The oldest fossils known to date have been discovered in 3.7-billion-year-old rocks in Greenland by an Australian-led team of researchers.

Key points: Evidence of ancient bacterial communities known as stromatolites uncovered in rocks in Greenland

Evidence of ancient bacterial communities known as stromatolites uncovered in rocks in Greenland Dating of rock above and below stromatolites places them between 3.7 and 3.68 billion years old

Dating of rock above and below stromatolites places them between 3.7 and 3.68 billion years old Analysis pushes back earliest known evidence of life on Earth by 220 million years

The discovery of the fossilised bacterial communities, known as stromatolites, could be the first clear biological evidence of the earliest known life on Earth, according to a paper published today in Nature.

Before this find, the earliest accepted evidence for life were 3.48-billion-year-old fossil stromatolites from the Pilbara region in Western Australia.

Lead author Professor Allen Nutman of the University of Wollongong has been investigating ancient rock formations in the Isua Supercrustal Belt in south-west Greenland for more than 30 years.

The rocks were known to contain a unique carbon signature, but until now, it was unclear whether the signature had been created by ancient life forms or changes in the rock caused by heat and pressure.

During one of their visits to the area four years ago, the team found a new outcrop had been exposed by increased melting of the summer snow.

"There were some new outcrops that, as far as I can see from historical aerial photographs, have not been exposed in at least the last 40 years or so," said Professor Nutman.

Those rocks bore unusual features that the group instantly recognised.

"We could see this very diagnostic layering where one side of the layering was completely flat and then rising out of that, we had this irregular distribution of bumps or hummocks along it," Professor Nutman said.

Within those bumps and hummocks, they could also see the fine layering that is distinctive of stromatolites, where calcified layers are gradually built up by bacteria over hundreds and even thousands of years.

Analysis of rock layers pins down date

Allen Nutman (left) and Vickie Bennet (right) with a specimen of 3.7-billion-year-old stromatolites from Isua, Greenland. ( Supplied: Yuri Amelin )

The team used sophisticated techniques to carefully date the layers of rocks above and below the fossils.

The analysis placed the stromatolites between 3.71 and 3.695 billion years of age.

"So we've really pinned it down," Professor Nutman said.

Evidence from the "molecular clock" — the mutation rate of genetic material — has long suggested that life first emerged on Earth around 4 billion years ago.

As the product of bacterial communities, stromatolites suggest some degree of sophistication already existed at 3.7 billion years ago, Professor Nutman said.

The discovery could also help guide the search for evidence of past life on Mars, he added.

"At that particular time on Mars, there was still water on the surface, so if life has taken this path already on Earth by 3.7 billion years ago, it does increase the possibility of finding evidence of life on Mars."

Scientists will be 'queuing up'

Commenting on the finding, biogeochemist Dr David Wacey, Future Fellow at the University of Western Australia, said the find would undoubtedly have paleobiologists "queuing up waiting for more snow to melt and reveal additional new outcrops in this region".

Dr Wacey said the discovery was exciting, but not necessarily surprising, given that microfossils and stromatolite fossils dated to around 3.48 billion years ago had previously been found.

"This shows that life had attained a certain degree of complexity at this point so it follows that some considerable amount of time had probably passed since the origin of life itself," he said.

Dr Wacey stressed that the Isua fossils would need to undergo the same rigorous scrutiny as previous ancient microbial fossils had been before this could be considered 100 per cent proof of life 3.7 billion years ago. But at this stage, he said, there were no other competitors for "world's oldest fossil".