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Scientific theory to explain the origin and formation of cells
In evolutionary biology, the term cellularization (cellularisation) has been used in theories to explain the evolution of cells, for instance in the pre cell theory, early evolution of life and primordial metabolism (see Iron Sulfur world hypothesis metabolism first scenario, according to Wächtershäuser) led to the early diversification of life through the evolution of a multiphenotypical population of pre cells, emerged successively. In this scenario the three domains of life did not originate from an ancestral nearly complete “first cell“ nor a cellular organism often defined as the last universal common ancestor (LUCA), but from a population of evolving pre cells. Kandler introduced the term cellularization for his concept of a successive evolution of cells by a process of evolutionary improvements. in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere. and Hanson (1977). This cellularization (syncytial) 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).

According to the syncytial theory, the ciliate ancestor, by several cellularization processes, evolved into the currently known turbellarian flatworms, which are therefore the most primitive metazoans. 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.

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 more complex animals) build up the embryo.

There is a lot of evidence against ciliates being the metazoan ancestor. Ciliates have two types of nuclei: a micronucleus which is used as germline nucleus and a macronucleus which regulates the vegetative growth. This division of nuclei is a unique feature of the ciliates and is not found in any other members of the animal kingdom. Therefore, it would be unlikely that ciliates are indeed the ancestors of the metazoans. This is confirmed by molecular phylogenetic research. Ciliates were never found close to animals in any molecular phylogeny.

Furthermore, the syncytial theory cannot explain the flagellated sperm of metazoans. Since the ciliate ancestor does not have any flagella and it is unlikely that the flagella arose as a de novo trait in metazoans,  the syncytial theory makes it almost impossible to explain the origin of flagellated sperm.

The term syncytium cellularization is used for instance for a process of cell development in the endosperm of the Poaceae, e. g. barley (Hordeum vulgare), rice (Oryza sativa).