There wasn’t always a breath of fresh air on Earth. Methane and nitrogen choked any potential life in Earth’s early atmosphere. But this doesn’t mean that life didn’t exist.
Long ago, tiny microscopic cyanobacteria flourished in ocean environments. They evolved to carry out photosynthesis anaerobically.
The key to their existence was that they didn’t need oxygen to survive. As a byproduct, these anaerobic organisms released oxygen which was toxic for them.
Eventually, oxygen filled the oceans. Then, it made it all the way to the atmosphere. This event was the great oxygenation event. This paved the way for aerobic organisms to carry the torch.
An oxygen crisis that almost ended cyanobacteria
For millions of years, cyanobacteria used heat from volcanoes or mid-ocean ridges, then released oxygen into the oceans and air. Eventually, they filled the oceans with oxygen.
The irony of cyanobacteria is that the oxygen they released was toxic to them. As a whole, the number of anaerobic organisms dropped in the Proterozoic Eon.
The reason why this event is called an “oxygen crisis” is because they threatened their own existence through their own waste oxygen.
Cyanobacteria didn’t completely vanish in the Proterozoic Eon. By hiding in low oxygen environments, cyanobacteria avoided their own mass extinction.
A struggle for constant oxygen
Banded iron formations (BIF) consist of alternating layers of red (oxidized) and (unoxidized) black iron. Because iron rusts when it reacts with oxygen, these layers were red.
So the red (oxidized) layers mark a time when there was oxygen. The alternating layers give a sneak peek of how oxygenation was just starting to get a foothold.