What caused the increase of oxygen in the Earth's atmosphere?

The increase of oxygen in the Earth's atmosphere is primarily attributed to the process of photosynthesis carried out by plants and algae. Through this process, these organisms convert carbon dioxide and water into glucose and oxygen, utilizing the energy from sunlight. This not only produces oxygen as a byproduct but also contributes to the overall balance of gases in the atmosphere.

During Earth's early history, oxygen was scarce, but the advent of photosynthetic organisms around 2.4 billion years ago led to what is known as the Great Oxidation Event. This dramatic increase in atmospheric oxygen levels was primarily driven by cyanobacteria, algae, and later terrestrial plants. This rise in oxygen was crucial for the development of aerobic (oxygen-using) life forms and allowed for more complex biological processes and the eventual evolution of larger organisms.

Other choices do not accurately reflect the predominant source of atmospheric oxygen. Volcanic eruptions can release various gases, but they do not contribute significantly to the production of oxygen. Similarly, while some minerals may release oxygen when they undergo chemical reactions, this process does not compare to the vast amounts generated by photosynthesis. Cosmic radiation can cause the splitting of water molecules, but this is a minor process and not the primary contributor to atmospheric oxygen levels.