Phylogenetic tree of eukaryotic Hsp70 protein 12. The tree includes the human HSPA12A and HSPA12B proteins and the homologs found in eighteen sequenced eukaryotic genomes. See legend for Figure 3 and text for methods and other details. Homologs of human HSPA12A and HSPA12B were found only in vertebrates. The evolutionary tree suggests that the human genes originated from a unique progenitor also appearing in fish possibly due to a duplication event that occurred in Tetrapoda before the divergence of Amphibia. Only one HSPA12 copy was found in reptiles and birds, suggesting gene loss (or extreme divergence) of one HSPA12 copy from this lineage.