By producing oxygen as a byproduct of photosynthesis, cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere, into an oxidizing one, causing the " rusting of the Earth " and the Great Oxygenation Event, that dramatically changed the composition of life forms and led to the near-extinction of anaerobic organisms.
42.
At the time of the Miller Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen ( e . g ., CH 4 and NH 3 as opposed to CO 2 and nitrogen dioxide ( NO 2 ) ).
43.
Their experiments demonstrated the production of biologically important organic compounds ( including amino acids ) induced by passing electric charge through a mixture of gases which were at the time considered to be the components of Earth s early, reducing atmosphere ( H 2 O, CH 4, H 2 and NH 3 ).
