Answer:
Evolution of cyanobacteria
Step-by-step explanation:
Early earth was reducing therefore there was no oxygen on early earth. Oxygen first came on the earth due to the evolution of cyanobacteria on earth. Cyanobacteria were the first aerobic bacteria on earth.
This bacteria was able to conduct photosynthesis and split water into Hydrogen and oxygen. This oxygen is released in the atmosphere and created the aerobic environment on the earth.
Cyanobacteria used sunlight energy to split water and by this, they are able to make their own food in the form of carbohydrates. Therefore the correct answer is cyanobacteria.