One of the most important milestones in the evolution of life occurred when paired fins (and later limbs) appeared, leading to new types of locomotion. Now, in the advanced online issue of Nature, scientists show that, although originating from different cell types, human limbs and median fins share a common developmental mechanism. These results support the idea that it was from median fins that all fins and limbs evolved, a hypothesis that has been around since the 19 century, but, until now, has never been proved.