Evidence for earliest Earth life is just rocks

Structures claimed to be evidence of the earliest life on Earth may not be biological in origin after all, a re-examination has found.

In 2016, researchers led by Allen Nutman from University of Wollongong in Australia published a paper in the journal Nature, which made the case for the emergence of life 220 million years earlier than previously thought.

Nutman and colleagues described features found in a freshly exposed region of metacarbonate rocks in a feature known as the Isua supracrustal belt in southwest Greenland, known to comprise the oldest rocks so far found.

The researchers suggested that some distinctive conical features, between one and four centimetres high, were the remnants of stromatolites – layered structures produced by colonies of microbes. Similar structures have been dated to 3.48 billion years ago and are regarded as the earliest evidence for macroscopic life.

Nutman’s team examined the internal layout and conducted chemical analyses of the Greenland structures and concluded that there was sufficient evidence to conclude that they were the remnants of stromatolites that had formed in a shallow marine environment 3.7 billion years ago – pushing back the start date for life.

Now, however, another team of researchers, led by Abigail Allwood of the California Institute of Technology, US, has challenged the findings, presenting evidence that suggests that the structures aren’t biological at all.

In a letter published in Nature, the researchers say that three dimensional analysis of the rocks reveal that the putative stromatolites in fact contain no internal layers, and are ridge-shaped rather than conical. Allwood and colleagues also say their own chemical analysis reveals nothing consistent with microbial activity.

On the basis of the fresh evidence, they write, the structures “are more plausibly interpreted” as the result of geological deformations occurring long after the rocks sunk underground.

The findings, they add, have ramifications for other investigations seeking to track the emergence of life.

“The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales,” they write.

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