Comment: Why it happened? The answer is ... (Score 1) 109
... evolution. That wasn't all that hard, was it?
Small errors leads to metabolisms that weren't just more resistant to oxygen (remember that it's a nasty poison to anything that's not used to it), but that could acutally use it to generate energy (in fact, more efficiently than by anaerobic metabolism). That opened up whole new habitats. Exponential growth ensues.