The correct option is Adding Oxygen.
Explanation
The evolution of Earth's atmosphere occurred in three major stages: the loss of the primordial atmosphere, the release of gases from the Earth's interior (degassing), and the modification of atmospheric composition by the biosphere through photosynthesis.
Detailed Analysis:
- The Process of Oxygenation: The early atmosphere produced by volcanic degassing was rich in carbon dioxide, water vapour, nitrogen, and methane, but it contained negligible free oxygen.
- Impact of Photosynthesis: Approximately 2.5 to 3 billion years ago, early photosynthetic organisms, specifically cyanobacteria (blue-green algae), began converting carbon dioxide and water into glucose and oxygen using sunlight.
- Atmospheric Transformation: Initially, the released oxygen reacted with iron in the oceans. Once the oceans were saturated, oxygen began to accumulate in the atmosphere. This biological process is solely responsible for the high concentration of oxygen found in the modern atmosphere.
- Incorrect Options:
- Nitrogen: Nitrogen accumulated primarily through volcanic degassing and its relative chemical inertness; it is not a byproduct of photosynthesis.
- Helium: Helium was part of the primordial atmosphere and was largely stripped away by solar winds before the atmosphere stabilized.
- Water Vapour: Water vapour was added mainly through degassing; photosynthesis consumes water rather than releasing it as a primary atmospheric component.
Key Takeaway: The onset of photosynthesis by early life forms was the critical factor that transformed Earth's atmosphere from a reducing environment to an oxidizing one by adding free oxygen.