Visual Purple is the Royal Blood of the Emergence of Cellular Life

Chris King SEC <> PDF 14 Dec 2025

Abstract: If a mathematician asked you to explain why human blood has a gently alkaline symmetry-broken ph of 7.4 you wouldn’t expect that they would take you back on a journey to your origin in primal alkaline sea floor vents four billion years ago, that shows how deep and far-reaching the symmetry-breaking of the forces of nature are. Nor would you expect that they would explain that the rhodopsin-held retinal we use to see in our retinas was key to a purple Earth that powered the first living cells using an almost identical microbial rhodopsin .

Insight as Primal Energy: They say that seeing is believing, that insight is the key to understanding, enlightenment and illumination. It turns out that such insight has a planetary and even cosmic origin. “Visual purple” is a purplish-red light-sensitive pigment present in the retinas of humans and many other animal groups and in archaea and bacteria. "Royal blood" refers to the idea of noble lineage, symbolised by the colour purple, attributed to the dark blood of Spanish nobles.

Fig 1: Operation of retinal opsins. Type 2 animal rhodopsin is the photoreceptor in our rod cells that mediates dim light vision and thus is extremely sensitive to light. When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is fully regenerated in about 30 minutes, after which the rods are again more sensitive. The colour is derived from the isoprene molecule retinal which reflects a purplish hue as it absorbs green light, midway in the visible spectrum, switching between the ground cis-state and the excited trans state, resulting in bleaching and regenerating as the cis form using an ATP-powered enzyme.

But rhodopsins and retinal go back to the very origin of cellular life in type 1 procaryote forms. Both extremophile haloarchaea such as salt-loving Halobacterium salinarum and bacteria, among which, free-living  Pelagibacter ubique, now called communis is the most ubiquitous abundant organism on Earth. Both the archaea and bacteria that use rhodopsin and retinal to entrap solar radiation, to pump protons out of the cell to generate ATP, the principal energy currency of nucleotide-based life, via the rotary ATPsynthase, are also able to use an aerobic metabolism.

Fig 2: Mechanism of proton pumping in procaryote rhodopsins involves a reverse trans to cis  isomerisation causing a charge separation on the retinal, releasing a proton to Asp85 and down a bucket brigade of water molecules to the extracellular medium. The return to the ground state results in a capture of a proton from Asp96, then replaced by the cytosol.

The microbial rhodopsin energy budget is thus one proton transferred from the cytosol to the extracellular medium for each absorbed photon – a proton for a photon!

Fig 3: Comparison of the microbial and animal forms of rhodopsin and retinal isomerisations (Ernst et al. 2014).

ATP Synthase and the First Living Cell

Complementing microbial rhodopsin in the generation of the energy-carrying molecule adenosine triphosphate (ATP) from the ensuring proton gradient is ATPsynthase, a molecular motor embedded in membrane that generates ATP from ADP and inorganic phosphate. It operates by using a proton gradient to drive a rotary mechanism: the flow of protons through the F_o part of the enzyme causes the central stalk to spin, which in turn drives conformational changes in the F₁ part, leading to the synthesis of ATP.

Fig 4: Rotation of ATPsynthase generates 3 molecules of ATP per revolution using 8-14 protons.

ATP synthase requires around 3 to 4 protons per ATP molecule, with a full rotation of its rotor (driven by 8-14 protons) typically generating three ATPs, meaning roughly 10-14 protons per full turn, but the precise number varies by organism (e.g., 8 protons for bovine, 10-14 for others) and includes transport costs, averaging out to about four protons per ATP in many cases. ATP synthase is a remarkably fast molecular motor, with observed rotation speeds varying from 100 to over 600 revolutions per second (rps), depending on conditions and organism, reaching peak rates around 300-350 rps at body temperature, though it can hit higher speeds (up to 650 rps) at elevated temperatures or in certain species, generating ATP as its central stalk spins, powered by proton flow. The rotational behavior of FoF1 resembles that of F1. This finding indicates that ‘‘friction’’ in the Fo motor is negligible during the ATP-driven rotation (Ueno et. al. 2005).

