Figure 4: Schematic evolution of cellular membranes.Distribution of proteins involved in phospholipid biosynthesis in the three domains, LUCA, and LACA are depicted according to Lombard et al., [18, 22], but my evolutionary interpretation is different from those proposed by these authors [22]. Bacterial/eukaryal type membranes are in green and archaeal type membrane in red. Green circles correspond to universal enzymes involved in phospholipid biosynthesis (glycerol phosphate dehydrogenases, enzymes linking polar head groups to glycerol). Pink circles correspond to the classical mevalonate pathway for isoprenoid biosynthesis that was probably present in LUCA and lost in bacteria. Red circle represents enzymes of the alternative mevalonate pathway, which are specific of archaea, and involves a mixture of eukaryotic-like and archaeal specific enzymes. Blue circles correspond to the nonhomologous methylerythritol pathway for isoprenoid biosynthesis present in bacteria. Black circles represent the fatty acid biosynthetic pathway, which is no longer used for membrane phospholipid biosynthesis in archaea. Circles corresponding to proteins involved in the biosynthesis of membrane phospholipids are encircled.