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This question is related to the following question on whether complex life is possible without oxygen but is not a duplicate. I was doing some searching and found this and this link, the first on whether complex life requires oxygen and the second on the first multicellular creature discovered that doesn't require oxygen. This led me to wonder whether oxygen causes intelligent life (humanoids) or if intelligent life depends on oxygen or if it is just as likely to use another gas?

Specifically, can intelligent life function without oxygen?

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Jax
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  • From the little I know about chemistry, oxygen usually helps finish off organic molecules. Just like with using silicon instead of carbon, you could use whatever gas is below oxygen on the periodic table for similar results. That said, oxygen will be more naturally abundant. – DaaaahWhoosh Feb 10 '15 at 16:17
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    Also, I think computers can function without oxygen. We'll be making them intelligent pretty soon. – DaaaahWhoosh Feb 10 '15 at 16:25
  • So an intelligent race that can breathe CO2 is out of the equation? – Jax Feb 10 '15 at 16:37
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    I recall that plants 'breathe' CO2. You realize there's oxygen in CO2, though, right? – DaaaahWhoosh Feb 10 '15 at 18:04
  • Yes but the gas itself is not oxygen. Even a third grader knows the 'O' in CO2 is Oxygen. – Jax Feb 10 '15 at 18:07
  • So, by 'oxygen', you mean oxygen gas, specifically O2? I apologize, I guess I misunderstood. – DaaaahWhoosh Feb 10 '15 at 18:08
  • @DaaaahWhoosh plants do not 'breathe' CO2, they use it for photosynthesis (which is a completely different process). Plants breathe O2 and expel CO2 just like animals do; it is just that at day the phosyntesis process more than compensates for the O2 used for breathing. – SJuan76 Feb 10 '15 at 18:48
  • Plant use of CO2 for photosynthesis is also biochemically really different from the use of oxygen in respiration. You could have an intelligent race that needed high CO2 for some other metabolic function and used not-oxygen (sulfur, nitrates) as their terminal electron acceptor for respiration. – plagueheart Feb 10 '15 at 23:53
  • @DaaaahWhoosh Computers would be an interesting case, though, because they'd be intelligent life that could function without oxygen, but required an oxygen-breathing precursor to come into being. – plagueheart Feb 10 '15 at 23:56

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No, oxygen is not required.

Assuming complex (multi-cellular) life can form then intelligence can evolve in those life forms.

We don't know yet what the constraints are on developing true intelligence and self-awareness as opposed to the far simpler systems used by more life forms. It maybe that true intelligence is rarely going to involve, but there is no reason to think that oxygen respiration would be requirement, so long as sufficient energy is available.

The development of a civilization without access to oxygen would be very interesting though, as fire was a very important part of human development.

Tim B
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  • +1 I like the part about fire. I had assumed that the biology just couldn't use oxygen, but you took it away from them completely. – Samuel Feb 10 '15 at 18:23
  • @Samuel Oxygen respiration is very effective for getting energy. If it is available it is unlikely anything else would be used (although I'm sure you could contrive a situation where it would be available but not used if you tried). – Tim B Feb 10 '15 at 21:10
  • @TimB Now I'm imagining radiation-hardened sulfur-breathers that harnessed the power of raw radionuclides to power their civilization, since fires don't happen in their nitrogen/sulfur atmosphere. (Or maybe they're aquatic, so no fire there either.) – plagueheart Feb 11 '15 at 01:01
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Not all intelligences would need to be biological, either. The point is raised above that computers may well become intelligent (depending on what your feelings on hard AI are, you either do or do not believe this) sometime soon, so that would be an example of non-biological intelligence. But that's a little bit of a tricky case, as I noted, because intelligent computers on Earth would require oxygen-breathing designers, unless there is an eensy-weensy sulfur-reducing bacteria civilization that we're overlooking that's created true AI.

However, this doesn't rule out (depending on the hardness of your sci-fi setting) the existence of organized forms of matter that are not organic (by our standards) becoming intelligent. A good example of this are the Qax from Stephen Baxter's Xeelee cycle--they're definitely intelligent, but because they're essentially a kind of standing waveform in chaotic fluid systems, they're sure not biological. And they do not need oxygen to be intelligent, just a system where their thoughts can be encoded. (There's also the photino birds and a whole bunch of other non-biological "organisms" in the Xeelee cycle, so I'd recommend that as reading for people interested in deep-space "life".)

So while oxygen might be necessary for complex, intelligent biological life--and we don't know this, having not checked for eensy-weensy civilizations involving sulfur-reducing bacteria, or theoretical interstellar civilizations involving sulfur-reducing bug-eyed monsters--it's not necessary for intelligence per se.

plagueheart
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  • You can argue that intelligent computers are still not intelligent life, however. – ArtOfCode Feb 11 '15 at 09:33
  • @ArtOfCode That's why I was careful to refer to them as "intelligences," rather than intelligent life. :) Although I imagine we might see the biological definition of life expanded (it already sort of has been in the field of "artificial life," where we talk about program "organisms" that behave and evolve like living animals) if we start discovering non-biological constructs that still fit some or all of the criteria of biological life. – plagueheart Feb 11 '15 at 19:37
  • @ArtOfCode Though I realize I wasn't so careful in my comment above on the main question, whoops! But my intent was to point out the separability of "intelligent" from "life", at least in a science fiction setting. – plagueheart Feb 11 '15 at 19:39
  • Indeed. The comment was a note to other readers, not a criticism :) – ArtOfCode Feb 11 '15 at 19:40
  • @ArtOfCode Ah, all right! Thank you. Didn't mean to come off as defensive or anything, it's an excellent point. – plagueheart Feb 12 '15 at 00:39