Tiny filaments thought to be ancient fossils are shown to be inorganic.
Twenty years ago the palaeontological community gasped as geoscientists revealed evidence for the oldest bacterial fossils on the planet. Now, a report in Nature Geoscience1 shows that the filament structures that were so important in the fossil descriptions are not remnants of ancient life, but instead composed of inorganic material.
Found in the 3.5-billion-year old Apex Chert rock formation in Western Australia, the fossils were initially accepted as being oxygen-producing cyanobacteria, but debates have raged over whether they are really fossils at all. A key argument against is that the formation, originally thought to represent an ancient shallow sea floor, is now thought to be a hydrothermal site. Hydrothermal sites often host complex inorganic chemical interactions, and the remains of these processes have sometimes been mistaken for biological fossils.
"Unhappily, the proposed cyanobacteria are in some of the most messed up chert I have seen in my life. With basaltic lava rich in tin, zinc and copper, criss-crossed with iron-oxide and quartz, the rock is a mess," says palaeobiologist Martin Brasier, from the University of Oxford, UK.
Reports on the fossils have varied in their findings. Some have argued that the branching, filamentous microstructures are made of non-biological carbon in the form of graphite; others say that they are kerogen, a compound that can be made biologically.
No life after all
Keen to explore the problem, Alison Olcott Marshall and her team at the University of Kansas in Lawrence collected new samples from the Apex Chert and ran an in-depth analysis.
The researchers cut 300-micrometre sections of the filaments, just as past teams have done, for study under the microscope. But they also cut 30-micrometre sections to allow more light into the sample. This revealed that the filaments were fractures within the matrix of rock and that they were filled with both a light, clear mineral and a dark, plate-like mineral. The researchers then turned to Raman spectroscopy, a process that uses a laser, to identify the minerals: the dark mineral was haematite and the light one quartz. Neither are biological materials.
Their finding is not stirring feelings of jubilation. "After nearly 30 years of effort at pushing evidence for life to or beyond 3.5 billion years ago, we are reminded that the ancient record is more fraught with complications than we ever thought," says geologist Stephen Mojzsis at the University of Colorado, Boulder.
Brasier, who had a key role in showing that the Apex Chert came from a hydrothermal vent, says he is not surprised by the finding. "We found a fossil called the red banana back in 2002 that led us to argue that many of the microfossil structures were haematite," says Brasier.
Still a spark?
Although the filamentous structures seem now not to be biological, the study did find an intriguing detail in the rock matrix surrounding the filaments — it contained carbonaceous material, which could, perhaps, have been formed by living things.
Olcott Marshall suggests that this carbonaceous material may have been accidentally sampled by other research teams and played a part in them identifying the filaments as biological. Brasier disagrees, saying instead that people wanted to find life so badly that they ignored the obvious. "There is a willful blindness about these structures that sometimes has more to do with local politics than global truth," he says.
So although the carbonaceous material may have created past problems, it keeps a spark of hope alive that the Apex Chert holds evidence of ancient life. "I remain optimistic as always, but we must remember to adhere to the highest standards," says Brasier.
Olcott Marshall agrees. "The question of whether or not these rocks contain life has just got much more complicated."
Marshall, C. P., Emry, J. R. & Olcott Marshall, A. Nature Geosci. advance online publication doi:10.1038/NGEO1084 (2011).
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Kaplan, M. Filamentous figments in the Apex Cherts. Nature (2011). https://doi.org/10.1038/news.2011.110