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Let’s say you have a planet called... Galileo. Galileo has a large level of biodiversity, with bacteria, plants, fungi, and animals. The animals are what I’d like to focus on today. Unlike Earth with its birds and insects, I intend on Galileo having no animals capable of flight. My question is, what could make the evolution of flight-capable animals never happen?

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  • $\begingroup$ To clarify: do you want it to be physically impossible for them to fly, or just not advantageous enough for it to become part of the evolutionary process? $\endgroup$ – Mr_Bober Mar 3 at 18:17
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    $\begingroup$ @Mr_Bober: The second thing $\endgroup$ – DT Cooper Mar 3 at 18:52
  • $\begingroup$ Then I stand by my asnwer below. $\endgroup$ – Mr_Bober Mar 3 at 19:20

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The first question you should ask yourself is: did it NEVER EVER happen, or maybe did it just not stick around (AKA the few who evolved this way went extinct)?

If you want it to never have happened, your best chance is to go with one of the answers above me, which make flight impossible. But if you want to consider why it wouldn't be a sound evolutionary choice, then bear with me.

Flight on Earth gives you a few advatanges, first and foremost mobility. This advantage relies on a few things:

  1. Suitable Weather: Flying on Earth is "easy". Most of the time we have clear days, maybe a light raint, and mild winds. But what if our concept of "extreme weather events" was the norm for Galileo? Constant rain and high winds would already make flight way less appealing, but we can go one step further: frequent hail, duststorms / sandstorms, even tornadoes and hurricanes would make flight very impratical. I don't know what your planet looks like, but you can have fun with this too. Maybe it rains acid instead of water. Electrical storms, or fire storms, could make flying extremely deadly. Maybe some rocky residue falls from the sky every so often, making it a sort of stone hail. And so on.
  2. Ladnscape and Resources: Another advantage of flying is getting to hard-to-reach places, especially for resources. Tall trees and mountains, but also canyons and islands are all safer spots to rest because it's harder for land predators to reach them. But what if the world was a flat plain, where food and water are so plentyful you don't need to travel far for it? What if you had no need to migrate, or if it was so easy to do it on land that it wasn't worth the effort to fly? Or you could go the other way: instead of making life on land easier, you can make life in the air MUCH harder. Maybe plants evolved to eat animals, making their branches a place of certain death instead of a safe haven. Maybe the gravity fluctuates so much that flying would be incredibly hard and dangerous. Or maybe flying is just useless: a world completely covered in water (or any other liquid) would make flight kinda pointless; any animal would have evolved to be better suited in the water, not in the air (remember, seabirds still need land, couldn't survive long without it). Or if your life forms only evolved underground, with no access to the surface world (for whatever reason), chances are they all evolved to climb incredibly well, maybe burrow, not fly.
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  • $\begingroup$ What about flying fish? We have those on Earth. Even if the world was entirely covered with water -- which from recent astronomical discoveries may be more the norm than desert planets -- conceivably flying fish could evolve. $\endgroup$ – llywrch Mar 4 at 16:06
  • $\begingroup$ Flying fish don't actually FLY. Their wing-like fins allow them to GLIDE once they jump out of the water, allowing them essentially to jump for longer distances in order to avoid predators. They are NOT able to fly in any given direction, as birds do. $\endgroup$ – Mr_Bober Mar 5 at 12:40
  • $\begingroup$ You're spitting hairs. The fact that a fish can leave the water to jump or glide means it is possible they cd evolve to fly. Evolution had taken many an odd turn to get to even stranger destinations. $\endgroup$ – llywrch Mar 6 at 18:03
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The absence of an atmosphere.

The development of higher life as we know it in a vacuum is kinda problematical. But life could exist in the sub-surface ocean of Europa.

These beings wouldn't ever develop flight, since they are swimming. OTOH this could be considered a form of under-water flight, i suppose.

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  • $\begingroup$ Penguins arguably fly underwater. $\endgroup$ – jamesqf Mar 4 at 3:44
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There are a bunch of factors that support muscle-powered flight, including atmospheric density, oxygen partial pressure and the strength of the local gravity well.

Tweak them to suit. Increased gravity seems like the easiest, making flight much harder and using lightweight hollow bones much more risky. Trees will shorter and squatter, and cliff formation will be less favourable so there will be slightly less incentive to fly, too.

