I’ve been giving some thought, once again, to Thalassa, my oceanic planet with a thick oxygen atmosphere orbiting Proxima Centauri, which I’ve featured in some stories…but it’s some stories I might want to revise, because the more I brainstorm the biosphere, the more alien and wondrous it gets.
Ironically, this development comes courtesy of deleting a category of life we’re familiar with on Earth. Foundationally, Thalassa has no land masses, just a deep water ocean below the surface and a high-pressure oxygen atmosphere above the surface. Therefore, rather than life emerging out of the water from crawling up onto land, it would emerge via lineages like flying fish, i.e. taking to the air directly.
I wanted something different from your average Earth tetrapod, and since there is no land for animal life to attach itself to anyway — most animals spend their entire lives on the wing — there’s no particular reason for it to even have legs. So the basic body plan could easily have two limbs, which both form wings…and nothing else. Instead of a tetrapod call it a “dipod” (the Greek word meaning “two-footed”, as contrasted with “tetrapod”, which means “four-footed”). Since Earth tetrapods evolved from lobe-finned fish which had four fins, we can only assume that Thalassan dipods evolved from fish that had two appendages. So instead of a lobe-finned fish, the first species to take to the air on this planet would have been something more like a manta ray. Ooh…I’m liking it already.
Like Earth terapods, this ancestral species would have radiated into a mind-boggling variety of ecological niches, acquiring a great deal of diversity, but also like Earth tetrapods the basic body plan would be genetically “locked in”. On Thalassa, due to adaptations to high radiation (which stimulates the development of DNA that’s more robust against mutations, damage, etc) and the vagaries of evolution, the basic developmental architecture is strikingly conserved to a degree not seen in Earth vertebrates.
All the dipods might have certain common traits, with all of them being biologically immortal (i.e. they can be killed but they don’t age), having one offspring at a time widely spaced in age which require parental care and learning, an XY sex-determination system, lifelong monogamy unless they reproduce parthenogenetically, feathers, a pair of camera eyes, a beak (without dentition, i.e. they have no true teeth), et cetera. So the net result is that compared to Earth veterbrates there’s more visible similarity in the basic bodily architecture and life cycle. Already slightly alien.
One striking element I’ve brainstormed recently is that if these creatures are on the wing, then live birth becomes advantageous, which would lead a worldbuilder to think of the mammalian placenta…but some species of shark, notably, simply retain the egg inside the body of the mother, yolk and all, until it hatches, and then the young emerges, no placenta required. Arguably more elegant and certainly more alien. If this trait is conserved and is common to all the dipods of Thalassa, it would be quite striking. Some dipods may well retain the young until hatching, but others, more heavily modified, might lay the egg before hatching (for wind dispersal…more on that later).
The oxygen atmosphere of Thalassa is 50 times Earth’s total pressure, and over two hundred times Earth’s partial pressure of oxygen. Far more energy is available for air-breathing organisms to utilize, and exchanging gas through the skin would presumably work far better. This would be extremely helpful to insectoid life, which presumably could attain hundreds of times the mass it does on Earth (which would lead to bugs in lower altitudes approaching or exceeding the mass of a human….yikes). But ironically, I’ve wondered lately, what if Thalassa had no arthropods?
It’s not as weird as it sounds; Earth animals consist of basically three great lineages: you have the vertebrates, like us and the dinosaurs; you have the molluscs, like octopuses and snails; and you have the arthropods, like the bugs and the crabs. There’s no particular reason arthropods would have to evolve. Indeed, if vertebrate analogues evolved first, they could well occupy the sort of ecological niches the ur-arthropod took in our world, thus choking out insectoid life from being a significant player. Notably, the arthropod body plan is far friendlier to minaturization than the vertebrate body plan is.
Vertebrates miniaturize more than one might think; some frogs can fit on a human fingernail, yet they have a complete skeleton and all the requisite equipment, all in miniature. However, whether it’s a frog or a fish the record-holders of smallness among the vertebrates seems to cluster around 7 millimeters or so in body length. Possibly implying some sort of structural limit. Birds might be more instructive as analogues here, and they bottom out in size with the bee hummingbird, at around 7 centimeters (an order of magnitude longer than the smallest frogs and fishes). Interestingly — and this is where I first got the idea — the body sizes involved here overlap with the larger and even some of the mid-sized arthropods of our world. A small hummingbird is, by descent, a theropod dinosaur, and yet it occupies the same niche as a large honeybee! Could an alien planet take the same principle much further? Possibly.
What would be the consequences of a world without arthropods? Especially considering that the all-dominant lineage is very strongly “K”-selected: they live a long time, they reproduce only very slowly, and each individual is extremely valuable for natural selection to preserve.
