Fossil tooth fractures and microscopic details of enamel offer new clues into human diet and evolution

Teeth can tell us a lot about the evolution of prehistoric humans, and our latest study of one of our species’ close relatives may finally solve a long-standing mystery.

Genre Paranthrope is closely related to ours, Homo, and lived about one to three million years ago. Both Paranthrope and Homo are often considered to have evolved from Australopithecus, represented by the famous Lucy and Mrs Ples fossils.

the Paranthrope This group stands out in our family tree because of its massive posterior teeth, several times the size of ours, and extremely thick enamel (the outermost layer of our teeth). This led to the hypothesis that they ate mostly hard foods, and one of the most complete Paranthrope specimens has been nicknamed the Nutcracker Man.

But our study shows Paranthrope had very low rates of enamel chipping (a common type of tooth fracture), comparable to those of living primates such as gorillas and chimpanzees, which rarely ate hard foods. This supports other recent research on the diet of this group and should ultimately end the nutcracker hypothesis.

Replenish food from teeth

Our understanding of diet and behavior during human evolution has changed dramatically over the past few decades, partly because of new technologies, but also because of some spectacular fossil discoveries.

Teeth are often at the forefront of this research. They are by far the most abundant resource because they survive fossilization better than bones. This is a happy circumstance because teeth also offer other information that helps us to reconstruct the environment of our fossil ancestors and loved ones.

We can glean a lot of information about the microscopic scratches created by the foods that scrape the surface of the teeth during chewing, the tiny particles retained in dental plaque, and the chemical makeup of the teeth themselves.

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Before such techniques were developed and refined, researchers relied on the overall shape and size of teeth, as well as the wear and chipping visible to the naked eye. Small sample sizes and lack of comparative material hampered these studies, but they provided bold claims about the diets of our fossil ancestors.

For most techniques, we need large datasets of extinct and living species for comparison. For example, a species that commonly eats a lot of hard seeds and nuts should theoretically exhibit high rates of tooth chipping. But without a large database of species, we wouldn’t know if 10% of teeth with fractures are normal for a hard object eater, or just an expected percentage caused by other factors.

Chipping of teeth

In our recent research, we studied a wide range of living primates and compared this information with data on fossil species. The results were surprising, with our species homo sapiens and the fossil relatives of our genus generally exhibit high rates of flaking, similar to living primates that usually eat hard foods.

Previous studies have consistently suggested that humans have developed smaller teeth over the past two million years in response to cooking and processing food and consuming more meat, while Paranthrope evolved from large, sturdy teeth at rest to eat a lot of hard foods.

But teeth can grow by more than just the overall size or thickness of the enamel. The microscopic structure and composition of dental tissue can also vary among species. Could such variation explain the differences in scaling and wear between species?

If so, this could explain why humans with small teeth have a lot of chipping on their teeth while large teeth Paranthrope barely has any.

Mechanical and structural properties of teeth

To answer these questions, we sectioned the teeth of several living primate species, including humans, to examine the variation in mechanical and structural properties between dental crowns. We have collected teeth from non-human primates in museums. Human teeth have been donated by patients during routine dental treatment.

Mechanical testing involved a tiny diamond-tipped probe, which produced readings of the hardness and elasticity of the enamel. We used high power microscopes and CT scans to analyze the structure and mineral density of the enamel.

The results show that the mechanical and structural properties are uniform among the primate groups. The surfaces most prone to fractures in primates – the inner side of the lower teeth and the outer side of the upper back teeth – have much harder enamel.

These patterns are similar regardless of the diet of the primates. This suggests that the internal structure of the enamel plays a crucial role in protecting the tooth, but these patterns have remained remarkably stable during the evolution of primates.

We argue that other dental properties, including overall tooth size and shape, change faster to cope with changes in diet. Therefore, evidence for the scaling patterns and tooth structure of living primates suggests Paranthrope rarely ate hard foods, and their huge back teeth likely evolved for other purposes, capable of chewing large amounts of very hard leaves.

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Why fossil humans have such high spalling rates requires further research, but we offer several explanations, including accidental ingestion of gravel and the use of front teeth as a “third hand” to hold non-food items. . For example, large fractures on the front teeth of Neanderthals may be due to this tool-use behavior, and small chips on the back teeth of Naledi man are probably related to chewing grain-laden foods.

But that flies in the face of the interesting idea that we developed smaller teeth when we started using fire and processing higher quality foods, as heavy wear and fractures remained. The notion of nutcracker group and cooking / eating meat was appealing in its simplicity. Based on how the shape and size of teeth changed over time, this seemed like a reasonable guess. But the actual wear and tear of fossil teeth tells a very different story that is slowly emerging.