Oxygen.
https://biology.stackexchange.com/questions/54762/did-cyanobacteria-commit-evolutionary-suicide-by-producing-oxygen
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Fun fact: humans aren't responsible for the first mass extinction caused by organisms polluting the atmosphere with poisonous gas. Blue-green algae did it first, with "a decrease in the size of the biosphere of >80%".
Nobody complains about trees blooming, it’s all unfair for algae :(
People with hayfever probably do
We're going for the speed-run not the first completion 👍
The 100% run.