Book excerpt: A.J. Jacobs has received some strange emails over the years, but this note was perhaps the strangest: “You don’t know me, but I’m your eighth cousin. And we have over 80,000 relatives of yours in our database.” And so begins A.J. Jacobs’s quest to build the biggest family tree in history. In an era of us-versus-them thinking, this book is a hilarious, heartfelt and profound exploration of what binds us all – where family begins, how far it goes, and the science that is revolutionizing the way we think about ethnicity, history and the human species. This book is about A.J. Jacobs’s family. But it’s also about your family. Because it is the same family.