Roughly 3 billion years ago, a single-celled photosynthetic bacterium began burping a new chemical that was poisonous to nearly every species on Earth. Over the following hundreds of millions of years, more microorganisms began producing this toxic gas, first saturating Earth's oceans and eventually its atmosphere. Up here, this chemical changed the composition of pre-existing gases so drastically that it caused a global ice age. And the name of this powerful, poisonous, world-changing gas? Oxyg