many of the foxes backed away and snarled. Others lunged. But out of a hundred, about a dozen were less aggressive. A few would gently take food from the assistants’ hands. As Dugatkin and Trut put it, the “foxes that didn’t bite the hands that fed them became the parents of the next generation.”

After three generations, Belyaev was delighted to see results — nothing dramatic, but still, they were real. Some of the pups of the selected foxes turned out to be calmer. At times, they seemed not to mind at all when their keepers approached them. At that point, Belyaev decided that the project was working and should be expanded. He needed new people and a larger location. In 1958, he recruited Trut, a young researcher who loved dogs. Belyaev told her that “he wanted to make a dog out of a fox” — and swore her to secrecy.

Belyaev found space in Lesnoi, a remote, brutally cold area in Siberia. By 1960, his original team had bred eight generations of foxes, and a dozen of the tamest were sent to Lesnoi. To Trut, two of them stood out. She found them astonishing. They would even allow her to pick them up. She named them Laska (“gentle”) and Kisa (“Kitty). Building on Belyaev’s design, she tested dozens of foxes daily, giving them a score of one to four for various interactions; those with the highest scores were deemed the tamest. The top 10 percent were selected as new parents for the next generation.

In 1963, Trut chose to visit the several dozen pups born to the tamest mothers several times a day, examining their behavior, temperament, and size. As she approached one pen, a small male pup named Ember began to wag its tail. As Dugatkin and Trut report, “It’s really true, she thought, the foxes are becoming more like dogs!” Until that day, no fox had been observed wagging its tail in response to human beings. Ember’s litter mates did not wag their tails, but they were visibly calmer than the previous generations — and the third litter sired by Ember included several tail-waggers. By 1966, the young pups had become increasingly dog-like. When Trut reached her hand into their pens, they licked it. When she walked away, they whined. When she approached, they rolled onto their backs, asking her to rub their bellies. Belyaev and Trut decided to denominate the most dog-like foxes as “the elite.” With every generation, there was an increase in the percentage of elite foxes.

Soon Trut decided that she should adopt a fox and keep it at home — just like a dog. “If she really wanted to know just how much social and emotional depth these tame foxes were capable of, she would have to give one of them the opportunity to live in the rich social environment of a home, with humans as its closest companions.” She selected a pup that seemed to crave human attention. She named it Pushinka, which translates into “tiny ball of fuzz.” When Trut brought Pushinka home, she became a playful and loyal companion. She loved to play with balls. And when she was allowed out in the yard, “she’d always come bounding right back inside when Lyudmila called her. Just like a dog.”

Pushinka and Trut formed a deep emotional bond. When Trut left the house, “Pushinka would greet her at the door excitedly when she got back, wagging her tail.” She also treated people who visited the house as individuals rather than as generic human beings. She even seemed to sense their personalities. Trut was stunned when an apparent intruder approached the house one night. Pushinka responded with a series of barks, sounding a lot like a guard dog. Trut’s stunned reaction: “[D]ogs bark to protect their humans, foxes do not.”

As the experiment continued, the foxes’ physical appearances started to change. Just as Belyaev expected, they developed floppy ears. Their fur showed white patches. But the most dramatic changes involved their personalities. To be sure, they were not dogs. But they were pretty close. Over the years, some have ended up as pets, living with people as we live with our dogs. Those domesticated foxes would sit on command. (“Good fox!”) They would fetch balls. They would go on walks. They would cuddle. The Russian Fox Domestication Experiment, as it is sometimes called, continues to this day. Trut, now in her 80s, remains in charge.

Some of Belyaev’s most provocative speculations, which he ventured lightly, did not involve dogs at all. In a brief comment at the end of an international address, he noted that his theories about domestication, established in the fox experiment, “can also, of course, apply to human beings.” What does that even mean?

The most elaborate answer comes from anthropologist Richard Wrangham, whose magisterial, eye-poppingly revelatory book The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution is focused on a single quality that Belyaev did not identify, and that Wrangham is the first to elaborate in this context: reactive aggression, understood as immediately aggressive responses to real or perceived threats. Wolves show plenty of that. Dogs show much less. Wrangham explores that difference.

In his pioneering work on dogs, Raymond Coppinger tells a story of visiting the cage of relatively tame wolves at Wolf Park, Indiana. He was afraid to approach the animals, but his friend Erich Klinghammer, director of the facility, casually said, “Just treat them like dogs.” Coppinger did, patting an adult female, named Cassi, on her side. As Coppinger put it: “That was when she became all teeth. Not a nip, but a full war — a test of my ability to stay on my feet and respond to Erich’s excited command, ‘Get out, get out! They’ll kill you!’” Pause over the phrase: “They’ll kill you!” Witnessing reactive aggression in action, Coppinger concludes, “never again did I think that tame wolves could be treated like dogs.”

