It's no surprise that a lack of oxygen can finish us off, but as hard as it is to believe, this precious gas is linked to one of the most massive extinctions Earth has ever seen. Part of that conflict is written in our own atmosphere. Simply put, oxygen came later, and everything that didn't adapt to its presence was doomed to vanish.
Our imaginary time machine takes us to the end of the Archean Eon, about 2.5 billion years ago. Planet Earth wasn't very hospitable (by human standards, of course), with a minimal amount of surface land, high iron concentrations in the seas, and an atmosphere almost free of oxygen.
It's speculated that the water had a green tint back then, due to the reaction of iron with sulfur and chlorine, while the atmosphere was dominated by nitrogen, methane, carbon dioxide, and water vapor. The first life forms on Earth were anaerobic, so things worked relatively well... until cyanobacteria appeared.
(Editor's note: Subtitles in Spanish)
Oxygen, the planetary killer
With them came photosynthesis, and the mechanism we already know: a bit of sunlight, carbon dioxide, and water, releasing oxygen as a result. The first drastic change appeared in the ocean, turning from the supposed green to a deep red, suggested by the banded iron formations. This oxygenation of the seas began to annihilate anaerobic life forms, poisoned by the new element. Then came the buildup of oxygen in the atmosphere, the reduction of carbon dioxide (used by cyanobacteria), and the reaction between oxygen and methane. The temperature drop was considerable, and what didn't die at the hands of oxygen fell prey to the Huronian Glaciation, the longest and oldest ice age documented so far.
The extinction process also included the cyanobacteria. Let's not forget they depend on CO2, and after using up the vast majority, their fate was none other than drowning in their own waste: oxygen. Still, no mass extinction is perfect, and the life forms that survived inherited a recalibrated planet, with a robust ozone layer and much more tolerable temperatures.
