We want to thank again our commentators for their stimulating ideas and insightful comments that we are sure helped to clarify and develop our ideas. Authors acknowledge funding from the European Research Council (ERC) under the European Union's Seven Framework Program grant agreement n∘ 340863 (JMG-R) and under the Horizon 2020 research and innovation program grant agreement n∘ 646894 (MvZ). JMG-R also acknowledges the Ministerio de Economía y Competitividad of Spain for funding the project CGL2016-78971-P . This is IPGP contribution n∘ 4161 .
[EN] The search for signs of life in the ancient rock record, extreme terrestrial environments, and other planetary bodiesrequires a well-established, universal, and unambiguous test of biogenicity. This is notably true for cellular remnantsof microbial life, since their relatively simple morphologies resemble various abiogenic microstructures that occur innature. Although lists of qualitative biogenicity criteria have been devised, debates regarding the biogenicity of manyancient microfossils persist to this day. We propose here an alternative quantitative approach for assessing thebiogenicity of putative microfossils. In this theoretical approach, different hypotheses—involving biology or not anddepending on the geologic setting—are put forward to explain the observed objects. These hypotheses correspond tospecific types of microstructures/systems. Using test samples, the morphology and/or chemistry of these systems arethen 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 differentsystems are then searched for empirically. The trends found are translated into ''decision spaces'' where the differentsystems are quantitatively discriminated and where the potential microfossil population can be located as a singlepoint. This approach, which is formulated here on a theoretical level, will solve several problems associated with theclassical qualitative criteria of biogenicity. Most importantly, it could be applied to reveal the existence of cellular lifeon other planets, for which characteristics of morphology and chemical composition are difficult to predict. ; The Ministerio de Economia y competividad of Spain for funding the project CLG2016-78971-P. This project has received funding from the EuropeanResearch Council, under the European Union's Horizon2020 research and innovation programme (grant agreementno. 694894) and the Seventh Framework Programme-FP7/2007-2013 (grant agreement no. 340863) ; Peer reviewed
Archean hydrothermal environments formed a likely site for the origin and early evolution of life. These are also the settings, however, were complex abiologic structures can form. Low-temperature serpentinization of ultramafic crust can generate alkaline, silica-saturated fluids in which carbonate–silica crystalline aggregates with life-like morphologies can self-assemble. These "biomorphs" could have adsorbed hydrocarbons from Fischer–Tropsch type synthesis processes, leading to metamorphosed structures that resemble carbonaceous microfossils. Although this abiogenic process has been extensively cited in the literature and has generated important controversy, so far only one specific biomorph type with a filamentous shape has been discussed for the interpretation of Archean microfossils. It is therefore critical to precisely determine the full distribution in morphology and size of these biomorphs, and to study the range of plausible geochemical conditions under which these microstructures can form. Here, a set of witherite-silica biomorph synthesis experiments in silica-saturated solutions is presented, for a range of pH values (from 9 to 11.5) and barium ion concentrations (from 0.6 to 40 mmol/L BaCl). Under these varying conditions, a wide range of life-like structures is found, from fractal dendrites to complex shapes with continuous curvature. The size, spatial concentration, and morphology of the biomorphs are strongly controlled by environmental parameters, among which pH is the most important. This potentially limits the diversity of environments in which the growth of biomorphs could have occurred on Early Earth. Given the variety of the observed biomorph morphologies, our results show that the morphology of an individual microstructure is a poor criterion for biogenicity. However, biomorphs may be distinguished from actual populations of cellular microfossils by their wide, unimodal size distribution. Biomorphs grown by diffusion in silica gel can be differentiated by their continuous gradient in size, spatial density, and morphology along the direction of diffusion. ; This project has received funding from the European Research Council(ERC) under the European Union's Horizon 2020 research and inno -vation programme (grant agreement nº 646894) and under the ERC Seventh Framework Programme FP7/2007-2013 (grant agreementn° 340863). JMG-R also acknowledges the Ministerio de Economía y Competitividad of Spain through the project CGL2016-78971-P. We acknowledge the analytical platform PARI and Stefan Borenstazjnfor SEM imaging. Prof. Y. Tsukii and The Protist Information Server (http://protist.i.hosei.ac.jp/) are acknowledged for the use of picturesof cyanobacteria. This is IPGP contribution n° 3912. We are gratefulto two anonymous reviewers for their helpful comments
In this study, we investigate the molecular structural characteristics of organic remains in various cellular organelles from a 180 Ma Jurassic royal fern belonging to the Osmundaceae family of ferns, and compare their carbon isotopic compositions to a now-living species of royal fern (Osmunda regalis). We discovered molecular structural variations indicated by Raman and infrared spectral parameters obtained from various fossilized cellular organelles. The organic remains preserved in the chromosomes and cell nuclei show marked structural heterogeneities compared to the cell walls during different stages of the cell cycle. The fossil and extant fern have similar δ13C values obtained from bulk samples, supporting evolutionary stasis in this plant lineage and an unchanged metabolic pathway of carbon assimilation since the Jurassic. The organic remains in the cellular organelles of the fossil seem to be less heterogeneous than those in the extant fern, likely due to the preferential preservation of certain cellular compounds during fossilization. Taphonomic processes appear to have diminished the subcellular isotopic heterogeneities. Our research sheds light on the functioning of ancient plant cellular organelles during mitosis, provides insights to the taphonomic processes operating at molecular and isotopic levels, and shows the practicability of in situ techniques in studying the evolution and behaviors of ancient cells. ; This project is also financially supported by the National Key R&D Program of China (2018YFC0309800), the Hundred Talent Program C of China (Y810011BRC), the Swedish Research Council (VR 2015-04264), the Bergen Research Foundation (Norway), the European Research Council under the European Union's Horizon 2020 research and innovation program (grant agreement 646894), and the Department of Science and Technology–National Research Foundation Centre for Excellence in Palaeosciences at the University of Witwatersrandthe (South Africa). The NordSIM laboratory at the Swedish Museum of Natural History is a Vetenskapsrådet (VR)-funded research infrastructure.
