Hypothesis about the evolution of higher eukaryotic organisms based on the dualistic structure of living systems

초록

Generally, simple paths from genes to trait in prokaryotes have been accepted as a basic system for genetic mapping. However, in multi-layered organisms such as mammals, mapping genes to phenotypes is regarded as a black-box. The evolution of an organism has been also considered the process by which the fitness of genes and the complexity of mapping paths increase. Various evidences and a number of inferences exhibit that the one-dimensional mapping of genes into phenotypes is not only insufficient but also incorrect for explaining both evolution and the biological system of higher organisms. Here, we propose a dualistic structure of the higher eukaryotic biological system, which divides the system into genomic and material spaces. Genomic space can be considered a place for a set of algorithms such as the scenario (or operating system) of life and a device for multitasking with RNAi. The scenario of life is a kind of "document" written with four nucleotide symbols based on a genomic "grammar" . It executes an interrupt triggered by environmental changes according to procedures included in the scenario, and it processes both development and maintenance of biological systems. Material space includes structural aspects of organisms as fixed structures and transient functional components. Both of them construct chemical circuits, in which the scenario of life can be embodied. Therefore, these chemical circuits, whose parts are shared by different species or different types of cells, seem to be a generalized tool for realization of the scenario. The evolution of genes in higher organisms has focused on the multi-usability rather than the fitness of related traits. In spite of humans` high phenotypic complexity, the human genome contains only 25,000 to 30,000 genes and a low proportion (about 1.7%) of coding DNA. While the correlation between the gene number and the degree of evolution is obvious in prokaryotes, the correlation does not exist in eukaryotic higher organisms. Rather, the proportion of non-coding DNA increases according to evolution. Many homologous genes in mammals are more similar to each other than those of lower organisms are. It has been shown that genes, involved in the expression of many different traits, have rarely evolved. As biochemical machinery, prokaryotes have evolved with the improvement of fitness of genes as a kernel of evolution because proteins or biochemical processes seize the initiative of life. In contrast, having a relatively small number and a low evolvability of genes, higher eukaryotes require another way to increase their fitness for environments. The scenario of life governs most life phenomena (e.g., development) composed of harmoniously and sequentially organized traits. The scenario of life has a multi-dimensional structure that can be changed by a small mutation of non-coding DNA. These inferences indicate that the pivot of evolution may be switched from genes to the scenario of life. In order to achieve the evolution to the various and flexible scenario of life, genes as words in the scenario should evolve into both a generalized and abstract form. In summary, a higher eukaryotic organism has evolved to have a dualistic structure and information machinery, while a prokaryote generally has a monistic structure and acts as biochemical machinery. The scenario of life is important for maintaining the robustness of the whole life system and to establish a foundation for directional evolution in higher organisms.