A fresh look at the fossil evidence for early Archaean cellular life

Abstract

The rock record provides us with unique evidence for testing models as to when and where cellular life first appeared on Earth. Its study, however, requires caution. The biogenicity of stromatolites and 'microfossils' older than 3.0 Gyr should not be accepted without critical analysis of morphospace and context, using multiple modern techniques, plus rejection of alternative non-biological (null) hypotheses. The previous view that the co-occurrence of biology-like morphology and carbonaceous chemistry in ancient, microfossil-like objects is a presumptive indicator of biogenicity is not enough. As with the famous Martian microfossils, we need to ask not 'what do these structures remind us of ?', but 'what are these structures?' Earth's oldest putative 'microfossil' assemblages within 3.4-3.5 Gyr carbonaceous cherts, such as the Apex Chert, are likewise self-organizing structures that do not pass tests for biogenicity.There is a preservational paradox in the fossil record prior to ca 2.7 Gyr: suitable rocks (e.g. isotopically light carbonaceous cherts) are widely present, but signals of life are enigmatic and hard to decipher. One new approach includes detailed mapping of well-preserved sandstone grains in the ca 3.4 Gyr Strelley Pool Chert. These can contain endolithic microtubes showing syngenicity, grain selectivity and several levels of geochemical processing. Preliminary studies invite comparison with a class of ambient inclusion trails of putative microbial origin and with the activities of modern anaerobic proteobacteria and volcanic glass euendoliths.

Curiosity’s Mars Hand Lens Imager (MAHLI) Investigation

“…If this interpretation is correct, then strata in the lower part of the mountain would date to a time corresponding with the earliest-and sometimes disputed-records of life on Earth (Mojzsis et al 1996;Altermann and Kazmierczak 2003;Brasier et al 2006;Schopf 2006).…”

Lifeness signatures and the roots of the tree of life

“…Unlike in some of its upper branches, populating the tree of the life at its base remains highly speculative: the fossil record is scarce and spurs lasting controversies when it comes to interpreting minute structures or molecular compounds that might, or not, be remains of primitive forms of life (e.g. Schopf 2006;Brasier et al 2006). It is even possible that no fossil older than 3.7 billion years will ever be found.…”

Facula, Faculae

“…For example, a combination of infrared spectroscopy (providing an aliphaticity index of the organic matter) and microscale measurements of the carbon isotopic compositions was used by Sangely et al (2007) to distinguish between biology and Fischer-Tropsch-type reactions as genetic processes for the bitumen found in the Cretaceous uranium deposits of Athabasca. Known abiotic products that can mimic life morphologies or chemistries include vesicles made in the laboratory from meteoritic kerogen or in other prebiotic chemistry experiments (e.g., Deamer et al 2006), fluid inclusions, carbonaceous filamentous shapes resulting from migrating organic matter (with carbon isotopic fractionation resembling life patterns) around minerals casts in hydrothermal environments (Brasier 2005;Brasier et al 2006), aggregates of silica spheres and rods in silica-rich waters of hydrothermal springs, migration of carbonaceous materials along microfractures (VanZuilen et al 2007), within or around silica (e.g., Jones and Renaut 2007;Lepot et al 2009b). Finally, mineralized pseudo-fossils have been produced using a mixture of barium carbonate and silica in laboratory experiments (Garcia-Ruiz et al 2003).…”