Published October 18, 2019
Two million years of eating meat and cooked food may have helped humans shift further from other great apes on the evolutionary tree. The evidence is in our saliva, according to new UB research.
The research discovered that the human diet — a result of increased meat consumption, cooking and agriculture — has led to stark differences in the saliva of humans compared to that of other primates.
Human saliva is unique in that it is waterier and contains a different mix of proteins. The findings came as a big surprise to the researchers, since humans are known to be genetically close relatives of the great apes, chimpanzees and gorillas.
“Salivary proteins in humans and other primates could be overlooked hotbeds of evolutionary activity,” says Stefan Ruhl, lead investigator and professor of oral biology in the School of Dental Medicine. “We knew already that evolutionary adaptations to a human-specific diet have resulted in obvious changes to jaws and teeth, as well as the oral microbiome. Our findings now open up the possibility that dietary differences and pathogenic pressures may have also shaped a distinct saliva in humans.”
The study was published Oct. 15 in Molecular Biology and Evolution.
Saliva is a crucial bodily secretion in humans. The fluid helps digest food, protects tooth enamel, polices microbes in the mouth, and forms a first line of defense against pathogens. Saliva plays an important role in speech and taste as well.
Spit’s many functions can be credited to the salivary proteome, the thousands of proteins within the fluid. These proteins may also reveal clues to how humanity diverged from the great apes throughout its evolution.
The researchers compared the salivary proteins of humans and our closest evolutionary relatives: gorillas and chimpanzees. Macaques — who share a common, more distant ancestor with humans and great apes — were examined as well.
Key findings include:
“Did human salivary glands evolve to produce a more watery saliva to accommodate a diet which drastically differs from that of great apes?” asks Supaporn “Nina” Thamadilok, a PharmD candidate at the Massachusetts College of Pharmacy and Health Sciences, and former postdoctoral research associate in Ruhl’s lab. “Great apes and Old World monkeys chew on their fiber-rich food for longer periods of time, while humans swallow food faster, an ability that is arguably supported by the cooking of food. A watery consistency of saliva could aid in faster digestion of dry food in the mouth, and easier swallowing,” she says.
“It might also be advantageous to keep the mouth moist in arid, savannah-like environments where early humans evolved, or be important to facilitate human speech and vocalization. Of course, those possibilities remain speculative.”
Adds co-investigator Omer Gokcumen: “The study’s findings provide a necessary basis for future studies to assess whether the differences in human salivary proteins were caused by natural selection.
“The challenge will be to decipher the genetic underpinnings of these changes, and understand the evolutionary mechanisms that caused them. For some of the genes, we have a good idea; for most of the others, however, we still have to find out,” says Gokcumen, associate professor of biological sciences in the College of Arts and Sciences.
Differences between the saliva of humans and great apes were also found in proteins involved in defense against disease.
The saliva of chimpanzees, gorillas and macaques possessed greater amounts of parotid secretory protein than human spit. Human and chimpanzee saliva contained higher levels of secretory immunoglobulin components than gorilla saliva. Each of the proteins play a role in defense against disease.
“Besides diet, pathogenic pressure is another important driving force for evolutionary adaptation,” Ruhl explains. “Whether any of the salivary proteins that show human-specific features were driven to change by diseases that came along with the evolution of humans into a top predator and the later shift toward agriculture and breeding livestock is an intriguing question worth examining.
“Little is known about the degree to which human saliva has adapted to dietary, technological (pounding, cutting and cooking), environmental and microbial pressures,” he says. “Hopefully, other labs will join with their expertises, and help us answer those fundamental questions.”
Additional investigators on the study include Kyoung-Soo Choi, associate consultant biologist at Eli Lilly and Company and a former research technologist at Roswell Park Comprehensive Cancer Center; Lorenz Ruhl, senior associate at Putnam Associates and a former summer research student at Roswell; Latif Kazim, emeritus faculty at Roswell; and Fabian Schulte, research fellow, and Markus Hardt, assistant member of the staff, both at The Forsyth Institute.
The research was supported by the National Institute of Dental and Craniofacial Research and the National Cancer Institute, both within the National Institutes of Health, as well as by Roswell Park Comprehensive Cancer Center.
I think it would be interesting to observe whether the saliva of people who eat only or predominately (e.g., 99% of calories) plants is different from the saliva of people who eat a lot of animal-based foods.