Final answer:
The Great Oxygenation Event took place during the Archaean Eon, initiated by cyanobacteria that released oxygen into the atmosphere through photosynthesis, leading to massive ecological changes.
Step-by-step explanation:
The Great Oxygenation Event took place during the Archaean Eon, which dates back approximately 2.5 billion years ago. Cyanobacteria were the primary drivers of this event, fundamentally altering Earth's atmosphere by producing and accumulating oxygen through photosynthesis. This increase in oxygen levels was catastrophic for many existing anaerobic lifeforms but ultimately beneficial for the long-term diversification of life, paving the way for more complex aerobic organisms. It represented a dramatic shift in Earth's history where oxygen levels in the atmosphere and ocean increased significantly, leading to the demise of many anaerobic species but also enabling the evolution of aerobic life forms capable of utilizing oxygen in their metabolism.