Poster Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Electron microscopy solves the controversy of earth’s oldest fossils (#274)

David Wacey 1 , Martin Saunders 1 , Charlie Kong 2 , Martin Brasier 3
  1. Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, WA, Australia
  2. Electron Microscope Unit, The University of New South Wales, Kensington, NSW, Australia
  3. Oxford University, Oxford, Oxfordshire, UK

Introduction: Filamentous microstructures from 3.46 billion year-old rocks in WA have been interpreted as remnants of Earth’s oldest cellular life, but the biological nature of these filaments has been robustly questioned leading to one of the longest running and most controversial debates in palaeontology.

Methods: TEM wafers were prepared using focused ion beam milling (FIB) on FEI Nova NanoLab and FEI Helios NanoLab G3 instruments. TEM data were obtained using a FEI Titan G2 80-200 TEM/STEM with ChemiSTEM Technology operating at 200 kV. 3D-FIB-SEM nano-tomography was carried out on a Zeiss Auriga Crossbeam instrument, with data processed using SPIERS and AVIZO software.

Results: TEM and SEM analyses reveal filament morphologies that are characteristic of a mineralic origin, with complex nano-scale intergrowths of mineral phases. Filaments comprise multiple plate-like grains of vermiculite, interspersed with quartz. Morphological features previously cited as evidence for cell compartments are shown to be carbon-coated stacks of vermiculite crystals. Carbon interleaved between mineral grains within the filaments may resemble ‘cellular compartment walls’ when investigated with lower spatial resolution but our EM analysis of supposed ‘cellular compartments’ instead reveals very inconsistent compartment lengths (<50 nm up to c. 1 micron), with length/width ratios that match crystal growth patterns and are unlike any known microbial cells. Carbon also frequently occurs at quartz grain boundaries and in micro-fractures away from the filaments, indicating significant redistribution of carbon in the vicinity of Apex filaments, in marked contrast to patterns found in bona fide fossils from younger rocks. 3D filament reconstructions reveal non-rounded cross sections, extreme changes in diameter and examples of branching incompatible with a filamentous prokaryotic origin.

Conclusion: At the nano-scale, the Apex filaments exhibit no biological morphology nor bear any resemblance to younger bona fide carbonaceous microfossils.  Instead, these microstructures likely formed during fluid-flow events within a hydrothermal system.