Earth’s Oldest Fossils
us in bona fide fossil microbes from younger rocks [ 16 , 19 ]. Carbon interleaved between phyllosilicates within the filaments may resemble “cellular compartment walls” when investigated with lower spatial resolution (for example, in light microscopy or Raman work [ 12 – 13 ]). Our higher spatial resolution analysis of supposed “cellular compart- ments” instead reveals very inconsistent compartment lengths (from <50 nm up to about 1 μ m), with length/width ratios that match crystal growth patterns and are unlike any known microbial cells.
Three-dimensional FIB-SEM data reveal further complexities to the filaments and additional insights into carbon distribution in their vicinity ( Figure 4 ). These data demonstrate how the morphology of the filaments changes quite significantly over spatial scales of only a few micrometers along the length of a filament. In some FIB slices, their filamentous nature is clear, and books of phyllosilicate crystals appear neatly stacked, whereas in other slices the filaments are seen to branch or suddenly thicken ( Figure 4 ). Furthermore, SEM highlights a number of nano-cracks within the chert matrix; these often feed right into the filaments and are filled with black material. This black material was identified as carbon from X-ray maps, but the raw data for Figure 5 comes from SEM morphology images, which can be acquired much more quickly than X-ray maps. The 3D reconstruction and visualization of the carbon from Apex filaments ( Figure 5 ) reveals additional features that are essentially hidden in 2D images. These include branches extending below the main body of filaments ( Figure 5c ) and patches of carbon outside the main body of filaments ( Figure 5e ), as well as emphasizing the rather linear, sheet-like nature of much of the carbon sandwiched between the phyllosilicate grains ( Figure 5f ). Like the TEM data, the 2D and 3D SEM data are incompatible with these filaments being fossils of filamentous organisms.
Discussion
Figure 4 : Variation in morphology of an Apex fi lament. BSE SEM images of 3 out of 200 successive FIB-milled slices through a fi lament. The space between individual slices is 200 nm, hence slice 89 is 12 μ m further along the fi lament from slice 29, and slice 119 is 6 μ m further along from slice 89. Uniform mid-gray is the quartz matrix, slightly lighter gray sheet-like material stacked together in complex book-like patterns is the phyllosilicates, black is carbon, and the brightest white is an Fe-rich mineral. Carbon coats many of the individual phyllosilicate sheets in the book-like stacks. Carbon also occurs in a number of nano-cracks that, in places, join the fi laments. The morphology of the fi lament changes considerably along its length, and it branches at depth below the surface of the thin section (see slice 89). The morphology and distribution of carbon are incompatible with a biological origin for the fi lament.
2016 January •
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These new electron microscopy data clearly show that the carbon distribution in the Apex filaments is not comparable with true cellular morphology. This means that an alternative explanation must be sought for the origin of these filaments. Our explanation builds upon previous theories of a non-biological mineral origin [ 4 – 6 ], but we are now able to be much more specific with our model. A three- stage model is now put forward: (1) hydration of mica flakes (abundant in the surrounding rocks) during widespread hydrothermal activity resulting in vermiculite-like phyllo- silicate formation; (2) continued heating plus expulsion of water from phyllosilicate crystal lattices, causing exfoliation (i.e., accordion-like expansion at right angles to the cleavage plane) and creating the initial worm-like filamentous morphological expression of microfossil-like artifacts; and (3) adsorption of later hydrocarbons (and locally additional iron) onto the phyllosilicate grains, mimicking cell walls. It is notable that exfoliated vermiculite has high adsorption capacity for hydrocarbons resulting from the strong capillary
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