Humanity has just discovered a massive, derelict space ark containing its own ecosystem. Its artificial gravity has long since failed, and it's been drifting undisturbed for millennia.

Assume that:

  • The environment in the ark is Earthlike and mostly forested, with a temperate climate and relatively mild weather. The Pacific Northwest is a good example if you're looking for one.
  • Food, water, oxygen, and other basic resources are in abundant supply.
  • There is a small food chain with both prey and predator animals.
  • The original animals were similar to Earth mammals.
  • The ark contains many small areas as well as extremely large, open spaces.
  • The ark has been drifting long enough for the species inside to become completely optimized for the environment.

The main thing I'm wondering is how an animal like this would move around.

  • $\begingroup$ Problem: The instant the gravity was turned off, the ecosystem would start to die. Everything would eventually drown - i.e. be unable to breath because of the layer of water growing in their lungs which would prevent oxygen exchange. Not to mention free-floating waste-products, dead-bodies, bacteria growing rife. Life might need to start from the single celled - in which case all bets are off what it might look like. $\endgroup$ Jul 26, 2022 at 23:57
  • 1
    $\begingroup$ "Millenia" is definitely insufficient for evolution of long-lived species. Yes, selective breeding can make changes over a few thousand years (eg sheep, dogs, horses - every domesticated species on Earth) but unguided evolution will require millions of years, quite apart from @AngryMuppet quite valid observation. $\endgroup$ Jul 27, 2022 at 0:02
  • $\begingroup$ Mobility: Flight (and possibly swimming) will still work. Climbing will still work. Walking/running will not work, but leaping may be more effective. Many of these modes of transport will require less energy. Fighting/Feeding: Most methods of attack will still work. To pounce effectively, however, a creature must launch itself at its target from the opposite side of a sturdy surface or catch the prey midair. Some exceptions include falcons, shrikes, archer fish, and antlions, who rely on gravity to hunt. $\endgroup$
    – Jafego
    Jul 27, 2022 at 0:04
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    $\begingroup$ However, some animals have digestive systems that rely on gravity. Cats, dogs, and many other mammalian predators rely on gravity to drink, since they cannot create suction. The same is true for birds. Bottom feeders would not do well, either, since they rely on gravity to bring them food. Trees and other plants could grow just fine; they provide their own structure and, without gravity, they could grow to unlimited height. Roots and branches might break the vacuum seal of the ship. Plants will grow their tops toward light, and their bottoms away from it. $\endgroup$
    – Jafego
    Jul 27, 2022 at 0:08
  • $\begingroup$ @AngryMuppet why would everything drown? You are assuming that no water dwelling creatures exist, and that nothing would be able to rid itself of fluid buildup, which seems unlikely considering plenty of animals have survived in orbit for periods of time. $\endgroup$
    – Topcode
    Jul 27, 2022 at 0:40

4 Answers 4


Fish, Birds and Quaddies:

Your species start terrestrial, but over time they will adapt to freedom from gravity. The ability to control vector and thrust intrinsic in the design of bats and birds mean these species are already virtually adapted to operate in an atmospheric zero-G environment. But without the relative restrictions of weight, your bird-like species will be able to become bigger without much of a cost. So giant eagle-like species may predominate.

There are other species that have a similar environment, namely fish. They thrust against the media they live in and are supported by that media. They would likely need bigger fins to provide thrust in a less dense media, so they could look more like really big tropical fish with flourishing wide fins.

And science fiction has similarly tried to answer this question with Quaddies, or long-limbed lightly built humans with essentially four arms. the ability to grip and push in three dimensions would be a logical extension of a ground-evolved species that re-evolved for zero gravity. This is fairly conservative, but I'd guess there would be roles for such species.

Grip would be critical, especially in a confined environment, to hold you to food and food sources, allow sudden rapid direction changes, and the like. Bird feet are extremely good at gripping, because of the nature of the environment they live in, and zero G would likely be similar.


