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?

The correct answer is cyanobacteria. This was an organism that is responsible for affecting the early atmosphere by photosynthesizing and releasing huge amounts of oxygen into the air because there was so much of it. It's also known as the blue-green algae, and versions of it exist to day though they're different than what happened then.