Rhodopsin is a pigment-containing sensory protein that converts light into an electrical signal. As a photoreceptor, rhodopsin is employed by a wide range of organisms, from bacteria to mammals.
Rhodopsin has 2 components: a protein called scotopsin and its covalently bound cofactor retinal. Scotopsin is an opsin. Retinal, a pigmented form of vitamin A, is the chemical agent that affords light-sensing via photoisomerization: changing isomer upon absorbing a photon. The configuration change provokes scotopsin to dissociate from retinal, resulting in photobleaching, which lessens opsin sensitivity to light. In humans, bleached rhodopsin-powered vision rod cells fully regenerate in ~30 minutes.
Animals have 2 kinds of photoreceptor cells that use rhodopsin: rhabdomeric and ciliary. In each case, light activates rhodopsin, but the subsequent stages of phototransduction diverge. In rhabdomeric receptors, light causes cation channels to open in the photoreceptor cell membrane, thereby depolarizing the cell, whereas in ciliary receptors light hyperpolarizes the cell by closing these channels.
The common ancestor of animals had both ciliary and rhabdomeric photoreceptors, and they continue to coexist. One puzzle is how these photoreceptors evolved to perform opposite roles in different animals.
Visual photoreceptors – those present in eyes – are ciliary rods and cones in vertebrates, but rhabdomeric in protostomes (segmented worms, arthropods, and mollusks). Conversely, vertebrate rhabdomeric receptors have non-visual roles such as regulating circadian rhythms, whereas protostome ciliary receptors are non-visual sensors.
Insects have rhabdomeric receptors and vertebrates have ciliary receptors, but both have excellent vision. ~ English neuroscientist Daniel Colasco
In bright light, ciliary receptors are superior to rhabdomeric receptors because they consume less energy and suffer less response variation, which would reduce signal reliability. Also, a higher photopigment density in ciliary receptors enhances their sensitivity.
The morphology and phototransduction cascade of the rods and cones enable them to count photons more efficiently in terms of the space they occupy, the materials and energy they use, and the accuracy with which they register photon hits. For this reason, the majority of vertebrates adopted a duplex retina with slow, high-sensitivity rods for efficient scotopic vision in dim light, and lower-sensitivity cones for fast and accurate photopic vision in bright light. ~ American physiologist Gordon Fain et al
To their credit, rhabdomeric receptors function over an enormous intensity range: from starlight to bright sunlight. By contrast, the ciliary mechanism has to trade off response speed against the rate of spontaneous photopigment activation in the absence of light. This spontaneous activation, or dark noise, creates a constant veiling pseudo-light effect, which, in cone photoreceptors, overwhelms vision at low intensities but is insignificant in daylight.
To overcome this ciliary functional deficiency, vertebrates have a duplex retina of rods and cones. Whereas retinal cones are responsible for photopic (bright-light) vision, rods afford scotopic (low-light) sight.
Low spontaneous activity allows rods to signal detection of a few photons, but rods suffer from slow response and take considerable time to recover from bleaching, which leaves them blind in daylight. Vertebrate cones give snappy responses, but dark noise makes them useless at night.
Rods burn out in bright light and cones create a haze in the dark, yet we have no awareness of these failings. Our vision seems seamless regardless of light level. Continuous signal clarity is a mystery given the biodynamics of photoreceptors.
Gene duplications provide raw material for functional adaptations. ~ Gordon Fain et al
To create the vertebrate photoreceptor complex, the genome duplicated early on in these animals’ descent. (The genome of the vertebrate ancestor was doubled twice at the dawn of these creatures. These massive gene duplication events led to many novel functions, not just vision.) The original acted as a base of gene conservation while the copy served as a workshop for innovation. New sets of opsin genes were generated.
Ciliary cone-based color vision evolved before the dim-light capability of rods developed – low-light vision being a refinement. Yet rods must have been on the evolutionary plan, as ciliary-based vision is inferior overall to rhabdomeric vision without low-light rod cells.
Ciliary photoreceptors come to dominate the chordates due primarily to the invention of the high-sensitivity rod. ~ Gordon Fain et al
The Sun’s brightness was gaining intensity as life developed on Earth. Early evolved animals stuck with the traditional rhabdomeric receptors, whereas the chordates that became vertebrates crafted the more complex but energy-efficient ciliary system. This adaptation anticipated sunnier days. (This book brushes past adaptive evolution as existence-proof of coherence and an argument against matterism, which cannot explain the observed dynamics of evolution. For an exploration of this subject, see Spokes 3: The Elements of Evolution.)