Wild Primates Challenge the Toothpick Grooves Theory in Human Evolution

For many decades, narrow grooves on ancient human teeth have been regarded as evidence of intentional tool use - people supposedly cleaning between their teeth with sticks or fibres, or relieving sore gums with improvised "toothpicks". Some scholars have even described this as humanity's oldest habit.

However, our new research, published in the American Journal of Biological Anthropology, disputes this enduring interpretation of human evolution. We discovered that comparable grooves can develop naturally in wild primates, while evidence that tooth-picking caused them is limited.

More notably, we examined over 500 wild primates from 27 living and fossil species and found no indication of a widespread modern dental condition: deep, V-shaped notches at the gumline known as abfraction lesions.

Taken together, these results may change how fossil evidence is understood and prompt new questions about the distinctively human ways in which teeth are affected today.

Why teeth are important in human evolution

Teeth are the hardiest elements of the skeleton, frequently enduring long after the rest of the body has decomposed. Anthropologists use them to piece together the diets, ways of life and health of ancient populations.

Even very small traces may be significant. A commonly observed feature is a narrow channel running across exposed tooth roots, particularly in the spaces between teeth. Since the early 20th century, these marks have been called "toothpick grooves" and treated as evidence of tool use or oral hygiene.

Such grooves have been recorded throughout relatively recent human evolutionary history, from fossils dating back 2 million years to Neanderthals. Yet researchers had not properly investigated whether they occurred in other primates as well.

Abfraction is a separate condition with a very different appearance: deep, wedge-like indentations close to the gumline. It is extremely common in contemporary dentistry and is often associated with teeth grinding, vigorous brushing or acidic beverages. Its apparent absence from fossil remains has long been difficult to explain. Could other primates truly be unaffected by it?

How we studied primate tooth lesions

We tested these ideas by assessing more than 500 teeth representing 27 extinct and living primate species. Our sample covered gorillas, orangutans, macaques, colobus monkeys, fossil apes and others.

Importantly, every specimen was sourced from wild populations. Their dental wear therefore could not have been shaped by toothbrushes, fizzy drinks or processed food.

We searched for non-carious cervical lesions, the term used for loss of tissue around the neck of a tooth that does not result from decay. Microscopy, 3D scanning and measurements of tissue loss allowed us to record even the most minor lesions.

What the primate teeth revealed

Lesions were present in roughly 4% of individuals. Several closely resembled the familiar "toothpick grooves" found in fossil humans, including tapered forms and delicate parallel scratches.

Other lesions were smooth and shallow, most often on front teeth. These were probably linked to acidic fruit, which many primates eat in substantial quantities.

One finding, however, was particularly conspicuous: there were no abfraction lesions whatsoever. Although the study included species eating exceptionally demanding diets and generating powerful chewing forces, not one primate had the wedge-shaped defects frequently observed in modern dental practices.

What do the findings mean?

Firstly, grooves that look like "toothpick" marks are not necessarily proof of tool use. Ordinary chewing, abrasive food or grit swallowed with food can create comparable patterns. Particular behaviours, such as using teeth to strip vegetation, may also play a role in some instances. We should therefore be careful not to interpret every groove on a fossil tooth as intentional tooth-picking.

Secondly, the total lack of abfraction lesions among primates provides strong evidence that they are a specifically human issue associated with modern practices. Vigorous brushing, acidic drinks and processed diets are much more plausible causes than the natural forces generated by chewing.

This puts abfraction alongside other dental problems, including impacted wisdom teeth and misaligned teeth. These conditions are uncommon in wild primates but widespread among people today. Together, the findings contribute to the emerging field of evolutionary dentistry, which draws on evolutionary history to explain present-day dental disorders.

Why this matters now

Grooves on fossil teeth may initially appear insignificant, but they have implications for both dentistry and anthropology.

For evolutionary research, they demonstrate why our closest relatives should be examined before a particular - or uniquely cultural - explanation is assumed. For current health, they underline the extent to which human diets and lifestyles change our teeth in ways that distinguish us from other primates.

Comparing human teeth with those of other primates helps separate what is universal - the unavoidable wear produced by chewing - from what is particular to humans: the effects of modern diets, behaviours and dental care.

What comes next for evolutionary dentistry?

Further work will include larger samples of primates, explore connections between wild diets and tooth wear, and use advanced imaging to examine how lesions develop. The goal is to improve interpretations of the past while identifying new approaches to preventing dental disease today.

A groove on a fossil human tooth that appears to result from tooth-picking may equally be an outcome of routine chewing. It could also reflect other cultural or dietary behaviours that produce similar traces.

Resolving these alternatives requires much larger comparative datasets on lesions in wild primates. Only then will it be possible to identify broader trends and sharpen interpretations of the fossil record.

In the meantime, the absence of abfraction lesions in primates indicates that some of the most prevalent dental problems may be uniquely human. It is a reminder that our evolutionary story is inscribed even in something as commonplace as toothache, yet is shaped as much by modern habits as by ancient biology.

Ian Towle, Research Fellow in Biological Anthropology, Monash University and Luca Fiorenza, Senior Lecturer in Anatomical Sciences, Monash University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.