Some years ago, when I first began writing about the evolution of human sociality, a colleague put to me the question: Why are humans still social? That “still” was weighty with meaning—the idea of a primal solitary state, to which humans might return, perhaps finally freed from group living by technological progress. I was dumbfounded. Humans have no choice but to live in groups. They are unable to reproduce and survive to reproductive age without a group, which makes them obligately interdependent. That interdependence is inscribed upon the body. We lack natural defenses such as impressive...