Scientists have discovered the oldest physical evidence for life on the planet in the form of fossils in Greenland rocks that formed 3.7bn years ago.
The researchers believe the structures in the rocks are stromatolites - layered formations, produced by the activity of microbes, that can be found in today in extremely saline lagoons in a few locations around the world.
The new fossils are 220 million years older than any previously discovered.
"Up until now the oldest stromatolites have been from Western Australia and they are roughly 3,500 million (3.5bn) years [old]," said Clark Friend, an independent researcher and co-author of the research. "What we are doing is pushing the discovery of life earlier in Earth's history."
The discovery, says Mr Friend, also raises questions about the possibility of life on other planets.
“If we have got life at 3,700 million (3.7 bn) years on Earth, did it exist on other planets - because Mars, for example, 3,700 million years ago was wet,” he said.
Writing in the journal Nature, Mr Friend, together with a team of Australian scientists, including his long-term collaborator Allen Nutman of the University of Wollongong, reveal how they discovered what they believe to be stromatolites in rocks of the Isua supracrustal belt in southwest Greenland.
With the Earth’s known rock record only extending around 4.2bn years, the site is home to some of the world’s older rocks.
But there is a hitch. Isua rocks are metamorphic - they formed from sedimentary rocks that have been subjected to intense heat and pressure - conditions that typically deform the rocks.
That, the authors say, made the discovery of stromatolites something of a surprise. “We were not exactly expecting to see them,” said Mr Friend, who with Mr Nutman has been studying Greenland’s rocks for more than 35 years.
Stroke of luck
Fortunately, the team had a stroke of luck. A snow patch had melted, exposing an outcrop that still contained evidence of features from its sedimentary past.
Visible on the surface of the rock, but extending deeper within it, the newly discovered stromatolites are peak-like structures 1-4cm in height; some dome-like, some more akin to the shape of a shark’s tooth.
These structure form from a build-up of very thin layers, produced by microbial communities as they trap and bind sediment or cause carbonate to be deposited from surrounding water. “Sediment is being trapped by algae or bacteria, or a combination of both,” said Mr Friend. “You can find similar things in Australia currently forming in Shark Bay.”
While the nature of the microbes in question remains unknown, the researchers say they would have had a watery existence. “We know that’s true because the sediments themselves wouldn’t form unless they were in water,” said Mr Friend, adding that other evidence from the rocks suggests that the water was most likely to have been shallow.
Nick Lane, an evolutionary biochemist from University College, London, said that the finding ties in with recent revelations that the Hadean - an eon that ended around 4bn years ago - was probably not the hellish environment it was previously thought.
“This idea of the Earth as being bombarded by meteorites and, basically, volcanoes and [having]no water, has kind of evaporated so now we have a rather modern-looking planet with global oceans and some land going back several hundred million years before these fossils were found,” said Mr Lane.
While Mr Lane says it is surprising to have found stromatolites in the Greenland rocks, he believes the conditions on Earth 3.7bn years ago make it far from astonishing that life could have been present.
Indeed, recent discoveries have offered tantalising suggestions that organisms could have been around 4.1bn years ago, while the authors note that so-called “genetic molecular clocks” also suggest that life is likely to have emerged around that time.
“This is really just another piece of evidence that suggests that the world was far more comfortable, if you like, for life as we know it on a global scale than you would perhaps read in even quite recent books,” said Mr Lane of the new discovery.
Unconvinced
However not everyone is convinced.
Robert Riding, of the University of Tennessee says while the similarities of the structures to younger, better-preserved, stromatolites from Australia are intriguing, the poor preservation of the rocks throws doubt on whether the structures were indeed produced by living organisms.
“These examples underscore the problem that we really have not yet found sufficiently well-preserved very old - ie, between 3500 and 4000 million (3.5 and 4bn) year - sediments to be confident that what we are seeing could definitely be regarded as stromatolites,” he said.
But, he said, “It is completely plausible that we could find stromatolites in rocks of Isua age, and perhaps even older, because available evidence suggests that the Earth’s surface at that time already had conditions suitable for microbial life. We just need to find well-preserved sediments of that age to be really sure.”
Lewis Dartnell, an astrobiologist at the University of Westminster, says the discovery could help researchers explore whether life was once present on other planets. “The Martian surface today is very cold and dry, but around the time that these ancient layered rocks formed in Greenland, Mars was itself a much warmer and wetter, and thus habitable planet,” he said. While finding stromatolites with robotic landers or even manned missions is likely to be challenging, says Mr Dartnell, if stromatolites are present, they could offer a wealth of information.
The Guardian