Another article from the the week's Proceedings of the National Academy of Sciences
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Apparently fish evolved eyes that were good for seeing in air before they evolved legs for walking on land.
The theory seems to be that the eyes stuck up out of the water - like a crocodile's - and the fish could see prey that was further away, and sneak up and eat it.
Visual range in air is longer than in water, so they got to see more prey, and eat enough of the extra to beat out the competition.