Question

Describe the organism that scientists believe was responsible for changing the composition of the early atmosphere. What is it called and how did it change the atmosphere? Why was this change to the atmosphere so important?

Answer 1

The early atmosphere was most likely principally CO2, with very little or no chemical element. The proportion of chemical element went up attributable to chemical change. The chemical change was conducted from little organisms.

Answer 2

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. The proportion of oxygen went up because of photosynthesis. The photosynthesis was conducted from tiny organisms.
cyanobacteria, or blue-green algae. 
They used sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. This change to the atmosphere was very important because the breathable air we enjoy today was created.