Rock and awe: Raman discredits microbial fossils

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  • Published: Mar 1, 2011
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Rock and awe: Raman discredits microbial fossils

Oldest fossils, just rocks

The oldest fossils of bacteria ever found were discovered in a rock formation in Western Australia, the discovery led to great excitement that has not abated for more than two decades. Until now. Raman spectroscopy now shows that what palaeontologists thought were pristine microbial fossils may not be anything more complex than chunks of ancient Australian rock.

The Pilbara craton in the northwest of Western Australia is one of only two remaining areas of pristine Archaean crust, thought to have been laid down between 3.6 and 2.7 billion years ago. The second is in Kaapvaal, South Africa. Pilbara is renowned for its deposits of apex chert, a fine-grained cryptocrystalline material, which is rich in silicon dioxide and features sedimentary rock with a microfibrous structure. Scientists have dated the Pilbara Apex Chert at 3.5 billion years and those filaments have been labelled as examples of the oldest fossilized cyanobacteria found anywhere in the world.

Filamentous doubts

Sceptical scientists, however, had always doubted that these filamentous features of the chert were indeed signs of ancient life that existed in a shallow sea billions of years ago. Support for those who disagreed was strengthened by revelations that the region may have been the site of hydrothermal vents rather than a balmy sea. The ancient marks of hydrothermal sites have been mistaken for fossils elsewhere, after all.

Now, assistant professor of paleobiogeochemistry, Alison Olcott Marshall and colleagues, assistant professor of geospectroscopy Craig Marshall and their graduate student Julienne Emry of the University of Kansas, in Lawrence, Kansas, USA, have turned to analytical chemistry to try and settle the argument once and for all by analysing new samples of the Apex Chert.

They cut thin, 300 micrometre sections of rocks collected from the same locality at which the putatively biological filaments had originally been collected, similar to what had been done in previous studies. However, they also produced 30 micrometre thick slides to allow more light to enter the samples. A close inspection revealed that the filaments were not the remains of cyanobacteria but rather nothing more than tiny fractures in the rock, filled with a dark mineral and a light, clear mineral. The team then used Raman spectroscopy to identify the chemicals within the "filaments" and revealed them to be two of the most widespread, and in some sense, mundane minerals hematite (dark, iron(III) oxide, rust one might say) and quartz (light, silicon dioxide). Neither mineral is in any way biological in origin. A possible last chance for the fossils was the discovery by Olcott Marshall and colleagues of carbonaceous material in the rock surrounding the filaments, could be indicative of ancient biochemistry. However, Olcott Marshall suggests that this material is, in and of itself, insufficient proof for the presence of living organisms at the time these rocks were formed, and it does not point to a biological origin for the filaments.

Rocky vindication

The work of Martin Brasier of the University of Oxford, England, is now vindicated. Brasier was the first to suggest that rocky filaments of this sort might be due to hydrothermal activity. He based his hypothesis on the 2002 discovery by him and his colleagues that a purported fossil, which was known colloquially among scientists as the "red banana", was nothing more than hematite rather than evidence of some ancient microbe.

The Kansas team concludes with a warning to other researchers: "We caution against identifying microstructures as biological in origin without a full morphological and geochemical assessment," they say.



The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The oldest fossils of bacteria ever found were discovered in a rock formation in Western Australia, the discovery led to great excitement that has not abated for more than two decades. Until now. Raman spectroscopy now shows that what palaeontologists thought were pristine microbial fossils may not be anything more complex than chunks of ancient Australian rock.
Between a rock and a hard place
(Credit: Olcott Marshall et al)

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