Phylogenetic tree of eukaryotic organisms based on Set8 orthologs. This tree based on Set8 shows a phylogenetic incongruence with respect to trees recovered by broad scale eukaryotic phylogenomics. Displayed in blue is the animal clade of Set8 denoted by a strongly supported node (large arrow). Apicomplexan Set8 groups within the metazoan/animal clade with strong posterior probability support that is unexpected given accepted eukaryotic relationships. Set8 is absent in other lower eukaryotes based on current genome sequences. Where the closest Set8 hits were found, the proteins group together in the yellow shaded box (with strong support). These are likely not Set8 as Tetrahymena, a representative ciliate known to lack Set8, is featured in this clade. Based on these observations, it is likely that the apicomplexan Set8 is derived from animals. A horizontal gene transfer provides the simplest, most parsimonious explanation.