Figure 1. The middle animal on the left is a depiction of the presumed bilateral, worm-like animal that gave rise to insects such as the Drosophila fruit fly (top left) and the mammal (bottom left). To the right of the fruit fly is a schematic of its Hox gene cluster. The arrows denote the presumed derivation of these genes from the Hox cluster of the bilateral worm-like ancestor. At bottom right is a schematic of the four Hox gene clusters of a vertebrate, with the arrows denoting homologous relationships between the ancestral Hox genes and the Hox genes in the current Drosophila genome. Different tints used denote which genes are expressed in which anterior–posterior regions of the fruit fly, bilaterian, or mammal body plans.

Figure 2. The phylogenetic tree shows a conventional branching order of animal groups during evolution. The number of Hox genes in the genomes of present-dxay animals of these different groups is listed on the right side, adjacent to the branch tips.