Evo-devo’s greatest revelation thus far has been that all bilaterally symmetric animals use largely the same genes to construct their bodies, including ours, despite stark differences in gross anatomy [969]. These shared genes have been traced to a multicellular ancestor that lived ~600 million years (MY) ago [405]. That ancestor has been dubbed the “Urbilaterian” [518], where “Ur-” denotes a progenitor, and “bilaterian” indicates that its two sides were mirror images. How did we come to this realization?

The first hint of a deep developmental unity among evolutionarily diverse animals came in 1976 when cockroaches and salamanders were shown to regenerate their legs according to the same cellular rules [707]. However, it seemed absurd to think that those rules might imply a leg-bearing common ancestor, because arthropods and chordates (their respective phyla) were thought to have evolved legs independently of one another [632].

Then, in the 1980s geneticists kept finding the same 180 base-pair motif in virtually all animal genomes [1432]. It was dubbed the “homeobox” (homeosis box) because mutations in the genes that contain it cause homeosis (transformation of one body part into another) [1756]. Even more surprising was that insects and vertebrates use homologous clusters of homeobox genes in the same colinear sequence on their chromosomes [1650] to organize the anterior–posterior (A–P) axis of their body (Fig. 1.1). These linked genes were termed “Hox” (Homeobox) genes [294]. Thus, by definition, all Hox genes are homeobox genes, but relatively few homeobox genes reside in a Hox Complex. Several Hox genes (Scr, Ubx, abd-A, and Abd-B) were instrumental in insect evolution (see Chapters 2 and 3).