Fig 5: (1) The H⁺-dependent ATP synthase (Guo et al. 2019, Mahendrarajah et al. 2023, Nakano et al. 2023) universal to the chemiosmotic coupling of electron and H⁺ ion transport to ATP production is a rotary motor which appears to have evolved from two separate subunits F0 and F1. It occurs in the F subtype in bacteria and the A/V subtypes in archaea where the V is an ATP synthetase generating a H⁺ gradient using ATP and A is a synthase like F, that may have arisen from recombination of V and F. The F1 subunit is purported to have descended from an ATP-dependent helicase, and the F0 from a passive ion channel  (Falk & Walker 1988, Nirody, et al. 2020). F1 unit has been proposed to share ancient structure with a  polynucleotide helicase (Doering et al. 1995, Gomis-Rüth et al. 2001). Hexameric helicases are found both in the SF3 superfamily in viruses and the MCM helicases are critical to replication forks in diverse organisms from humans to archaea. The viral SF3 superfamily helicase tree shows variants active on both RNA and DNA substrates, consistent with an origin in the RNA era (Caprari et al. 2015). (2) Heptahelical rhodopsin receptor in the G-protein linked family,  also critical for brain function in diverse neurotransmitter and olfactory receptors. Rhodopsin binds to purple retinal, inducing a photo-activated H⁺ ion gradient in Haloarchaea, providing voltage gradient excitability and generating ATP. Oceanic Pelagibacter, the most abundant organism on Earth, also uses retinal photo-activation, via proteorhodopsin, evolutionarily related to both bacteriorhodopsin and visual rhodopsins (3) with both species absorbing in the optimal green region (5) complementing the absorption of chlorophyll-based cyanobacterial photosynthesis, e.g. of of the abundant keystone species Prochlorococcus marinus (Sephus et al. 2022), in turn leading to the “Purple Earth” hypothesis, that an epoch of rhodopsin-ATPsynase cellular chemiosmosis was critical to the first life on Earth before electron transport evolved (DasSarma et al. 2018). (5) shows Heliobacteria growing on Australian saline pans.

The Symmetry-breaking Connection

Lost city vents, which have existed from the first liquid oceans on Earth to the present day, are formed by a chemical garden reaction between basic olivine and acidic sea water with dissolved CO₂ . Olivine is cosmologically abundant on asteroids, Earth and the Moon and was far more abundant on the early Earth. Resulting H₂ and CO can drive the formation of organics including C2-5 hydrocarbons in reactions (Proskurowski et al. 2008) such as noted below.

(Mg,Fe)₂SiO₄ + H₂O + C → Mg₃SiO₅(OH)₄ + Mg(OH)₂ + Fe₃O₄ + H₂ + CH₄ + C2-C5

Lost-city vents have been found to form carbonate columns with pores which have been demonstrated to be able to concentrate organics and in particular nucleotide molecules by a factor of over 1000 (Baaske et al. 2007), bringing them up to molar concentrations where a reactive metabolism and informational replication could be sustained. This provides a prospective nursery environment for life to emerge as a far from equilibrium complex dissipative systems becoming a cooperative progenote of replicating nucleotide and polypeptide molecules, with cell membranes arising later.

Fig 6: (1)  Lost city vents formed by serpentinisation, a chemical garden reaction between basic olivine (lower) and acidic sea water with dissolved CO₂ . Resulting H₂ and CO can drive the formation of organics including C1-4 hydrocarbons. These can be concentrated x1000 to biological levels (lower). (2) Emergence of archaea and bacteria as complementary cellular life forms with differing membrane structures, from the common progenote (Lane & Martin 2012). (3) Primitive metabolism involves ion transport. (4, 5)The divergence between archaea and bacteria occurred before the membrane components (4) and (5) evolution of DNA polymerases (Leipe et al. 1999). Olivine is cosmologically abundant on asteroids, Earth and raining out of a young star in the Orion Nebula, attesting to an origin in cosmic symmetry-breaking, as are the organics.

The end result is the chemiosmotic origin of life where membrane electrochemistry set the whole process going. This gave rise to two fundamentally complementary life forms, archaea and bacteria as well as a lot of viruses emerging from a cooperative milieu called a progenote in which viruses and transposable genetic elements provided a sexual complementation to emerging cellular life forms. The archaea are geological organisms in salt pans, hot smoker ocean vents, methane generating swamps, and volcanic hot pools. They don't cause diseases. Bacteria are fast metabolic organisms that decompose, photosynthesise and respire and can be pathogenic. Retinal is itself an isoprene, consistent with an archaean origin.

