Published online by Cambridge University Press: 29 April 2021
Body-plan features that have been discussed so far include symmetry, segmentation, skeletons, and limbs. When these are encountered in different phyla, are they homologous or convergent? There are examples of both of these, plus examples where the answer is not yet clear. Bilateral symmetry of the overall body plan seems to have originated just once. So the fact that vertebrates and arthropods are both bilaterally symmetrical is due to their having inherited that body layout from their last common ancestor; in other words, their bilaterality is homologous. However, although vertebrates and arthropods both have skeletons (whereas animals belonging to many other phyla do not) these represent convergent rather than homologous skeletons – this is clear from the fact that one is ‘endo’, the other ‘exo’. Turning to segments and limbs, the fact that both vertebrates and arthropods have these component parts is hard to interpret with certainty one way or the other. The reason for this is our lack of knowledge of that ancient animal that we call the urbilaterian, or ‘Urby’ for short. Direct evidence of this creature will probably never be forthcoming, since it was almost certainly small and soft-bodied, and has left us with no fossils from which to infer its living form. Instead, we can only make rather indirect inferences based on the point in the animal evolutionary tree at which we think bilaterality arose. However, indirect inference is better than nothing, so here goes.