Yet, we have evolved almost the same model of eye. In 1976, the famous classificationist Ernst Mayr and his colleague Salvini-Plawen , decided to survey the animal world to see how many times eyes had evolved independantly. They identified image-forming eyes in at least 40 separate lineages of animals. The astonishing conclusion was that the eye had evolved from scratch somewhere between 40 and 65 times. Apparently, despite human incredulity, its no big deal to evolve an eye.
Sight is obviously a huge survival advantage for a mobile animal.
And perhaps the number of ways of efficiently producing an image is limited. So animals, through trial and error end up converging on the same basic designs over and over again. Just another case of convergent evolution, like wings in bats and birds, or fins in dolphins and fish. Yet in 1976, Salvini-Plawen and Mayr were not entirely convinced by the conclusions they'd drawn from their paper. Surely it would have been as important for the prototypical animals of the Pre-Cambrian seas, as for animals in later eras.