This provides the following sequence of steps in the evolution of cellular life. Archaea have differing isoprene cell membranes from bacteria, so it is possible that each emerged from the progenote and became cellular as they escaped fig 6, gaining metabolic autonomy via the membrane energetics. The DNA polymerases of archaea and bacteria are also different, so it also looks as if they evolved and diverged before DNA-based life had stabilised, at a point where DNA and RNA were flipping cyclically using the reverse transcriptase that is still in the endogenous retroviruses in our genomes and in our RNA-primed telomerase.

Caveat: Life stands to this day in the fossilised reflection of its origin in serpentinisation fig 6. The pH of the cytosol in a typical healthy human cell is tightly regulated to between 7.2 & 7.4, and that of human blood is even more tightly regulated between 7.35 & 7.45, with 7 and less leading to metabolic lysis. This is illustrated in carbohydrate restriction of a high protein ketogenic diet where the ph falls to 7.35 and in diabetic or severe starvation keto-acidosis where, instead of glucose, acidic ketones like acetoacetate are generated by the liver, converted into acetyl-CoA and become used for energy in the citric acid cycle, causing the ph to fall below 7.3. A ph below 7.1 requires urgent medical care to avoid coma or death. The mitochondrial matrix ph is even higher at 7.78, reflecting the H⁺ ion gradient across the membrane from LUCA the last universal common ancestor, in the first archaea and bacteria – a relic of the  alkaline lost city vents which are higher still – 9 to 11. Nevertheless organisms have adapted to extremes, from 3 to 13, and the human lysozyme, involved in digestive dissolution, is around 5. This is a testament to the far-reaching power of cosmic symmetry-breaking of the forces of nature.

The Universality of Proton Transport

This overall picture is consistent with a photo-activated origin of ATP synthase driven by rhodopsin-retinal H⁺ pumping to generate nucleotide energetics, coincident with, or immediately following,  the topological transition to cell formation, upon which this process depends. This primary source of nucleotide energy powering the primal cell would have been complemented by lithoautotrophy, providing organics from the energetic products of serpentinisation, fig 6, including H₂ and CO, arising in a collective progenote, followed by electron transport, firstly two forms of anaerobic 1-photon, e.g. H₂S-based  photosynthesis independently involving each of photosystems 1 & 2, then cyanobacterial 2-photon aerobic photosynthesis, as a fusion of the two fig 6b, with photosystem 1 also enabling cyclic photosynthesis coupled only to ATP production via H+ pumping. After leaving PS1, cyclically flowing electrons travel back to the cytochrome complex (Cyt) or plastoquinone (Pq) in the first leg of the electron transport chain. After the great cyanobacterial oxygenation event, respiratory electron transport arises. Finally 2 bya later, we have the eucaryote endosymbiosis of an archaeon and an alpha-proteobecterium. Although electron-driven processes of proton pumping are not fully elucidated due to the complexity and sensitivity of the the electron transport cytochromes (Belevich et al. 2007, Yoshikawa et al. 2011) and both photosynthesis and respiration involve quinones linking e^- transport and H⁺ pumping the picture of electron transport providing the energy for proton pumping is key to both energy processes and the continuity of life and confirmed by the ph differences. While the photosynthetic interior lumen is at an acidic  ph of 4 - 5, the outer stroma is at an alkaline 8 demonstrating electron induced proton pumping. The same polarised picture occurs in mitochondria, where the inner matrix is at an alkaline 7.78, while the inter-membrane space is slightly more acidic than the cytosol, measured at 6.88.

Fig 6b: (1) Two photon photosynthesis with H⁺-induced ph values. (2) Origin of 2-photon photosynthesis by fusion of 1-photon systems (Xiong et al. 1998, Xiong & Bauer 2005). (3) Experimentally analysed H⁺ pumping in respiratory cytochrome C oxidase (Belevich et al. 2007, Yoshikawa et al. 2011, Kaila et al. 2011, Wikström & Sharma 2018). (4) The respiration electron transport chain with  H⁺-induced ph values.

Why Pelagibacter ubique is a key candidate to investigate origin of life scenarios.

