Distribution of the duplication times of the human orthologs of 195 CDY/CD groups calculated by the molecular clock approach (red line). The times were averaged over intervals of 50 Myr. The mean value of 91% of all duplication times is at 488 Myr. Cephalochordates have diverged from vertebrates around 650 Mya (see Suppl. Fig. S2 and Table S5). The duplication time measured as the expected number of nucleotide substitutions per codon (blue line) follows the distribution of the molecular clock duplication times. The distribution of synonymous substitutions per synonymouse site (the top right corner of the plot) shows 3 peaks, at 4 (the majority of the values of 1414 pairs of human genes), at 2 and at 0–0.2.