An Alternative Approach for Assessing Biogenicity

Abstract

The search for signs of life in the ancient rock record, extreme terrestrial environments, and other planetary bodies requires a well-established, universal, and unambiguous test of biogenicity. This is notably true for cellular remnants of microbial life, since their relatively simple morphologies resemble various abiogenic microstructures that occur in nature. Although lists of qualitative biogenicity criteria have been devised, debates regarding the biogenicity of many ancient microfossils persist to this day. We propose here an alternative quantitative approach for assessing the biogenicity of putative microfossils. In this theoretical approach, different hypotheses-involving biology or not and depending on the geologic setting-are put forward to explain the observed objects. These hypotheses correspond to specific types of microstructures/systems. Using test samples, the morphology and/or chemistry of these systems are then characterized at the scale of populations. Morphologic parameters include, for example, circularity, aspect ratio, and solidity, while chemical parameters could include elementary ratios (e.g., N/C ratio), isotopic enrichments (e.g., d13C), or chirality (e.g., molar proportion of stereoisomers), among others. Statistic trends distinguishing the different systems are then searched for empirically. The trends found are translated into ''decision spaces'' where the different systems are quantitatively discriminated and where the potential microfossil population can be located as a single point. This approach, which is formulated here on a theoretical level, will solve several problems associated with the classical qualitative criteria of biogenicity. Most importantly, it could be applied to reveal the existence of cellular life on other planets, for which characteristics of morphology and chemical composition are difficult to predict.

Ultra-small cells and DPANN genome unveiled inside an extinct vent chimney

“…We exclude the possibility that the small spheres are abiotic carbonaceous particles such as recently discovered particulate graphite (C) in hot and cold vent fluids 14 . In addition to the circularity and narrow size distribution of the spheres, the lack of sharp edges typically found in abiotic carbonaceous particles further supports the biogenicity of the small spheres 15 . Taken together, we conclude that the small spheres are ultra-small cells.…”

“…The gene prediction and annotation of the contigs and the near-complete genome were annotated using Prokka 54 and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway tool 55 with the Blast-KOALA tool 56 . Initial analyses of near-complete genomes from Crystal Gyser 15 were performed by ggKbase (https://ggkbase.berkeley.edu). For selected functional genes, closely related sequences and their source organisms were initially accessed using NCBI Protein BLAST Program 57 .…”

False biosignatures on Mars: anticipating ambiguity

“…A number of protocols and strategies have been devised to assess the credibility of candidate biosignatures (e.g. Buick 1990;Brasier and Wacey 2012;McLoughlin and Grosch 2015;Vago et al 2017;Neveu et al 2018;Rouillard et al 2021). Most of these schemes use multiple, nested criteria to assess biogenicity: did the object (or population of objects) form in a demonstrably habitable ( palaeo) environment, with appropriate evidence of endogeneity and syngenicity?…”

Dubiofossils from a Mars‐analogue subsurface palaeoenvironment: The limits of biogenicity criteria

“…To avoid such an impasse, more work is still needed to advance our understanding of abiotic mineral morphogenesis, microbial taphonomy and micro‐analytical paleobiology. The resulting data will inform the development of new, more definitive and robust protocols for biogenicity determination, for example, multiparametric statistical comparisons that show quantitatively whether particular dubiofossil assemblages more closely resemble true fossils or pseudofossils (Rouillard et al., 2021). In the meantime, we recommend that fossil‐like structures whose biogenicity cannot be determined should be reported as such, and not overinterpreted either as biological or non‐biological objects.…”

“…McLoughlin et al., 2010). In general, there are good reasons to doubt that “biogenicity criteria” can be reliably formulated on our present state of knowledge about the formation of lifelike microstructures in geochemical systems; more work in this area is needed to facilitate life‐detection in the rock records of early Earth and Mars (Garcia‐Ruiz et al., 2002; McMahon, 2019; Rouillard et al., 2021).…”