HL only: Subtopic 6.1: Introduction to the atmosphere. The evolution of life changed Earth's atmosphere mainly through photosynthesis, which removed carbon dioxide and released oxygen. Life and the atmosphere then co-evolved: organisms altered atmospheric conditions, and those conditions influenced subsequent evolution.
The Mechanism
Earth's early atmosphere contained almost no free oxygen. It had relatively high concentrations of carbon dioxide, water vapour and gases released by volcanic activity.
The evolution of photosynthetic microorganisms, particularly cyanobacteria, introduced oxygenic photosynthesis:
At first, much of the oxygen reacted with dissolved iron and other reduced materials. Once these oxygen sinks became less effective, oxygen accumulated in the atmosphere during the Great Oxidation Event, beginning approximately 2.4 billion years ago.
| Biological development | Atmospheric effect |
|---|---|
| Oxygenic photosynthesis | Removed carbon dioxide and released oxygen |
| Burial of organic carbon | Prevented carbon from being rapidly returned to the atmosphere as carbon dioxide |
| Aerobic respiration | Consumed oxygen and released carbon dioxide |
| Methanogenic microorganisms | Produced atmospheric methane under anaerobic conditions |
| Expansion of algae and land plants | Increased photosynthesis and influenced the carbon and oxygen cycles |
Rising oxygen enabled the formation of stratospheric ozone, which absorbs harmful ultraviolet radiation. This helped organisms survive closer to the surface and eventually colonize land, creating another feedback between atmospheric change and evolution.
A common misconception is that life only increased atmospheric oxygen. In reality, biological processes both produce and consume gases: photosynthesis releases oxygen, while respiration and decomposition consume oxygen and release carbon dioxide.
Exam Technique
For an IB ESS explain question, give a linked causal sequence: photosynthetic organisms evolved, carbon dioxide was removed, oxygen accumulated, ozone formed, and new environmental conditions influenced further evolution. Include the Great Oxidation Event as a named example and recognize that geological processes also affected atmospheric composition.