What process led to the increase in oxygen levels?

The increase in oxygen levels was primarily caused by photosynthesis carried out by early life forms, particularly cyanobacteria.

Photosynthesis is a process used by plants and other organisms to convert light energy, usually from the sun, into chemical energy that can be later released to fuel the organisms' activities. This process is crucial for the increase in oxygen levels on Earth. The first organisms to perform photosynthesis were likely simple, single-celled microorganisms like cyanobacteria. These early life forms began to produce oxygen around 2.4 billion years ago, during a period known as the Great Oxygenation Event.

Cyanobacteria are a type of bacteria that obtain their energy through photosynthesis. They are often referred to as 'blue-green algae', although they are not true algae. These organisms are thought to have been the first to perform oxygenic photosynthesis, a process that produces oxygen as a byproduct. This process involves the use of sunlight to convert water and carbon dioxide into glucose, a type of sugar that the organism can use for energy, and oxygen.

The oxygen produced by cyanobacteria and other photosynthetic organisms began to accumulate in the Earth's atmosphere, leading to a significant increase in oxygen levels. This increase in oxygen levels allowed for the evolution of more complex life forms that rely on oxygen for respiration, a process that releases energy from glucose.

It's important to note that this process took a very long time. The Great Oxygenation Event, which marked the first significant increase in Earth's oxygen levels, occurred over a period of hundreds of millions of years. Even after this event, it took a further billion years for oxygen levels to reach a point where they could support the large, complex life forms that we are familiar with today.