Hey @suesa, did you know that O2 concentration could have been as high as 34% during some time periods of pre-historic Earth?
This of course affected the respiratory scale limit of insects and other species without lungs, among other things leading to spiders of more than 1m size.
Ewww...
I am really glad to have 21% O2 in the atmosphere nowadays 😅
Ha! Next time someone tells me I'm a bad person for smoking, I'm going to say "I'm doing it to keep the spiders small!"