Cooperation was prominent among the suite of behaviors that marked the emergence of behaviorally modern humans in Africa. Those living 75,000--90,000 years ago at the mouth of what is now the Klasies River near Port Elizabeth, South Africa, for example, consumed eland, hippopotamus, and other large game. The rock painting of hunters and their prey on the jacket of this book is from the nearby Drakensberg Mountains. The Klasies River inhabitants, and their contemporaries in other parts of Africa, cooperated in the hunt and shared the prey among the members of their group. Even earlier evidence of trade in exotic obsidians extending over 300 kilometers in East Africa is another unmistakable footprint of early human cooperation. Other primates engage in common projects. Chimpanzees, for example, join boundary patrols and some hunt cooperatively. Many species breed cooperatively, with helpers and baby sitters devoting substantial energetic costs to the feeding, protection and other care of non-kin. Social insects, including many species of bees and termites, maintain high levels of cooperation, often among very large numbers of individuals. But Homo sapiens is exceptional in that in humans cooperation extends beyond close genealogical kin to include even total strangers, and occurs on a much larger scale than other species except for the social insects. In A Cooperative Species, we show that people cooperate not only for selfish reasons but also because they are genuinely concerned about the well-being of others, try to uphold social norms, and value behaving ethically for its own sake. People punish those who free-ride on the cooperative behavior of others for the same reasons. Most of this evidence comes from behavioral experiments in which individuals have the opportunity to divide up substantial sums of money between themselves and others and also to pay for the opportunity to punish those who act selfishly. We took our experiments out of the lab and into societies of hunters and gatherers in Africa, Asia and Latin America. One of us even hunted with the Hadza people of Tanzania to get some idea of the kinds of lives our ancestors might have led. We concluded from this research that among economics majors in the lab and hunter-gatherers in the forest, contributing to the success of a joint project for the benefit of one's group, even at a personal cost, evokes feelings of satisfaction and pride. Failing to do so is often a source of shame or guilt. Cooperation thus is sustained by altruistic motivations that induce people to help others when not helping would result in their having higher fitness or other material rewards. These experimental results contradict the assumption common to both economics and biology, namely that individuals are self-interested and act to maximize their personal gains whether it be biological fitness or material wealth. The scientific challenge, then, is not that addressed by biologists and economists who have studied cooperation, namely to explain why selfish people would nonetheless cooperate. Rather the challenge is to explain how the unforgiving calculus of natural selection could have produced a species in which a substantial fraction of individuals are willing to sacrifice their own gains to help others, to uphold moral principles, or to advance their group. To address this challenge we assembled archaeological, genetic, climatic, and other data on the distant past as well as from recent societies of hunters and gatherers. We then used models of natural selection and computer simulations based on these data to generate literally millions of possible histories of the biological and cultural evolution of our species over the last 100,000 years. Our conclusion is that Homo sapiens came to have these "moral sentiments" because our ancestors lived in environments, both natural and socially constructed, in which groups of individuals who were predisposed to cooperate and uphold ethical norms tended to survive and expand relative to other groups, thereby allowing these pro-social motivations to proliferate.