The past 12,000 years represent the only time in the sweep of human history when there has been only one human species. How did this extraordinary proliferation of species come about – and then go extinct? And why did we emerge such intellectual giants? The tale of our origins has inevitably been told through the stones and bones of the archaeological record, yet Robin Dunbar shows it was our social and cognitive changes rather than our physical development which truly made us distinct from other species.