Sit down with an anthropologist to talk about the nature of humans, and you are likely to hear this chestnut: “Well, you have to remember that 99 percent of human history was spent on the open savanna in small hunter-gatherer bands.” It's a classic cliché of science, and it's true. Indeed, those millions of ancestral years produced many of our hallmark traits—upright walking and big brains, for instance. Of course, those wildly useful evolutionary innovations came at a price: achy backs from our bipedal stance; existential despair from our large, self-contemplative cerebral cortex. As is so often the case with evolution, there is no free lunch.