Scientific discoveries are sometimes made in the most extraordinary way. Horizon reveals how a car radiator helped one scientist realise why humans walk upright.

For decades it had been thought that we began walking the way we do when our brains became more complex and we needed to free our hands to use tools. However, two fossil discoveries in the 70s - the Laetoli footprints and "Lucy" - proved that these two developments happened millions of years apart.

More recently, while a British scientist was using a model called Boris to demonstrate that big brains - like computers - need to be kept cool, a palaeontologist was inspired by her car's cooling system to investigate blood flow to the brain.

Following on from the two recent programmes entitled In Search of Our Ancestors, Chris Hale's film shows how these two independent strands of research came to the same conclusion.