Final answer:
The Earth's atmosphere became oxygen-rich through photosynthesis by plants and the evolution of oxygen-producing organisms. Studies show that the atmosphere lacked oxygen until about 2 billion years ago. The increase in oxygen led to the formation of Earth's ozone layer.
Step-by-step explanation:
The Earth's atmosphere became oxygen-rich through the process of photosynthesis by plants and the evolution of photosynthetic and oxygen-producing organisms, such as cyanobacteria. Studies of ancient rocks show that the atmosphere was anoxic, meaning there was no oxygen, until about 2 billion years ago. As the plant population grew, more oxygen was produced and accumulated in the atmosphere, eventually leading to the formation of Earth's ozone layer which provided protection from harmful solar ultraviolet light.