Saliva offers new insight into human evolution

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A new study suggests the diet of our ancestors has led to substantial differences in our saliva compared to other great apes. The research, published in the journal Molecular Biology and Evolution, suggests this may relate to a variety of dietary changes, such as more meat consumption or cooking.

Saliva helps digest food and protects teeth, but also plays an important role in speech, taste and protection against pathogens. The makeup of saliva has evolved in response to these different factors, leading to substantial species differences. By comparing human salivary proteins with those of other primates this new study aimed to provide further insight into human evolution.

“Salivary proteins in humans and other primates could be overlooked hotbeds of evolutionary activity,” said Dr Stefan Ruhl, lead author and professor of oral biology in the UB 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.”

Human saliva is unusual compared to other primates, containing a different mix of proteins, and, perhaps surprisingly, is more watery than in other species. The overall protein content in human saliva was less than half of the amount found in chimpanzee, gorilla and macaque saliva.

“Did human salivary glands evolve to produce a more watery saliva to accommodate a diet which drastically differs from that of great apes?” asks Dr Supaporn “Nina” Thamadilok, PharmD candidate at the Massachusetts College of Pharmacy and Health Sciences. “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.

Human saliva is more adept at breaking down starch, modifying fat, and detecting key flavors in human diet. The researchers discovered in humans higher amounts of amylase (an enzyme key to digesting starch into sugars) and carbonic anhydrase VI (an enzyme involved in taste perception) than was present in chimpanzees or gorillas.

Another intriguing possibility discovered in this research is that losing our fur has led to the lack of latherin in human saliva. Latherin, a detergent-like protein that helps fluids become frothy, was only found in the saliva of the non-human great apes. The researchers suggest that humans, who no longer grow fur coats and don’t engage in social grooming, lost the need to produce the protein.

“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,” said Dr Gokcumen, associate professor of biological sciences in the UB 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 in humans. 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,” said Ruhl. “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 towards 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. Hopefully, other labs will join with their expertise, and help us answer those fundamental questions.”

Research article: Human and Non-Human Primate Lineage-Specific Footprints in the Salivary Proteome

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