|Cyanobacteria were responsible form the Great Oxidation Event|
The Great Oxidation Event, as it is known, is immensely important in the history of the planet. The accumulation of oxygen in the atmosphere required some kind of photosynthetic microorganism and so the geochemical signature of oxygen is taken as evidence of cyanobacteria living at least 2.5 billion years ago. Of course the Great Oxidation Event did not occur overnight. It took tens and possibly hundreds of millions of years to transform the atmosphere, suggesting that cyanobacteria were even older still.
Whiffs of oxygen from 2.9 billion years ago suggested that cyanobacteria may have had an evolutionary history extending back at least as far. A recent discovery, however, has pushed that history back again by an incredible 300 million years. Analysis of a 3.23 billion year old sediment core sample from South Africa show a small but definite signature of having been deposited in an oxygenated environment. The core consisted of several different rock types each of which were analysed using mass spectroscopy with the purpose of detecting the ratios of iron isotopes.
|The core sample from South Africa|
Confirmation of the iron results came from studies of uranium and its decay products in the samples, Uranium is only soluble in the oxidized form, so the uranium in the sediment had to contain oxygen when the rock solidified,' said Brian Beard from the University of Wisconsin-Madison. Measurements of lead formed from the radioactive decay of uranium showed that the uranium entered the rock sample 3.2 billion years ago. 'This was an independent check that the uranium wasn't added recently. It’s as old as the rock; it’s original material.'
Various forms of photosynthesis exist but only cyanobacteria perform oxygenic photosynthesis. 'There was evolutionary pressure to develop oxygenic photosynthesis,' said Clark Johnson, also from the University of Wisconsin-Madison. 'Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live.' Scientists have long suspected that oxygenic photosynthesis and cyanobacteria pre-dated the Great Oxidation Event. Conclusive fossil evidence of the antiquity of these things is hard to come by, but the geochemical record is much harder to deceive,