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The Wiki article Life on Mars states the following:

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.

Is it generally believed that multicellular life could exist within the confines of the Solar System (outside of Earth, of course)? Or is there consensus that any life that could possibly exist would be unicellular, similar to extremophiles on our own planet?

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    $\begingroup$ "If life exists in our Solar system, is it believed to be unicellular or multicellular?" As a multicellular organism... $\endgroup$
    – Anton Hengst
    Jul 29, 2021 at 16:35
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    $\begingroup$ Who says life elsewhere will have any cell structure at all? Just because that's how it works here doesn't mean it has to be that way elsewhere, you have to consider what other forms it may take. $\endgroup$
    – GdD
    Jul 30, 2021 at 7:37
  • $\begingroup$ @GdD fair point. Care to add an answer? $\endgroup$
    – JonathanReez
    Jul 30, 2021 at 14:52
  • $\begingroup$ I have no data to give an empirical answer @JonathanReez. I'm not trying to be Spock here, but logic suggests that unicellular life is far more likely than multicellular life as it's easier for it to evolve. However there's people on this site with much better information on this subject than me. $\endgroup$
    – GdD
    Jul 30, 2021 at 15:21

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As we have yet to find life beyond Earth, one can only speculate what life would be like elsewhere. However, there are good arguments that unicellular life would be more common than multicellular life.

  • Single-celled life is simpler than multicellular life. The latter requires not only all of the properties of the former, but also the ability of the cells to stay together and cooperate.

  • Simple systems are more likely to randomly originate than complex systems. Because of the preceding point, life is more likely to originate in single-celled form than multicellular.

    This is supported by the history of life on Earth. Single-celled life originated about 3.7 billion years ago. Multicellular life appeared about 1.7 billion years ago. Thus, single-celled life has been around more than twice as long as multicellular.

  • Being simpler and generally smaller in size, single-celled life requires less materials and energy than multicellular life.

  • Reproduction of single-celled organisms is simpler (fewer parts to replicate, fewer reproductive mechanisms needed). Single-celled organisms should require less materials, energy, and time to reproduce than multicellular (this is certainly true of Earth organisms).

  • On Earth, single-celled organisms vastly outnumber the multicellular ones. Given the arguments above, it's likely that this would happen to other forms of life, too.

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