I wonder how that first primitive lense came about? - Juila Orwell
Great question.
About 90% of the proteins in the human lens are called crystallins. It turns out that they are not unique to vision. Many crystallins are also enzymes that perform other functions around the body. The most abundant is α-crystallin shields other proteins from damage and can be found in the brain, liver, lung, spleen, skin and small intestine.
11 crystallins have been discovered 3 of which are found in the eyes of all vertebrates. The others vary from species to species. All of them also perform other functions.
Almost all proteins are transparent. As is so often the case, a protein that was already common has simply been pressed into service for a new function.