section_path: Biological production and role of O/Build-up in the atmosphere
section_name: Build-up in the atmosphere
paragraph_text: Build-up in the atmosphere
  
  Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the Paleoproterozoic era (between 3.0 and 2.3 billion years ago). Even if there was much dissolved iron in the oceans when oxygenic photosynthesis was getting more common, it appears the banded iron formations were created by anoxyenic or micro-aerophilic iron-oxidizing bacteria which dominated the deeper areas of the photic zone, while oxygen-producing cyanobacteria covered the shallows. Free oxygen began to outgas from the oceans 3–2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago.
  
  The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant anaerobic organisms to extinction during the Great Oxygenation Event (oxygen catastrophe) about 2.4 billion years ago. Cellular respiration using O enables aerobic organisms to produce much more ATP than anaerobic organisms. Cellular respiration of O occurs in all eukaryotes, including all complex multicellular organisms such as plants and animals.
  
  Since the beginning of the Cambrian period 540 million years ago, atmospheric O levels have fluctuated between 15% and 30% by volume. Towards the end of the Carboniferous period (about 300 million years ago) atmospheric O levels reached a maximum of 35% by volume, which may have contributed to the large size of insects and amphibians at this time.
  
  Variations in atmospheric oxygen concentration have shaped past climates. When oxygen declined, atmospheric density dropped, which in turn increased surface evaporation, causing precipitation increases and warmer temperatures.
  
  At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire O in the present atmosphere.
  
  It is estimated that oxygen on Earth will last for about one billion years.