The rise of the urancestor. A geological timeline defined by a molecular clock of domain structure at FSF level is used to date the FSF repertoires of the urancestral sets. Oxygen levels are indicated as percentage of present day atmospheric levels (PAL) . Colored circles indicate FSF used for clock calibration. Black and red arrowheads labeled a and b indicate major and second transitions in ribosomal evolution, respectively , and lines indicate the appearance of FSFs associated with ribosomal proteins (table 2). Arrows show the discovery of crucial FSFs linked to membrane glycerol ester and ether lipid chemistries and sn1,2 and sn2,3 lineages. Time is given in billions of years (Ga).