Oxygen is toxic stuff until you evolve to use it. Just look at what it does to iron
Although the event is inferred to have constituted a mass extinction,[7] due in part to the great difficulty in surveying microscopic organisms' abundances, and in part to the extreme age of fossil remains from that time, the Great Oxidation Event is typically not counted among conventional lists of "great extinctions", which are implicitly limited to the Phanerozoic eon. In any case, isotope geochemistry data from sulfate minerals have been interpreted to indicate a decrease in the size of the biosphere of >80% associated with changes in nutrient supplies at the end of the GOE.
[0] https://en.wikipedia.org/wiki/Great_Oxidation_EventMakes you think of the sulphur dioxide in Venus' atmosphere, and whether life could still evolve in those conditions.