While it is probable that the first photo-activation cycle began with an archaeon , or even before LUCA,  Pelagibacter ubique, now named communis, is literally ubiquitous and is the most abundant species on Earth, as well as rhodopsin photo-cyclic ATP generation. It is an abundant member of the SAR11 clade in the phylum Alphaproteobacteria. SAR11 members are highly dominant organisms found in both salt and fresh water worldwide and were originally known only from their rRNA genes, first identified in the Sargasso Sea in 1990 and later found in oceans worldwide. "Ca. P. communis" and its relatives may be the most abundant organisms in the ocean, and quite possibly the most abundant bacteria in the entire world. It can make up about 25% of all microbial plankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water. The total abundance of "Ca. P. communis" and relatives is estimated to be about 2 × 10²⁸ microbes.

Pelagibacter ubique is the smallest free-living cell that is capable of synthesising all amino acids on its own. It is a marine bacterium that has a diameter of only about 0.1 to 0.2 micrometers, making it one of the smallest known organisms. It is able to produce all 20 standard amino acids on its own, which is a unique feature among free-living bacteria. This is in contrast to many other bacterial species, which rely on external sources of amino acids for growth and survival. The ability of P. ubique to synthesise all its own amino acids is thought to be a key adaptation for its survival in nutrient-poor environments, such as the open ocean. Iit has one of the smallest known genomes of any free-living organism. However, even with its small genome size, P.ubique has been shown to have a complex gene regulatory network that allows it to respond to changes in its environment, such as nutrient availability and temperature. The bacterium has been found to have a number of regulatory genes, including transcription factors and signal transduction proteins, which are involved in controlling the expression of its genes. These regulatory genes allow P. ubique to adjust its metabolism and other cellular processes in response to changing environmental conditions.

The genome of P. ubique strain HTCC1062 was completely sequenced in 2005 showing that P. ubique has the smallest genome (1,308,759 bp) of any free living organism encoding only 1,354 open reading frames (1,389 genes total). Genome data indicate that this bacterium possesses the complete pathways required to generate energy by respiration and also to harvest energy from the sun using proteorhodopsin, a light-dependent, retinylidine proton pump. P. ubique is also equipped with numerous high affinity nutrient transporters, consistent with a competitive strategy for nutrient scavenging. Most are ABC transporters, a large and ancient protein family noted for providing high substrate affinities at the cost of copious ATP. Clearly, the cells invest in transport to outcompete other microbes in nutrient acquisition and, based on their abundance and efficient assimilation of organic matter, they have succeeded! In their level of core genome conservation, the members of SAR11 are outliers, the most conserved free-living bacteria known. This is consistent with keystone procaryote species streamlining for compact efficient genomic phenotypes.

Evolutionary Kaleidoscopes of how Animal Rhodopsin got Back into the Picture

While animal type 2 and microbial type 1 rhodopsins (and type 3, with inverted membrane orientation), share strong structural similarities (heptahelical and retinal-binding), their high sequence divergence has made the link unclear, suggesting an ancient common ancestor, with animal opsins evolving from a microbial rhodopsin precursor, by horizontal transfer, possibly after a period of functional loss (Shen et al. 2013). Bulzu et al. (2022) have compiled an evolutionary tree of microbial types they term "kaleidoscopic" but note that "Type-1 and Type-2 share similar, seven-helical topological conformation and membrane orientation with the N terminus in the extracellular space and a Schiff base linkage from a conserved lysine to retinal in the seventh helix. However, while the overall fold is the same, there is no detectable sequence similarity between these two types. Kojima & Sudo (2023) have suggested convergent evolution, but the ubiquity of microbial rhodopsins and their mulifunctional horizontal transfer into eucaryote Dinoflagellates (Rhiel et al. 2022) and Cryptophyta (Govorunova 2016) would imply horizontal transfer, as hypothesised by Shen et al. However, they have noted the combination of a functional loss and horizontal transfer, particularly to an ancestral eucaryote would erase virtually all local sequence correspondence.

Fig 7: The diversity of type 1 & 3 microbial rhodopsins

(Bulzu et al. 2022).

