AP BIOLOGY

Detailed explanation-1: -So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff? The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

Detailed explanation-2: -Eventually, a simple form of bacteria developed that could live on energy from the Sun and carbon dioxide in the water, producing oxygen as a waste product. Thus, oxygen began to build up in the atmosphere, while the carbon dioxide levels continued to drop.

Detailed explanation-3: -Photosynthesis sustains a large planetary biomass on Earth. Specifically, oxygenic photosynthesis has created an oxygen-rich atmosphere and allowed the evolution of large metabolically demanding creatures.

Detailed explanation-4: -Eventually, oxygen started to accumulate in the atmosphere, with two major consequences. Oxygen likely oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water.

Detailed explanation-5: -Bosak says the answer to that lies in vivid green bacteria called cyanobacteria. Their ancestors were the first organisms to develop a special evolutionary ability, photosynthesis, that changed the world as we know it. “Cyanobacteria are the very first organisms that figured out how to make oxygen.