For most of the approximately 200,000 years that our species has existed, we shared the planet with at least four other types of humans. They were smart, they were strong, and they were inventive. Neanderthals even had the capacity for spoken language. But, one by one, our hominid relatives went extinct. Why did we thrive? In delightfully conversational prose and based on years of his own original research, Brian Hare, professor in the department of evolutionary anthropology and the Center for Cognitive Neuroscience at Duke University, and his wife Vanessa Woods, a research scientist and award-winning journalist, offer a powerful, elegant new theory called "self-domestication" which suggests that we have succeeded not because we were the smartest or strongest but because we are the friendliest.