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    $\begingroup$ Insects and some arachnids are rather small, and the way they fly is different from how larger animals fly. Especially really small insects or arachnids can (and actually do) fly using quite strange strategies, for example small spiders do it by hanging from long silk threads floating in the air. $\endgroup$ – AlexP Feb 26 at 14:05
  • $\begingroup$ @AlexP ballooning isn't quite in the same category as winged or gliding flight, so I'm not going to worry too much about that (because you can't stop stuff blowing away unless you get rid of most of your air pressure, after all). Limiting the effectiveness of low-reynolds-number flight techniques will be harder... I'm not sure how much they'd suffer from increased gravity, but it'd certainly make it less energetically advantageous. $\endgroup$ – Starfish Prime Feb 26 at 14:37
  • $\begingroup$ I've actually heard decreased gravity is better to prevent the evolution of flight. Flapping flight is dependent on how hard your wings apply downward force to keep you aloft, and that's a lot harder in a low-gravity planet with less dense atmosphere than a high-gravity one with more dense atmosphere. On a low gravity planet flapping is less efficient and there is less pressure to develop true flight instead of just gliding. $\endgroup$ – user2352714 Mar 1 at 5:48
  • $\begingroup$ @user2352714 decreasing gravity by itself is insufficient, as Titan will show you. Low density regardless of gravity will indeed make flight more difficult, though, yes. $\endgroup$ – Starfish Prime Mar 1 at 9:28
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Aerodynamic analysis

There's four main components that determine flight capabilities:

  1. Lift: the upwards force of the wings
  2. Weight: the downward force of the body
  3. Power: the force propelling the body forward
  4. Drag: the force stopping the body from going forward

Forces on airplane

If these forces are too large/small flight becomes impossible. Examples for how this could become the case:

  1. Low air density, resulting in low lift
  2. Large gravitational pull, resulting in large weight
  3. Few resources available so not enough energy is available
  4. Large friction requires more power making flight too expensive

I think options 1 and 2 speak for themselves and have been covered in other answers. I'll explain 3 and 4 in more depth.

If there are only few resources available, flight may be too expensive to be used. Resources could mean edible food, but it could also be oxygen. If there's little to no oxygen available, it could cause all animals to be constantly gasping for air. Although this would leave room for critters jumping out of trees and gliding.

An increased amount of drag could also cause animals to never fly. Drag is made up of two main components: induced drag and parasitic drag. Induced drag is necessary and is strongly correlated to lift. However parasitic drag consists of the air "slushing" against the animal. This would however require some very strange gasses.

More interesting options

There are other options still. For instance, flight could be very dangerous. Imagine the planet being covered in constant dust storms or incessent lightning. It could also be that plant have covered the air with spores which stick to anything and everything causing wings to get stuck resulting in any animal trying to fly to get stuck in a big ball of fuss and crashing to their deaths.

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  • $\begingroup$ There's no trust as such for gliding (that's still flying, right?) Gravity does that one too. $\endgroup$ – Adrian Colomitchi Mar 1 at 7:35
  • $\begingroup$ @AdrianColomitchi indeed, gliding does not require thrust $\endgroup$ – Nathan Mar 1 at 12:09
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Nip it in the bud

Evolutionarily, flight evolved from humble beginnings. Species developed membranes under its arms that, with a single flap, let it abruptly alter its path and evade predators’ attacks.

If your planet’s early predators were more successful than earth’s, then these early wings would never confer a evolutionary benefit and the proto-flyers would go extinct, and birds would never evolve.

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  • $\begingroup$ Bats, bugs, and birds all evolved flight independently, so you'd have to quash several evolutionary paths to not have flight. You've pointed out limitations on flight for prey creatures, but there are plenty of flying predators who might still benefit from wings. $\endgroup$ – Nuclear Hoagie Mar 9 at 15:36
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Flight:

  • aerodynamic (winged) vs aerostatic (buoyancy)
  • propulsion yes/no (e.g wing flapping/gliding, zeppelin/balloon)

Putting the things this way, one discovers that conditions believed impossible or hard for flight may actually be easy (depending on other factors)

  1. no air or too low pressure will surely make any kind of flight impossible or too taxing. note: even at 0.1% of Earth's atmospheric pressure, flight is still possible - except large wingspan and light body may prove too high an evolutionary cost for a biologic organism. Too costly for buoyancy too, it will require very large volumes.

  2. violent atmosphere dynamics (read "storms") may be a serious impediment for flight evolution. This is where a higher atmospheric pressure may contribute - higher gas densities will trap more energy - just look at the Jupiter's storms. Otherwise, high gravity/high atmospheric pressure by themselves...

  3. ... are not an impediment for flight. If life already evolved at high gravity and if there is a dense enough atmosphere (with high gravity, that's to be expected), it's likely flight will evolve in one form or another. Example - even if only jumping insects using wings to prolong their jump (like grasshoppers) will have an evolutionary advantage. One weird way that I can imagine for flight in dense atmosphere/high gravity would exploit the behavior of some gasses at the critical point. E.g. imagine a balloon-squid drinking liquid CO2 from a pool, heating it only 2-3K and soaring upwards by the CO2 passing into super-critical fluid phase - any excess is ejected, squid-rocket style, for propulsion purposes (CO2 critical point is at 72.8 atm and 304.13K = 31.0°C = 87.8°F). So no, high gravity by itself won't necessarily stop flight.