Quoting Coppinger’s tale and influenced by Belyaev, whose research he discusses in detail, Wrangham contends that domesticated animals “have changed genetically from their wild ancestors; they are less easily stimulated into producing reactive aggression.” As compared with wolves, dogs are defined above all by reductions in that form of aggression. Such reductions are what make all other canine characteristics possible, most notably their distinctive social intelligence. It is because dogs are less likely to want to fight or flee that they are so open to reading human signals. Importantly, this is not a matter of intelligence or ability to learn. In Wrangham’s view, it is the key to understanding Belyaev’s experiments. Among foxes, lower levels of reactive aggression are what characterized the elite.

Wrangham offers a detailed treatment of gentle, female-led bonobos and more fiery, male-dominated chimpanzees, showing that the extraordinary differences between the two closely related species precisely parallels the differences between dogs and wolves. (Brian Hare, Wrangham’s collaborator and former student, calls bonobos “the Dog of the Apes.”) Of these differences, the most important is that bonobos have much lower levels of reactive aggression. In effect, bonobos are self-domesticated chimpanzees. To make this claim plausible, Wrangham offers an account of the likely origins of bonobos, involving the movements of an ancestor of chimpanzees, and also of bonobos, into a geographically distinct region south of the Congo River. According to that account, the abundant food in the region allowed females to travel in stable subgroups, and selection ended up favoring their choosing less aggressive males as mates.

With reference to both dogs and bonobos, Wrangham also draws attention to the four characteristics of domesticated animals. They usually have smaller bodies than their wild ancestors. Their faces tend to be shorter and do not project as far forward. The differences between males and females are less highly developed, with domesticated males being, in a sense, feminized. They tend to have smaller brain cavities and thus smaller brains (but not necessarily lower intelligence). All four characteristics can be found in dogs as compared with wolves and in bonobos as compared with chimpanzees.

But what about Homo sapiens? “The key fact,” Wrangham writes, “is that within our social communities we have a low propensity to fight; compared to most wild mammals we are very tolerant.” Wrangham offers highly suggestive evidence that the human species that died out were, essentially, wilder versions of Homo sapiens. As dogs are to wolves, and as bonobos are to chimpanzees, so is Homo sapiens to those extinct species. For at least 250,000 years, human evolution has been dominated by two distinct kinds of Homo. The first is a series of more archaic and robust types; the second is “the more lightly built, gracile Homo sapiens.” As with bonobos, a process of self-domestication led the latter species in a more peaceable direction. And starting about 40,000 years ago, Homo sapiens was all that remained.

Our own species unquestionably shows three of the four features of domestication. Compared to us, previous species of Homo had broader and heavier skulls and thicker skeletons. “Their archaic looks were of a species that differed from Homo sapiens rather as a chimpanzee does from a bonobo, or a wolf from a dog.” Fossil evidence shows that, over time, the size of the face and the brow ridge diminished, as did sex differences; male faces became more feminine. Anatomical characteristics of domestication like small bodies, shorter faces, and reduced sexual dimorphism are likewise all found during the evolutionary history of Homo sapiens. The story is more complicated for brain size, but for the last 35,000 years, Homo sapiens “experienced a reduction […] of around 10 to 15 percent to reach today’s levels.”

Because of a comparative decrease in reactive aggression, Homo sapiens had a variety of significant advantages over other human species. Wrangham argues that across the Homo species, the level of intelligence did not greatly vary, but “exceptional cooperation and social learning seem to have been unique to Homo sapiens.” Our success had a great deal to do with the transmission of culture from one generation to the next, which these made possible. In his account, “[d]ocility should be considered as foundational of humankind, not just because it is unusual, but because it seems likely to be a vital precondition for advanced cooperation and social learning.” Brian Hare’s name for what happened: “Survival of the Friendliest.”

In a time of division, distrust, polarization, tribalism, and violence, Wrangham’s demonstration that our species is defined by attenuated reactive aggression, and by an unprecedented ability to cooperate with and learn from one another, is inspiring, even moving. And there is something that feels both cheering and mischievous in his contention that Homo sapiens is, in a deep sense, the dog among the diverse family of human species — and that this is precisely why we alone survived. You can read Wrangham as offering a quiet call to action here, which is to honor and celebrate our own domestication, and to cultivate it.

Work on evolution tends to be intriguing. It can be profoundly revelatory. But it usually doesn’t touch the deepest and most tender parts of the human soul. Miraculously, these books manage to do that.

Like a dog.

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The Robert Walmsley University Professor of Harvard University, Cass R. Sunstein is the founder and director of the Program on Behavioral Economics and Public Policy. He is the author of many articles and books.