To give an idea of the degree of diverse horizontal transfer, and repeated mutation, Bulzu et al. (2022) note:

We expanded the typical rhodopsin blueprint by showing that a highly conserved and functionally important arginine residue (i.e., Arg82) was substituted multiple times during evolution by an extensive amino acid spectrum. We proposed the umbrella term Alt-rhodopsins (AltRs) for all such proteins that departed Arg82 orthodoxy. AltRs were present in nearly all phylogenetic clades of rhodopsins in bacteria, archaea, eukaryotes, and viruses (insofar as taxonomic origin could be reliably ascribed). However, by far the vast majority (n = 102) were found in the eukaryotic Cryptophytes. Altogether, we classified 399 sequences as AltRs. At least 121 of these sequences originate from previously described channelrhodopsins found in unicellular algae and giant viruses, xenorhodopsins, and potassium pumps, all classified as Type-1 rhodopsins. The previously undescribed sequences (n = 278) encompassed both Type-1 and HeRs with 93 of them being of confident taxonomic origin, with 30 from eukaryotes (26 cryptophytes, 1 fungus, 1 ciliate, and 2 unclassified), 34 of nucleocytoplasmic large DNA viruses (NCLDVs), 25 bacterial (12 proteobacteria, 9 actinobacteria, 3 cyanobacteria, and 1 Verrucomicrobiota) and 4 archaeal (3 Halobacteriota and 1 Thermoplasmatota).

To add to the diversity, Yoshida et al.(2017) also note in the discovery of a microbial rhodopsin in the choanoflagellate Salpingoeca rosetta, other microbial rhodopsintransfers into the green alga Chamyldomonas reinhardtii, the choanoflagellate group Proterospongia, fungal Allomyces macrogynus, and cryptomonad alga Guillardia theta.

Just in case you aren't familiar with G-protein coupled receptors, or GPCRs, they are an absolutely key set of diverse membrane receptors that govern all intercellular communication in multicellular animals, that have the same hepta-helical structure as microbial rhodopsins but span all the neurotransmitters that govern our conscious lives, from the amine dopamine to polypeptide endorphins, and in addition to visual opsins, include both all the olfactory receptors that have to be able to bind to virtually any type of molecule, as well as adhesion receptors that regulate cell to cell contact, and the "frizzled" receptors crucial to embryonic development. GPCRs likewise play key roles in inter-cellular communication in single celled eucaryotes to maintain the survival of the collective organism.

Fig 8: Evolutionary tree of the human G-protein linked receptors (Stevens et al. 2013), with examples highlighted in colour. On the α branch are amine receptors - serotonin 2A HTR2A, dopamine D1, adrenergic (ADRB1), muscarinic acetylcholine (CHRM2), as well as rhodopsin (RHO) on the opsin branch and the melatonin receptor MTNR1A with their ancestral gene duplication *. On the glutamate branch top are metabotropic glutamate mGluR2 GRM2 and GABA GABBR1. On the β branch is adenosine ADORA2A surrounded by vasopressin receptors and Ghrelin. On the γ branch are opioid κ and μ (OPRK1, OPRM1). G-proteins also occur in Asgard archaea, where the Arf family are involved in processes akin to eukaryotic organelles (Zhu et al. 2025).

Notably animal rhodopsins are surrounded by an evolutionary tree of diverse neurotransmitter, adhesion and olflactory receptors with rhodopsin itself an exception, consistent with horizontal transfer of a heptahelical-protein rather than a photosensitive rhodopsin. This would also have had to happen with the founding single-celled eucaryotes as the free-living excavate Naegleria gruberi possess GPRCs and a very wide diversity of G-linked proteins (Fritz-Laylin et al. 2010, de Mendoza et al. 2014). and choanoflagellates have a diversity of GPRC families (De Las Bayonas & King 2025).

Supporting horizontal transfer, Shalaeva et al. (2015) note:

Microbial rhodopsins and G-protein coupled receptors (GPCRs, which include animal rhodopsins) are two distinct (super) families of heptahelical (7TM) membrane proteins that share obvious structural similarities but no significant sequence similarity. Comparison of the recently solved high-resolution structures of the sodium-translocating bacterial rhodopsin [Kovalev K et al. 2019] and various Na⁺-binding GPCRs revealed striking similarity of their sodium-binding sites. This similarity allowed us to construct a structure-guided sequence alignment for the two (super)families, which highlighted their evolutionary relatedness. Our analysis supports a common underlying molecular mechanism for both families that involves a highly conserved aromatic residue playing a pivotal role in rotation of the 6th transmembrane helix.

Fig 9: Above: Hypothetical alignment between the Krokinobacter eikastus, sodium pump and the human delta-opioid receptor. Below: Simplified model of the horizontal transfer (Shalaeva et al.)

