Archaeological and geological evidence has shown that 10,000 years ago, the Severn estuary didn't exist. Then the sea level rose rapidly, forcing our Mesolithic ancestors back inland. How did they cope with this fast-changing coastline? Quentin Cooper discusses with Professors Martin Bell and Nigel Nayling the challenges of working in areas that are often exposed for a mere hour-and-a-half a day, at low tide. Producer Pamela Rutherford