A phylogeny of animals. Evolutionary relationship, numbers of genes and divergence times of selected animals whose genomes have been sequenced. Along with the increasing overall gene number, the occurrence of single HOX gene cluster in invertebrate genome and four or more clusters in vertebrate genome is taken as evidence for two rounds of whole genome duplication at the base of vertebrate lineage (2R hypothesis). The double headed arrow underneath shows the proposed time windows of genome amplification events. Features not drawn to scale.