Dr Alice Roberts explores how our species, Homo sapiens, developed its large brain and asks why humans are the only ape of its kind left on the planet today. The evolution of the human mind is one of the greatest mysteries. It is the basis of religion, philosophy and science. We are special because of our extraordinary brains, and to understand why we think and act the way we do, we need to look at where and why our brains evolved. The Rift Valley in Kenya is thought to be the crucible of human evolution, and here Alice examines the fossils in our family tree which reveal our brains have more than quadrupled in size since our ancestors split from chimpanzees. Research investigating sediments and rocks laid down during the period of greatest brain growth suggests a fluctuating environment may have played a part. Drawing on research on social politics in chimpanzees, the cognitive development of children and the tools that have been found littered across the Rift Valley, Alice explores how and why our ancestors brains became so big. Successive species of increasingly large-brained humans migrated around the world - from Homo erectus to heidelbergensis, the Neanderthals to us. It has always been assumed the reason that Homo sapiens succeeded where others failed is to do with our large brains. Comparing skulls it's clear Neanderthals had just as big a brain as us, so why is there only us left? Alice goes to meet Svante Paabo, who is decoding the Neanderthal and human genome, and Clive Finlayson, who is unearthing the Neanderthals' final settlement, to try to find out.