Fossils and astrobiology: new protocols for cell evolution in deep time

Abstract

The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.

Calcium phosphate preservation of faecal bacterial negative moulds in hyaena coprolites

“…On the other hand, experimental studies have successfully produced abiotic spherical apatite precipitates consisting of bundles of small crystallites (Busch et al 1999) very similar to the ones in our specimens. Any biogenic origin of these microspherulites therefore remains questionable (Brasier et al 2012). …”

Identifying microbial life in rocks: Insights from population morphometry

“…In these studies, the absolute range of sizes and the relative width of size distributions (quantified in our study by the mean/ SD parameter) were particularly discussed. Qualitatively, the similarity and continuity of shapes (shape distributions in our study) in a population were also proposed as a criterion of biogenicity (Brasier & Wacey, ; Buick, ; Schopf et al, ). However, as noted by Brasier and Wacey (), the morphospaces occupied by biological and relevant abiogenic populations have hardly, if ever, been compared quantitatively.…”

“…In order to answer these difficulties, specific criteria have been proposed to assess the biologic origin of potential microfossils in rocks (Brasier & Wacey, ; Buick, ; Schopf, ; Vago et al, ). These biogenicity criteria require that (a) microstructures occur in a sedimentary rock of constrained age, (b) are hollow and contain a mineral filling, (c) include traces of altered organics, (d) display a biologic‐like shape, and (e) occur spatially associated with similar specimens reflecting a biologic population.…”

Contributions of Professor Martin Brasier to the study of early life, stratigraphy and biogeochemistry

“…For example, their work led to the discovery of sulphurmetabolizing microfossils from the 3.4 Ga Strelley Pool Chert (Wacey et al 2011); the description of putative microbial mat fabrics in seafloor cherts of the 3.46 Ga Apex Chert described in this book (Hickman-Lewis et al 2016) and the hypothesis of Archean volcanic pumice as a possible prebiotic substrate for life (Brasier et al 2011b). Much of this early Archean research had implications for the emerging field of astrobiology, a discipline that excited Martin and inspired several of his studentsleading, for example, to work discussing the protocols for testing for signs of life in Martian samples Brasier & Wacey 2012) and to developing biogenicity criteria tailored for samples from both the early Earth and extraterrestrial materials (McLoughlin et al 2007). Page from Martin Brasier's notebook relating to his investigations of thin sections from the c. 3.5 Ga Dresser Formation in the North Pole Dome, Pilbara Craton, collected from the 'Awramik' site.…”