(A) Summary of current knowledge about CMGC group kinase specificity in the context of the maximum likelihood phylogeny of protein sequences. Major groups, such as IME2, MAK, ICK, etc, have been collapsed for simplicity. Branch lengths express the number of amino acid substitutions per protein sequence site. Branch support values are approximate likelihood ratios (aLRs), expressing the ratio of the likelihood of the maximum likelihood phylogeny to the next-best phylogeny lacking the indicated branch. For example, an aLR value of 10 indicates that the branch is 10 times more likely than the next-best phylogenetic hypothesis. The position of the common ancestor of CDK, MAPK, CDKL, GSK, and the IME2/LF4/RCK superfamily (AncCMGI), is indicated by a purple circle. (B) Position scanning peptide libraries were used to determine the primary specificity of the maximum likelihood resurrected AncCMGI kinase. Raw peptide data are shown in Figure 2—figure supplement 1. A complete repeat of the PSPL for a Bayesian sampled alternative reconstruction of AncCMGI (AncCMGI-B2) is shown in Figure 2—figure supplement 2. A structural model of AncCMGI is shown in Figure 2—figure supplement 3 in phylogenetic context along with structures and models for extant kinases that were derived from this ancestor.