We have shown that various behaviors of organisms emerge from the dynamics of biomolecules. In Chap. 2 , we saw that multiple steady states of the gene regulatory system produce different properties of cells. Here, we will further promote this idea. In other words, we will generally consider the diversity of cell types as the diversity of the steady states of a regulatory system. Cell diversity has been considered as a measure of the complexity of multicellular organisms. Therefore, considering the origin of cell diversity also leads us to consider the origin of the complexity of organism’s body.