Did a gene enhancer humanise our thumbs?

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.