A comparison of the wearing effect of plant-derived silica and desert dust on tooth enamel suggests that extreme wear on teeth might not be caused by food. The findings may change some thoughts about the diets of human ancestors.
Chewing is much like using a pestle and mortar, but upside down. Your jaw muscles supply the power and move your lower teeth (the pestle) up and across your upper teeth (the mortar). As long as any food caught between them is not especially hard or tough, it will be broken into smaller pieces and the teeth should remain intact. But, unlike the granite or marble pestle and mortar in your kitchen, teeth are gradually worn down. It has been widely assumed that interactions with food are the cause of this destructive damage. But, writing in the Journal of the Royal Society Interface, Lucas et al. suggest1 that the culprit may not be the food we chew, but the dust or grit we ingest along with it.
When teeth are newly erupted into the jaw, the enamel surface is relatively pristine. However, even though enamel is the hardest tissue in the body, once teeth have been used to chew on food, tiny grooves, scratches and pits soon pockmark the enamel. Living animals with different diets produce different patterns of dental microwear2,3, so it was natural to assume that these patterns can be used to infer the types of food eaten by our ancestors and close evolutionary relatives4. However, a consideration of the physical processes that cause wear to hard surfaces calls some of these assumptions into question.
Materials scientists define wear as what happens when an object loses some of its volume. If you dance in high heels on your neighbour's softwood floor, you will annoy them by marking it, but marking merely deforms the surface of the floor — technically it is not worn. But if you were to dance in golf shoes, you will abrade the floor and eventually wear it away, because abrasion results in loss of volume. It seems unlikely that any food built primarily from either cellulose (deriving from plants) or collagen (from animal soft tissues) is going to abrade enamel, yet these are the foods we have in mind when we think about diet. Food acids can eventually dissolve enamel, but this is only a problem in some modern human populations. So what is it that wears teeth down? A basic understanding of wear in industrial processes suggests that it is probably something really hard, such as silica.
In the context of chewing, resistance to indentation — or hardness — is the crucial variable. In their experiments, Lucas et al. used an orang-utan molar tooth as a proxy for the thick-enamelled fossil teeth of our ancestors and close relatives. They measured its hardness (which ranged between 4.08 and 5.72 gigapascals), and compared it with the hardness of two sorts of phytolith, deriving from squash (which had a hardness range of 0.43–1.74 GPa) and grass (1.35–4.24 GPa), and of Kuwaiti desert dust (10.1–14.1 GPa). Phytoliths are silica particles produced by plants as a defence mechanism, but they are non-crystalline and, as can be seen from these hardness measurements, they are softer than both quartz grit and enamel.
The authors mounted individual microscopic particles of the phytoliths, quartz dust and chips of enamel from a tooth on a nanoindenter (a device used for measuring wear effects on materials at the nanoscale) and then slid these across the surface of tooth enamel at a known force and angle. They found that neither the phytoliths nor the enamel chips resulted in any loss of volume of the tooth; the particles marked it, but they did not abrade it. Only the quartz dust resulted in the volume loss that would eventually accumulate to produce the type of wear on a tooth that is visible to the naked eye (Fig. 1).
The authors also found that the enamel was abraded only if the quartz particles have the right angle of 'attack'. The critical attack angle is set by the toughness of the tooth tissue; it is low for enamel, but higher for dentine, which is softer but tougher. Without the right geometry, even very hard particles will not abrade enamel.
All methods for reconstructing evolutionary history begin with observations about pattern, but what really matters are the processes that generate the pattern. Interpretations based on dental microwear are at the stage of shifting from questions about the patterns to more fundamental questions about the basic physics of the processes that determine them, and these findings suggest that dental wear results from the dust and grit that is ingested along with food5,6, rather than by factors intrinsic to the food.
So what do these results mean for attempts to reconstruct the diet of our ancestors and their close relatives? As well as dental microwear, the shape and size of the skull and teeth can be used to generate hypotheses about the types of food (hard versus soft, tough versus weak) that these primates were adapted to eat. The chemicals that accumulated in their bones and teeth also reflect the types of vegetation (dry tropical grass versus leaves) consumed7. And dental macrowear can be used to interpret how quickly the enamel covering has been removed from the teeth. In many cases, these lines of evidence lead to similar conclusions, but in one of our close evolutionary relatives, Paranthropus boisei, the microwear on the teeth8 is not consistent with the extreme dental macrowear seen in this taxon (Fig. 1). Lucas and colleagues suggest that this extreme macrowear is not caused by diet, but instead is most likely to be the result of these creatures literally 'gritting their teeth'.
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Distinguishing dietary indicators of pastoralists and agriculturists via dental microwear texture analysis
Surface Topography: Metrology and Properties (2015)