Why did algae Change the atmosphere?

Algae, particularly cyanobacteria, played a crucial role in changing the Earth's atmosphere by producing oxygen through photosynthesis. Around 2.4 billion years ago, this process led to the Great Oxidation Event, significantly increasing atmospheric oxygen levels. As a result, oxygen became abundant in the atmosphere, enabling the evolution of aerobic life forms and transforming Earth's ecosystems. This fundamental shift laid the groundwork for the development of complex life.