EVOLUTION OF PROTOZOA TO METAZOA

The evolving of Metazoa from their protozoan ancestors was considered until recently the most puzzling enigma of phylogeny. The emergence of metazoans has been explained by two major theories; the syncytial the ory, or the colonial theory. However, in both views a di(poly)phyletic origin of Metazoa was assumed. Based on constituent characters of the sponges a monophyletic origin of the Porifera could be deduced. After having analyzed those genes especially from the marine sponge Geodia cydonium which are typical for multicellularity, e.g. those coding for adhesion molecules and adhesion receptors or elements involved in sig nal transduction pathways it has to be concluded that all animals, inc luding sponges, are of monophyletic origin. It is apparent that one ma jor event which allowed evolution of Protozoa to Metazoa was the inven tion of the genetic apparatus to insert introns and hence to facilitat e the creation of mobile modules; this made the subsequent process of exon-shuffling possible. With the isolation of the first sponge genes, especially the one coding for receptor tyrosine kinase, it is now est ablished that modular proteins, composed by exon-shuffling, are common to all metazoan phyla. This mechanism of exon-shuffling is apparently absent in plants and protists. Ii this view can be accepted then that ''burst of evolutionary creativity'' during the period of Cambrian ex plosion which resulted in the big bang of metazoan radiation was drive n by the process of modularization. We propose that during the transit ion from Protozoa to Metazoa the formation of domains was in the cente r of evolution. After having reached a critical number of domains the mechanism of modularization allowed a rapid formation of a series of m osaic proteins by exon-shuffling.