Early microorganisms, through the process of photosynthesis, produced Earth's abundant oxygen.
The Role of Photosynthesis
Photosynthesis, a process where organisms use sunlight, water, and carbon dioxide to create energy, is the primary reason for Earth's oxygen-rich atmosphere. As a byproduct of this energy production, oxygen is released. This is a process you may remember from elementary school. [Source: Multiple sources including the Planetary Society and Ask An Earth and Space Scientist.]
The Great Oxidation Event
The increase in atmospheric oxygen wasn't instantaneous. The Great Oxidation Event marked a significant shift, with cyanobacteria playing a crucial role in producing and releasing vast quantities of oxygen into the atmosphere. [Source: ASM's article on the Great Oxidation Event.]
The Ocean's Contribution
It's important to note that the ocean also plays a significant part in oxygen production. Scientists estimate that roughly half of Earth's oxygen comes from oceanic plankton, through photosynthetic processes in the water. [Source: NOAA's information on ocean oxygen production.]
A Simpler Explanation than Expected
Recent simulations suggest that the rise of oxygen in Earth's atmosphere may have been surprisingly straightforward, potentially easier than previously thought. [Source: New Scientist's article on oxygen's rise.]
The Uniqueness of Earth
While oxygen-producing photosynthesis occurs on Earth, it's important to recognize that this is not a common feature across other planets in the known universe. Earth's unique combination of factors, including the presence of life and the specific geological and atmospheric conditions, allows for this extensive oxygen production. [Source: Multiple sources, including Quora and a Stack Exchange geochemistry discussion.]
Sustaining Oxygen Levels
It's not simply the production of oxygen that is important, but the balance between oxygen production (primarily through photosynthesis) and oxygen consumption (through respiration). [Source: Stack Exchange geochemistry discussion.] The continued existence of oxygen-producing organisms maintains the current high levels of oxygen in the Earth's atmosphere.
