The body of the human animal is astoundingly complex. There are, for example, about one hundred trillion cells, plus one thousand trillion micro-organisms, in each of us. This micro-universe more or less behaves as a unity, which is another miracle. Most of us think, it seems, in terms of being a personal self, as if we were truly single individuals. Each of us could, theoretically, behave as an isolated creature, grubbing around for food, maybe occasionally mating as mindlessly as we foraged and fed. We could each be a castaway on the endless ocean of meaninglessness. Yet we seek out other...