Sunday, 26 April 2015

Solving The Controversy Of The World's Oldest Fossils

Filamentous structures from the Apex Chert
In 1993 Bill Schopf, from UCLA, found bizarre filamentous microstructures in the Archaean Apex Chert in the Australian outback. They appeared to be the remains of bacterial cells. Using volcanic ash layers above and below the strata from which they came, the fossils were radiometrically dated to an incredible 3.46 billion years old. In the following decades no microfossils - either putative or indisputable  - were found which were older. The Apex Chert fossils were heralded as the earliest traces of life on the planet. In 2002, however, this was challenged.

The fossils were quite complex for their proposed place in the history of life. Additionally their colouration was unusual - amber rather than black, typical of microfossils. This led Martin Brasier from the University of Oxford to posit that they were inorganic mineral structures. Chemical analysis of the surrounding rock showed that it had formed in a hydrothermal vent system. Therefore the supposed microfossils could have been inorganic minerals covered in a skin of accreted carbon. The organic and inorganic hypotheses have competed against each other for the past 13 years, but now a recent study is set to settle the debate.

Dr David Wacey, working in conjunction with Martin Brasier before his death earlier this year, conducted an elemental analysis of a range of microfossils using transmission electron microscopy. The analysis included specimens from the Apex Chert. The results were used to compile nanoscale maps of the distribution of carbon in the structures. Previously, imaging techniques did not have the sub-micrometre resolution to make this kind of study possible. These new maps, however, were detailed enough to allow the origin of the microstructures to be identified. The deposition of layers of clay within the hydrothermal vent systems resulted in the formation of filamentous structures. Dissolved carbon in the vent fluids then adsorbed onto the edges and in between the clay layers, giving a false chemical signature of an organic origin.

An elemental map of a filamentous structure from the Apex Chert,
showing aluminium rich clay layers interspersed with bands of carbon
'It soon became clear that the distribution of carbon was unlike anything seen in authentic microfossils', said Dr Wacey. 'A false appearance of cellular compartments is given by multiple plates of clay minerals having a chemistry entirely compatible with a high temperature hydrothermal setting. We studied a range of authentic microfossils using the same transmission electron microscopy technique and in all cases these reveal coherent, rounded envelopes of carbon having dimensions consistent with their origin from cell walls and sheaths. At high spatial resolution, the Apex 'microfossils' lack all evidence for coherent, rounded walls. Instead, they have a complex, incoherent spiky morphology, evidently formed by filaments of clay crystals coated with iron and carbon.'

While it is sad to see these hallowed 'fossils' deposed, the late Martin Brasier had this to say: 'this research should, at long last, provide a closing chapter for the Apex microfossil debate. Such discussions have encouraged us to refine both the questions and techniques needed to search for life remote in time and space, including signals from Mars or beyond. It is hoped that textbooks and websites will now focus upon recent and more robust discoveries of microfossils of a similar age from Western Australia, also examined by us in the same article.' While the oldest known fossils are not 3.46 billion years old, we can be rest assured that lie is still incredibly ancient.