Fig. 13. Summary of proposed scenario on the origin of saccharides. The figure is constructed originally on the basis of the previous paper [19]. As a result of chemical evolution on the prebiotic earth, formose reaction generated small saccharides including GA and DHA, which subsequently undergo aldol condensation yielding few ketohexoses. Among the products, only fructose could be converted into the most stable aldohexose glucose and also mannose, and thus, these saccharides formed the first triplet. As the first metabolic pathways were developed by the first living organisms, glucose and mannose were maximally utilized, and a number of "bricolage" saccharides were produced that included galactose as a "recognition" saccharide (glycotope) and ribose as a frame saccharide in nucleic acids. In this context, galactose should be regarded as "late-comer" saccharide relative to glucose and mannose, both classified into the same aldohexose group.