How long has life flourished on our planet?
A new study suggests it could go back more than 3.7 billion years.
In a study published Wednesday in Nature, a team of Australian researchers describe small conical structures that may have been built by microorganisms less than a billion years after the planet was born.
The work adds to a growing body of evidence that suggests life has flourished on Earth since its infancy.
“If these really are the figurative tombstones of our earliest ancestors, the implications are staggering,” Abigail Allwood, a geologist at the
The search for the earliest signs of life on our planet has taken many different forms. Some researchers scour ancient minerals for chemical signatures that likely have a biological origin. Others have looked for physical remnants left behind by the planet's earliest organisms.
In this study, the research team examined recently exposed rocks from the Isua Greenstone Belt in southwestern Greenland, which has some of the oldest rocks on the planet, dating back as much as 3.7 billion years. They were looking specifically for stromatolites — structures produced by microorganisms that trap and bind sediment.
As it turns out, they may have found some. After cracking open rocks from this area, the researchers report the discovery of stromatolites from two sites where the rocks have remained relatively undisturbed for billions of years. Further analysis revealed that the structures likely emerged in a shallow marine environment.
"Seeing stromatolites in such a setting would hardly be surprising — if the rocks were half a billion years younger," Allwood wrote.
But because they are so old — formed less than a billion years after the birth of the planet — the research team lead by Allen Nutman of the University of Wollongong in New South Wales, Australia, will have to work hard to convince other scientists that the structures really do constitute evidence of early life.
In the absence of organic or cellular remains, the authors point to four clues that suggest the small mounds were built by ancient organisms.
These include the conical shape of the structures, a layered internal structure, and the fact that sedimentary layers between the cones appear to have formed as sediment piled up against the cones as they stuck out of the sea floor.
They also note a difference in abundance of both titanium and potassium between the stromatolites and the surrounding rock.
"These four pieces of evidence are not as clear cut as you'd ideally want for such an extraordinary claim," Allwood said. "Nonetheless, the Isua structures are clearly not folds or dewatering structures."
She said it is certainly possible that they are biological, but said she cannot absolutely refute the possibility that they formed by localized mineral precipitation from seawater.
"If we found these on Mars, would we plant a flag and declare that we had found life on Mars? I think not, but we would definitely get very excited and continue looking around for more information," she said. "And I suspect that's exactly what will happen in Isua."
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