Final answer:
Oxygen in Earth's atmosphere primarily came from the activity of photosynthetic organisms like cyanobacteria, which began producing oxygen as a by-product around 2.4 billion years ago. This process played a significant role in the formation of the ozone layer and enabled life to extend beyond the oceans to land. Volcanic activity contributed to the early atmosphere, but life was the driving factor in oxygenating it.
Step-by-step explanation:
Our atmosphere and oceans were initially devoid of oxygen. The early atmosphere contained ammonia, methane, water vapor, and carbon dioxide, but only trace amounts of oxygen. Over time, as the planet cooled, volcanic activity released gases that contributed to the formation of the atmosphere, and later, rainwater and impacts from comets and asteroids helped form the oceans. However, the significant accumulation of oxygen in the atmosphere originated from the activity of cyanobacteria and other photosynthetic life forms.
Approximately 2.4 billion years ago, photosynthesis by cyanobacteria started to release oxygen as a by-product, leading to an increased concentration of atmospheric oxygen. This process, known as the Great Oxygenation Event, drastically changed Earth's environment, leading to the formation of the ozone layer which protected life from harmful UV radiation, and enabled life to thrive on land as well as in the oceans. As photosynthetic organisms proliferated, they played a key role in further oxygenating the atmosphere.
Through the eras, the interaction between biological activity, the planet's geology, and external factors like comet and asteroid impacts, shaped the oxygen-rich environment we know today. The accumulation of free oxygen in the atmosphere was not a sudden event but a gradual process influenced primarily by the emergence and evolution of life on Earth, particularly photosynthetic organisms like cyanobacteria.