Final answer:
Earth's atmosphere became oxygen-rich around 350 million years ago due to the accumulation of oxygen by early organisms.
Explanation:
Earth's atmosphere reached its current oxygen-rich state around 350 million years ago. Initially, oxygen began accumulating in the atmosphere about 2.4 billion years ago due to the photosynthetic activities of early organisms like cyanobacteria, eventually leading to the formation of ozone that protected the Earth's surface.
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