<<< Return to Spanish version Oxygen (O 2 ) is approximately 21% of the Earth's atmosphere, but this has not always been the case: the primitive Earth's atmosphere contained only small amounts of this compound, probably as a result of the reaction of sunlight with steam water from volcanoes. One only could find oxygen in water molecules (H 2 O) or attached to the forming iron oxides, but never freely. The oxygen-rich atmosphere of today (eon up, eon below) is the result of one of the deadliest weather disasters in the history of our planet. About 2.5 million years ago the Earth was very different: there were no green plants neither, of course, animals or insects. The only forms of life were bacteria and they lived in the old way, ie, under anaerobic conditions. These primitive organisms thrived in the absence of oxygen, using sulphur for their energy needs. Under these conditions, iron is capable of dissolving in water, resulting in oceans rich in thi...