Lucas Brouwers discusses research into the origins of animal vision, based on the history of opsins. Opsins exist in almost all animals, however they are not always photosensitive. Brouwers discusses the research of several scientists who are trying to trace photosensitive opsins throughout history. Brouwers says, “Apparently, opsins only evolved after sponges had diverged from other animals, but before the split between Bilateria and Cnidaria.”