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Rare type of tooth decay found in a 2 million-year-old hominin fossil

Carious lesions, or cavities as they are commonly called, are often thought of as a new disease that is a consequence of our modern diet. However, caries has now been identified in a variety of primate species, including our fossil ancestors. In a new study published in the Journal of Human Evolution, researchers from Liverpool John Moores University and the University of Florence describe a rare type of carious lesion in a fascinating relative of ours, Paranthropus robustus.

The Paranthropus genus were remarkable members of our family tree. They had extremely large back teeth, along with massive jaws and cheeks. These features are thought to have evolved so they could cope with a diet rich in tough vegetation. Paranthropus robustus was one of several species in this group and their remains have been found in abundance in several South African locations, all situated within the 'cradle of humankind' about 50km from Johannesburg.

The new cavity was found on the root surface of a third molar, also known as a wisdom tooth. The root would have been exposed in the mouth, likely through gum disease. Given that third molars are the last teeth to erupt, and the tooth was extremely well worn, this suggests the lesion likely directly related to the individuals old age. This is supported by evidence in humans today, in which these lesions are typically associated with ageing.

The remarkable fossil is from the site of Drimolen and dates to around 2 million-years-ago. A micro‐CT scan of the tooth was used to see how far the lesion had extended into the tooth. If a cavity reaches the tooth's pulp chamber then an abscess can form in the jaw, and without dental treatment can have severe consequences. However, in this case the lesion had not reached deep enough into the tooth for this to occur.

The CT scan also highlighted an interesting band of higher density material on the outside of the lesion. This at first sounds odd since caries is a destructive disease. However, although enamel can do little in response to caries, the underlying dentine can produce new material, called reparative or tertiary dentine. Often this forms surrounding the pulp chamber to protect the tooth. This type of dentine only usually occurs on the outside of the tooth if the oral environment has changed and the cavity is no longer progressing. This suggests this individuals diet may have changed, or perhaps the adjacent tooth was destroyed due to caries or wear, meaning there was no longer a place for cariogenic food and bacteria to accumulate.

This new carious lesion takes the total number described in Paranthropus robustus teeth to 10. Beyond the consumption of caries‐causing food, such as honey or certain plants and fruits, cariogenic bacteria were also clearly common in our distant ancestors mouths. This new lesion highlights a rare type of cavity (root caries) and is also the first in the fossil record that shows signs of 'healing'. Given the high prevalence of lesions in this species, comparable to many human populations, it is likely this species ate substantial amounts of sugary foods. More broadly this research highlights that tooth decay is an ancient disease and has been afflicting our ancestors for millions of years.

Lead author of the research Ian Towle adds: "this adds to our understanding of caries in the hominin fossil record. It is becoming clear that carious lesions were relatively common throughout human evolution and therefore cariogenic foods must have been regularly consumed."

More information:
Ian Towle et al. Root caries on a Paranthropus robustus third molar from Drimolen, American Journal of Physical Anthropology (2019). DOI: 10.1002/ajpa.23891

Provided by Liverpool John Moores University