Answer:
If cyanobacteria ceased to exist there would not have been enough oxygen in the atmosphere.
Step-by-step explanation:
Cyanobacteria can be described as free living bacteria which have the ability to produce their food by carrying out the process of photosynthesis.
In the early earth conditions, free oxygen was not present in the Earth. The element oxygen either was present in the water molecules or in other minerals.
In such conditions, cyanobacteria began the process of photosynthesis and made enough free oxygen for supporting the eukaryotic life.