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

The rise in oxygen levels in Earth's atmosphere is primarily attributed to the process of photosynthesis carried out by plants and algae. During photosynthesis, these organisms use sunlight to convert carbon dioxide and water into glucose and oxygen. This process not only produces oxygen as a byproduct but also plays a crucial role in reducing carbon dioxide levels, balancing atmospheric gases over geological time scales.

Initially, the Earth's atmosphere had very little oxygen. However, the evolution of photosynthetic organisms, such as cyanobacteria, around 2.5 billion years ago marked a significant turning point. As these organisms proliferated, they released substantial amounts of oxygen into the atmosphere, leading to what is referred to as the Great Oxidation Event. This increase in atmospheric oxygen was essential for the development of aerobic life forms and played a critical role in shaping the planet's environment as we know it today.