The strange history of atmospheric oxygen

Many of us think a lot about oxygen. This includes how the normal body handles oxygen in health, but particularly how this is complicated by lung disease. Few of us are aware that as human inhabitants of the earth, we have a unique privilege. This is that as air breathers, we and most other animals on Earth, are the only living creatures in the known universe that have unlimited supply of oxygen. This situation came about through one of the greatest miracles of nature, that is photosynthesis, the ability to release oxygen from water using the energy of sunlight. One consequence of this was that the first atmospheric oxygen came from the metabolism of microorganisms, the cyanobacteria, that used photosynthesis, but for which oxygen was an unwanted by-product. In fact, the oxygen had to be discarded for the organisms to thrive. When a major increase of oxygen concentration in the atmosphere occurred some 2 billion years ago, and the partial pressure of oxygen in the air rose to perhaps 200 mmHg, this Great Oxidation Event as it was called, was a death sentence for the large population of anaerobic animals for whom oxygen was toxic. Today much of the oxygen in the atmosphere comes from photosynthesis in microorganisms, including the cyanobacteria, and the recently discovered Prochlorococcus, that discard this unwanted by-product. The result is that the PO2 in our atmosphere at sea level remains nearly constant at about 150 mm Hg, although the factors responsible for this are not understood.