Final answer:
The Earth's atmosphere has seen fluctuating levels of oxygen, which started accumulating around 2.4 billion years ago due to photosynthesis, leading to the creation of the ozone layer and promoting the emergence of life on land. Geological and biological processes contributed to this sustained increase in atmospheric oxygen.
Step-by-step explanation:
The amount of oxygen in the Earth's atmosphere has changed significantly over time. About 2.4 billion years ago, free oxygen began to accumulate in the atmosphere, thanks to the process of photosynthesis by ancient plants and cyanobacteria. The rise in atmospheric oxygen eventually led to the formation of Earth's protective ozone layer, which shields the surface from harmful ultraviolet light and allowed life to emerge onto land.
However, the presence of free oxygen was not steady, as it was initially removed as quickly as it was produced through reactions with Earth's crust. Over time, the increase in plant life and geological changes that led to the burial of organic carbon allowed for a more sustained increase in atmospheric oxygen, which in turn influenced the development of life on Earth.