At the edge of an icecap in July 2012, Allen Nutman of Australia’s University of Wollongong and a group of fellow geologists picked through the oldest known rocks on Earth. The team of Australian and British scientists was surveying the landscape of the Isua supracrustal belt in remote southwest Greenland when it noticed a newly exposed outcrop of ancient rocks that had been previously covered by year-round snow.
According to a study published in the academic journal Nature by Nutman and his fellow geologists, one of the 3.7-billion-year-old rocks revealed by the melting icepack contained the oldest fossil known to date. If confirmed, the discovery will push back the earliest fossil evidence for life on Earth by 220 million years and cause scientists to rethink ideas about the origins of life on Earth and the possible existence of life elsewhere in the universe.
The research team reports that upon studying one of the ancient rocks found in Greenland, they discovered fossilized stromatolites—sedimentary formations created by the layered growth of microorganisms—between one and four centimeters high. The stromatolites are believed to have been created 3.7 billion years ago by microbial communities living in shallow waters that trapped and bound together thin layers of sediments. While the characteristic dome and cone shapes of stromatolites are present, the primordial microbial colonies that created them have not survived.
It’s rare to find any rocks as old as 3.7 billion years that would have escaped the intense heat and tectonic forces that typically twisted, warped and obliterated ancient rocks and destroyed any fossil evidence in the process. Previously, 3.48-billion-year-old stromatolites found in sedimentary rocks in the Pilbara region of Western Australia contained the world’s oldest known fossil.
The Washington Post reports that a laboratory analysis of the stromatolites by the research team found “chemical signatures consistent with a biological origin for the conical structures.” Other scientists, however, caution that it’s possible the wavy formations in the rocks could have occurred through natural, rather than biological processes, and a conclusive determination can be difficult in rocks so old. Abigail Allwood, a scientist at the California Institute of Technology’s Jet Propulsion Laboratory, writes in an article accompanying the study in Nature that “the problem is that structures that look similar to stromatolites can form without the action of living organisms, and so the interpretation of stromatolite-like structures has been notoriously difficult in Earth’s oldest rocks.”
Still, Allwood writes that “if these are really the figurative tombstones of our earliest ancestors, the implications are staggering.” It could mean that life not only started earlier than previously thought, but its evolution was more rapid than earlier believed as well. Nutman says that the organisms that created the stromatolites were relatively complex, which means they were descendants of earlier life forms, pushing the origins of life on Earth even further back than 3.7 billion years ago to a time when, in geological terms, the planet was a mere child in its formative stages.
Scientists believe the Earth formed around 4.5 billion years ago from debris orbiting the Sun. If life began hundreds of millions of years earlier than previously thought, it could bump up against the Hadean eon, the time up to 4 billion years ago when volcanoes spewed lava, oceans boiled and asteroids and comets blasted the planet’s surface, or the Late Heavy Bombardment stage at the beginning of the ensuing Archean eon between 3.9 billion and 3.8 billion years ago when asteroids continued to pummel the Earth.
If confirmed, the discovery could mean that perhaps the Hadean eon wasn’t as hellish as currently believed or that life was able to start in conditions previously thought inhospitable. “If life could find a foothold here,” Allwood writes, “then life is not a fussy, reluctant and unlikely thing. Give life half an opportunity and it’ll run with it.”
The finding could also have implications for our understanding of the possibilities of life on other planets, particularly on Mars, which featured standing bodies of surface water similar to Earth’s 3.7 billion years ago. “Suddenly, Mars may look even more promising than before as a potential abode for past life,” Allwood writes.
“Three thousand seven hundred million years ago, Mars was wet,” Nutman told the Washington Post. “If life had managed to evolve to produce structures like stromatolites by 3,700 million years ago on Earth, there is an increased probability—certainly not a certainty—that the same type of process might have happened on Mars before it dried out.”