Two elephants walk together at night. (No, this isn’t a joke—it’s a scene from a wildlife reserve in Thailand.) There is heavy rain and the older elephant slips and falls in the mud. She’s unable to get up. The younger elephant, unrelated to her companion, stays with her for most of the night. The next day a group of mahouts, elephant caretakers from the wildlife reserve, try to hoist the elephant up to her feet with braces and ropes. In all the commotion—a crowd has gathered to watch the rescue—the younger elephant remains by the side of her fallen friend. The mahouts and the crowd shout for her to move out of the way, so they can get better leverage. But she won’t budge. Instead, she burrows her head under the body of the other elephant and tries to lift her up. She does this several times, risking injury in the attempts. Incredibly, the elephant appears to recognize that the mahouts want to help rather than hurt her friend. She times her pushes, or so it seemed to me, with the hoisting of the mahouts.

Until recently, biologists thought such complex behavior—behavior with an undeniable moral dimension—was exclusive to human beings. As much as anyone in the world, the primatologist Frans de Waal is responsible for changing this perception. Starting with Chimpanzee Politics[1]—de Waal’s fascinating account of the intrigues and machinations of a chimpanzee troupe in the Arnhem Zoo—and continuing through recent books like Good Natured and Our Inner Ape, de Waal has illustrated the uncanny similarities between human beings and our primate relatives. De Waal has not restricted himself to descriptions of behavior, however. He is famous for his willingness to enter into the largely taboo world of animal emotions, where research is routinely dismissed as “anthropomorphizing.” The result is an impressive array of evidence suggesting that we are not the only species to have moral feelings.

De Waal’s research is no friend to human vanity. In the grand tradition of Galileo and Darwin, de Waal provokes those who seek to draw a clear line between human beings and everything else. But his message is an optimistic one. If human morality has deep roots in our evolutionary past, then we can expect it to be more resilient, less susceptible to the contingencies of history. Seeing morality in this light also undermines the view of human beings as inherently selfish—a view that de Waal terms “veneer theory.” Morality, according to veneer theory, is merely a recent cultural invention, a thin veneer that masks our “true” selfish animal nature. De Waal’s criticisms of this...