RNA-based processes traceable to the Last Universal Common Ancestor.

Universal Rfam families that show evidence of vertical inheritance (Table S1 in Text S1) are all associated with the processes of translation (rRNAs, tRNAs, RNase P) and protein export (SRP RNA). A previous study examining the antiquity of protein coding genes [21] identified only 37 universally distributed proteins which show evidence of vertical inheritance. The majority of these vertically inherited proteins are associated with translation and protein export; numbers of such proteins associated with each of the depicted processes is given in grey (original data are from Harris [21]). The proteins associated with RNase P are not universally conserved, with archaeal and eukaryotic RNase P proteins being unrelated to their bacterial counterparts [72]. While tRNA synthetases are universal, they have undergone ancient horizontal gene transfer events [73], which complicates establishing the timing of their origin.