Evolution and Diversity of Glycomolecules from Unicellular Organisms to Humans

The emergence of glycomolecules in nature coincided with the dawning of the ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) world, along with the development of a colossal assortment of related monosaccharide building blocks, which vastly exceed the building blocks for any other class of macromolecules. The elaboration of glycomolecules by unicellular organisms, inside cells and on their surfaces and secretions, led to an explosion of diversity, in which unique types of glycomolecules evolved to be highly expressed in all kingdoms of life – monera, protists, fungi, plant and animal. In addition to nucleic acids, glycomolecules include glycoproteins, glycolipids, oligo- and polysaccharides, glycosylated RNA (glycoRNA), and many others. The glyco-modification of amino acids in proteins overcame the limitation inherent in the genetic code, which restricts protein structures to 20 amino acids. Thus, the virtually unlimited nature of protein glycosylation greatly expands the structures and functions of proteins. A highlight in the evolution of glycomolecules is the diversity in glycan structures, as each cell type in nature, from unicellular organisms to cells in metazoans and humans, generates incredibly complex and varied glycan structures. Thus, a central paradigm of glycomolecular evolution is that the collective glycan structures are the defining features unique to each organism. This article touches on the marvelous diversity of glycans and glycomolecules in nature that led to their dominance in the landscape of biology, as well as thoughts on evolution that likely gave rise to their elaboration, diversity, and evolutionary success.