I just found this site, so go easy on me if this isn't the typical answer. :) I would say they would probably be a bit blobby and fatty, like the humans in Wall-E, maybe like a big old amoeba floating around. It would also be neat if they developed a type of internal propulsion system, maybe like a sea creature that uses tubes to propel themselves with water. It could also be combustible like in the third Tremors movie. They could have suction cups or slime, like an octopus or slug, that helps them grip to surfaces as well. Oooo, or they would be like free floating dung beetles and they just rolled as much matter into a giant ball as they could to live in. And all of the balls are connected by vines and things, like a floating Dip N Dots <3


This is a Frame Challenge

An Earthlike zero-G environment has a problem: nothing sticks to the ground. The water, the dirt, the leaves and needles that fall off trees, dead creatures. Everything would be floating around inside the Ark. In fact, the slightest change in trajectory of the Ark would result in whole trees and boulders lifting from the inner surface of the Ark to float around in the void. And once the grasses and trees left a bed of soil (without regular water, no less), they're dead.

Water would break apart into droplets, rarely forming more than a cup or two in volume, especially as it's broken apart by the passage of more massive objects. Like dirt. Objects would move around and collide and, in my opinion, little life beyond molds and fungi would survive.

The only way to avoid this is gravity, whether created artificially with spin or using Clarkean Magic like gravity plates.

But, let's ignore that and answer the question

Things like flight and swimming rely on gravity. It's easiest to see when considering how a bird glides. The mass of the bird is pulling it toward the Earth. The outstretched wings, like a parachute, are pressing against the atmosphere. The atmosphere compresses, providing a "surface" that the wings can glide on. This is a long way of saying that birds when not flapping their wings are really falling with style.

Fish are a bit different because they use various means to create buoyancy. If a fish had the density of a rock and, for example, no air bladder to offset that density, it would sink to the proverbial ocean floor unless it had strength to swim upward at a constant acceleration to avoid falling.

You don't have gravity.

Once again, ignoring my frame challenge (not a small thing), fish in water would begin to stop using whatever mechanisms they once depended on for buoyancy. Air bladders, for example, would no longer be needed and would begin to atrophy. Without the force of gravity, the muscles needed just to swim are not needed to the same degree (although, ignoring my frame challenge, they'd still be pushing against the medium of water... it's really hard to ignore my frame challenge). So I can believe there would be some overall muscle atrophy as well. They'd be really soft eating.

Birds, I don't believe, would change all that much. Rather than gliding against gravity, they'd learn to glide against their own forward motion. They'd take a flap or two, not down, but forward... then they'd do nothing and simply blob along like a water balloon until they needed a course correction or to stop their forward motion, then they'd bring out those wings and do what they did before, just rotated 90 degrees.

Thousands of years isn't enough time to evolve anything (that would take millions and millions of years). But it is enough time to adapt, and adapt the creatures would do. I suspect most animals would become more playful in zero-G. Could you imagine what a squirrel would do in that environment? They'd have the time of their lives launching themselves across the Ark chasing one another. The wonder is that they'd learn (by rote of death if nothing else) to judge the force of their jumps to avoid splatting against the ground on the other side.


A very broad question but a couple of thoughts rather than a complete answer.

Evolution takes a long time, for the ecosystem to adapt the change from having some gravity to zero gravity would need to occur slowly. One possibility is that orgionally the habitat was spinning to provide its gravity and the spinning slowly stopped, or perhaps there was some artificial gravity that slow decreased over many thousand years. Longer lived generations would adapt more slowly that shorter lived generations, but that could provide be a lot of stress to adapt.

Also even some microgravity is a little different than true zero gravity, it is hard to completely stop something from rotating in space, although internal energy dissipation does slow rotation.

Big issues are things like not being able to keep fires going easily, Surface tension resulting in blobs of water, and the lack of mixing due to convection and changes in density. Perhaps very big blobs become little ecosystems, but would be very dependent on how they get nutrients or oxygen.

However, perhaps there could be other parts of the design that still work that can help with such things for example blowers that mixed the atmosphere or something else.

With locomotion, if it is a big open area, becoming stuck in the middle becomes more of a problem and that might also drive evolution with longer limbs or ways to jet or throw materials to get some reaction to push them in some direction.


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