The extraordinary success of Homo sapiens is a result of four things: intelligence, language, an ability to manipulate objects dexterously in order to make tools, and co-operation. Over the decades the anthropological spotlight has shifted from one to another of these as the prime mover of the package, and thus the fundament of the human condition. At the moment co-operation is the most fashionable subject of investigation. In particular, why are humans so willing to collaborate with unrelated strangers, even to the point of risking being cheated by people whose characters they cannot possibly know? Evidence from economic games played in the laboratory for real money suggests humans are both trusting of those they have no reason to expect they will ever see again, and surprisingly unwilling to cheat them—and that these phenomena are deeply ingrained in the species’s psychology. Existing theories of the evolution of trust depend either on the participants being relatives (and thus sharing genes) or on their relationship being long-term, with each keeping count to make sure the overall benefits of collaboration exceed the costs. Neither applies in the case of passing strangers, and that has led to speculation that something extraordinary, such as a need for extreme collaboration prompted by the emergence of warfare that uses weapons, has happened in recent human evolution to promote the emergence of an instinct for unconditional generosity.