For three billion years, microorganisms were the only inhabitants of the earth

Abstract

Microorganisms were the sole inhabitants of our planet for almost 3 billion years. They have survived the intense geological upheavals that have marked the history of the Earth. They profoundly shaped their environment, thus participating in a true co-evolution between the biosphere and the geosphere. Through their activity, they also created favourable conditions for the emergence of multicellular aerobic organisms (particularly with an intense production of oxygen released into the atmosphere). Among past microorganisms, LUCA occupied a central position in the evolutionary history of life. The possible origin and thelarge uncertainties about the nature of LUCA are discussed: where and when did LUCA live? Was it a hyperthermophilic, thermophilic or mesophilic organism? How did its genome look like? Scenarios and hypotheses regarding the emergence and the relationships of the three domains of life – Archaea, Bacteria and Eucarya – as well as the transition from a prokaryotic to eukaryotic cell organisation are discussed in the light of the most recent data. Possible major steps in the evolution of microorganisms are deduced from genomic investigations and from the geological record (fossils, isotopic ratios, biomarkers). Although the early steps of microbial metabolic evolution are still hotly debated, it is possible to speculate on the occurrence of the first living entities, from the primordial metabolisms to the advent of photosynthesis.