All animal eyes and eye-spots contain opsin, a protein that captures light. This is the compound eye of Antarctic krill. Photo by Gerd Alberti and Uwe Kills
Gaze deep into any animal eye and you will find opsin, the protein through which we see the world. Every ray of light that you perceive was caught by an opsin first. Without opsin there would be no blue, no red, no green. The entire visible spectrum would be.. just another spectrum.
But opsins haven’t always been the sensitive light detectors that they are today. There is one critter, obscure and small, that 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!.
Opsins lie embedded in the outer membrane of the cell, where retinal (grey molecule in the middle) can trap photons.
Scientists have known about the existence of opsins (or rhodopsin, as the retinal-bound form is also called) ever since the 19th century. The German physiologists Wilhelm Kühne and Franz Boll first discovered and isolated rhodopsin in 1876 and 1878, respectively. It took another fifty years before the American biochemist would George Wald discover retinal in 1933.
Since these early days of visual chemistry, scientists have uncovered opsin’s light detecting tricks and resolved its molecular structure in atomic detail. It is safe to say that the physical and chemical nature of opsin are better understood than its history. Many questions about the evolution of opsins have remained unanswered in the past 130 years of opsin research. In which of our many ancestors did opsins evolve? How old is opsin? How old is vision?
The short answer is ‘ancient’. Since almost every animal carries opsins of some sort, these proteins must have appeared early in our evolution. The long and more more precise answer involves an evolutionary reconstruction of opsin’s earliest history, such as the one that was published by Roberto Feuda and others in PNAS three weeks ago. Feuda and his colleagues gathered opsin sequences from all corners of the animal kingdom, hairy, scaly and squishy, and calculated how related these genes were to each other.
First of all, Feuda confirmed the existence of three distinct opsin types within bilateria (bilaterians are animals with left-right symmetry). These three opsin types are called R-opsins, C-opsins and RGR-opsins. For a long time biologists thought C-opsins were exclusively found in animals with a spine (the vertebrates) and that R-opsins were limited to protostomes, a diverse group of animals that includes mollusks and arthropods. (The third type of opsin, the RGR-opsin, is a bit odd compared to the other opsins. Instead of detecting light, they play a role in regenerating ‘spent’ retinal molecules.)
The division was so stark and neat that vertebrates and protostomes must each have evolved their own light detecting opsins from an ancestral template. Or so scientists thought. The tidy story unraveled once opsins started to pop up in unsuspected places. The brain of the ragworm Platynereis dunerlii, a protostome, was found to contain C-opsins. R-opsins were identified in nerve cells in the human retina. These discoveries forced opsin biologists back to the evolutionary drawing board. In their new scenario, the common ancestor of vertebrates and protostomes, the ur-bilaterian, already had three types of opsin. The two lineages later recruited C-opsins or R-opsins for their visual systems, respectively.
Now, Feuda and his colleagues push back the origin of this opsin cluster farther still. The first animal to carry three opsins was not the bilaterian ancestor, but the last common ancestor of Bilateria and Cnidaria (jellyfish, anemones, corals and their kin). Feuda found all cnidarian opsins belong to one of three different groups, each of which correspond to the three basic opsin types in Bilateria.
Cnidarians are plain weird, from a bilaterian perspective. Their anatomy differs radically from our symmetrical bauplan. Cnidaria don’t have brains for example; their thoughts and decisions are born in a decentralized net of nerves instead. For hundreds of millions of years, our evolution and development have followed vastly different path. We became jaguar, they became jellyfish. They are coral, we are crab. Yet Feuda’s results bear one mind-boggling implication: the c-opsins in your cones and rods are more closely related to the corresponding opsins in the eye-spots of a jellyfish, than either of them is to the r-opsin in your retinal nerves. The roots of animal vision run deep indeed.
To see how deep, Feuda’s team leapt to another branch of the family tree, and scoured the genomes of two sponges, Oscarella and Amphimedon, for opsin sequences. No dice. Apparently, opsins only evolved after sponges had diverged from other animals, but before the split between Bilateria and Cnidaria. Fortunately for Feuda, there exists one animal lineage in this sweet spot between sponges on one side and cnidarians/bilaterians on the other. Meet the placozoans. Small, simple and flat, placozoans resemble shapeshifting pancakes more than anything else. They drift along the sea floor, searching for detritus to scavenge. Below, you can see how one Trichoplax (the only defined placozoan species) becomes two:
Sure enough, the placozoan genome harbours two opsins. But here’s the catch: these opsins cannot detect light. Remember retinal, the molecule that changes shape when it is struck by light? The placozoan opsins cannot bind retinal, because they lack the amino acid to which retinal binds (amino acids are the building blocks of proteins). Without ‘lysine-296′, it is unlikely that the placozoan opsins can detect light. But if not light sensors, what then? “Surely placozoans use these opsins. How? I cannot tell. Your answer would be as good as mine I am afraid”, David Pisani, the lead author of the study, writes in an e-mail.
Feuda and colleagues are not the first to notice these placozoan opsins. On this wiki about opsin evolution, a UCSC researcher wrote that “these [placozoan] genes retain uncanny similarities to opsins in otherwise rapidly changing regions. Perhaps these genes should be considered opsins in spite of lacking [lysine-296].” However, Feuda’s team is the first to investigate how these ‘uncanny opsins’ relate to the other opsins. This is how they visualized the scenario they came up with:
Pondering this figure, it hit me that our opsins really had two origins. One is the birth of opsin itself, the other is the mutation that turned opsin into a light sensing protein. The opsin lineage itself arose between 755 and 711 million years ago, from the duplication of a single GPCR. The last common ancestor of Bilateria and Cnidaria lived between 711 and 700 million years ago. This leaves a short window of time (evolutionary speaking) in which opsin acquired the light sensing mutation and split into the three opsin families we still carry today.
This probably won’t be the final word on opsin evolution. Branches will shift as more opsin sequences become available and researcher probe further into the earliest history of animals. Also remember that a single light sensing protein does not make a functional eye or eye-spot. The roads that animals took towards vision are myriad, with each eye and eyelet evolving along its own trajectory, towards splendid colour or dreary monochrome, eagle-eye vision or simple on/off light detection.
But although the differences are many, the starting point was the same. A single opsin. A flash. Then there was light.