Photosynthesizing cyanobacteria were responsible for the creation of Earth's oxygen atmosphere during the Archaean and Proterozoic Eras, and in doing so these relatively primitive organisms played a tremendously important role in shaping the course of succeeding evolution and ecological change throughout Earth's history. The oceans and atmosphere had a very different chemistry lacking free oxygen, unsuitable for life as we know it today, prior to the appearance and activity of cyanobacteria. They were the dominant lifeform on Earth for over 2 billion years and the builders of Earth's first reefs.