Unveiling the Secrets of Human Saliva's Evolution: A Journey Back to Our Primate Roots
Did you know that the humble saliva in your mouth has an incredible story to tell about human evolution? Despite its mundane nature, saliva plays a crucial role in our daily lives, aiding digestion, maintaining dental health, and defending against oral diseases. However, the evolution of human saliva has remained a mystery—until now.
Thanks to the groundbreaking work of two University at Buffalo faculty members and two graduate students, we're finally unraveling this fascinating tale. Stefan Ruhl, a professor and chair of the Department of Oral Biology at the School of Dental Medicine, and Omer Gokcumen, an associate professor of biological sciences, have discovered that the protein genes responsible for creating human saliva have undergone a complex journey of duplications, losses, and regulatory changes, especially evident in our primate ancestors.
"Our research highlights how evolutionary adaptations to diet and disease have shaped the biology of primates, including humans," explains Ruhl, a veteran in the field of saliva biology. Their findings, recently published in the esteemed journal Genome Biology and Evolution, offer a unique perspective on the evolution of saliva and its impact on our species.
But here's where it gets controversial... The scientists, along with Petar Pajic, a former PhD student now at Yale University, and Luane Landau, a current PhD student, used DNA and RNA datasets to compare species. They found that secretory calcium-binding phosphoprotein (SCPP) genes underwent significant changes and expansion at pivotal moments in evolution. These moments include the development of skeletons in early animals, the appearance of tooth enamel in fish, and the emergence of milk production in mammals.
"We hypothesized that saliva, constantly interacting with food, microbes, and pathogens, might evolve more rapidly than other systems," says Gokcumen, an expert in evolutionary anthropology. "This locus could serve as a model for understanding that dynamic."
This study sets the stage for further exploration, as Ruhl, Gokcumen, and Pajic have collaborated on multiple projects to understand saliva's functions better. Ruhl notes, "Saliva contains almost everything that also appears in blood, composed of over 3,000 components, yet only a dozen are highly abundant. These abundant proteins, produced by the salivary glands, are crucial for oral health, as the glands have evolved to protect our teeth."
Initially, the group believed human saliva would be identical to that of apes, given their high genetic similarity to humans. "If you look at their blood, it's pretty identical to ours. We thought it would be the same for saliva, with maybe one or two different components. How wrong we were! There were many substances that were different."
This revelation led the team to compare human saliva with that of other animals, and they found that saliva protein composition is influenced by diet. Ruhl explains, "The environment an animal lives in and its dietary preferences shape the composition of saliva proteins over time."
For instance, nonhuman primates have low amylase levels in their saliva, while humans have much higher levels. This change occurred because humans became early starch consumers, unlike apes. Upon further investigation, the researchers discovered other genes encoding very abundant salivary proteins in humans, found in the same cluster as milk caseins, which provide growing infants with calcium for bone growth.
"The real development of saliva genes resembling those in humans occurred in the primate lineage," Ruhl says. "This is intriguing because nonhuman primates are picky eaters, mostly choosing from a variety of fruits and veggies. We believe the diversity of saliva proteins in primates is linked to their ability to distinguish between different tastes or protect against harmful substances in plants."
Similar phenomena have occurred in other branches of the phylogenetic tree, such as bats, which have diverse diets ranging from fruit to insects to blood. Ruhl predicts that a similar diversification of saliva proteins evolved in bats as in primates.
Another frontier to explore is the composition of saliva in different cultures with distinct traditional diets. This could provide a better understanding of saliva and why individuals are differently prone to oral diseases. Ruhl emphasizes, "To find reliable biomarkers for disease and disorders, we must first establish a robust baseline. We know there are biomarkers among different individuals, but we don't know their normal baseline levels in saliva, whether it's related to our genetic backgrounds or our lifestyles and diets."
He adds, "While medical doctors have blood and urine as diagnostic fluids, dentists and researchers should claim saliva as their biofluid, as it can indicate a lot about oral health and should be utilized more often."
Gokcumen further highlights that the rapid evolution of genes important to oral health may make some individuals more susceptible to certain conditions, such as cavities or metabolic variations under specific environmental circumstances. "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 rapidly novel genes can emerge and diversify across species."
This research not only sheds light on the evolution of human saliva but also opens up exciting possibilities for future studies and applications in personalized medicine and oral health.
