There are many missions (and future missions) searching for extraterrestrial life in the Solar System and on exoplanets. The stakes are high with enormous consequences ensuing bona fide detections – for the researchers involved, for the decision makers who fund research and/or space missions, as well as for the general public. While exoplanet research is particularly geared to traces of evolved life, ranging from organisms having developed some form of photosynthesis to SETI and technosignatures, Solar System studies are more “modest” and realistic in the sense that we know we are looking for life resembling (but not identical to) terrestrial anaerobic microorganisms.

Evolved life on Earth (from oxygenic phototrophs onward) co-evolved with a geological and environmentally changing world that has no counterparts in the Solar System (unless some form of geological evolution took place on Venus). The early Earth, on the other hand, was relatively simple geologically speaking: mostly ocean planet, paucity of emergent landmasses, mostly CO2 atmosphere, environment dominated by volcanism and hydrothermal activity etc. When life emerged on Earth more than 4 billion years ago (Ga), and for about a billion years, it was totally controlled by geological processes. The appearance of anoxygenic photosynthesis and the gradual instauration of global plate tectonics leading to the formation of continental landmasses paved the way for the greater influence of life on the environment and geological processes.

However, apart from possibly Venus, plate tectonics did not occur on other bodies in the Solar System and technologies are not yet at a stage to detect it on exoplanets. What was life like on the very early Earth before the emergence of photosynthesis?
I will show our research of nearly 30 years into some of the oldest traces of life, concentrating specifically on chemotrophs, organisms that obtain their energy from oxidation of inorganic substances (chemoautolithotrophs) or from oxidation of pre-existing organic matter (chemoorganotrophs), as examples of the kinds of primitive life forms that might inhabit or have inhabited planets or icy satellites in the Solar System. Chemolithoautotrophs are believed to have been the first forms of life on Earth and microorganisms using similar kinds of metabolisms are likely to be the most common in the Universe – they do not leave detectable atmospheric traces.

In particular, I will underline the challenges of studying their fossilised remains to prove both biogenicity and syngenicity, even here on Earth where we have access to unlimited and highly sophisticated equipment. As a fore taste, our recent PhD student Laura Clodoré has just published a paper detailing the painstaking steps necessary for such studies: Clodoré, L. et al., 2024. Multi-Technique Characterization of 3.45 Ga Microfossils on Earth: A Key Approach to Detect Possible Traces of Life in Returned Samples from Mars. Astrobiology 24. Published Online: 20 Feb 2024, https://doi.org/10.1089/ast.2023.0089.