The main bases of Biology suggest that all extant organisms share a last common ancestor, namely the cenancestor. As soon as the comparison of molecular characters of organisms representative of the whole diversity of life became possible, hot debates emerged about the environmental conditions in which the cenancestor lived, its closeness to the origins of life, its single or community nature and its relationships with the three domains of life (Archaea, Bacteria and Eucarya). However, available information about current organisms was for a long time inadequate to precisely describe the biology of this organism. For instance, the observation that the main archaeal membrane components, called phospholipids, are synthesized by different means than their bacterial/eukaryotic counterparts was proposed to reveal that modern phospholipid biosynthesis pathways emerged late in independent lineages and were, therefore, absent in the cenancestor. This hypothesis argued that the cenancestor had no lipid membranes, so it could not be a cellular organism although other indirect clues indicated the opposite. These contradictions raise the question of the presence in modern organisms of traces that the cenancestor had a phospholipid biosynthesis machinery.In this dissertation, I took advantage from the recent accumulation of genomic data to address this issue. Previous work had shown that the members of two universal protein superfamilies could be present in the cenancestor to carry out the non-specific synthesis of the glycerol phosphate enantiomers that are the backbones of modern phospholipids. Bacterial and eukaryotic phospholipids use fatty acids whereas archaeal phospholipids are made up of isoprenoids. Thus, I studied the evolution of the metabolic pathways that synthesize these molecules and build up the phospholipids from their components. My results show that the eukaryotic isoprenoid biosynthesis pathway and a hypothetical archaeal fatty acid biosynthesis pathway are likely to have had less specific ancestors in the cenancestor. In addition, the phospholipid assembly machinery was also probably present in the cenancestor.These results suggest that the cenancestor was likely able to enzymatically synthesize its phospholipids by means less specific than modern ones. Dissimilarities in modern membrane phospholipids would result from the specialization of each biosynthesis system in each lineage. My work also stresses the fact that the cenancestor should be described on the basis of the comparison of modern organisms to avoid frequent confusions between the cenancestor and the origins of life.
