But opsins haven’t always been the sensitive light detectors that they are today. There is one critter, obscure and small, carries opsins that are blind to light. These opsins aren’t broken, like they are in some cave dwelling species. They never worked to begin with. They are the relics of a distant past, a time in which our ancestors still dwelt in darkness.
Opsin is a member of large family of detector proteins, called the ‘G-protein coupled receptors’ (GPCRs). Like a needle and thread, all GPCRs wind themselves through the outer membrane of the cell seven times. Halfway between cell and outside world, these tiny sensors are perfectly positioned to monitor the surroundings of the cell. Most GPCRs detect the presence of certain molecules. When a certain hormone or neurotransmitter docks their outward facing side they become activated and release signalling molecules on the inside of the cell. But opsin is different. It doesn’t bind molecules physically. Instead, it senses the presence of a more delicate and ephemeral particle: the photon itself, the particles (and waves) that light is made of.
Opsins trap photons with a small molecule in the heart of their architecture, called retinal. In its resting state retinal has a bent and twisted tail. But as soon as light strikes retinal, its tail unbends. This molecular stretching exercise forces the opsin to change shape as well. The opsin is now activated and eventually will cause a nearby nerve to fire, which will relay its message to the brain: light!.