However comparison with human GPCRs is not the best transition point. Fig 10 shows that although we have well established GPRCs in choanoflagellates, here are only microbial rhodopsins present. At the next evolutionary step with sponges, we have diverse GPCRs but only cryptochrome light detection and it is only with cnidarian jelly fish that we actually find animal rhodopsin, now clearly in evolutionary relationship with our own form.  The fact that rhodopsin is at the tip of a branch on the human GPCR tree fig  8 and even the gene duplication fig 11 goes back to the first multicellular animal ancestor and the incidence of GPSRs in founding excavates like Naegleria implies a horizontal transfer would have to be into the founding eucaryote LECA.

Fig 10: Left: aGPCR subfamilies differ in their phylogenetic distribution across the choanoflagellate diversity. Subfamilies H, M, K, I, Q, G, and J are detected in diverse craspedid and acanthoecid choanoflagellates – therefore they were probably present in stem choanoflagellates. Notably, the subfamily H is the most widespread of all choanoflagellate aGPCR subfamilies and is conserved in filastereans and metazoans (De Las Bayonas & King 2025). However the only rhodopsins so far discovered in choanoflagellates are microbial versions (Yoshida et al. 2017). Centre: Sponges have a diverse set of GPCRs spanning the adhesion, glutamate and rhodopsin branches as shown in red titles (Krishnan et al. 2014), but the only light sensing protein they have is cryptochrome (Rivera et al. 2012). Right: Jellyfish represent the first evolutionary incidence of GPCR animal rhodopsins (Koyanagi et al. 2008, Musio et al. 2001).

Fig 11: Left: Placozoa place the origin of opsins from the duplication of the common ancestor of the opsin and melatonin gene, which we know is coupled to circadian light rhythms and indeed, humans also have a melanopsin receptor in our retina’s to detect circadian light levels, probably in the common ancestor of the placozoa and ctenophores.. The melatonin receptor genes MTRN are close to the base of the opsin branch in fig 8 and the duplication can also be seen at *, showing the human evolutionary GPRC tree indeed reflects evolutionary history.. These then by successive gene duplications give rise to three key opsin types: ciliary (C–), rhabdomeric (R–), and etinal Go protein-coupled receptor (Go/RGR) opsins. Rhabdomeric opsins (r-opsins) are light-sensitive proteins in invertebrate photoreceptors, located in microvilli (rhabdomeres) of the eye, distinct from vertebrate ciliary c-opsin photoreceptors (Feuda et al. 2012). Right: The evolutionary tree including ctenophores.

We can now provide even more focus on the emergence of animal opsins. Opsins have also been found in ctenophora, or comb jellies, and in placozoans, leading to a gene duplication of the melanin gene in placozoa and then subsequent duplications to three key types of opsin. Placozoans (e.g., Trichoplax adhaerens) lack specialized, dedicated photoreceptor organs or eyes, but they are able to sense light and exhibit light-avoidance (negative phototaxis) behaviour. This function is thought to be mediated by the presence of opsin-like proteins called placopsins, which are evolutionary related to visual opsins but typically lack the retinal-binding site.

Fig 12: Mnemiopsin is a light-emitting photoprotein unrelated to GPCR opsins

but it shares their origin with the first multicellular animals (Aghamaali et al. 2011).

.

In ctenophores, luminescence photoprotein transcripts are co-expressed with two of the putative opsins in developing photocytes – specialized light-producing cells in comb jellies, containing photoproteins like mnemiopsin, unrelated to GPCR opsins, that generate bioluminescence when activated, by calcium influx, and are located under the comb rows, playing a dual role in both light production and potentially light reception (phototransduction) as part of the organism's sensory system both dating back to the first multicellular animals (Schnitzler et al. 2012).

The sheer variety of the evolved GPCRs’ capacity to selectively bind to virtually any type, size or complexity of molecule, leading to both G-protein neurotransmitter types, from simple amines like serotonin to polypeptide endorphins, adhesion and diverse olfactory receptors and visual rhodopsins, means that it is only a matter of time and need  for these receptors to find their way back to the retinal that gave them their original key to life’s H⁺ pumping ATP energetics.

This overall analysis proposes horizontal transfer of prokaryote heptahelical rhodopsins at the eucaryote root with Naegleria which has both GPCRs and diverse G-proteins and the later evolution of the animal opsins in a gene duplication in the ancestral melatonin receptor in the Ediacarian in the placozoan/ctenophore ancestor.

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