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Let's set up our planet and aliens:

  • The planet is reasonably Earth-like; similar size, orbiting similar star at a similar distance, plenty of water and land.
  • Our aliens are carbon-based, oxygen breathing animals, roughly the same size as humans.
  • They are intelligent; I define intelligence by having been to space. They haven't been to the stars, but have set foot on the closest planet.

On Earth, we have two symmetries when it comes to animals; bilateral and radial, former being the most common.

My question is, given the above context, is it possible for a land-faring radially symmetrical animal to evolve? I would like the answer to focus on few things listed below, but any feature they have must be justified as a reasonably possible evolutionary adaptation.

  1. Again, to stress the point, they must be radially symmetrical.
  2. What would their 'legs' (or equivalent) be? Assuming legs are a better adaptation to land, how many legs would make sense given their symmetry, and can legs even work given that there's no primary 'direction'? I know many insects have 2+ legs, but they still are bilaterally symmetrical and there's a 'front' and a 'back'. If not, can they move like starfish do, and still be effective on land?
  3. I want them to be primarily vision-based. Can multiple eyes evolved to see every direction?
  4. Given that they've evolved to the point of space-faring, I think hand-like appendages and opposable thumbs (or an equivalent) are a reasonable assumption? If not, what other design would permit them to advance to a point where they can create technology?

And a bit of fun:

  1. I think Hollywood has given very little - if at all - thought into what alien clothing might look like. In our world, what would their clothing be like?

Given all that, can we have a radially symmetrical alien flying to other planets?

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    $\begingroup$ Are you familiar with Lovecraft? $\endgroup$
    – DKNguyen
    Sep 24, 2020 at 21:03
  • $\begingroup$ Please remember to restrain your questions to a single line of inquiry. More than that is a red-flag for needs more focus. $\endgroup$
    – Frostfyre
    Sep 25, 2020 at 12:39
  • $\begingroup$ Isn't a radially symetrical creature just a bilaterally symetrical creature with just one additional section instead of two? $\endgroup$ Sep 25, 2020 at 15:22
  • $\begingroup$ @JustinThymetheSecond, technically true. But there is no 'primary' axis when you are radially symmetrical, ergo, no one 'forward' or 'backward' direction as we do have. $\endgroup$
    – Sach
    Sep 25, 2020 at 19:16
  • $\begingroup$ So, technically, the head, eyes, ears, nose also have to be radially symetrical, other wise it is a hybrid radial-bilateral arrangement, with a defined primary axis? $\endgroup$ Sep 25, 2020 at 21:21

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The first thing you need to understand is why a creature would have radial symmetry. It must provide some form advantage to the creature in its living environment and not many disadvantage. On Earth, most if not all radial symmetric creature are ocean based. These creatures, like jellyfish mostly, don't need to move fast. The water environment also provides buoyancy which counteracts gravity making moving easier. They are also equally sensitive in all direction or more like equally less sensitive. Also not many obstacles in an ocean.

So on an alternate world the reasons for a sufficiently complex organism to have radial symmetry would be as follows:

  1. the initial environment at least does not have obstacles making navigation easier
  2. Evolved from squid/octopus like water dwelling creatures in terms of motion, so has radially symmetric arrangement of limbs
  3. Lack of predators that are faster or more mobile, otherwise they would have been wiped out long ago
  4. Sensory organs should be uniformly distributed kind
  5. The predators is probably the type that swarms and attacks from all direction, which necessitates omni-directional sensitivity. Like how Tigers like to attack from the back, so in India's Sundarban, villagers wear a mask behind the head to trick the tigers.
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I'm not going to say it's impossible, but I will point out that on Earth, most radially symmetric organisms are either sessile (non-moving), or are evolved from sessile organisms. Radial symmetry is particularly useful for organisms that do not move, as it allows them to interact equally well with the environment on all sides, even if the organism never moves. Creatures like starfish and jellyfish are not actually sessile, but they also do not have highly complex sensing organs. Complex body structures are costly to develop and maintain, so an organism will tend to have as little complexity as it can get away with. Humans, for example, do not need eyes in the back of their head, because they have the ability to simply turn their head and look. This setup is far more efficient than adding more eyes. More eyes would actually be a detriment, since they would have additional complexity and energy requirements, and achieve very little that someone who can simply turn their head cannot.

I find it unlikely that an intelligent, mobile species would develop and maintain complex, redundant body structures that repeat around the body. If the organism can simply turn around, it doesn't really have a strong need to have the same structures on all sides of the body. If they did develop, I would expect them to have evolved from radially symmetric sessile creatures and never lost their radial symmetry. But if our current evolutionary track is any guide, radial symmetry tends to be found in not-very-complex organisms that do not display high levels of intelligence. But hey, evolution has produced some weird, unlikely creatures, so nothing is impossible.

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    $\begingroup$ Starfish and jellyfish do have eyes. $\endgroup$
    – rek
    Sep 24, 2020 at 21:14
  • $\begingroup$ I agree with @rek that jallyfish etc. do have eyes, so some of your argument may not stand. But there may be a point about simply turning around is far easier than having repeated organs all over the body. Since the question is closed I may post one with more focus. $\endgroup$
    – Sach
    Sep 25, 2020 at 19:19
  • $\begingroup$ @Sach You can edit this question to be more focused and it will likely be reopened. $\endgroup$
    – rek
    Sep 25, 2020 at 21:11

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