About 2.7 billion years ago, primitive plants and algae began to grow in the oceans. They carried out photosynthesis, using carbon dioxide and water to make glucose and releasing oxygen.
carbon dioxide + water → glucose + oxygen
The word equation shows that photosynthesis takes carbon dioxide out of the atmosphere, so it helped the amount of carbon dioxide to fall further. It also released oxygen, which entered the atmosphere. Because the plants released oxygen over a very long time, the amount of oxygen in the atmosphere gradually increased until it reached the level of about one fifth that we have today.