Hey, environmental destruction isn't something to take lightly. Spewing huge amounts of corrosive chemical pollution into the air as a byproduct of energy generation, irrevocably altering the environment, causing mass extinctions world-wide, all because of--eh? What? Humans? Oh, I thought we were talking about cyanobacteria.
(For details, see: http://en.wikipedia.org/wiki/Oxygen_catastrophe )