Around 2.4 billion years ago, Earth’s atmosphere began to change due to chemical reactions conducted by cyanobacteria in the oceans, which split water molecules and released oxygen as a byproduct. Initially, this oxygen was absorbed quickly by available sinks. When these sinks became saturated, oxygen accumulated in the seas and the atmosphere, becoming toxic to many existing life forms.
This period is known as the Great Oxidation Event, sometimes referred to as the oxygen catastrophe. It is considered one of the earliest mass extinctions, caused not by external factors like impacts or eruptions, but by the actions of life itself modifying Earth’s environment.
How we know the air changed
Evidence for the timing of this atmospheric transition primarily comes from the study of sulfur isotopes. In rocks older than 2.4 billion years, these isotopes exhibit a pattern that forms only when ultraviolet light interacts with sulfur dioxide in an oxygen-free atmosphere. This signature was first documented by James Farquhar and others in a 2000 paper in *Science*. After approximately 2.4 billion years ago, this signature disappears, marking the arrival of free oxygen.
A second line of evidence comes from the presence of iron. The oceans initially contained high levels of dissolved iron, which reacted and precipitated as oxygen levels rose, contributing to the formation of banded iron formations that are still mined today.
Why oxygen was a poison
Oxygen is a reactive element that can create reactive oxygen species, which damage cellular structures in organisms that evolved in its absence. This buildup of oxygen caused a significant decline in anaerobic life forms that lacked the necessary defenses against it.
Although some species retreated to environments where oxygen was less prevalent, many were unable to survive the increasing oxygen levels. The cyanobacteria responsible for producing the oxygen continued to thrive, despite the adverse effects on neighboring organisms.
The cold may have done as much as the oxygen
In addition to the toxicity of oxygen, another significant factor was the decline of methane, a potent greenhouse gas that kept the Earth warm when the Sun was less bright. The rise of oxygen led to the destruction of methane, resulting in the Huronian glaciation, a series of ice ages occurring roughly between 2.4 and 2.1 billion years ago. Research published in *PNAS* by Robert Kopp, Joseph Kirschvink, and others suggests that the emergence of oxygenic photosynthesis may have triggered this global cooling.
What the record can and cannot tell us
The fossil record from this time is sparse, making it difficult to determine the specific lineages that were lost. The American Society for Microbiology notes that the microbial life of this period did not produce abundant fossils for analysis. While oxygen is confirmed to be poisonous to many early forms of life, classifying this as “the first mass extinction” is based on limited evidence.
The increase in atmospheric oxygen was not a straightforward process. Early oxygen levels were much lower than today and fluctuated over about 200 million years before becoming stable. Studies in 2021 and 2017 highlight this gradual, uneven transition rather than a single event.
Despite these complexities, the fundamental change in the planet’s chemistry is clear. Life itself triggered this shift, and many forms of life were unable to adapt. The oxygen that proved deadly for early organisms became essential for the development of complex life. Ongoing research seeks to better understand when these transitions occurred and what was lost during this period.
Source: spacedaily.com via Google News.
