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Imagine an Earth-like world with oceans, some complex vegetative and animal life, but with an extreme low amount of oxygen in the atmosphere, let's say one third of what we are accustomed to, like the death zone of Everest, but on the sea level.

Can such a world plausibly exist? I mean oxygen originates from organic life. There were initially some bacteria that produced oxygen, than some plants etc. So does it mean that if life has evolved to a level like we know it, it should have also produced the amount of oxygen in the atmosphere as we know it? Or could complex life do well at lesser oxygen levels?

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    $\begingroup$ Please note: one question per post. Anything else is too broad. $\endgroup$ – L.Dutch Jan 24 '20 at 10:32
  • $\begingroup$ Is this really a terraforming question, if you're asking whether a world can naturally evolve a certain way, which is the opposite of terraforming? $\endgroup$ – KeizerHarm Jan 24 '20 at 14:15
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There are organisms that respirate in low-oxygen environments, like yeast. They do this through lactic acid fermentation, and humans can actually do this as well, but it's painful and not very efficient. So, hypothetically, a more complex organism could evolve to respirate in that way, but since it is so inefficient (lactic acid fermentation produces about 2 ATP molecules for every 32 produced by cellular respiration) the organism would need to consume very high amounts of food in order to stay alive. If there was some oxygen in the environment, they would most likely use a mix of both forms of respiration, like we do but to a greater extent. So yes, it is possible, especially if the first forms of life on the planet were in a low-oxygen environment.

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  • $\begingroup$ Unfortunately this does not work for complex life forms. The anaerobic respiratory cycle creates lactic acid build up that requires additional oxygen to break down when you are done exerting yourself. While the anaerobic respiratory cycle might become a more commonly required system for complex organisms in a low oxygen environment, it does not actually reduce total oxygen needs. $\endgroup$ – Nosajimiki Jan 27 '20 at 15:04
  • $\begingroup$ @Nosajimiki-ReinstateMonica read the whole thing $\endgroup$ – Alexandra Eagle Jan 27 '20 at 21:31
  • $\begingroup$ I said "might", but the more I reconsider this in the context of oxygen debt, the more I think complex organisms in a low oxygen environment would need to rely less on the anaerobic respiratory cycle. Lactic Acid build up is toxic; so, you need to be able to quickly break it down when you are done over exerting yourself. The less oxygen you have at your disposal, the longer this takes. That would mean that anaerobic respiration becomes more harmful to rely on in low oxygen environments. Out running a predator does you no good if you just die of exhaustion anyway. $\endgroup$ – Nosajimiki Jan 28 '20 at 15:39
  • $\begingroup$ That's why I said they would probably use both ahaha $\endgroup$ – Alexandra Eagle Jan 28 '20 at 23:03
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Complex life could certainly exist as long as the world isn't also cold like the Everest death zone. Cold blooded reptiles have metabolic rates that average 7.2 times less than that of placental mammals. This means that reptiles would still have plenty of oxygen to spare in the thinner atmosphere.

Plants would also tend to grow slower if they cannot get as much CO2 but without mammals around eating obscene amounts of plant matter, this should not be a particularly limiting factor. In short, life could still be pretty complex, just... slower.

Anaerobic respiration works for simple organisms, but it can not replace aerobic respiration in complex life forms because it creates a toxic buildup of lactic acid over time. Simple life forms can compensate for this by pushing the acid out of themselves and using non-reactive cell membranes to keep it out, but flushing acid out of a single cell and flushing it completely outside of a complex organism are two different feats. While many complex organisms are designed to be able to do anaerobic respiration on the short term, it creates an oxygen debt. This means we need to eventually breath in more oxygen to be able to break the lactic acid down when we are done exorcising making this an unviable mechanism.

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