From the slime coatings of slugs to the saliva in our mouths, many animals contain mucus. How did this marvel of biology evolve?

In mammals, the answer is many times and often in surprising ways, according to a new study on proteins called mucins. These molecules have a variety of functions, but as a family, they are components of mucus, where they contribute to the substance's gooey consistency.

Through a comparison of mucin genes in 49 mammal species, scientists identified 15 instances in which new mucins appear to have evolved through a process that transformed a non-mucin protein into a mucin. The U.S. National Science Foundation-supported research is published in the journal Science Advances.

The scientists propose that each of these "mucinization" events began with a protein that wasn't a mucin. At some point, evolution tacked a new section onto this non-mucin base consisting of a short chain of building blocks called amino acids with sugar molecules attached. Over time, this new region was duplicated, with multiple copies added on to elongate the protein, making it a mucin.

The doubled regions, called "repeats," are key to a mucin's function, say University at Buffalo researchers Omer Gokcumen and Stefan Ruhl, the senior authors of the study, and Petar Pajic, the first author. The sugars coating these regions protrude outward like the bristles of a bottle brush, giving mucins the slimy property that's vital to the many important tasks these proteins fulfill.

"I don't think it was known that protein function can evolve this way," said Gokcumen. "A protein that isn't a mucin becomes a mucin just by gaining repeats. It's an evolutionary trick, and we document this happening over and over again."

Leslie Rissler, acting director of NSF's Division of Environmental Biology, added that "the study shows that studying genetic mechanisms in a phylogenetic and comparative framework can result in unexpected information on how evolution works."