Cellularization

Key: 1 Reductive formation of organic compounds from CO or CO2 by Me-sulfur coordinative chemistry 2 tapping of various redox energy sources and formation of primitive enzymes and templates 3 elements of a transcription and translation apparatus and loose associations 4 formation of pre-cells 5 stabilised circular or linear genomes 6 cytoplasmic membranes 7 rigid murein cell walls 8 various non-murein rigid cell walls 9 glycoproteinaceous cell envelope or glycokalyx 10 cytoskeleton 11 complex chromosomes and nuclear membrane 12 cell organelles via endosymbiosis".According to Otto Kandler's pre-cell theory,[1][2][3] early evolution of life and primordial metabolism (see Iron-Sulfur world hypothesis - metabolism first scenario, according to Wächtershäuser[5][6]) led to the early diversification of life through the evolution of a multiphenotypical population of pre-cells,[1][2][3] from which the three founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life[7] emerged successively.

[1][2][3][8] 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[9][10][11]), 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.

[8] According to Kandler, the protection of fragile primordial life forms from their environment by the invention of envelopes (i.e. membranes, walls) was an essential improvement.

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.

[14] 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.

This is in contrast to germ layer theory in which ectoderm, endoderm and mesoderm (in more complex animals) build up the embryo.

Here two examples: In the embryonic development of Drosophila melanogaster, first 13 nuclear divisions take place forming a syncytial blastoderm consisting of approximately 6000 nuclei.

[23] 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),[24] rice (Oryza sativa).