The major lines of metazoan evolution: Summary of traditional evidence and lessons from ribosomal RNA sequence analysis

Contrary to a widespread belief among biologists, and to diagrams still found in many elementary biology or zoology textbooks, the general pattern of the phylogeny of Metazoa (the multicellular animals) is far from being settled. While there is a strong body of essentially congruent morphological, paleontological and molecular data concerning the branching pattern within some phyla or classes, most notably the vertebrates, the problem of the overall relationships of the invertebrate phyla is much more open (see Fig. 1 for a summary of conflicting schemes). These 32–36 phyla, however, account by far for most of the biological diversity of animals. They include some huge groups such as Arthopoda or Mollusca, to which many of the favorite experimental organisms discussed at this meeting belong as well as many additional groups also containing experimentally important species (e.g. Nematoda: Caenorhabditis; Echinodermata: the various sea urchins, both extensively used models in developmental biology; Annelida and Platyhelminthes, classical organisms for the study of embryology and regeneration, etc.). As illustrated in detail in books recently devoted to the subject (Barnes, 1987; Brusca and Brusca, 1990; Willmer, 1990), summarizing and updating over 200 years of comparative anatomy and embryology, defining a phylum is usually straightforward, but it is the establishment of the evolutionary relationships linking the different phyla that is difficult to achieve.