I stumbled upon (not literally) this Science commentary that says that fish may have evolved their gills initially as agents for ion exchange. The researchers, and I'm not sure how animal friendly this is, put fish into little boxes that separated their back ends from their front ends and measured ion levels and oxygen levels in the separate compartments. Little fishes are able to exchange ions and oxygen through their thin skin, and the idea was that as the fish age, the gills would start to exchange ions and oxygen faster than the back end of the fish. The gills exchanged ions earlier in development than oxygen, so the researchers are saying that this indicates that they were evolved for this function. This is not really a very compelling argument and the researchers are talking about doing future work with gene expression to see which set of machinery is expressed in development first, but still... That doesnt quite make the argument either, because when something appears in development earlier isnt necessarily correlated with earlier evolution.
It is apparently hard to take pictures of baby fishes because there are no good pictures out there. Fish gills were too gross and drawings are boring. Here is another hit I got with baby fishes though :)
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