Stromatolite fossils push back date for start of life on Earth

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A team of scientists claim to have discovered the world's oldest fossils that are around 3.7 billion years old, pushing back the fossil history to near the start of Earth’s geological record(Credit: University of Wollongong)

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In a remote, isolated region of Greenland, where ancient rocky outcrops have recently been exposed after a perennial snow thaw, scientists claim to have found fossil proof of the oldest life ever discovered. Consisting of fossilized blue-green algae colonies known as stromatolites, the 3.7 billion year old petrified remains are more than 200 million years older than the world's previous oldest stromatolite fossils and are set to rewrite the record books on the age of life in Earth's earliest seas.

Located along the edge of Greenland's icecap in an area known as the Isua Greenstone Belt, the stromatolites are clear evidence of an early shallow sea that once existed in the area many thousands of millennia ago. Their discovery pushes back the point at which life began to little more than a few hundred million years after the Earth was first formed some 4.543 billion years ago.

"The significance of stromatolites is that not only do they provide obvious evidence of ancient life that is visible with the naked eye, but that they are complex ecosystems," said Professor Allen Nutman of the University of Wollongong, and leader of the team. "This indicates that as long as 3.7 billion years ago microbial life was already diverse. This diversity shows that life emerged within the first few hundred millions years of Earth's existence, which is in keeping with biologists' calculations showing the great antiquity of life's genetic code."

Stromatolites (meaning, "layered rock") are slow-growing rigid structures formed by single-celled cyanobacteria microbes (blue-green algae) that trap sediment on their surfaces. Over very long periods of time the captured sediment reacts with calcium carbonate in sea water to form limestone, which grows at a rate of around 5 cm per 100 years.

The previously oldest known fossils are also stromatolites found embedded in Archaean era (4 billion to 2.5 billion years ago) rocks of Western Australia, and are around 3.5 billion years old. As cyanobacteria are among the easiest microfossils to recognize, especially as their form changed very little over millions of years, these same organisms were clearly distinguishable in the latest fossil discovery.

"The structures and geochemistry from newly exposed outcrops in Greenland display all of the features used in younger rocks to argue for a biological origin," said Professor Van Kranendonk, another member of the team from the University of New South Wales Australian Centre for Astrobiology.

"This discovery turns the study of planetary habitability on its head," said Associate Professor Vickie Bennett, from Australian National University's Research School of Earth Sciences. "Rather than speculating about potential early environments, for the first time we have rocks that we know record the conditions and environments that sustained early life. Our research will provide new insights into chemical cycles and rock-water-microbe interactions on a young planet."

Though the discovery of the Greenland fossils should help provide a much greater comprehension of the early variety of life on Earth, the researchers also believe that the findings may have implications for understanding possibilities of life on an early Mars.

"This discovery represents a new benchmark for the oldest preserved evidence of life on Earth," said Professor Van Kranendonk. "It points to a rapid emergence of life on Earth and supports the search for life in similarly ancient rocks on Mars."

The findings will be published today (September 1st) in the journal Nature.

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