The biggest consequence is that there are simply fewer trophic levels: without tiny and even microscopic animals consuming everything in sight and reproducing in huge numbers all the time, the ecosystem is just going to be far more quiet and sedate from an Earth point of view…but by the same token, plants are going to grow without nearly as much obstruction, and microbial life-forms like fungi would have far more freedom. Instead of tiny insects munching on microbes, you would see tiny vertebrates feasting on fungal mats and the like. Above the microbial level, you might see plants and then small herbivores, and then mid-sized predators, and then large predators…and that would be about it. Given the K-strategist nature of the dominant lineage as well as the far greater energy levels available, you could see fewer levels of the food chain and far fewer individual animals in the total biosphere, but each animal would live far longer and would likely attain far greater size and mass. Very fun if big game is what you’re after.
Also fun is the consequences of each individual being valuable; for some species sheer size or superior cognition would be protective against predation, but for many others chemical defenses would become widespread, much like we see in some poison frogs today. Toxins the likes of which Earth’s biosphere has scarcely even dreamed of would be extremely widespread, and this perhaps would lead to a biosphere where advertising “don’t eat me or my body will kill you” becomes the norm. “Aposematism” is what it’s called: warning colors. Camouflage might pale into insignificance; animal life where it exists might be extremely obvious to the unaided eye, brightly and vividly colored, perhaps even flashing with bioluminescence and pulsing with intimidating displays. Compared to Earth, where the vast majority of species try to hide, this would be quite alien.
We can readily imagine flying forms similar to birds and pterosaurs in an environment like this, and even, given the high-pressure atmosphere, buoyant photosynthesizing forms that float in the air and possibly attain gigantic sizes (what I affectionately call “gasbags”, since most of their volume would be lifting gas generated biochemically). But an interesting question is what happens when sessile gasbags aggregate for mutual advantage, forming a colony floating in the atmosphere, which then could support secondary symbiotic plant life…and then animal life. In the same fashion as a coral reef in the ocean. Hardly an original idea in science fiction, but in my opinion, a lush thick atmosphere would likely support such a biological structure.
Once you have a surface like this, flying forms would no doubt visit it and establish niches as users of these airborne reefs…perhaps even, as seen on birds who take to the land on Earth, leading to the evolution of flightless forms, as lifestyles become more and more terrestrial. One would ordinarily think that the wings of the dipods in question would then transform into legs over geologic time, but as birds show the genetic equipment for such a development has been largely lost. More likely for evolution would be deleting the limbs entirely, by gradually reducing the wings, leading to a limbless creature which presumably would locomote like an Earth snake. Only with feathers and a beak. These creatures might be very small — remember, Earth frogs demonstrate a vertebrate body plan can miniaturize to as little as a centimeter or less in body length — and these reefs could easily extend miles, so for them their world might be pretty big. Fungal analogues and various forms of detritus might provide quite a feast for a slithering dipod descendant.
One wonders how, without flight, these dipods would reproduce and spread to ensure their survival beyond what one reef offers, but these dipods might be the ones to evolve an adaptation of laying the egg before the egg hatches, perhaps forming a symbiosis with an appropriately sized gasbag to take it away, in the manner of a hot air balloon, and let it disperse on the wind, landing on a reef some distance away, ensuring the survival of the lineage.
The fundamental issue with this concept is how the young would be without its mother when it hatches, which according to the dipod life plan is going to be a serious problem…but I’ve had a brainstorm about that too. Suppose that dipods retain the ability to reproduce asexually, by birthing a clone, i.e. parthenogenesis. Given a whole lineage of such creatures, we could suppose that colonies of the species on nearby reefs might be made up of literal clones of the original mother; so from the perspective of the young, of evolution, and of the dipod “life plan”, it’s all good. We could even suppose that sexual reproduction, as seen in some of the smaller simpler creatures on Earth, might be infrequent or episodic, with clonal reproduction being favored the vast majority of the time. Thus the entire colony is made up of clones and very close relatives (and would be overwhelmingly females, with males being much less common). Thus there is a fitness advantage to raising young who drift on the wind from outside and hatch, since genetically they are young of their own kind. No doubt there would be sophisticated mechanisms to instinctively recognize a compatible juvenile and vice versa.
This is reminiscent of the strategies employed by Earth’s eusocial insects, but unlike the r-strategy favored by eusocial insects, these creatures are K-strategists, with a life cycle of slow births, slow growth, long lifespan, and intensive care, much more reminiscent of humans. Also unlike the eusocial insects, there is no sterile versus reproductive caste distinction; the creatures are more or less equals. Fascinating, eh?
