M�LLER, W.E.G.: The Urmetazoa: The hypothetical ancestral animal

The origin of Metazoa remained the most enigmatic of all phylogenetic problems. Sponges [Porifera] as “living fossils” positioned at the base of multicellular animals have been used to answer basic questions in metazoan evolution by molecular biological techniques. During the last few years genes coding for informative proteins have been isolated and characterized from sponges, especially from the marine demosponges Suberites domuncula and Geodia cydonium. The analyses of their protein sequences allowed a molecular biological resolution of the monophyly of Metazoa [W.E.G. M�ller (1995) Naturwiss. 82, 321-329]. Molecules of the extracellular matrix/basal lamina, with the integrin receptor, fibronectin and galectin, cell-surface receptors, elements of nerve system/sensory cells, homologs/modules of an immune system as well as morphogens classify the Porifera as true Metazoa. As “living fossils”, provided with simple, primordial molecules allowing cell-cell- and cell-matrix adhesion as well as processes of signal transduction as known in a more complex manner from higher Metazoa, sponges also show peculiarities. It is concluded that molecular biological studies with sponges as models will help to understand the evolution to Metazoa. The hypothetical ancestral animal, the URMETAZOA from which the metazoan lineages diverged had the following characteristics: cell adhesion molecules with intracellular signal transduction pathways – morphogens/growth factors forming gradients – a functional immune system – a primordial nerve cell/receptor system. The structure and function of the molecules are evolutionary novelties that were developed in the hypothetical ancestral Metazoa, the URMETAZOA [W.E.G. M�ller (2001) Comp. Biochem. Physiol. (A) 129, 433-460].