The inevitability of only five nitrogenous bases, twenty amino acids, and various proteins in living beings is systematically revealed on the basis of physics as the weft and molecular biology as the warp. First, we focus on fact that symmetry and asymmetry coexist in biomolecules. The nitrogenous bases that form nucleic acids have two large purines (A and G) and three small pyrimidines (C, T, and U), for an asymmetric ratio of molecular types of around 2:3, while those forming DNA have a symmetric ratio of 1:1, with two purines of A and G and two pyrimidines of C and T. Size ratios of around 2:3 are seen in base pairs such as A:T and G:C in DNA and also 1:1 in RNA. Chargaff's rule shows that the frequencies of purines and pyrimidines in double-strand DNA are deterministic with a symmetric ratio of 1:1, while the asymmetric frequency ratios of purines and pyrimidines in RNA are between 1:1 and approximately 2:3, which is fairly stochastic. Thermo-fluid dynamics clarifies the reason why a 1:1 deterministic symmetric ratio and a slightly stochastic asymmetric ratio of around 2:3 exist together. This analysis also reveals the further reason why molecules such as the five bases and twenty amino acids are naturally selected in living beings. Finally, it is shown that microorganisms such as yeast also undergo symmetric and asymmetric cell divisions, with size ratios between 1:1 and 2:3, and that human beings also have left-right asymmetric organs such as the liver and the heart and symmetric ones such as the lungs and kidneys. The physical principle is clarified for determining whether organs become asymmetric or not.