A simple chipped stone from the Rift Valley in Tanzania marks the emergence of modern humans. Faced with the needs to cut meat from carcasses, early humans in Africa discovered how to shape stones into cutting tools. From that one innovation, a whole history of human development springs. Neil MacGregor, Director of the British Museum, tells the story with contributions from flint napper Phil Harding, Sir David Attenborough and African Nobel Prize winner Dr Wangeri Maathai. (Duration: 15 mins)