Obviously, no one can know what life on another planet would look like until we discover such life. However, scientists have speculated that (if I'm not mistaken) all living things must be carbon-based. I think it's also assumed that all life requires water.

Are you aware of a published list of theoretical "universal life guidelines"? For example, I would speculate that locomotory organs (e.g. legs) tend to evolve in pairs (one-legged animals unlikely), and that microscopic creatures have no legs at all (replaced by cilia, tentacles, etc.).

Again, this isn't hard science; I'm just interested in knowing about the speculations of various scientists.

Let me add a few details to make my question more clear. On Earth, most life can be divided into a handful of kingdoms (e.g. plants and animals), and animals are commonly divided between invertebrates and vertebrates. Vertebrates are generally bigger and are bilaterally symmetrical (as are some invertebrates).

I would speculate that the laws of physics dictate that really tiny organisms have locomotory organs like cilia, and some may have just one cilia.

However, larger, more complex animals possess bilateral symmetry. This includes not just vertebrates but arthropods and even some mollusks (e.g cephalopods). There are some obvious advantages to bilateral symmetry among larger life forms.

So, if we discovered a planet with a similar size to Earth, we probably would not expect to find fifty-feet amoebas crawling on land. We might speculate that most large terrestrial creatures would have paired leg-like organs - and probably one or two pairs, not 100. Similarly, paired eyes help with navigation and depth perception.

But those are just very general personal guesses. Lots of alien life forms have been created for science fiction movies, but I wondered if the scientific community has created some more rigorous theories about what alien life forms would really be like.

  • $\begingroup$ On what basis do you distinguish legs from other locomotory organs (e.g. cilia, tentacles, etc)? $\endgroup$ – benjimin Jan 26 '19 at 22:46
  • $\begingroup$ Size. Larger organisms with inner skeletons have limbs. Smaller organisms (e.g. insects) have jointed legs. The tiniest organisms (especially aquatic) have cilia, etc. Tentacles aren't generally used in locomotion, as far as I know. $\endgroup$ – David Blomstrom Jan 27 '19 at 2:14

On Earth, many animals only have one locomotory organ (such as the tail of a fish or a legless lizard). Some have five limbs (starfish). It just happens that the presently dominant group of large animals has bilateral symmetry (a distinct head and tail, underside and back, and medial and lateral, but usually no left and right distinction, except in internal organs), and hence often has paired anatomical features.

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  • $\begingroup$ Good comment, but you're focusing on exceptions more than the rule. I'll modify my question to elaborate. $\endgroup$ – David Blomstrom Jan 27 '19 at 2:03

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