Answer:
If cyanobacteria would not have evolved during the Earth's history, there would be little oxygen in the atmosphere. As a result, aerobic life on Earth might have been difficult to exist on Earth. The oxygen in the primitive Earth conditions was not present in the free state. The Early Earth possessed oxygen bonded in the water molecules or bonded to other minerals.
Cyanobacteria were photosynthetic and were able to provide free oxygen in the environment which made aerobic life possible to exist on Earth.
