History of protein domain structures. A. The Venn diagram shows distribution of FSFs in the taxonomic groups. Viral families included in the analysis: Adenoviridae, Ascoviridae, Asfarviridae, Corticoviridae, Iridoviridae, Marseilleviridae, Mimiviridae, Phycodnaviridae, Poxviridae, Rudiviridae, and Tectiviridae (see Table 2). B. Phylogenomic tree of protein domain structure describing the evolution of 1,739 FSFs in 1,037 proteomes (4,63,915 steps; consistency index CI = 0.051; retention index RI = 0.795; tree skewness g1 = −0.127). Taxa are FSFs and characters are proteomes. Terminal leaves were not labeled, as they would not be legible. C. Distribution index (f, the number of species using an FSF/total number of species) of each FSF plotted against relative age (nd, number of nodes from the root/total number of nodes) for the four supergroups and individually for sampled viruses, Archaea, Bacteria, and Eukarya. D. Boxplots displaying distribution of FSFs in viral and cellular taxonomic groups with respect to age (nd). Vertical lines within each distribution represent group median values. Dotted vertical lines represent important evolutionary events in the evolution of proteomes.