Sweet spot How the coating on our cells shaped humankind

The sugar chains covering our cells have interacted over the ages with our environment and the diseases that plague us, influencing our evolution

- Mark Honigsbaum

According to the latest estimates, Covid-19 may be responsible for more than 18 million deaths worldwide. While infectious diseases like this have devastated humanity, it may be wrong to assume they are always antithetical to our survival and flourishing as a species. Otherwise, why would ancient pathogens such as malaria (of the falciparum type), cholera, typhoid, measles, and influenza A persist as human-only diseases - and why have we not evolved immunity to them? That is a question Professor Ajit and Nissi Varki (a husband and wife team) and colleagues at their lab at the University of California, San Diego, have been asking for several decades. The answer, they believe, lies in the complex array of sugar chains called glycans that decorate the surfaces of cells, and the sugar molecules known as sialic acids that cap most of these chains. These terminal sugar chains are involved in everything from the regulation of immune responses to adaptations that may have played a key role in human evolution, such as the ability of our early hominin ancestors to run for longer without becoming fatigued advantage when pursuing prey.

Ajit Varki first became interested in sialic acids and glycobiology in the early 1980s, when he was treating a patient who had suffered an adverse immune response to a therapeutic horse serum used to treat a type of anemia. Rather than the immune response being directed against the presence of foreign proteins - then the standard explanation in biology textbooks - Varki discovered it was because of the sialic acids on the horse proteins, which was surprising as every vertebrate, including humans, can make sialic acids.