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., δ13C), 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.
Highlights
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
Several of the early reported Archean microfossils were later reinterpreted as abiotic features [see review in Schopf (1983)], and some of the more recent claims of discovery of ancient microfossils—such as those occurring in the wellstudied Apex Chert, Pilbara, Western Australia, or in the Nuvvuagittuq Greenstone Belt, Quebec—are still extensively debated (Brasier et al, 2002; Schopf et al, 2002, 2018; Garcıa-Ruiz et al, 2003; Schopf and Kudryavtsev, 2009; Marshall et al, 2011; Wacey et al, 2015; Dodd et al, 2017; McMahon, 2019)
It is the probability that the specific object considered is a true cellular remnant, which is always higher: a potential microfossil is being evaluated because it has been selected as a promising candidate
Summary
A large number of scientific studies have explored the boundaries of life, be they environmental [extreme settings ( Jannasch, 1985; Belilla et al, 2019)], temporal [ancient rocks (Noffke et al, 2006; Sugitani et al, 2006)], or spatial [extraterrestrial environments (Oyama et al, 1976; Des Marais et al, 2008; Schwieterman et al, 2018)] In these different contexts, the search for life relies on the detection of biosignatures, that is, features characteristic of life: living organisms, the evidence of life activity in its environment, or fossils of past organisms and traces of their past activity (Des Marais et al, 2008). It must be possible to assess its biogenicity
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