Molecular evidence for dim-light vision in the last common ancestor of the vertebrates

Animal vision is mediated through pigments belonging exclusively to the opsin family. These are members of the G-protein-coupled receptor family that bind retinal [1]. Based on function and phylogenetic relationship, vertebrate visual opsins can be clustered in five groups: Rhod photoreceptors (Rh1), Rh1-like (Rh2), Short Wave Sensitive (SWS1), SWS1-like (SWS2), and Long (LWS) or Medium (MWS) Wave Length Sensitive (LWS/MWS). Rh1 is used for seeing under dim light conditions (scotopic vision), while the others permit full colour (photopic) vision in bright light 2, 3, 4 and 5. Opsins have diversified by a series of gene duplications, and the inferred order of these duplications indicates that photopic vision predated scotopic vision in vertebrates 2, 3, 4 and 5