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Animals which fly evolved not only in terms of wings, but many other adaptations that make it possible to fly... in case these adaptions are rendered useless due to environment it would be difficult to fly and a combination of these factors would make it impossible ...

Few of the fact and how they can be affected are listed below:

Feathers - provide insulation, waterproofing, and a lightweight means to become airborne. So, If weather conditions happen which make fight difficult despite ability to adapt for fight like excessive moisture (making waterproofing impossible) or some drastic changes of atmospheric conditions (making it impossible to fly ... adaptations happen over time...with relatively constant conditions .. where adaptation happens and then its possible to see if they are useful for natural selection or not ) .

Bone- honeycombed or hollow bones,beak made of keratin instead of bone reducing body weight ... Suppose the climate is so harsh that you need a hard exoskeleton to survive ... this exoskeleton would make it really difficult to fly as it increases body weight.

Food-Most birds consume energy-packed foods rich in calories – like seeds, fruits, and meat, which add as little as possible to a bird’s payload. And what they eat is processed rapidly, so they aren’t weighed down by waste. ... Suppose the food that they eat needs lots of time to be processed so it persists in the body for a long time increasing the body weight.

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Because Galileo is so far away from its star, virtually no light reaches it. The animals and plants get energy from heat vents similar to volcanoes. Because there is no light, all Galilean animals are blind. They "see" by heat sensing organs. Because of their lack of vision, flight would be pointless.

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  • $\begingroup$ Most kinds of bats are virtually blind, but are very adept flyers. Vision is not necessary to make flight worthwhile. $\endgroup$ – Nuclear Hoagie Mar 9 at 15:38
  • $\begingroup$ Maybe they are uncapable of using echolocation: perhaps the atmosphere is too thin for sound to travel efficiently. $\endgroup$ – CheetSheatOverlode Mar 9 at 21:13
  • $\begingroup$ If the atmosphere is that thin, flight is likely impossible in the first place. According to physics.stackexchange.com/questions/192996/…, there's still enough atmospheric pressure at 100km altitude for sound to propagate over a bat's echolocation distance, even though that pressure is only 1 one-millionth of an atmosphere, and that altitude far exceeds the max altitude achieved during flight. If your atmosphere can't carry a sound, it definitely can't carry an animal. $\endgroup$ – Nuclear Hoagie Mar 11 at 13:24
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Just make a planet with no dryland but fully covered in waters.

On Earth any flying animal is descended from a land dwelling organism, that took advantage of flying versus crawling on the solid surface.

Without a portion of dryland, there would be no incentive, not food nor shelter, for an organism to permanently venture outside water and consequently also to develop flight.

You might still have species which use short jumps and glide out of water, though.

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The question is missing some clarity, as is shown from the diversity of answers.

The first issue is that "flight" is not defined. Does that mean any locomotion through the atmosphere? Is jumping prohibited? Is jumping high and then using control surfaces to control the landing point prohibited? Is jumping off a high point with controlled landing prohibited? I could draw many other nuanced variations on flying, and it is not clear what is not permitted.

Because there are so many modalities of flight, it seems unlikely that anything would prohibit it.

Suppose we increased the G-force. Surely then it would take more work to stay aloft. I suspect that the updrafts would also be more intense, giving one form of flight the advantage.

Suppose we lower the G-force. Some answers have said that wing flapping is less effective is there is lower G-force. In the limit, as G approaches zero, flying becomes more like swimming through air. Flight isn't impossible -- it just changes.

I suggest that similar changes in density, atmospheric depth, weather, oxygen content, temperature, and other factors would similarly not prohibit flight, but would constrain or shift the envelope of what modalities work effectively.

If there is a fluid, life will find a way to use it.

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it's not that different from planets that we would never be able to leave with current rocket technologies.

After a planet gets large enough, the amount of fuel it would take to leave the gravitational field ramps in a way that even if we found unlimited "rocket-fuel" we simply can't escape. Black Holes obviously being the extreme of Infinite energy at the speed of light STILL won't make it out.

SO, The idea of too much gravity for any species short of a nuclear-fueled carbon-fiber-boned bird to get into the air wouldn't even need that much other than typical gravity to fight against...

That Said, Evolution just Not yet figuring out flight on the planet (or how to use oxygen on land instead of aquatic species) wouldn't be too much of a reach. Flight is hard, and you have to hollow out your bones and develop all sorts of aerodynamic mods to your skin to get into the air, first flight being a sad crash and insects or others giving up wouldn't be unlikely.... it took a LOT for humans (not built for it) to get off the ground!

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