The Great Oxygenation Event [0]. Arguably the planet's first mass extinction.
To say that nothing we have done to the planet is as bad for the environment as the Great Oxygenation Event would be to pat ourselves on the back for not literally poisoning the atmosphere.
Obligate anaerobic photoautotrophs: they photosynthesize, and need light to survive, but don't produce (and are killed by) O2. Unfortunately for them, post-GOE, there aren't many habitats that are anoxic but still have much light!
PBS Eons is a great youtube channel on Earth's biological history. It's very well done stuff. Their 'Purple Earth' video is a good layman's intro to some of the per-oxygenation earth: https://www.youtube.com/watch?v=IIA-k_bBcL0
The early biological history of Earth has a few 'Colored Earth' phases: Red-Purple-Green-White-Blue(hypothetically).
To say that nothing we have done to the planet is as bad for the environment as the Great Oxygenation Event would be to pat ourselves on the back for not literally poisoning the atmosphere.
[0] https://en.wikipedia.org/wiki/Great_Oxygenation_Event