For years, we’re taking saliva for granted, a fluid crucial for digestion, oral health, and disease defense. However, the evolutionary journey of human saliva has remained largely unknown—until now. A recent study by researchers at the University at Buffalo has shed new light on how human saliva evolved, revealing frequent protein gene duplications, losses, and regulatory changes, particularly within the primate lineage. These discoveries offer valuable insights into how diet and disease have shaped primate biology and human evolution.
Key Findings from Genomic Analysis
The research, published in Genome Biology and Evolution, builds on previous collaborative work between Stefan Ruhl, DDS, Ph.D., professor and chair of Oral Biology, and Omer Gokcumen, Ph.D., associate professor of biological sciences. The study, led by former Ph.D. student Petar Pajic and current Ph.D. student Luane Landau, utilized DNA and RNA datasets to compare species, demonstrating how secretory calcium-binding phosphoprotein (SCPP) genes changed and expanded throughout evolution—a process linked to the development of skeletons, the emergence of tooth enamel in fish, and the production of milk in mammals.
Human Saliva: Surprisingly Distinct
Initially, the researchers hypothesized that human saliva would closely resemble that of apes, given their high genetic homology (over 98%). However, they were surprised to discover numerous compositional differences. This revelation prompted a wider comparative analysis across various animal species, highlighting a strong correlation between diet and saliva protein composition.
Diet Drives Saliva Evolution
The findings indicated that animal diets play a key role in shaping their saliva. For instance, nonhuman primates have relatively low levels of amylase—the enzyme breaking down starch—in their saliva, a contrast to humans who have significantly higher levels. This difference stemmed from the early adoption of starch consumption by humans while apes did not.
Evolutionary Links to Milk Production
Further investigation revealed that some key genes encoding abundant salivary proteins in humans are clustered with those responsible for milk casein production. These genes provide calcium for infant bone growth, mirroring the role of saliva in protecting teeth through mineralization. The researchers concluded that the diversification of saliva genes primarily occurred within the primate lineage—a consequence, they believe, of the diverse diets consumed by nonhuman primates, helping them distinguish tastes and defend against harmful substances.
Potential for Future Research and Personalized Medicine
This research opens several avenues for future exploration. Comparing saliva composition across cultures with distinct traditional diets could illuminate links between diet, saliva, and susceptibility to oral diseases. Researchers also suggest examining the saliva of bats, known for their diverse diets, to see if similar protein diversification has occurred.
“If you want to find reliable biomarkers for disease and disorders, you first have to establish a robust baseline,” says Ruhl. “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.”
Ultimately, the study underscores the potential of saliva as a diagnostic tool, suggesting that dentists and dental researchers should embrace saliva as a valuable biofluid. Additionally, the rapid evolution of genes influencing oral health could contribute to personalized medicine approaches addressing both oral and systemic health. This research provides a fascinating glimpse into how novel genes emerge and diversify across species, offering valuable insight into the intricate relationship between evolution, diet, and oral health.
