We derive the orthology relationships from the eggNOG database (32). Based on these relationships, we assign the evolutionary appearance of each cancer gene, defined as the deepest branch of the tree of life where an ortholog for that gene can be found. Overall, we divide the tree of life into seven main branches: Last Common Ancestor (LCA), which identifies the ancestral cellular organism, Eukaryotes, Opisthokonts, Metazoans, Vertebrates, Mammals and Primates. For example, a human gene whose orthologs are traceable in prokaryotes is considered to have appeared in the LCA, while a human gene with orthologs only in fungi and metazoans, but not in plants, is assumed to be born with Opisthokonts.
