>>1036366
Oxygen was rapidly absorbed by the oceans that there highly Ferric at the time. Became rust and sunk to the bottom.
It took about 800 million years before the oceans were 'cleared' and O2 could seriously build up in the atmosphere.

>danger to life itself.
Which consisted of cynobacteria, and.. just them.
Still, Oxygen allowed proper Organic molecular chemistry to happen, being the middle-man for carbon and phosphorous/sodium/potassium/sulfur/etc.