Final answer:
The breakdown of water molecules into oxygen and hydrogen by ultraviolet light in the early atmosphere is called photodissociation. This process, alongside oxygenic photosynthesis by cyanobacteria, contributed to the rise of atmospheric oxygen and the concept of the ozone layer.
Step-by-step explanation:
Process Responsible for Early Atmospheric Oxygen
The breakdown of water molecules into oxygen and hydrogen by ultraviolet light in Earth's early atmosphere is known as photodissociation. This process, along with the significantly later development of biological oxygenic photosynthesis by ancestral cyanobacteria, contributed to the gradual increase of free oxygen in the atmosphere - a pivotal change that led to aerobic respiration and the diversification of life on Earth.
Before the oxygenation of the atmosphere, the environment was anoxic, lacking molecular oxygen (O₂), thus only anaerobic organisms could survive. The accumulation of atmospheric oxygen eventually led to the formation of the ozone layer, which protects life from harmful solar ultraviolet radiation, allowing organisms to thrive in previously uninhabitable environments such as landmasses.