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There must be a rise in oxygen, but too much O2 might poison humans so I thought the increase was a gradual change and both invertebrates and humans evolve the necessary respiratory systems and other adaptations, and I would like to know the max amount of O2 that can naturally occur in an alien atmosphere

I need to know if giant ants are possible as they're the star of the story, a medium-sized ant would not be bigger than a normal-sized dog, and the other creatures are scaled up accordingly to that proportion. enter image description here

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    $\begingroup$ Welcome to worldbuilding. Though I appreciate having an accepted answer, we usually advice to wait at least 24 hours before choosing an answer, as this increases the chances of getting more and diverse options. Please give a read at our help center to find more about us. $\endgroup$
    – L.Dutch ♦
    Commented Jun 25, 2022 at 5:16
  • $\begingroup$ You might want to look into space flight. They use almost pure O2 for humans to breathe. I'm unaware of this poisoning humans. The only thing that we even notice is a build up of CO2, that I'm aware of at least. I could be wrong though. $\endgroup$
    – Alexandra Williams
    Commented Jun 25, 2022 at 11:39
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    $\begingroup$ @Mr.D My answer to your other question goes into detail abut the possibility of giant ants and humans interacting in the same environment. worldbuilding.stackexchange.com/questions/231891/… $\endgroup$
    – M. A. Golding
    Commented Jun 26, 2022 at 2:22
  • $\begingroup$ Depends on how giant you mean. Humans and quite a few species of Bugs Of Unusual Size are coexisting in Florida right now. :-) Car sized or bigger might take a bit more effort or a large dose of handwavium. $\endgroup$
    – Escaped Lunatic
    Commented Jun 26, 2022 at 5:08
  • $\begingroup$ @AlexandraWilliams Oxygen toxicity is very much a thing. Dependent on partial pressure. However It will almost never be reached in a typical atmosphere due to oxygen's reactivity. Which is where the toxicity comes from, oxygen's reactivity. $\endgroup$
    – Gault Drakkor
    Commented Jun 27, 2022 at 23:08

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For a species to evolve there also need to be an ecological niche available to be occupied.

In a world where there are humanoids there will be other mammals, small and large. If there is competition between the small mammals and the large bugs, the possibility of large bugs evolving depends on their capability of coming ahead in the competition, which again depends on the environment and the features of each species.

Possible, not granted.

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