Molecular Evolution: Evidence for Monophyly of Metazoa

This volume focuses on the origin of multicellular animals, Metazoa. Fossil records are scarce and morphological, biochemical or cytological evidence has not demonstrated an unequivocal phylogeny. Therefore, the question remains: Did Metazoa evolve from Protozoa only once, or several times? Is the origin of animals monophyletic or polyphyletic? Especially the relationships between the existing lower metazoan phyla, particularly the Porifera, are uncertain. In addition, phylogenetic analyses of nucleotide sequences of small and large ribosomal RNA also revealed no conclusive results with regard to the evolu tionary dichotomy of diploblasts (Porifera, Cnidaria and Ctenophora) and triploblasts in general and the diploblastic lineages in particular. During the evolution of unicellular eukaryotes to multicellular animals, complex molecular systems were established which allow a tuned and control led interaction between cells among each other, and cells and extracellular molecules. Hence, cell surface-associated adhesion and growth hormone receptors, as well as their corresponding extracellular ligands, have evolved. In addition, transcription factors had to be introduced that control gene func tions in the variety of cell types in developing and adult organisms.