Scientists often stick genes into organism in order to create something new. Remote-controlled flies, for example, or photographic E. coli. But by creating new kinds of life, scientists can also learn about the history of life. Give a mouse human vision, for example, and you may learn something important about how our own eyes evolved.

As mammals go, we have unusual eyes. Most mammals produce two kinds of pigments for catching light. One is sensitive to short wavelength light (at the blue end of the spectrum). The other is sensitive to a longer wavelength, in the green or red part of the spectrum. Each pigment is encoded in a single gene, and any given photoreceptor in the retina uses