Fig. 7.

A possible scenario of evolution of forward and reverse photoreceptors from an ancestral bistable opsin. a Phylogenetic view of opsin family and photochemical properties of the groups in the family. As many members in most of the groups show the bistable nature, such bistability should be an ancestral type of the photochemistry of opsins. b Schematic view of possible evolutionary branching into forward and reverse photoreceptors. Loss of ability to directly bind all-trans-retinal and undergo its trans-cis photoisomerization produced forward photoreceptors. On the other hand, reverse photoreceptors lost the ability to bind 11-cis-retinal along with the ability to undergo the photo-conversion reaction from the inactive state to active state