Out on a Limb Parallels in Vertebrate and Invertebrate Limb Patterning and the Origin of Appendages

Recent discoveries of similarities in the developmental geneticsunderlying the formation of insect and vertebrate eyes, hearts,segments, and other structures have fueled new speculation anddebate about the origins of these features and the morphologicalcomplexity of early bilaterians. The pivotal issue concerningthese developmental similarities is whether they represent convergenceof pattern-forming mechanisms or reveal developmental regulatorymechanisms or even physical characteristics derived from a commonancestor. Here, we set forth an explicit hierarchical set ofcriteria for assessing developmental genetic similarities amonganimals. We suggest that interpretations of convergence versusdescent from common ancestors should be weighed by the number,type, and phylogenetic distribution of genetic regulatory similarities.We then apply these criteria to the analysis of appendage evolution.We conclude that there has been no continuity of any structurefrom which the insect and vertebrate appendages could be derived,i.e., they are not homologous structures. However, there isabundant evidence for continuity in the genetic informationfor building body wall outgrowths and/or appendages in severalphyla which must date at least to the common, potential appendage-bearingpre-Cambrian ancestor of most protostomes and deuterostomes.In order to further trace the origin of this genetic informationand of appendages, it will be essential to analyze more primitivetaxa such as the Cnidaria and to obtain a much better fossilrecord of pre-Cambrian animals.