Final answer:
The Great Oxygenation Event refers to the significant increase in atmospheric oxygen caused by cyanobacteria's photosynthesis around 2.5 billion years ago, leading to the first mass extinction and setting the stage for complex life.
Step-by-step explanation:
The Great Oxygenation Event was a pivotal turning point in Earth's history when atmospheric oxygen began to significantly increase due to the biological activity of early photosynthetic bacteria, predominantly cyanobacteria. This event marks the transition from an anoxic to an oxygen-rich atmosphere approximately 2.5 billion years ago. While it initiated the first-known mass extinction by harming anaerobic life forms that could not survive in oxygenated conditions, it also laid the groundwork for the evolution of aerobic and more complex life forms, including multicellular organisms. This process of oxygenation was gradual but had profound long-term effects on the planet's climate, life forms, and biological evolution.