We’re not the first organisms to alter the atmosphere and cause a mass extinction. The first ones were cyanobacteria and the gas they released was oxygen. They caused
a decrease in the size of the biosphere of >80%
We’re descended from the survivors that adapted to a high-oxygen environment but to this day there are many microorganisms that die if exposed to even small amounts of oxygen.
I saw a guy once with no gap between his beard and his chest hair.