When we think of our human ancestors, we think that they mainly ate meat. But hard plant foods may have made up a larger part of the diet of early human ancestors than is currently assumed, according to a new study. This theory is based on the study of modern dental enamel at Washington University in St. Louis.

Hard plants were the diet of Australopithecines
Scientists often look at microscopic damage to teeth to infer what an animal was eating. This new research – which draws on experiments investigating the microscopic interactions between food particles and tooth enamel – shows that even the hardest plant tissue wear very little on primate teeth. These findings have implications for the reconstruction of the diet, and potentially for our interpretation of the fossil record of human evolution, the researchers said.

“We found that hard plant tissues such as nut and seed shells hardly influence the micro-textures of teeth,” said Adam van Casteren, senior lecturer in biological anthropology and first author of this new study.

Traditionally, it is believed that the consumption of hard foods damaged the teeth by producing microscopic cracks. “But if the teeth do not have pits and elaborate scars, that does not exclude the consumption of hard foods,” said Van Casteren.

Humans diverged from non-human apes about seven million years ago in Africa. This new study examines an ongoing debate about what some of man’s earliest ancestors ate; australopithecines. These hominid species had very large teeth and jaws, and probably enormous muscles to chew food well.

Australopithecines could produce strong bites
“All of these morphological attributes seem to indicate that they had the ability to produce strong bites, and therefore likely to feed on hard foods such as nuts, seeds or underground resources like tubers,” van said. Casteren.

Previous mechanical experiments had demonstrated how gravel – literally, pieces of quartz rock – produced deep scratches on the flat surfaces of teeth, using a device that mimicked the microscopic interactions of particles on the teeth. But there was little or no experimental data on what happened to tooth enamel when it came into contact with actual woody plant material.

For this study, the researchers attached tiny pieces of seed shells to a probe and rubbed it on the enamel of a Bornean orangutan molar. They made 16 “slides” showing the contacts between the enamel and three different seed shells from woody plants that are part of modern primate diets. The researchers rubbed these seeds against the enamel with forces comparable to those of chewing.

There were no signs of scratches on the enamel
The researchers found that the seed fragments did not make large holes, scratches or fractures in the enamel. There were a few shallow grooves, but scientists saw nothing that indicated that hard plant tissue could contribute significantly to tooth micro-wear. However, the seed fragments themselves showed signs of degradation by rubbing against the enamel.

This information is useful to anthropologists who have only fossils to try to reconstruct ancient diets. “Our approach is not to look for correlations between the types of microscopic marks on the teeth and the foods eaten – but rather to understand the mechanisms underlying the formation of these scars on the surface of the teeth,” said Mr. van Casteren. “If we can probe these fundamental concepts, we can generate more accurate images of what ancient humans ate. “

The seeds did not damage their teeth
So those large jaws of Australopithecus could have been used to chew large amounts of seeds – without damaging their teeth. “And that makes perfect sense when it comes to the shape of their teeth,” says Peter Lucas, a co-author of the Smithsonian Tropical Research Institute, “because the blunt, low-curvature shape of their molars is ideal for doing this kind of work. “