Pilbara Microfossils

Perhaps the most well known report of microfossil evidence for earliest life comes from the 3,460 Ma 'Apex chert'. These Apex 'microfossils' (e.g., Fig. 2f) were described in detail in a series of high profile papers (Schopf and Packer, 1987; Schopf, 1992a, b, 1993; Schopf et al., 2002) and became the benchmark for bona fide early Archean 'microfossils', the only ones to gain wide acceptance by the scientific community during the 1980s and 1990s (e.g., Buick, 1990; Knoll and Walter, 1996; McClendon, 1999; Schopf, 1999). The eleven putative species of microfossils described in those papers are from 700 to 1,000 million years older than putative cyanobacterial biomarkers (Summons et al., 1999), genomic arguments for dating the appearance of cyanobacteria (Hedges et al., 2001) and an oxygenic atmosphere (Catling et al., 2001). Whilst the similarities between the Apex 'microfossils' and other more primitive bacteria were acknowledged, the morphology and size range of the supposed cells has been taken to suggest that oxygen-releasing cyanobacteria may have been present at least 3,460 Ma ago (Schopf, 1992a, b, 1993, 1999). For almost two decades, many of the key issues concerning the evolution of the early Earth hinged on this one discovery. For example, if these 'Apex chert' microfossils are accepted then they not only imply an early start for the contribution of photosynthetic oxygen to the atmosphere, they also imply that high levels of biological diversity were achieved at a very early stage in Earth history (Schopf, 1993), remarkably soon after the end of massive meteoritic bombardment of the inner solar system at -3,800-3,900 Ma (cf. Kamber et al., 2001). A period of relative evolutionary stasis would then be required, as there is little evidence for further diversification in the fossil record until the emergence of widespread eukaryotes nearly 2 billion years later (Knoll, 1994, 2003).

Such important claims clearly require watertight evidence. Unfortunately, the veracity of these reports has come into question (Brasier et al., 2002, 2005, 2006; Garcia Ruiz et al., 2003). Brasier et al. (2002, 2005, 2006) re-examined both the geological context and morphology and geochemistry of 'Earth's oldest microfossils' and came to some surprising conclusions. The 'microfossils' are not in a seafloor chert as originally reported, but rather come from a chert breccia that lies some 100 m down a hydrothermal dyke system and well below the palaeo-sea floor. The microfossil-like structures are not confined to early formed clasts. They also occur in recrystallized, late stage hydrothermal fabrics. The 'microfossils' are chaotic and incoherent, not simple and unbranched as previously reported, and the 'stromatolite clasts' are in fact botryoidal cavity infills. In addition, Brasier et al. (2006) found no correlation between inferred 'cell shape', filament diameter and taxon-specific terminal cell morphology. Instead, they found that filament shape, septa and subdivisions could be best explained as self organizing structures resulting from silica re-crystallization of glass to spherulitic chalcedony that caused displacement of amorphous carbonaceous matter towards spherulitic margins. This created a morphological spectrum of arcuate to dendritic microstructures (see Brasier et al., 2006, Fig. 2) that includes microfossil-like artefacts and must therefore lead to the rejection of the biological nature of these putative fossils. Structures that resemble these supposed microfossils have also been produced abiogenically in the laboratory by Garcia Ruiz et al. (2003).

With these cautionary findings in mind, we go on to discover that other claims of putative microfossils from the Pilbara area have also been re-examined, and similar concerns about their biogenicity and antiquity have been expressed. For example, minute spheroids were described from the chert barite unit of the Dresser Formation (Dunlop et al., 1978) whose size distribution and kerogenous composition were used to infer biogenicity. However, both the syngenicity and biogenicity of these structures were questioned and they were re-interpreted as either viscous bitumen droplets in secondary megaquartz and chalcedony laminae (Buick, 1990) or simple mineralic non-biogenic spheroids (Awramik et al., 1983). Tubular filaments were also described from the Dresser Formation (Awramik et al., 1983; Buick, 1984) and were initially interpreted as being bio-genic based upon their morphology, carbonaceous composition and orientation within the rock, distinguishing them from more angular, crystalline micro-pseudo-fossils found in the same rock unit. On close examination they are remarkably similar to the 'Apex chert microfossils'. This means that an origin as self organizing abiogenic structures must also be considered for these structures. In addition, an origin as ambient inclusion trails (AIT's) has also been advanced (Buick, 1990). AIT's are formed by the propulsion of a mineral grain, often pyrite, through a partially or totally lithified chert substrate. They were once thought to form purely abiogenically (Tyler and Barghoorn, 1963) but Knoll and Barghoorn (1974) highlighted the occurrence of organic matter in association with the AIT's and hypothesized that the propulsion of the mineral grains through the chert host is aided by decomposition of the biological matter, producing a gaseous driving force and acidic products that can etch the chert. Although the structures described by Awramik et al. (1983) and Buick (1984) are unlikely to be filamentous microfossils, it is possible there may be a biogenic component to their formation via this organically mediated AIT hypothesis. Further examples of filamentous structures found in the North Pole area by Ueno and co-workers (Ueno et al., 2001) have not been re-examined independently and still await a consensus on their biogenicity.

Perhaps more robust microfossil evidence for early life may come from the rather younger Sulphur Springs Group, a sequence of komatiites, basalts, dacites and rhyolites erupted at about 3,240 Ma (Van Kranendonk, 2006). Pyritic filaments (Fig. 2c) from a 3,235 Ma deep sea, volcanogenic, massive sulfide deposit were reported by Rasmussen (2000) and interpreted as the fossilized remains of thread like thermophilic, chemotrophic prokaryotes. The filaments are 0.5-2.0 |im in width and up to 300 |im long, can be straight, curved or sinuous and exhibit putative biological behaviour including preferred orientations, clustering and intertwining. They come from a subsurface drill core and only occur in the para-genetically early chert and coarse grained quartz that are cross cut by later fractures. This report appears to conform both to the criteria for syngenicity and many of the criteria for biogenicity. It awaits independent verification and supporting lines of geochemical evidence. Even so, it is an intriguing discovery that appears broadly consistent with the hypothesis of a thermophilic origin for life in the vicinity of sub-marine hydrothermal vents. It is, perhaps, the most convincing of the case histories that we have reviewed thus far.

0 0

Post a comment