Importance of Prokaryotes in the Functioning and Evolution of the Present and Past Geosphere and Biosphere

On a volcanic and anaerobic planet characterized by abundant hydrothermal activity, physicochemical gradients and disequilibria at the local scale would have been fundamental for the emergence of life on Earth. Unfortunately, the early rock record pertaining to this existential process no longer exists, and, while chemists attempt to recreate life in a test tube, two other approaches can provide some information about early life and its metabolic processes. In the first place, phylogenetic, geological, thermodynamic, and microbiological settings suggest that disproportionation of reduced sulfurous compounds might have been essential for microbial evolution by delivering both sulfide and sulfate on Earth’s surface. These processes would have been fueled by serpentinization reactions in hydrothermal systems. Another approach is to study ancient and modern microbialites in order to better understand primitive microbial metabolic traits that occurred more than 3 billion years ago. The combination of all of these approaches to understanding early terrestrial life is of relevance to the emergence of life on other planets and satellites in the solar system, especially, for example, Mars.