Also fascinating is that if these creatures are slithering under the surface, their sense of vision might become reduced or even eliminated in some lineages, as we see in some Earth cave creatures, but the genetic machinery would still be there, so later derived forms might effectively re-evolve a camera eye. Interestingly, physics has a say here: camera eyes need to pick up a certain number of photons to work effectively at high resolution, and at smaller and smaller sizes this task becomes progressively more difficult, since less photons can be collected with a smaller eye. Interestingly, shorter wavelengths are more forgiving in this respect than longer wavelengths.
As in Earth birds, I suppose that the ancestral dipod might have possessed a sense of vision that extended into the near-ultraviolet, perhaps even, like Earth birds, being tetrachromats: red, green, blue, and a UVA color channel. Proxima Centauri emits most of its light in the infrared, but an oxygen atmosphere has a “window” of transparency similar to Earth’s atmosphere, and Proxima Centauri emits significant UV (especially during solar flares), so UV vision would still be as useful here as it is on Earth. Perhaps near-infrared is used as a fifth color channel, since there’s so much of it in this environment, or the “red” channel is shifted to these longer wavelengths. It might be delightfully alien anyway to suppose five color channels in the ancestral dipod.
In any event, given a smaller eye and reduced vision, we could see a reduction to only UVA vision, in monochrome fashion, in the smaller dipods, particularly in flightless versions who live on reefs. It’s even possible that multi-channel vision might re-evolve in some of these lineages that re-emerge into the sunlight. Imagine two or even three color channels, but all in the UV range, owing to their small size making it a higher-resolution way to gather information.
You could also see delightful adaptations like beaks extending outward toward the front, serving as a “feeler” for the sense of touch, as well as eyes being more spherical and grading “outward” for better vision, up to and including mounting them on stalks.
Another intriguing aspect suggested by the physics is that infrared vision might become more prominent. Near-infrared is already a significantly longer wavelength than red light, and a creature the size of a human would get relatively poor resolution from it. But an animal whose eyes are just much larger by default (because it has a hundred-foot wingspan anyway, e.g.) might be able to see deep into the infrared range at high resolution. The retention of an additional near-IR channel is thus highly plausible, or even two or more “color” channels shifted into the infrared. Most exotically, the very largest creatures might be able to see effectively in the “thermal window”, much deeper into the infrared and at much longer wavelengths. Snakes on Earth (of all creatures) can often already do this to a limited degree, but it’s large physical size of the aperture that really would open up thermal-infrared vision. Proxima Centauri’s planet is tidally locked to its sun and has an entire side that’s in permanent night (bar aurora outbreaks), so the floating life (including yummy “gasbags”, potentially) might be quite thermally bright against the cold background. A perfect environment for infrared vision…
Another alien aspect of a biosphere without tiny animals everywhere and with microbial life running relatively wild and red-hot is that parasites would have fewer microscopic predators, and there simply are a lot fewer tiny arthropods cleaning layers of dust and detritus away. And especially in a world full of aeroplankton and where creatures are flying through huge volumes of air, that means any animal is going to tend to get very dirty…
But one animal’s dirt is another animal’s meal. In the oceans we see the famous “cleaning symbiosis”, where “cleaner fish” sometimes congregate in specific “stations” where “clients” drift in and relax and let someone else clean the gunk off. For the big creature that’s been roaming around, it’s like checking into a spa, and for the small creatures doing the cleaning, that “gunk” is digestible as a yummy meal. On Thalassa, we could expect marine-style cleaning symbiosis to become extremely widespread, intense, and perhaps sophisticated if arthropods are absent…only with bird analogues rather than fish.
Tying in to how smaller dipods might form colonies and reproduce clonally, we could even see the evolution of true eusociality among the cleaner birds, so to speak. Imagine a species that reproduces clonally, but some clones could be reproductive, others sterile, with the sterile clones going out harvesting gunk and gorging themselves, and then returning to a fortress-like colony floating in the air to feed to the young as well as to the reproductive members of the colony, which would be better protected from predators than if they were roaming around outside. This is one key way eusociality could evolve in a K-strategist species, where individuals are not expendable but rather where the survival of every reproductive individual matters enormously for the survival of the group. Since dipods are locked in to the strategy where one offspring at a time hatches only infrequently, it seems extremely unlikely that an insectoid-style queen that has hundreds to thousands of young at once is ever going to be a “thing”, but an entire class of (to use the social-insect term) “princesses” doing the reproducing is certainly a viable evolutionary development.
The gunk that’s stored as fat or some such — and in this biosphere storage of energy as fat would, due to successful predation being extremely infrequent but potentially extremely high-reward per kill, be extremely prominent and important — would no doubt accumulate in the interiors of these fortresses, forming a substance akin to honey. No doubt coveted by various other specialized flying creatures, which in turn might prompt these colonies to form mutualistic relationships with their own cleaner-bird-type friends…and develop elaborate chemical defenses against their enemies (perhaps even electrical…after all, various biochemical liquids can be highly conductive…).
