c. 2.4 billion years ago Β· Planet-wide β evidence preserved at sites such as the Hamersley Range, Australia
The Great Oxidation Event
Oxygen released by photosynthetic cyanobacteria began to accumulate in the atmosphere about 2.4 billion years ago, permanently transforming the planet's chemistry.
For Earth's first two billion years, free oxygen was almost absent from the atmosphere and oceans. Cyanobacteria, which had appeared by about 3 billion years ago, were performing oxygenic photosynthesis β splitting water and releasing oxygen β but the oxygen they produced was, for a long time, rapidly consumed as it reacted with iron dissolved in the seas.
Around 2.4 billion years ago this balance collapsed. As dissolved iron began to run out, oxygen rose from the oceans into the atmosphere. The shift in surface chemistry was vast: banded iron formations laid down all over the world today preserve a direct geological signature of this transition.
Oxygen was poisonous to most existing life. Many anaerobic organisms went extinct or retreated to deep, oxygen-poor refuges β the first major mass extinction known on Earth. But a new door also opened: oxygen-based respiration releases far more energy than anaerobic metabolism. Complex cells, multicellular life and ultimately animals could not have emerged without this energetic windfall.
Location
Planet-wide β evidence preserved at sites such as the Hamersley Range, Australia Β· OpenStreetMap β
Sources
- The Great Oxidation Event β Nature Education
- Banded iron formation β Encyclopaedia Britannica
- Geological evidence of the Great Oxidation Event β United States Geological Survey