could venture into a larger variety of habitats, even into microaerobic habitats in shallow, illuminated surface waters. The continuous changes in the physical environment on the aging and cooling Earth led to further diversification of habitats and favored opportunistic radiation of primitive life into numerous phenotypes on the basis of each of the different chemolithoautotrophies. Concomitantly, with the accumulation of organic matter derived from chemolithoautotrophic life, opportunistic and obligate heterotrophic life may also have developed”. For more theories of the formation of first cells see main article History of life (chapter “Replication first”). Formation of cells in extant organisms (Bacteria, Archaea, Eucarya) The syncytial theory The syncytial theory of cellularization, also known as the syncytial theory or ciliate-acoel theory, is a theory to explain the origin of Metazoa. The idea was proposed by Hadži (1953) and Hanson (1977). The cellularization theory states that metazoans evolved from a unicellular ciliate with multiple nuclei that went through cellularization. Firstly, the ciliate developed a ventral mouth for feeding and all nuclei moved to one side of the cell. Secondly, an epithelium was created by membranes forming barriers between the nuclei. In this way, a multicellular organism was created from one multinucleate cell (syncytium). Arguments (synclital theory) Turbellarian flatworms By several cellularization processes, the ciliate ancestor evolved into the currently known turbellarian flatworms, which are therefore the most primitive metazoans according to the theory. The theory of cellularization is based on the large similarities between ciliates and flatworms. Both ciliates and flatworms have cilia, are bilaterally symmetric, and syncytial. Therefore, the theory assumes that bilateral symmetry is more primitive than radial symmetry. However, current biological evidence shows that the most primitive forms of metazoans show radial symmetry, and thus radially symmetrical animals like cnidaria cannot be derived from bilateral flatworms. By concluding that the first multicellular animals were flatworms, it is also suggested that simpler organisms as sponges, ctenophores and cnidarians would have derived from more complex animals. However, most current molecular research has shown that sponges are the most primitive metazoans. Germ layers are formed simultaneously The syncytial theory rejects the theory of germ layers. During the development of the turbellaria (Acoela), three regions are formed without the formation of germ layers. From this, it was concluded that the germ layers are simultaneously formed during the cellularization process. This is in contrast to germ layer theory in which ectoderm, endoderm and mesoderm (in