Final answer:
Cyanobacteria produced oxygen on ancient Earth through photosynthesis, resulting in significant changes in the atmosphere and evolution. They played a critical role in the formation of Earth's atmosphere.
Step-by-step explanation:
Cyanobacteria, also known as blue-green algae, produced oxygen on ancient Earth through the process of photosynthesis. They used energy from sunlight to convert carbon dioxide into organic compounds, releasing oxygen as a byproduct. The accumulation of oxygen in the atmosphere caused significant changes in the planet's atmosphere and the direction of evolution. These bacteria are believed to have played a critical role in the formation of Earth's atmosphere.