IIRC our atmosphere contains 78% nitrogen, 21% oxygen, 1% percent everything else (figures approx). How would life be different if those figures were altered? More nitrogen? More oxygen? More “other stuff”? I seem to recall reading that humans could not survive if the oxygen level was above 30%.
I dont know how helpful this is, but NASA somewhat dealt with this when determining what gas to use for astronauts in space. Originally they used 100% oxygen, but it was too flammable and was very hazardous. So they then switched to a near earth ration of 80% nitrogen, and 20% oxygen which was much safer and was still practical for space missions. So, if the oxygen level was higher, the Earth would be a little more “burnt” and fire would be much more dangerous and volatile
It’s not the percentage that’s important, it’s the partial pressure of each gas. If you have a pressure of 1 atmosphere with 20% oxygen, it means you have 0.2 atmospheres worth of oxygen. If you keep that constant and remove all other gases, you end up with a 100% oxygen atmosphere at a pressure of 0.2 atmospheres. This is what they did on some spacecraft - it’s still the same amount of oxygen so you have no problem breathing, but the spacecraft structure only needs to withstand 0.2 atmospheres. I believe the Russians still do this, as do the American space suits used for spacewalks.
I don’t know if the reverse is true for scuba diving - do they use lower concentrations of oxygen for deep dives?
In scuba it’s the nitrogen that’s a danger. For shallow dives they simply compress air into a tank, but decompression is required past a certain point to keep the nitrogen from doing Bad Things when you come back up.
For really deep dives, they replace the nitrogen with helium, resulting in a weird speaking voice for the divers, but preventing nitrogen narcosis, which is a form of delirium caused by too much nitrogen affecting the central nervous system.
Different combinations of gasses would have resulted in different forms of life and, most likely, in no life at all.
Too much oxygen is a problem – it promotes burning readily and anything on the surface that was inflammable would go up in smoke. Some have hypothesized that there is an equilibrium at the current 20% – if it goes higher, fires turn enough of it into carbon dioxide to reduce the percentage. There’s also the Gaia hypothesis that the Earth is somehow monitoring the oxygen levels.
Without oxygen, things get dicey. While anerobic life does exist, it’s hard to know if it could evolve into anything more than simple forms (the earliest life on Earth was anerobic, but was killed off as the oxygen level rose). There are some exotic combinations of atmospheres that are theoretically possible to support life, but it’s anybody’s guess if that theory could be put into practice.
If oxygen is somewhere in between, it’s possible for some form of life, but it would have to have greater lung capacity to use the oxygen available.