THE OXYGEN DILEMA

Oxygen or O2 for short is a molecule that today makes 21% of our atmosphere an is normaly called "the breath of life".
But O2 has her downsides: is very reactive an corrosive. The first microbes on earth didn´t use oxygen and when the iron rich seas of the time were bombarded with oxigen molecules the mayority of the microbes died by quemical reactions in wath´s called the great oxigenation event.
Luckily  some of them sneaked through and became animals and plants.
they used oxigen to create organic molecules more complex and make them faster to the point of becoming vital.