We have just seen a remarkable example of convergent evolution at the molecular level. Two membrane proteins independently evolved to carry out the same function. The two proteins have the same pattern of protein domains, but in reverse. Remarkable sequence similarity also exists, but again, seen only in reverse.
So how did this occur? We can think of this example of convergent evolution as the unfolding of the preadapted state. This original preadapted state channeled the blind watchmaker, much as a seeing eye dog can lead a blind man. And if it wasn’t for the lucky fact that the proteins are in reverse, an example of convergent evolution would be scored by everyone as an example of common descent.