The slimy, hydrated gel called mucus lines all wet epithelia in our bodies – over 200 square meters, including the eyes, lungs, and gastrointestinal and urogenital tracts. Healthy individuals produce several liters of mucus daily. Mucus was long thought merely a lubricant and physical barrier against pathogens. Mucus also acts as a communication filtering device to potentially nefarious microbes.
Mucin is the chemically active ingredient in mucus. Mucins are a family of sugar-coated (glycojugated) proteins. The proteins employ specific glycans (sugars) to selectively change bacteria behaviors: discouraging them from attaching to tissue surfaces or producing toxins and disrupting communications which would allow the bacteria to mob up into a potent biofilm. The glycans are molecules of communication.
German-American biochemist Katharina Ribbeck: “Mucins have regulatory effects and can cause behavioral switches in a whole range of pathogens. The pinpoint molecular mechanism responsible for this are the glycans. These glycans have biological functions that are very broad and sophisticated. They have the ability to regulate how microbes behave and really tune their identity.”
From an evolutionary perspective, mucins act as lion tamers: facilitating the domestication of foreigners and so give them the opportunity to join the host microbiome. This is an instance of the force of natural coherence promoting cooperation through chemistry. The intelligence behind such evolution can only be witnessed in molecular artifacts; in this instance, glycans bandied by proteins, which are sentient macromolecules.
“Nature evolved the ability to disarm difficult microbes instead of killing them. This would not only help limit selective pressure for developing resistance, because they are not under pressure to find ways to survive, but it should also help create and maintain a diverse microbiome. This is a theme that is likely at play in many systems where the goal is to shape and manipulate communities inside the body, not just in humans but throughout the animal kingdom,” reasoned Ribbeck.
The energetic force for evolution is beyond the material plane that constitutes the exhibition of existence we call Nature. That force is called coherence, and it is responsible for the perceived order in Nature as well as ongoing natural processes, of which evolution is one. As contrasted to mundane physics and chemistry activity, which renders Nature as ostensibly reliable, evolution is the creative adaptive dynamic of coherence.
An example of mucus evolution is found in saliva, which helps digest food, protects tooth enamel, and polices microbes in the mouth. Saliva also plays an important role in taste and speech enunciation as well, as evidenced by the smacking sound sometimes made while talking.
Human saliva is notably distinct from that of other apes: waterier, with less than half the protein content, and yet more adept at breaking down starch, modifying fat, and detecting key flavors. It seems that the proteins in saliva became more efficient as humans descended.
Losing fur led to less latherin in human saliva. Latherin is a detergent-like protein that helps turn fluids frothy, and so facilitate cleaning.
Kelsey M. Wheeler et al, “Mucin glycans attenuate the virulence of Pseudomonas aeruginosa in infection,” Nature Microbiology (14 October 2019).
“How mucus tames microbes: specialized sugar molecules called glycans can disarm opportunistic pathogens and prevent infection,” ScienceDaily (14 October 2019).
Supaporn Thamadilok et al, “Human and non-human primate lineage-specific footprints in the salivary proteome,” Molecular Biology and Evolution (15 October 2019).
“A secret in saliva: food and germs helped humans evolve into unique member of great apes,” ScienceDaily (15 October 2019).