The fossils, tiny tubules etched into ancient rocks in South Africa, were initially thought to be formed by ancient bacteria boring through volcanic glass in the seafloor — a process called bioalteration — during the Archean Eon, about 3.4 billion years ago.
But the new study, published yesterday (May 26) in the journal Proceedings of the National Academy of Sciences, suggests these tiny tunnels were actually formed by the cooling of the volcanic rock nearby, just 2.9 billion years ago.
"Our new data challenges this complex 'bioalteration model' proposed to have occurred in the Archean pillow lava rims," study co-author Eugene Grosch, an earth scientist at the University of Norway, wrote in an email to Live Science.
Several fossils have vied for the title of Earth's oldest life. Geologists thought rippling, wavy textures imprinted into rocks in the Dresser Formation in western Australia may have been formed by microbial mats about 3.4 billion years ago. At another formation in western Australia known as Strelley Pool, domelike structures called stromatolites may also have been formed by microbes nearly 3.5 billion years ago.
And in 2004, researchers digging at the Barberton Greenstone Belt in South Africa identified the newly analyzed microscopic filament structures, made of a mineral called titanite, that they believed were formed by ancient microbes in oceanic crust about 3.49 billion years ago.
But finding the signature of tiny microbes that lived billions of years ago is extraordinarily difficult, and geologists hotly debate which of these specimens is truly the earliest hint of life on Earth.
Grosch and his colleague Nicola McLoughlin, an earth scientist at the University of Norway, weren't convinced that the Barberton textures were formed by ancient microbes. To test that idea, the team drilled 590 feet (180 meters) into the rock where the textures were found.
They measured hundreds of the textures throughout the core and analyzed their size and shape distribution. The filaments had huge diameters and a very large size distribution compared with those of the miniscule tunnels formed by microbes in oceanic crust today, Grosch said.
The tiny trace fossils were formed between 2.9 billion and 2.8 billion years ago, so they're about 650 million years younger than the formation as a whole.
The team also used a mathematical model of the cooling conditions in nearby pillow lava and found that the titanite structures were likely formed by the prevailing conditions in the cooling rock at that time.
Read more at Discovery News
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