The image on the right is both beautiful and exciting. Let me explain why. It’s the paw of an embryonic mouse and a team of geneticists have inserted a fragment of human DNA into its cells. The fragment contains an “enhancer” element, a short span of DNA that switches other genes on and off; in this case, they put the enhancer in control of a gene whose activity creates a blue chemical.

This particular enhancer is called HACNS1. Throughout the course of animal evolution, its sequence has gone relatively unchanged in almost all back-boned animals, but it has evolved rapidly in the human genome since we split away from chimpanzees. So the blue patch in this image shows where this rapidly-evolved,