\section{The Great Oxidation Extinction}

\lettrine[lraise=0.1]{A}{bout} 2.4 billion years ago, bacteria developed photosynthesis --- the ability to split water using sunlight and release oxygen as a byproduct.

At the time, oxygen was poison to virtually all life. 
Photosynthesis was so successful that it brought about a mass-extinction of life --- caused by life itself. The atmosphere was permanently transformed.

And yet: that catastrophe was the precondition for an oxygen-based metabolism far more efficient than anything that came before. Without it, complex animal life is thermodynamically impossible.

Eventually an equilibrium was reached between carbon-consuming plants and oxygen-breathing animals. Today, about 21\% of Earth's atmosphere is oxygen.

Every breath you take is downstream of that crisis.