The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between 2500 to 1600 million years ago. This is when the continents first stabilized. This is also when the atmospheric oxygen increased. Cyanobacteria were responsible for the increase.

The significant increase in atmospheric oxygen was poisonous to almost all life that existed at this time; before this time all was ananerobic, that is, the metabolism of life depended on a form of cellular respiration that did not require oxygen. The presence of large amounts of free oxygen is poisonous to most anaerobic bacteria, and at this time most life on Earth died out. The only life that survived was either life that was resistant to the oxidizing and poisonous effects of oxygen, or life that spent most or all of its life-cycle in an oxygen-free environment.