Tuesday 28 February marks the 70th anniversary of – in my view – the most important day in the history of science. On a fine Saturday morning with crocuses in flower along the Backs in Cambridge, two men saw something surprising and beautiful. The double helix structure of DNA instantly revealed why living things were different: a molecule carries self-copying messages from the past to the future, bearing instructions written in a four-letter alphabet about how to synthesise living bodies from food.