We May Have Found Earth’s Oldest Fossils

From Scientific American:

In this week’s Nature, Australian and British researchers report finding layered structures called stromatolites in 3.7-billion-year-old rocks from Greenland. Stromatolites, which look a bit like geological cauliflowers, form when microbes trap sediment and build up layer after dome-shaped layer.

But the discovery involves some of the most physically tortured rocks on Earth, which have been squeezed and heated over billions of years as crustal plates shifted. The pressure and heat recrystallizes the rocks, erasing much of the fine-scale detail that researchers normally use to identify fossilized stromatolites—so the work is already triggering heated debate.

“I’ve got 14 queries and problems that need addressing before I’ll believe it,” says Roger Buick, a geobiologist at the University of Washington in Seattle.

The rocks hail from Isua, Greenland, where researchers have laboured to tease out potential signs of life dating back billions of years. Previous work on the rocks’ chemistry, such as a 1999 paper analysing carbon isotopes, suggested that they contain ‘biomarker’ traces of early organisms. But various claims over the years have remained contentious.

Now, melting snow has revealed new clues. A team led by Allen Nutman, a geologist at the University of Wollongong in Australia, visited a rock outcrop that had been buried under a perennial snow patch until warmer temperatures melted it away. They sawed out a chunk of 3.7-billion-year-old rock and took it back to Australia to study.

In it they found the purported stromatolites, along with other clues to ancient life. “It’s a combination of different types of evidence that makes the story so compelling,” says team member Martin Van Kranendonk, a geologist at the University of New South Wales in Kensington, Australia.

The structures are tiny bumps, just 1–4 centimetres tall, whose shape and internal layering strongly resemble ancient and modern stromatolites, Van Kranendonk and his colleagues say. The texture of the surrounding …

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