An international team of Scientists have made this claim in the ‘Nature’ magazine. Researchers have said that elements in three billion year old samples show evidence for oxidative weathering. Geological data shows that this occurred some 700 million years before the Great Oxidation Event. This event was where geological data shows a dramatic increase in the amount of Oxygen in the atmosphere.
Oxygenic Photosynthesis is very complicated for an organism to do and it would seem as if this would take billions of years to happen and it may not have occurred until this Great Oxidation Event. But now we see that oxygen was present in the Earth’s atmosphere a lot earlier than we first thought, this tells us that even very complex organisms can evolve very fast. Although according to a lead scientist at the University of Copenhagen these organisms did take a lot of time for geological and biological processes to evolve to how they do today. 
The team of Researchers looked at remnant soils, dated from about 2.95 billion years ago which were locked up in rocks in what is now known as Kwazulu-Natal Province in South Africa. They studied different types of isotopes of chromium atoms in the palaeosoils.
These particular isotopes were very sensitive to reactions involving oxygen. A heavier form of the atom chromium 53 became slightly more soluble when oxidised than the lighter chromium 52 atom.
This means that over time the soils that have become oxidised should become depleted in Chromium 53 as the rain water washes away these atoms. So the conclusion of the experiment is that ancient soils would have contained 0.03% of the oxygen it does now which is about one – 10,000th of the present level of the oxygen.
This is a very significant finding because some of the abundant oxygen would have been converted to the ozone layer This three atom oxygen molecule would have filtered damaging ultraviolet light from the Sun. This then would have let many forms of life thrive with the oxygen and lack of ultraviolet light.
This theory is hoped to be tested further by looking at other samples in countries such as Greenland and Australia. It’s also hoped that with the continuing technical advances it will eventually enable scientists to look for even lower levels of oxygen even earlier in time.