From Carl Zimmer

The more scientists study the eye, the more they recognize that Darwin was right. This is not to say that they know everything about how the eye evolved. Evolutionary biology is not an automatic answer machine that can instantly tell you every detail about how eyes–or any other organ–evolved. Instead, scientists study eyes of different animals, the proteins they are made of, and the genes that store their recipe. They come up with hypotheses about how evolution could have produced these results. Those hypotheses then point the way to new experiments. In this way, evolutionary biology is no different from geology or meteorology, or any other science that illuminates the natural world.

To be precise, I should say that scientists study the evolution of “the eye.” There are millions of different eyes (and other light-detecting organs), each built by a different species from its own unique set of genes. Closely related animals tend to have similar eyes, because they descend from recent ancestors. Some scientists study how eyes can adapt over a few million years to the special circumstances of a particular species. Other scientists step a little further back, to look at how the different types of eyes have evolved from simpler precursors. And other scientists step even further back in time, to find clues about where those simpler precursors came from. In this post, I will move back through time through these different stages of eye evolution (a la Richard Dawkins’s The Ancestor’s Tale (link in original).)