A new study tracks evolution of human saliva back to primates.

Saliva is a bodily fluid most of us take for granted despite the significant roles it plays: aiding in digestion, maintaining strong teeth, and defending against oral disease.

However, the evolution of human saliva has been largely unknown—until now.

Stefan Ruhl, professor and chair of the oral biology department in the School of Dental Medicine at the University at Buffalo, and Omer Gokcumen, associate professor of biological sciences, discovered that the protein genes that create human saliva have undergone frequent duplications, losses, and regulatory changes, which became particularly evident in the primate lineage.

“Our work highlights how evolutionary adaptations to diet and disease may have influenced primate biology, including humans,” explains Ruhl, who has studied different aspects of saliva biology for years.

The findings appear in the journal Genome Biology and Evolution.

Petar Pajic, a former PhD student in biological sciences who is now a National Science Foundation (NFS) postdoctoral research fellow at Yale University, contributed to the study and is the first author on the paper.

Using DNA and RNA datasets to compare species, the researchers found that secretory calcium-binding phosphoprotein (SCPP) genes changed and expanded at pivotal moments over the course of evolution—when early animals first developed skeletons, when tooth enamel appeared in fish, and when mammals began producing milk.

“Our idea was that saliva, as a biological fluid that constantly interacts with food, microbes, and pathogens, may evolve more rapidly than other systems,” says Gokcumen, an expert in evolutionary anthropology. “We thought this locus might serve as a model for understanding that evolutionary dynamic.”

This study preceded others in which Ruhl, Gokcumen, and Pajic have collaborated, and it stemmed from a desire to understand more about the functions of saliva.

“We know that saliva contains almost everything that also appears in blood,” says Ruhl, explaining how saliva is composed of more than 3,000 components, yet only a dozen or so exist in high abundance.

“Those abundant proteins, produced by the salivary glands, are probably the ones that really matter for keeping the mouth healthy because the salivary glands have evolved to protect the teeth,” he says.

“Teeth are the only place in the body where a mineralized substance is exposed to the environment. And it’s constantly being challenged by acids from food and those produced by bacteria that cause dental caries, along with the simple mechanical attrition of chewing.”

When the group began their research, they initially thought that human saliva would be identical to that of apes, which are more than 98% genetically homologous to humans.

“If you look at their blood, it’s pretty identical to ours in its composition. We thought it would be the same for saliva, with maybe one or two different components we could study,” Ruhl says.

“How wrong we were. It turned out there were not one or two but many substances that were different.”

That revelation prompted the team to compare human saliva to that of other animals.

“We have proven that saliva protein composition is influenced by diet,” Ruhl says. “The environment a certain animal lives in and what it prefers to consume will shape, evolutionarily speaking, the composition of saliva proteins.”

For instance, nonhuman primates have relatively low amounts of amylase, the enzyme that breaks down starch into simpler sugars, in their saliva, while humans have a great deal more of it. That change happened because humans became consumers of starch early on, while apes did not.

Upon further review, the researchers realized that there are a handful of other genes that encode very abundant salivary proteins in humans, and they were found in the same cluster of genes as milk caseins. Those genes provide growing infants with calcium for bone growth, much as saliva protects teeth through mineralization.

“The real development of the saliva genes that resemble those in humans occurred in the primate lineage,” Ruhl says.

“That was interesting to us because nonhuman primates are picky eaters, and they mostly choose from a variety of fruit and veggies. We believe that the diversity of saliva proteins in primates must have something to do with them being able to distinguish between different taste varieties or to protect from harmful substances in the plants they eat.”

There are other branches of the phylogenetic tree where similar things have happened.

“We know that bats are very diverse in their diets. There are some that eat fruit, some that eat insects, and some that suck blood,” Ruhl says. “It would be interesting to study their salivary composition. I would predict that a similar diversification of saliva proteins evolved as in primates.”

Another possible frontier to study is the composition of saliva in different cultures around the globe that traditionally follow distinct diets. This could provide a better understanding not only of saliva itself but also why individuals are differently prone to oral diseases.

“If you want to find reliable biomarkers for disease and disorders, you first have to establish a robust baseline,” Ruhl says. “We know there are biomarkers among different individuals, but we don’t know what their normal baseline levels in saliva are, whether it has to do with our genetic backgrounds or where and how we live and eat.”

He adds that while medical doctors have blood and urine as diagnostic fluids indicative of health, dentists and dental researchers should claim saliva as their biofluid, which can indicate a great deal about the oral cavity and should be used more often.

Gokcumen adds that the rapid evolution of genes that are important to oral health may make some individuals more susceptible to certain conditions.

“This could be cavities or metabolic variation, under particular environmental circumstances,” he explains.

“In that sense, our results open the door to exploring personalized medicine approaches related to oral and systemic health. More broadly, the study provides new insight into how novel genes can emerge and diversify across species.”

Funding for the work came from the National Science Foundation, the National Institute of Dental and Craniofacial Research, and the National Cancer Institute.

Source: University at Buffalo