Fig. 2
From: Opening the “Black Box”: The Genetic and Biochemical Basis of Eye Evolution
Opsin originated by mutation of a “serpentine protein” [also called G-protein-coupled receptor (GPCR)]. Protostomes, deuterostomes, and cnidarians possess opsins, but they are absent from porifera and non-animals. Yet, these organisms possess non-opsin GPCR proteins. This indicates that opsin originated by a GPCR gaining light sensitivity, before the common ancestor of Cnidarians, protostomes (including flies), and deuterostomes (including humans)