THE ROLE OF carbohydrates that are

attached to proteins and lipids has been a changing one1,2. Carbohydrates on the surface of yeast and other free-living eukaryotes probably play largely struc-tural roles, while those on the surface of multicellular eukaryotes can have information-bearing functions in cell– cell recognition and adhesion. Recent work on glycoprotein biosynthesis, which occurs as the molecules make their way to the cell surface, indicates that the carbohydrates also have an im-portant role in intracellular sorting – specifically, in quality control3,4. Con-sideration of the sets of sugars that are involved in these different processes suggests that these functions evolved sequentially and that there were four distinct evolutionary stages, during which different selection pressures se-lected for distinct types of sugar struc-ture. Here, we summarize evidence from studies of oligosaccharide structure, biosynthesis and recognition that sup-port such a sequential model for the evolution of glycoprotein oligosaccha-rides (see Fig. 1). Cores and elaborations in oligosaccharide structures Given the number of hexoses and the possibilities for assembling them into oligosaccharides by using multiple dif-ferent linkages, the potential for diver-sity in oligosaccharides is staggering5. However, the number of consitituent sugars that are actually present in most groups of oligosaccharides is not large, and the linkages that are used tend to be restricted. In mammals, for instance, projections suggest that synthesis of structures attached to proteins by N-and O-linkages, together with the glyco-sidic portions of glycolipids, probably can be accounted for by fewer than 500 glycosyltransferases, each specific for creating a single type of linkage6,7. The sugars in mammalian protein-linked oligosaccharides can be divided roughly into two groups: (1) the core sugars (such as N-acetylglucosamine, mannose and possibly glucose), which establish the basic branching pattern and are complemented by extensions (usually polylactosamine chains consisting of re-peated galactose-N-acetylglucosamine disaccharides); and (2) a variety of terminal elaborations, which include galactose, N-acetylgalactosamine, L-fu-cose, sialic acid and sulphates (see Fig. 2)