Fundamental to our success is our large brain that makes us very smart, both individually, but more importantly, collectively. Yet our brains are energy hungry, frequently go wrong, and cause difficulties in childbirth, so this bizarre adaptation must have given our ancestors a considerable advantage. Mark Maslin brings together the latest evidence from astronomy, geology, hydrology, climatology, and palaeoanthropology to show how the rapidly changing landscape of East Africa made the Rift Valley the cradle of humanity.