Spectral tuning of the eye generally depends on key substitutions of amino acid sites in opsin proteins.

Eyes universally depend on rhodopsins, proteins that play other important roles in sensory processing and belong to a much larger family of proteins. Spectral tuning of the eye generally depends on key substitutions of amino acid sites. Knowing both the molecular sequences of opsins in extant vertebrates and also their phylogeny it is possible to make reasonable assumptions as to the molecular composition of ancient opsins and also to construct artificial genes that will enable one to test the spectral sensitivity of an extinct animal. Just this has been done for the archosaurs, a group that ultimately gave rise to the dinosaurs and birds. In the archaic archosaurs there are good arguments to suggest they were adapted for vision in dim light.