Their findings could also prove useful in ridding the earth of excess methane, one of the greenhouse gases many scientists believe is responsible for global warming.

Traditional views of early life on earth centre on plants which converted carbon dioxide to oxygen.

"These (plant lifeforms) date back to between three and 3.5 billion years ago... We have found biomass (large cluster of organisms) using methane that geologists show could have existed around four billion years ago," Professor Antje Boetius, joint author of the study, told Reuters.

The two-year research by the scientists from Hamburg University, the Alfred Wegener Institute in northern Bremerhaven and the Max Planck Society centred on coral-forming micro-organisms in the Black Sea at depths where no oxygen and no light is present.

The Black Sea contains the largest oxygen-free basin in the world.

The lifeforms were able to process methane together with sulphates within the water, producing carbonates, in the form of coral, as waste.

That they were able to do so without oxygen suggests they may have been around before plant life.

"Perhaps micro-organisms like those found in the Black Sea were the original inhabitants of the earth during a long period of the earth's history," said Boetius.

She believes the findings could prove useful for climate control.

Previously, scientists had thought that methane, found in abundance in the sea and produced through agriculture, could only be broken down with oxygen.

The German researchers believe the discovery of a pool of organisms that process methane without oxygen could lead to a way of cutting down potentially harmful greenhouse gases without burning oxygen and producing similarly damaging carbon dioxide.