2/3. Another re-match of a classical debate, hosted by Allan Little. Carla Pinto-Correia and John Durrant restage the 19th-century arguments between Albrecht von Haller and Caspar Wolff about how life develops from a fertilised egg to a human being. They also look at how little we still know in today's age of the genome. Producer Kirsten Dwight Repeated on Christmas Day at 10.15pm