Courtesy Striving to a goalSomewhere, deep in the recesses of animal evolution, a mass of molecules known as opsin mutated from a run-of-the-mill protein into a detector protein with great vision. Not vision in the figurative way, but vision in the literal way. Opsin is the protein in the photopigments of your eye that interacts with light, and allows you to see all the wonderful things visible in the universe. If you’re reading this post, you have the opsins in your eyes to thank.
Here’s how it works. When a particle or wave of light (a photon) enters your eye, the light sensitive opsin traps it using a small chromophore molecule in it architecture called retinal. Normally, retinal’s tail is all twisted and bent, tensed up, and waiting for something to happen. That’s just the way retinal is when it’s chilling out. But when a photon hits it, the light particle interrupts retinal’s naptime, and the molecule reacts by straightening out its tail. The tail’s movement starts a chain reaction of sorts activating the opsin, which in turn, activates a nearby nerve that shoots out a signal that your brain perceives as light.
Three types of opsin can exist in the eye: R-opsins (rhabdomeric), C-opsins (ciliary, and Go/RGR-opsins (Go-coupled/retinal G protein-coupled receptor). The R and C opsins, depending what type of animal you are (e.g. vertebrate or jellyfish), are used for detecting light. Go/RGR-opsins don’t detect light but are used instead to help regenerate retinal cells and regulate an animal’s inner clock or biological rhythms. Scientists have known about opsins since the 19th century, but haven’t known much on how they evolved, or how they became designated light detectors.
In a recent study published in the journal PNAS, Roberto Feuda of the Department of Biology, National University of Ireland Maynooth, and colleagues reported on their detailed examination of the genetic trail of opsins in all kinds of animal life, from sponges and jellyfish to reptiles, birds and mammals. And while their results warrant further study, they did add new knowledge to our understanding how the eye evolved.
The study negated a long-held idea among scientists that only certain light-designated opsins were present in certain animal types. Generally, C-opsins were thought to be present only in vertebrates, and R-opsins only in invertebrates. But the study showed otherwise. It postulated that all three forms of opsins probably existed in the earliest common ancestor right from the beginning. Later, somewhere along their respective evolutionary lines each group designated the C or R opsins for light detection. The leftover opsins (whether C or R) were used for other non-visual purposes such as setting biological rhythms.
It also pushed the origins of light-sensitive organs back a couple hundred million years from about half a billion years ago to three-quarters of a billion years ago, a time not long after sponges had diverged from other animals and before they split into Bilateria and Cnidaria. Within that evolutionary timeline opsins were found in the gene sequence of the tiny and transparent shape-shifting microorganisms called placozoa. However, because the genome lacks a critical retinal-binding amino acid - lysine 296– it’s unlikely these opsins were able to detect light. (It should be noted that placozoan phylogeny is still under debate). But somewhere along the evolutionary line, these non-visual opsins mutated into a light sensing protein. After just two more gene duplications the three opsins, R, C, and Go/RGR we find in our eye’s photopigments today, were already present in the genome.
Why or when opsins developed into part of the eye’s photopigment is anyone’s guess. This research doesn’t solve all the mysteries surrounding them, particularly their non-visual functions but it does fill in some of the gaps in our understanding of key components of vision evolution.