How did oxygen (O2) get into Earth's atmosphere? How did oxygen (O2) get into Earth's atmosphere? It was captured from the solar nebula. It came from chemical reactions with surface rocks. It was outgassed from volcanoes. It was released by life through the process of photosynthesis.

Option (4)

Explanation:

The primitive atmosphere was comprised of lesser or no amount of oxygen. With the progressive time, primitive organisms appeared such as the primitive aquatic plants, algae, and cyanobacteria, that carried out the process of photosynthesis in the presence of sunlight, water, and CO₂, and in return produced the food and oxygen. This is how the rapid photosynthesis process led to the increasing amount of oxygen in the atmosphere that facilitates the growth and evolution of different life forms on earth.

Thus, the correct answer is option (4).