After millions of years of gloriously successful life on Earth, a dangerous new organism arose and spread rapidly across the planet. With unprecedented efficiency, this revolutionary life form pumped noxious fumes into the air, destroyed ecosystems and exterminated a substantial fraction of its fellow species. And the gasses it added to the atmosphere drastically altered global temperatures so that, between habitat destruction and climate change, the world was changed forever.
Mankind? No! These events happened 2 billion years ago. The delinquent organisms were cyanobacteria, the first photosynthetic life forms to give off pure oxygen gas.