How do new genes appear? For more than a century, researchers have thought that, from time to time, new gene functions evolved after cells accidentally made a copy of one of their existing genes. According to this theory, the ‘extra’ gene copy could then evolve freely. In this way, it could acquire a new function, while the original gene ensured that the original function was retained. No direct observation supporting this theory has been reported so far. Now, however, researchers at TU Delft have observed this evolutionary mechanism in action for the first time in yeast cells.