Hard plant foods may have made up a larger part of early human ancestors’ diet than currently presumed, according to a study of modern tooth enamel from Washington University in St. Louis.
Scientists often look at microscopic damage to teeth to infer what animals have eaten. The new research, using experiments looking at microscopic interactions between food particles and enamel, demonstrates that even the hardest plant tissues scarcely wear down primate teeth. The results have implications for reconstructing diet and potentially for our interpretation of the fossil record of human evolution, the researchers said.
“We found that hard plant tissues such as the shells of nuts and seeds barely influence microwear textures on teeth,” said Adam van Casteren, PhD, MSc, lecturer in biological anthropology in arts and sciences and first author of the study, who also noted that many people believe that hard foods can damage teeth by producing microscopic pits.
“But if teeth don’t demonstrate elaborate pits and scars, this doesn’t necessarily rule out the consumption of hard food items,” said van Casteren.
Humans diverged from non-human apes about 7 million years ago in Africa. The new study addresses an ongoing debate surrounding what some early human ancestors, the australopiths, were eating. These hominin species had very large teeth and jaws, and they also likely had huge chewing muscles.
“All these morphological attributes seem to indicate they had the ability to produce large bite forces, and therefore likely chomped down on a diet of hard or bulky food items such as nuts, seeds, or underground resources like tubers,” van Casteren said.
But most fossil australopith teeth don’t show the kind of microscopic wear that would be expected in this scenario. The researchers, then, decided to test it out.
Previous mechanical experiments had shown how grit, comprising pieces of quartz rock, produces deep scratches on flat tooth surfaces using a device that mimicked the microscopic interactions of particles on teeth. But there was little to no experimental data on what happens to tooth enamel when it comes in contact with actual woody plant material.
For the study, the researchers attached tiny pieces of seed shells to a probe that they dragged across enamel from a Bornean orangutan molar. They made 16 “slides” representing contacts between the enamel and three different seed shells from woody plants that are part of modern primate diets. The researchers then dragged the seeds against the enamel at forces comparable to any chewing action.
The seed fragments made no large pits, scratches, or fractures in the enamel. There were a few shallow grooves, but the researchers saw nothing that indicated that hard plant tissues could contribute meaningful to dental microwear. The fragments themselves, though, showed signs of degradation from being rubbed against the enamel. The researchers said this information is useful for anthropologists who are left with only fossils to try to reconstruct ancient diets.
“Our approach is not to look for correlations between the types of microscopic marks on teeth and foods being eaten, but instead to understand the underlying mechanics of how these scars on tooth surface are formed,” said van Casteren. “If we can fathom these fundamental concepts, we can generate more accurate pictures of what fossil hominins were eating.”
Those big australopith jaws could have been used to chew large amounts of seeds without scarring teeth, the researchers said.
“And that makes perfect sense in terms of the shape of their teeth,” said Peter Lucas, coauthor of the study and member of the Smithsonian Tropical Research Institute, “because the blunt low-cusped form of their molars are ideal for that purpose.”
“When consuming many very small hard seeds, large bite forces are likely to be required to mill all the grains,” said van Casteren. “In the light of our new findings, it is possible that small, hard objects like grass seeds or sedge nutlets were a dietary resource for early hominins.”
The study, “Hard Plant Tissues Do Not Contribute Meaningfully to Dental Microwear: Evolutionary Implications,” was published by Scientific Reports.
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