Your arms and toes began as tiny buds that sprouted from your sides when you were just a four-week-old embryo. By six weeks, these limb buds had grown longer and five rods of cartilage had appeared in their flattened tips. By week seven, the cells between the rods had died away, sculpting five small fingers or toes from once-solid masses of flesh.

Now, a team of scientists led by James Sharpe from the Centre for Genomic Regulation in Barcelona has discovered that these events are orchestrated by three molecules. They mark out zones in the embryonic hand where fingers will grow, and the spaces in between that are destined to die. Without this trinity, pianos and keyboards wouldn’t exist,