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Abstract
The early diverging metazoan lineages have highly disparate adult body plan geometries,
which can be characterised in terms of five major types of symmetry (asymmetrical,
spherical, cylindrical, n-radial, bilateral). Patterns of evolutionary changes in
symmetry types and the homology of body axes across lineages are discussed here by
confronting evidence from comparative anatomy, phylogeny, genomics and evo-devo. The
conventional scenario, postulating a graded complexification from asymmetry to radial
and finally bilateral symmetry, is considered untenable. Cylindrical symmetry is likely
to be the ancestral type from which derived all remaining types through multiple convergences.
Recent proposals prompted by molecular data that the bilateral anatomies of many cnidarians
and of the Bilateria are homologous are clearly not supported. The Hox-based patterning
system operating along the antero-posterior axis of the Bilateria does not seem to
predate their divergence with the Cnidaria, but intercellular signalling systems,
notably the Wnt pathway, could have been involved in generating the main body axis
in the last common ancestor of the Metazoa.