Why do we hold hands? If you think about it, it's a peculiar behavior. What is its function? What does it accomplish? Why do so many people all around the world do it? I hadn’t given it much thought until I embarked on the scientific study of how—at the level of brain function—people soothe each other’s fears and anxieties. In my early work, hand holding was little more to me than a convenient way to study social support in the restrictive environment of the brain scanner. But as the years, and studies, have gone by, a deeper understanding of simple hand holding has unlocked for me many of the secrets of our shared humanity—and helped me explain why, for humans, social isolation is the quickest route to misery, poor health, and even early death. We’ll use the mystery of hand holding as our point of departure on a scientific journey toward understanding the way social relationships affect our earliest sensory experiences, the length of our lives, and everything in between. We’ll also explore the likeliest theories about the evolution of Homo sapiens, and how that evolution is reflected in the structure and function of the human brain.