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Scientists have discovered what may be the oldest species on the planet - a deep sea organism that has remained unchanged for more than two billion years.

The microscopic bacteria were found living in mud off the west coast of South America and appear to be identical to fossils found in rocks in Western Australia.

Researchers believe the filament-like microorganisms have survived unchanged by living off the sulphurous compounds in sediments that were first laid down around 2.3 billion years ago.

The scientists compared bacteria found living off the coast of South America (far left and centre) with similar fossils of organisms found in rocks in Western Australia that were once under the ocean two billion years ago

They believe the bacteria first evolved during the early Proterozoic eon of the Earth, just after the continents first stabilised and before an oxygen levels in the atmosphere increased.

The scientists behind the research say their discovery also helps to support a central tenet in Charles Darwin's theory of evolution - that if there is no change in the physical environment of a well-adapted organism, its biology will remain unchanged.

Professor William Schopf, a palaeobiologist at the University of California Los Angeles who led the research, said: 'It seems astounding that life has not evolved for more than 2 billion years—nearly half the history of the Earth.

'These microorganisms are well-adapted to their simple, very stable physical and biological environment.'

The international team of researchers, whose work is published in the Proceedings of the National Academy of Science, examined preserved fossils of ancient deep-sea bacteria that lived in the ocean mud off that now forms part of Western Australia around 1,800 million years ago.

These fossils, found in an area now known as Duck Creek, close to Nanutarra, Western Australia, were found to be identical to another fossilised bacterial community that lived 500 million years earlier in what is now Turee Creek, in Karijini National Park, Western Australia.

These were compared to modern microbes found living in mud off the coasts of Chile, Peru, the Galapagos Archipielago, Ecuador, Panama and Costa Rica by researchers from the University of Concepción in Chile.

The scientists concluded that the modern bacteria were identical to the fossilised organisms found off the coast of Australia.

This suggests the species was hugely successful around two billion years ago, flourishing in the sulphur rich environments that emerged before oxygen levels increased around the globe.

The dark areas in this section of 1.8 billion year old rock are fossilised bacteria that have been found to be identical to microorganisms found living in the deep sea much off the west coast of South America

The fossils in this rock, marked by dark areas, are at least two billion years old, according to researchers

They say as bacteria capable of photosynthesis, which uses the power of the sun to react carbon dioxide with water to produce oxygen, started to increase oxygen levels, the sulphur loving bacteria were able to survive deep in the oxygen poor environments of the deep ocean floor.

Professor Schopf said that it was likely the molecular biology of these organisms would be very similar as the environments they lived have remained largely unchanged.

The researchers used several techniques to analyze the fossils, including Raman spectroscopy — which enables scientists to look inside rocks to determine their composition and chemistry — and confocal laser scanning microscopy — which renders fossils in 3-D.

Professor Schopf said that it would be necessary to examine other ancient bacterial fossils from similar environments to see whether the organisms there had changed at all.

He said: 'Such findings may eventually be regarded as having confirmed the null hypothesis required of Darwinian evolution, but such an assessment would be, at present, premature.

'If they were in an environment that did not change but they nevertheless evolved, that would have shown that our understanding of Darwinian evolution was seriously flawed.

'The rule of biology is not to evolve unless the physical or biological environment changes, which is consistent with Darwin.'