A theoretical approach to the large-scale evolution of multicellularity and cell differentiation

In contrast to Darwinian evolution in which organisms have been selected by the instantaneous judgment of advantage or disadvantage for a mutated gene, the large-scale evolution of multicellular organisms by drastic changes in their genomes to produce new genes is theoretically formulated on the basis of the new concept of ‘biological activity’. The ‘biological activity’ of an organism is a macroscopic quantity determined by its whole genome and the environment, consisting of three terms; the energy acquired from the outside, the energy stored in the form of bio-molecules, and the systematization of multicellularity as well as of organizing genes and their products. The acquired energy minus stored energy is lost as heat, and the entropy production by the heat must compensate for the entropy reduction owing to the systematization in the organism. Under the boundary determined by this thermodynamic law, the organisms, which experienced gene duplication to produce new genes for multicellularity and cell differentiation, first decline to be minor members in a population by the increase in the energy to be stored and by the advanced systematization of cell differentiation. If the acquired energy is raised by the cooperative action of newly differentiated cells with the pre-existing types of cells, however, the ‘biological activity’ of this new style of organism can be recovered. The new style of organism generated through this evolutionary process does not necessarily expel the old style of organism to extinction but can coexist by choosing different material and energy resources. Moreover, this theory of large-scale evolution not only explains the punctuated mode of evolution indicated by paleontology but also reproduces the divergence of body plans observed in Triploblastica and Tracheophyta.