Whats the great oxygenation event??
Oh boy! So, this all takes place around 2.3 billion years ago. Up until then, life had been pretty much chilling for a billion or so years. Pretty much everything was a single-celled organism back then. Also, the planet was a whole lot different. There was no ozone layer, so UV radiation was constantly reaching the surface. That kills stuff, by the way, in high concentrations. Methane gas was also being spewed into the air by various volcanic eruptions. So, not a very nice place. Most importantly, oxygen wasn’t much of a thing. The little there was existed in the ocean and bonded with the iron molecules that were floating around to make rust.
The organisms that were alive got along just fine without oxygen, for the time being. These are called anaerobic organisms, and some of them still exist today. A lot are in your stomach right now. Anyways, along comes this little thing called a cyanobacteria. It’s unclear how they evolved, but the point is, they can do this neat trick we like to call photosynthesis. As you probably know, a byproduct of photosynthesis is oxygen.
Cyanobacteria were incredibly successful - they could make up to 16 times as much energy as anything else. So, they started multiplying. Pretty soon, there was a bunch of extra oxygen floating around, and not enough iron to bond with it. Then things started dying. See, to a lot of anaerobic organisms, oxygen is incredibly deadly. And now there were billions of cyanobacteria constantly spewing it out.
Long story short, almost everything besides cyanobacteria died. The survivors either adapted to be able to live with oxygen, or went and lived in places without it, like underground or in sulfur vents (those guys are still around today!). The cyanobacteria were literally so successful that they changed the chemical makeup of the atmosphere. This also led to a decrease in greenhouse gases and started the longest ice age the world has ever seen. Go cyanobacteria!