Between two and six thousand years ago something unprecedented began happening in about six separate places around the world: The people of small chiefdoms in places like the Nile Valley, Mesopotamia, the Peruvian Andes, Mexico, China, and the Indus Valley began uniting to form the first large centralized states. It’s not known what triggered these transformations. There were no scribes yet to record the transition from villages to states and, until very recently, there wasn’t enough information about these first complex civilizations to make detailed comparisons