'Smoking Gun' Evidence Dates Some of Earth's Earliest Life to 3.5 Billion Years Ago

Photomicrograph of stromatolites (golden streaks in the image) from Australia's 3.5-billion-year-old Dresser Formation.
Photomicrograph of stromatolites (golden streaks in the image) from Australia's 3.5-billion-year-old Dresser Formation. (Image credit: UNSW Sydney)

Just a billion years after Earth took shape, microbial life was already thriving, scientists say.

Analysis of organic traces preserved in ancient Australian rocks — among Earth's oldest — revealed a "perfect snapshot" of microbial life 3.5 billion years ago, the researchers said in a statement

Though individual microbes are too small to be seen with the naked eye, millions of microorganisms can fossilize together to form larger features embedded in rock, known as stromatolites. Many of these structures are preserved in Western Australia's Dresser Formation. Though some geologists aren't convinced that stromatolites represent ancient life, a new study presents "exceptional evidence" of stromatolites' organic origins, researchers recently reported.

Related: In Images: The Oldest Fossils on Earth

Since their discovery in the 1980s, the Dresser Formation stromatolites have posed a maddening puzzle that scientists have sought to resolve: Did living organisms create the structures? Or was some non-biological hand at work? 

Because the stromatolites have been exposed for billions of years, weathering has taken a heavy toll, erasing chemical information that could link stromatolites to once-living organisms, said lead study author Raphael Baumgartner, a research associate with the School of Biological, Earth and Environmental Sciences at the University of New South Wales (UNSW) in Australia. 

What's more, certain geologic processes can shape mineral structures that closely resemble those left behind by ancient organisms, and even experts may be hard-pressed to tell them apart, Baumgartner told Live Science in an email.

So the scientists dug deep. They drilled dozens of meters below the rocky surface to extract stromatolite samples that were unaffected by weathering, according to the study. In those samples, they found filaments of organic material associated with mats of microbes. The researchers also detected veins of pyrite — a mineral also known as fool's gold — that held tiny particles of organic matter. This typically only happens when organisms decompose and the organic matter gets replaced by pyrite, Baumgartner said.

"This is all backed up by chemical analyses, including organic carbon isotope analysis clearly pointing to biomass," he said.

"High-quality, smoking gun evidence"

What in Earth's primordial soup combined to cook up early microbial life? Graphite traces in a zircon dating to 4.1 billion years ago hint that at least one important component of life — carbon — was already in place within just a few million years after Earth's formation. Another ingredient was probably cyanide, which may have traveled to a young Earth on primitive meteorites, sparking chemical reactions that eventually produced living cells, Live Science previously reported.

While microbes may have begun to appear billions of years ago, animals took somewhat longer to evolve. The oldest evidence of animal life — preserved chemicals from long-vanished soft bodies — dates to between 635 million and 680 million years ago, and is thought to belong to an ancient relative of modern sponges.

As old as the Australian stromatolites may be, other preserved evidence may represent life that's even older, Baumgartner said. In 2017, another team of researchers identified fossilized microbial evidence in Canada that may be between 3.77 billion and 4.29 billion years old. Study co-author and UNSW professor Martin Van Kranendonk is also investigating stromatolites in Greenland that may be 3.7 billion years old — but whether or not they were produced by living organisms "is highly disputed in the literature," according to Baumgartner.

"The problem is that these rocks of Greenland have experienced a lot of transformation and deformation at high temperatures," he explained. Over billions of years, any trace of organic material resembling what was found in the Australian stromatolites has likely been completely destroyed, making it difficult to prove that the Greenland structures were shaped by microbes, he said.

"What is so important about our Dresser Formation stromatolites discoveries is that we have set a benchmark with high-quality smoking gun evidence, in exceptionally preserved samples that have suffered little since they formed," Baumgartner said. 

The findings were published Sept. 25 in the journal Geology.

Originally published on Live Science.

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Mindy Weisberger
Live Science Contributor

Mindy Weisberger is an editor at Scholastic and a former Live Science channel editor and senior writer. She has reported on general science, covering climate change, paleontology, biology, and space. Mindy studied film at Columbia University; prior to Live Science she produced, wrote and directed media for the American Museum of Natural History in New York City. Her videos about dinosaurs, astrophysics, biodiversity and evolution appear in museums and science centers worldwide, earning awards such as the CINE Golden Eagle and the Communicator Award of Excellence. Her writing has also appeared in Scientific American, The Washington Post and How It Works Magazine.