Major Transitions in Animal Evolution: A Developmental Genetic Perspective

SYNOPSIS. Several phases of animal evolution have undergoneradical change in developmental mechanisms. I refer to theseas major transitions in animal evolution. The six most importanttransitions in the lineage leading to humans are proposed tobe: the origin of multicellularity, the origin of two-germ layersand radial symmetry, the origin of three-germ layers and bilateralsymmetry, dorsoventral axis inversion, the origin of vertebrates,the origin of gnathostomes. Here I discuss the genetic changesthat may have underlain these transitions. The last two transitionswere accompanied by, and possibly facilitated by, large increasesin gene number. This probably occurred by tetraploidy, withsome of the duplicate genes being subsequently lost. The originof three germ-layers, bilateral symmetry and a through gut alsoprobably involved gene duplication; in this case, duplicationof an ancestral ProtoHox gene cluster to yield two paralogoushomeobox gene clusters, Hox and ParaHox, with roles in patterningdifferent germ layers along the anteroposterior body axis. Thisevent may provide a partial genetic explanation for the Cambrianexplosion.