How can fruit flies and mice share the same gene that says to build an eye if they evolved separately?

Question

I saw a documentary where they inserted the gene of a mouse that basically is the starting "build an eye" command into a fruit fly, and a fruit fly eye grew. My question is, if eyes of different types of animal had different evolutionary beginnings (I don't know much about arthropod eye evolution, but for a different example, as I understand it, cephalopod eyes evolved from skin cells while vertebrate eyes evolved from brain cells), then **how is it that the same gene controls the growth of two such evolutionary different eyes**?

Answer 1

This goes back pretty far evolutionarily. Organisms adapt to their environment. If there is a signal (touch, hear, heat, light) then the organism will develop a way to detect it. Prokaryotes developed photoreceptors on their cell surface and this was passed along. Those same photoreceptors were put to use, found advantageous, and continued to be refined for that particular organisms purpose. A fly has no need to see miles out, so the eye they have evolved much different than a human. The basis of both are photoreceptors, but the application is very different.