Solving the Controversy of Earth’s Oldest Fossils Using Electron Microscopy
David Wacey , 1 , 2 * Martin Saunders , 1 Charlie Kong , 3 and Martin Brasier 4 † 1 Centre for Microscopy , Characterisation and Analysis , T e University of Western Australia , 35 Stirling Highway , Perth , WA
6009 , Australia 2 School of Earth Sciences , University of Bristol , Queen’s Road , Bristol BS8 1RJ , UK 3 Electron Microscopy Unit , T e University of New South Wales , Kingsford , NSW 2053 , Australia 4 Department of Earth Sciences , University of Oxford , South Parks Road , Oxford OX1 3AN , UK ; † deceased
*
david.wacey@uwa.edu.au
Abstract: In the early 1990s it was claimed that “microfossils” in Apex chert were the remains of fi lamentous organisms resembling cyanobac- teria. Analysis of new material from the Apex chert “microfossil locality” using high spatial resolution electron microscopy has demonstrated that fi lamentous microstructures, previously thought to be Earth’s oldest microfossils, are in fact mineral artifacts comprising stacks of phyllosilicate grains onto which carbon later adsorbed. Rather than being fossilized microorganisms, we interpret the fi laments as resulting from the alteration and exfoliation of fl akes of mica, plus the redistri- bution of barium, iron, and carbon during repeated episodes of fl uid movement within a hydrothermal system.
Introduction
Most of our understanding of the evolution and early history of life on Earth comes from the fossilized remains of microorganisms. As one goes back in time, however, microorganisms take on more simple morphologies, and are often more poorly preserved, so it becomes extremely difficult to differentiate signs of life from co-occurring mineral artifacts. The most famous case in point is that of filamentous microstructures in 3.46 billion-year-old rocks (the “Apex chert”) from Western Australia. These filaments have been claimed to represent the oldest morphological evidence of life on Earth [ 1 – 3 ], but their biological nature has been questioned on numerous occasions [ 4 – 8 ], and this has led to one of the longest running and most controversial debates in palaeontology.
The Apex chert “microfossils” first entered the literature
in 1987 [ 1 ] and were described in detail in the early 1990s [ 2 , 3 ], where it was claimed that they were the remains of at least eleven different species of filamentous prokaryotes. During the 1990s these objects became entrenched in textbooks as the earliest evidence of life on Earth, were taken by many to signal an early evolution of cyanobacteria [ 9 ], and were used as classic examples of what ancient microfossils should look like when claims of life in the Martian meteorite ALH 84001 [ 10 ] were being rejected. With the technology available in the late 1980s, an interpretation of the filaments being the remains of primitive bacteria was reasonable; after all, the filaments are carbonaceous and appear to be segmented in a way that resembles chains of cells ( Figure 1 ). In 2002 the authenticity of the Apex chert microfossils was seriously challenged [ 4 ]. New geological mapping showed that the geological context of the microfossils had been misinterpreted. They do not occur in a sedimentary unit as previously claimed but within hydrothermal veins, a
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high-temperature environment much less conducive (but not impossible) for life. In addition, computer-aided montage images of the type microfossils showed clear evidence of branching (a trait not present in very primitive bacteria) and distribution of carbon around the margins of mineral crystals ( Figures 1 a and 1 e) [ 4 ]. It was also noted that the filaments do not exhibit biological behavior; instead they are solitary, irregular, and randomly orientated. A number of the filaments are rather light in color, with yellow, orange, and light brown examples [ 5 – 6 ]. This is in contrast to other reports of early Archean carbon that illustrate a dark brown to black color, perhaps casting doubt on their age. Furthermore, the claimed diversity of species is particularly vast, with comparable diversity not seen in the geological record until some 1.5 billion years later [e.g., 11 ], which makes it difficult to reconcile the microfossils with accepted evolutionary theory. T e combined evidence led to a new hypothesis that the microfossils had a geological rather than biological origin, that is, they were in fact carbonaceous mineral rims that formed around recrystallizing grain margins during a complex series of hydrothermal events [ 4 ]. For the past 13 years, the debate over these microfossils has been vigorous. Additional petrographic and geological mapping appears to support a mineral rim [ 5 – 6 ] or related non-biological [ 7 – 8 ] hypothesis, whereas more sophisticated analyses of the carbon in the microfossils using confocal laser Raman microspectroscopy and confocal laser scanning microscopy have produced data consistent with (but not uniquely attributable to) a biological formation mechanism [ 12 – 13 ]. T e main analysis problem has been one of a lack of spatial resolution. Previously it has not been possible to really get to the bottom of what these microstructures are made of, in particular how the carbon is distributed at the sub-micrometer scale, and whether this is compatible with cellular life. In this article we use a combination of focused ion beam (FIB) milling, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) to decode the nano-scale morphology and chemistry of the Apex fi laments and determine their origin (also see [ 14 – 15 ]).
Materials and Methods T e material studied here comes from the original “microfossil” locality described in [ 2 , 4 – 6 ]. Analysis of the
doi: 10.1017/S1551929515001170
www.microscopy-today.com • 2016 January
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