/sci/ - Science & Math

>>14592460
The smallest organisms which react to light are bacteria. It's extremely common, and is not something that only evolved once. What you'd need for a light response to evolve is for a photopigment to become connected to a signal cascade.
Important to note here is that the proteins typically employed as photoreceptors in both animals and bacteria (and plenty of other organisms) have the same basic structure as those involved in chemical signalling. What makes them different is that they bind to retinals (ie.vitamin A); which change their conformation when they capture proteins.
Light reaction processes are not something that should be understood separately from the evolution of signal-response pathways in cells in general. They use many of the same pathways and motifs and clearly evolved together (or from each other).