And speaking of electrical senses, we can imagine electrical sensing and even attack being much more widespread as a weapon in the arsenal, though even thick air is a very poor conductor relative to water. No, the dipods that stay — or, rather, return — to the ocean would be the primary users of that strategy.
And here might be the most exotic dipods of all. As air-breathing creatures, their energy input would be fantastical compared to their water-breathing counterparts; they’d enjoy hundreds of times the oxygen supply of an analogous Earth creature. You’d see aquatic vertebrate dipods expand to far larger than the largest blue whale that ever swam in Earth’s oceans, with the largest creatures probably being rather whale-like in terms of lifestyle: filter-feeding in the deep waters while occasionally surfacing for air.
The adaptations would be striking, since Thalassa’s oceans extend about 30 miles deep under the surface, multiples deeper than Earth’s Mariana Trench extends, and we can reasonably suppose an air-breathing creature might migrate from the surface to take a breath and then all the way back down, over an astonishing range of pressures. How big a breath might this be?
At some point you run into structural or ecological limits, but there may well be enough food for a scaled-up analogue of a whale to eat in this ocean. Per the square-cube law (2x the length equals 4x the surface area and 8x the mass), a whale that’s ten times longer would have a thousand times as much volume, and thus mass. That’s a creature that’s 1000 feet long and masses 100,000 tonnes. Which sounds enormous, but keep in mind water effectively cushions the weight, so as long as neutral buoyancy is maintained (low density…) and the gas pockets don’t get structurally crushed or torn apart by turning etc. the mechanics would actually work out quite nicely. Obviously there would be adaptations in the body plan, as opposed to simply a whale or a penguin scaled up, but it illustrates the range at which we’d be working, when worldbuilding such a creature.
Frankly given a more favorable environment for gigantic filter feeders as well as this thick oxygen atmosphere, I imagine size may well be an advantage, so you could even see another order of magnitude in bodily dimensions. An animal a hundred times longer than a blue whale would be two miles long, and would mass 100 million tonnes. Yes, really. I suspect some sort of structural limit given vertebrate-style biomaterials would be reached before this size was reached, but I also suspect that the basic dipod body plan might actually be capable of sustaining a creature in this mile-long range. It’s an air-breathing animal, so the volume of air it would take in is proportionally scaled up from a whale. How much?
A blue whale inhales 5000 liters or so of air in a deep breath, and can stay under the water for about half an hour. Scaled up, our leviathan of Proxima Centauri would be gulping, in the 10x’ed version, 5 million liters of air. In the 100x’ed version, it’d have to gulp a whopping 5 billion liters of air. Keep in mind that likely the mile-long version would be lower in body mass as well as metabolism compared to a mammal, but also keep in mind that diving up to 30 miles underwater suggests it’d going to have to take on far more than a 30-minute air supply. Billions of liters of air might sound like a lot, and it is by animal standards, but in atmospheric terms it’s not really an absurd amount. It’s within the range of what your average small tornado occupies on Earth.
So again, mechanically, it’s extreme, but it does work. So the net result is you have this creature that basically never turns and has all the agility of a lumbering brick, but filter-feeds just fine, perhaps — owing to its enormous scale and thus the potential for charge differences — from electricity generated by an electrical organ which manipulates organic particles around it in the water as opposed to using muscle in any meaningful way beyond supporting its own body, but every few months comes up for air and takes such a huge gulp as to cause a mesocyclone disturbance in the atmosphere. Think a large thunderstorm, of roughly the same scale as a pyrocumulonimbus caused by a wildfire on Earth, only caused by the breath taken by Proxima Centauri’s greatest leviathan.
The wind forces involved could easily be comparable to a small hurricane for any life that is unfortunate enough to be chilling out at low altitude when such a leviathan emerges from the deep, which would be bad enough, but when you consider that the exhalation of any aerobic animal is going to be greatly enriched in carbon dioxide, there starts to become a real risk of asphyxiation and carbon-dioxide toxicity in the local atmosphere. The net result? Natural selection would favor a response of flying away if at all possible at the first sign of such a beast breaching the surface. Simpler life-forms would experience a mechanistic reflex, more complex ones possessed of brainpower might soar to a safe distance and take in the view, including, perhaps, human visitors, who, after a decades-long journey in the interstellar void and perhaps months to years of floating in a zeppelin as if observing from heaven, would at last feel that they had truly arrived on a strange new world…
Makes me want to write some more letters from the airy deep…