Answer
See work below for details.
Work Step by Step
Today, about 21% of Earth’s atmosphere is composed of $O_{2}$ , but this was not always the case. Earth’s early atmosphere was reducing (rather than oxidizing) and contained hydrogen, methane, ammonia, and carbon dioxide. About 2.7 billion years ago, cyanobacteria (blue-green algae) began to convert the carbon dioxide and water to oxygen by photosynthesis. It took hundreds of millions of years to reach the current oxygen levels.
