The Great Oxidation Event is thought to have had a profound impact on the evolution of bioenergetics on Earth. We aim at getting more insight regarding how bacteria adapted their energetic metabolism to this massive change. We focus on a family of molecules with a key role in respiratory chains of most living organisms: the isoprenoid quinones (hereafter simply referred to as quinones). In respiratory chains, quinones shuttle electrons and protons between proteins in the membrane. According to their mid-point redox potential, quinones can be classified as low potential (LP) such as menaquinone (MK) or high potential (HP) quinones such as ubiquinone (UQ). The traditional view is to consider that LP quinones are involved in anaerobic processes and that HP quinones emerged to cope with rising O2 levels. We believe that quinone biosynthetic pathways have captured capital information regarding the evolution of energy metabolisms. Several pathways can lead to the production of the same quinone, and enzymes from different pathways often share homologies. Our group recently showed that UQ, a HP quinone only present in Proteobacteria and Eukaryotes, is synthesized via two biosynthetic pathways: the classical O2-dependent one, and the O2- independent that uses a source of oxygen other than O2 (1). To reconstitute the evolutionary history of the quinone pathways, we combined several ap- proaches: 1) analysis of the pathways and enzymes distribution; 2) follow up of the dynamics of genetic architecture of the pathways; 3) phylogenetic approaches. Our first observations in Proteobacteria showed a widespread distribution of the O2-independent pathway across this phyla which indicates that it could have existed in the common ancestor of UQ-producing Proteobacteria. This would suggest that high O2 levels in the atmosphere were not a pre- requisite for the emergence of UQ (1). In addition, we recently showed that UQ produced by the O2-independent pathway can be used for anaerobic respiration (nitrate respiration in Pseudomonas aeruginosa) (2). Our systematic investigation of quinone biosynthetic path- ways across bacterial genomes allowed us to gather evidence to decipher their relative order of appearance and to get a better understanding of their respective role. Altogether, our study questions the relative timing between the appearance of HP quinones in the context of rising O2 levels. (1) L. Pelosi, et al., Ubiquinone Biosynthesis over the Entire O2 Range: Characterization of a Conserved O2-Independent Pathway, MBio. 10 (2019) 21. (2) C.-D.-T. Vo, et al., The O2-independent pathway of ubiquinone biosynthesis is essential for denitrification in Pseudomonas aeruginosa, Journal of Biological Chemistry. 295 (2020) 9021–9032. https://doi.org/10.1074/jbc.RA120.013748.