Our beliefs about human nature shape almost everything we do as a society. Theories about whether some people are innately good or evil influence whom we imprison and for how long. Theories about whether some groups of people are more worthy than others influence our economic policies. Theories about whether some people are born smarter than others, and what this intelligence looks like, influence how we teach our children. Arguably, no folk theory of human nature has done more harm--or is more mistaken--than the "survival of the fittest." The idea that the strong and ruthless will survive while the weak perish became cemented in the collective consciousness around the publication of the fifth edition of Charles Darwin''s Origin of Species in 1869, in which he wrote that as a proxy for the term "natural selection," "Survival of the Fittest is more accurate, and is sometimes equally convenient." But somewhere along the way, "fitness" became synonymous with physical fitness. In the wild, the logic goes, the bigger you are, and the more willing you are to fight, the less others will mess with you and the more successful you will become.

You can monopolize the best food, find the most attractive mates, and have the most babies. Over the past century and a half, this mistaken version of "fitness" has been the basis for social movements, corporate restructuring, and extreme views of the free market. It has been used to argue for the abolition of government, to judge groups of people as inferior, and to justify the cruelty that results. But to Darwin and modern biologists, "survival of the fittest" refers to something very specific--the ability to survive and leave behind viable offspring. It is not meant to go beyond that. Darwin was constantly impressed with the kindness and cooperation he observed in nature, and he wrote that "those communities, which included the greatest number of the most sympathetic members, would flourish best and rear the greatest number of offspring." He and many of the biologists who followed him have documented that the ideal way to win at the evolutionary game is to maximize friendliness so that cooperation flourishes. The idea of "survival of the fittest" as it exists in the popular imagination can make for a terrible survival strategy.

Research shows that being the biggest, strongest, and meanest animal can set you up for a lifetime of stress. Social stress saps your body''s energy budget, leaving a weakened immune system and fewer offspring. Aggression is also costly because fighting increases the chance that you will be hurt or even killed. This kind of fitness can lead to alpha status, but it can also make your life "nasty, brutish and short." This is a book about friendliness and how it came to be an advantageous evolutionary strategy. It is a book about understanding animals--and here dogs play the starring role--because in doing so, we can better understand ourselves. It is also an exploration of the flip side of our friendliness: the capacity to be cruel to those who aren''t our friends. If we can develop an understanding of how this dual nature evolved, we can find powerful new ways to address the social and political polarization that endangers liberal democracies around the world.

We tend to think of evolution as a creation story. Something that happened once, long ago, and continued in a linear fashion. But evolution is not a neat line of life forms progressing toward the "perfection" of Homo sapiens. Many species have been more successful than we are. They have lived for millions of years longer than we have and spawned dozens of other species still alive today. The evolution of our own lineage, since we split from our common ancestor with bonobos and chimpanzees around 6 to 9 million years ago, produced dozens of different species within our genus, Homo . There is fossil and DNA evidence that for most of the approximately 200,000 to 300,000 years that Homo sapiens has existed, we shared the planet with at least four other human species. Some of these humans had brains that were as big as, or bigger than, our own.

If brain size was the main requirement for success, these other humans should have been able to survive and flourish as we did. Instead, their populations were relatively sparse, their technology--though impressive compared to that of nonhumans--remained limited, and at some point, all of them went extinct. What allowed us to thrive while other humans went extinct was a kind of cognitive superpower: a particular type of friendliness called cooperative communication. We are experts at working together with other people, even strangers. We can communicate with someone we''ve never met about a shared goal and work together to accomplish it. As you would expect, chimpanzees are cognitively sophisticated in many of the ways humans are. But in this sea of similarity, there is a gaping hole--they struggle to understand when communication is intended to help them accomplish a shared goal. This means that as smart as chimpanzees are, they have little ability to synchronize their behavior, coordinate different roles, pass on their innovations, or even communicate beyond a few rudimentary requests.

We develop all of these skills before we can walk or talk, and they are the gateway to a sophisticated social and cultural world. They allow us to plug our minds into the minds of others and inherit the knowledge of generations. They are the foundation for all forms of culture and learning, including sophisticated language, and it was dense groups of these cultured humans who invented superior technology. Homo sapiens were able to flourish where other smart human species didn''t because we excel at a particular kind of collaboration. When I began studying animals, I was so focused on social competition that it never occurred to me that communication or friendliness could be important for cognitive evolution, not just in animals but in ourselves. I thought increased skill in manipulation or deception could explain the evolutionary fitness of an animal. What I discovered is that being smarter is not enough. Our emotions play an oversized role in what we find rewarding, painful, attractive, or aversive.

Our preferences for solving certain problems over others plays as important a role in shaping our cognition as our computing abilities. The most sophisticated social understanding, memory, or strategy will not facilitate innovation unless it is paired with the ability to communicate cooperatively with others. This friendliness evolved through self-domestication. Domestication over the generations does not, as was once thought, decrease intelligence; it increases friendliness. When a species of animal is domesticated, it undergoes many changes that appear completely unrelated to one another. This pattern of changes--called the domestication syndrome-- can show up in the shape of the face, the size of the teeth, and the pigmentation of different body parts or hair, and it can include changes to hormones, reproductive cycles, and the nervous system. What we discovered in our research is that it also can increase a species'' ability to coordinate and communicate with others. All these seemingly random changes are tied to development.

The brains and bodies of domesticated species develop differently than those of the less friendly species from which they evolved. The behaviors that facilitate social bonding, such as play, appear earlier and are retained longer--often through adulthood--in domesticated species than in other closely related species. Studying domestication in other species has allowed us to understand how our own cognitive superpower probably evolved. Domestication is not just a result of artificial selection accomplished by humans choosing which animals to breed. It is also the result of natural selection. In this case, the selection pressure would be on friendliness--either toward a different species or toward your own. This is what we call self-domestication. Self-domestication gave us the friendly edge we needed to succeed as other humans went extinct.

So far, we have seen this in ourselves, in dogs, and in our closest cousins, bonobos. This book is about the discovery that linked our three species together and helped us understand how we became who we are. As humans became friendlier, we were able make the shift from living in small bands of ten to fifteen individuals like the Neanderthals to living in larger groups of a hundred or more. Even without larger brains, our larger, better-coordinated groups easily outcompeted other groups of humans. Our sensitivity to others allowed us to cooperate and communicate in increasingly complex ways that put our cultural abilities on a new trajectory. We could innovate and share those innovations more rapidly than anyone else. Other humans did not stand a chance. But our friendliness has a dark side.

When we feel that the group we love is threatened by a different social group, we are capable of unplugging the threatening group from our mental network--which allows us to dehumanize them. Where empathy and compassion would have been, there is nothing. Incapable of empathizing with threatening outsiders, we can''t see them as fellow humans and become capable of the worst forms of cruelty. We are both the most tolerant and the most merciless species on the planet. This book is our best effort at what E. O. Wilson called "consilience," a synthesis of seemingly disparate pieces of knowledge that creates a unified explanation. In this case, we want to show that the principles of a friendlier society and more successful democracy are informed by everything from dogs to bonobos.

The self-domestication h.