Animals exhibit a wide range of behaviors. They forage. They defend territory and flee predators. Sometimes they play. They reproduce, at times with spectacular displays. They learn. But perhaps the most striking from an evolutionary perspective is that they sometimes cooperate or help others, even at a cost to themselves. Why? Morality seems to defy the image of natural selection as "selfish," favoring only traits that benefit the individual in a competitive "struggle for existence." Given the importance of morality for human society, this puzzle is a major challenge for science. How do biologists interpret such behavior? How could morality originate in an evolutionary context?