Final answer:
Cyanobacteria, ancient photosynthetic organisms, existed at least 3.5 billion years ago and played a crucial role in Earth's oxygenation through the Great Oxygenation Event.
Step-by-step explanation:
It is true that cyanobacteria, one of Earth's oldest life forms, already existed at least 3.5 billion years ago. These photosynthesizing organisms are credited with transforming the climate through the Great Oxygenation Event around 2.5 billion years ago, releasing oxygen into the atmosphere and paving the way for the emergence of multicellular life. Cyanobacteria are observed in stromatolites, which are ancient, solid, layered rock formations that provide strong fossil evidence of early life on Earth. These stromatolites, like those found along the shores of Lake Thetis in Western Australia, were formed by the layering of cyanobacteria in shallow waters.