Whenever you pick up a book or take a step, you’re relying on a genetic legacy that’s been handed down since your ancestors swam about in prehistoric oceans. Your hands and feet– intricate bundles of bones and muscle – are your versions of the fins of fish. They may look very different, but they were sculpted from embryonic flesh using similar genetic programmes.

The appendages of all four-limbed animals (tetrapods) are governed by a cluster of genes called HoxD. This group oversees the development of a whale’s flippers, a finch’s wings, and a chimp’s arms. They control how big a limb becomes, the number of the bones within it, and the shape of those bones.

During an embryo’s early days, the HoxD