12 - Evolution of Metazoans and Their Control System

Abstract

This chapter provides an account of the evolution of metazoans and describes the development of the control system of the metazoans. It begins with describing the principles of organization of the multicellular structure. The ability of unicellulars to form colonies, loose aggregates of relatively autonomous cells, seems to have appeared as early as prokaryotes of the type of modern bacteria, with considerable progress shown in the eukaryotic colonies of the Volvoxtype. The development of complex structures such as metazoan organisms of this primitive group from unicellular gametes, egg or zygote, implies the presence of a developmental program at the supracellular level, which is necessary not only for the individual development from the unicellular stage to adulthood but also for maintaining the unavoidably eroding adult structure. Metazoans, and multicellular organisms in general, evolved from unicellulars, which clearly lack any “coordinating genes” for they had no cells to coordinate. Evolution does not work prophetically. Under the evolutionary account of metazoans, the study describes unicellular antecedents of metazoan life, unicellular premises of coordination of cell activity in metazoans, and explains the unicellular precursors of the metazoan epigenetic informational structure. The epigenetic information in unicellulars is also explained, leading to the description of the origin of the eumetazoan life. The chapter then takes a turn to describe the diffuse control system and the neural controller: the eumetazoan’s eureka and describes centralization of the neural control system, finally presenting an account of the evolution of the neuroendocrine control in vertebrates.