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Special report from Nairobi, Kenya. Researchers explain why a 2.4 million-year-old stone-tool 'factory' at Lake Turkana shows sophisticated thinking, and how our earliest-known ancestors were walking 4.1 million years ago. In the Majengo district, doctors are working with prostitutes who are immune to Aids to develop a vaccine. Plus, what what locals think about the prospect of the first genetically modified crop – a disease resistant sweet potato – being planted in Kenya.