As the catalytic production of life increased, proto-organisms evolved. The inherent instability of RNA would lead to its being replaced by the more complex DNA in later organisms. For a period of about two billion years, bacterial organisms were the dominant life-form on Earth. During this period, the Earth's atmosphere experienced a rapid transformation, as early cyanobacteria photosynthesized oxygen from the oceans, loosing hydrogen into space. Oxygen pollution of the atmosphere resulted in further evolution, producing an explosion of new types of bacteria, and provoking the first massive genocide as the terraformation of the atmosphere destroyed almost all organisms unable to survive an oxygen rich environment.