In the final episode Dr Alice Roberts explores how our species, homo sapiens, developed our large brain; and asks why we are the only one of our kind left on the planet today?

The evolution of the human mind is one of the greatest mysteries. We are special because of our extraordinary brain, 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. 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. It's thought their need to understand each other and share knowledge has fundamentally shaped the brains we live with today.

But our big brains came at a cost. Alice visits a maternity hospital to explore how our difficulties in giving birth are a result of our large brained babies, and travels to meet the women of the Hadza tribe to explore how looking after our large-brained children has shaped the life histories of all of us. Research suggests old age isn't a consequence of modern medicine, but an evolutionary adaptation to allow grandmothers to help raise their grandchildren.

Successive species of increasingly large brained humans migrated around the world - from Homo erectus to heidelbergensis, the Neanderthals to us. It's always been assumed the reason that Homo sapiens succeeded where others failed is to do with our large brains. But 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, decoding the Neanderthal and human genome, and Clive Finlayson, who is unearthing the Neanderthals final settlement, to try to find out...