The identification of cellular life in the rock record is problematic, since microbial life forms, and particularly bacteria, lack sufficient morphologic complexity to be effectively distinguished from certain abiogenic features in rocks. Examples include organic pore-fillings, hydrocarbon-containing fluid inclusions, organic coatings on exfoliated crystals and biomimetic mineral aggregates (biomorphs). This has led to the interpretation and re-interpretation of individual microstructures in the rock record. The morphologic description of entire populations of microstructures, however, may provide support for distinguishing between preserved micro-organisms and abiogenic objects. Here, we present a statistical approach based on quantitative morphological description of populations of microstructures. Images of modern microbial populations were compared to images of two relevant types of abiogenic microstructures: interstitial spaces and silica–carbonate biomorphs. For the populations of these three systems, the size, circularity, and solidity of individual particles were calculated. Subsequently, the mean/SD, skewness, and kurtosis of the statistical distributions of these parameters were established. This allowed the qualitative and quantitative comparison of distributions in these three systems. In addition, the fractal dimension and lacunarity of the populations were determined. In total, 11 parameters, independent of absolute size or shape, were used to characterize each population of microstructures. Using discriminant analysis with parameter subsets, it was found that size and shape distributions are typically sufficient to discriminate populations of biologic and abiogenic microstructures. Analysis of ancient, yet unambiguously biologic, samples (1.0 Ga Angmaat Formation, Baffin Island, Canada) suggests that taphonomic effects can alter morphometric characteristics and complicate image analysis; therefore, a wider range of microfossil assemblages should be studied in the future before automated analyses can be developed. In general, however, it is clear from our results that there is great potential for morphometric descriptions of populations in the context of life recognition in rocks, either on Earth or on extraterrestrial bodies. ; European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program, Grant/Award Number: 646894; ERC Seventh Framework Programme FP7/2007-2013, Grant/Award Number: 340863; Ministerio de Economía y Competitividad of Spain, Grant/Award Number: CGL2016-78971-P
The identification of cellular life in the rock record is problematic, since microbial life forms, and particularly bacteria, lack sufficient morphologic complexity to be effectively distinguished from certain abiogenic features in rocks. Examples include organic pore-fillings, hydrocarbon-containing fluid inclusions, organic coatings on exfoliated crystals and biomimetic mineral aggregates (biomorphs). This has led to the interpretation and re-interpretation of individual microstructures in the rock record. The morphologic description of entire populations of microstructures, however, may provide support for distinguishing between preserved micro-organisms and abiogenic objects. Here, we present a statistical approach based on quantitative morphological description of populations of microstructures. Images of modern microbial populations were compared to images of two relevant types of abiogenic microstructures: interstitial spaces and silica–carbonate biomorphs. For the populations of these three systems, the size, circularity, and solidity of individual particles were calculated. Subsequently, the mean/SD, skewness, and kurtosis of the statistical distributions of these parameters were established. This allowed the qualitative and quantitative comparison of distributions in these three systems. In addition, the fractal dimension and lacunarity of the populations were determined. In total, 11 parameters, independent of absolute size or shape, were used to characterize each population of microstructures. Using discriminant analysis with parameter subsets, it was found that size and shape distributions are typically sufficient to discriminate populations of biologic and abiogenic microstructures. Analysis of ancient, yet unambiguously biologic, samples (1.0 Ga Angmaat Formation, Baffin Island, Canada) suggests that taphonomic effects can alter morphometric characteristics and complicate image analysis; therefore, a wider range of microfossil assemblages should be studied in the future before automated analyses can be developed. In general, however, it is clear from our results that there is great potential for morphometric descriptions of populations in the context of life recognition in rocks, either on Earth or on extraterrestrial bodies. ; This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program (grant agreement no. 646894 to MVZ) and under the ERC Seventh Framework Programme FP7/2007‐2013 (grant agreement no. 340863 to JMG‐R). JMG‐R also acknowledges the Ministerio de Economía y Competitividad of Spain through the project CGL2016‐78971‐P. We are grateful to seven anonymous reviewers for their helpful comments. We thank four anonymous reviewers for their useful comments which greatly improved a previous version of the manuscript. This is IPGP contribution n°4098. ; Peer reviewed
