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Ancient Skeletons Reveal the Cost of a Sweet Tooth

Sweet tooth

People today have far higher frequencies of decay-associated bacteria than during the agricultural or hunter-gatherer periods.

By Christina Adler

A genetic study of ancient oral bacteria in the calcified dental plaque of human skeletons shows that our ancestors had healthier mouths than us.

Oral diseases such as dental decay and gum disease are extremely common in most countries around the world. For example, 60–90% of school-aged children in developed countries have cavities.

While dental diseases are now commonplace worldwide, the frequencies of dental decay and gum disease were rare in the not so distant past.

We can estimate the frequency of oral diseases though human history by looking at skeletal remains. By studying teeth and bones, we can look for the presence of tooth decay in the form of cavities and gum disease that is evident from pitting in the jaw bones around the teeth.

Skeletal remains from individuals that lived a hunter-gatherer lifestyle have shown that these populations had very good oral health, with low frequencies of dental disease. However, the presence of dental decay and gum disease in the skeletal record started to become more frequent approximately 10,000 years ago, and again even more prevalent around 150 years ago.

The increased frequency of oral diseases among human populations over the past 10,000 years is thought to be associated with two major changes to our diet. The first big change to diet during human evolution was caused by the development of agriculture, which involved the farming of grains such as wheat and barley. The farming and consumption of these grains meant that humans switched from a more meat-based diet they had been eating as hunter-gatherers to a more carbohydrate-based diet.

The next major change occurred only around 150 years ago during the Industrial Revolution, and was associated with the addition of concentrated sugar to our diet. It was during this time that refined sugar was produced from sugar beet and cane. The consumption of refined sugar and grain forms the basis of modern diets in developed countries, and increasingly in developing countries.

Although we know from the dentitia of skeletons that the prevalence of dental diseases has altered as diet changed, this form of analysis does not tell us how past diet changes have impacted on the bacteria that actually cause these diseases. To examine this we need a source of preserved oral bacteria on the skeleton. Fortunately, the human body has made such a source for us to analyse – dental calculus.

Bacteria in Dental Calculus from Skeletons

Dental calculus is basically calcified dental plaque. Dental plaque covers our teeth and is made up of many hundreds and thousands of bacteria. These bacteria help to keep your mouth healthy, but can also cause decay and gum disease.

Dental plaque can turn into calculus when a persons’ saliva, which is normally at a neutral pH, becomes basic so that calcium and phosphate ions precipitate out of saliva into the plaque, resulting in calcification. As the plaque calcifies, the bacteria within it become encased in crystals.

The crystalline structure of dental calculus makes it as tough as bone. Hence, calculus can be found on many human skeletons that are thousands of years old (Fig. 1).
Adler 1Figure 1. The skull of an individual being excavated at an archaeological site. Molar and premolar teeth are exposed, from which we can remove any dental calculus present for genetic analysis.

In a recent study published in Nature Genetics, my colleagues and I used dental calculus to obtain genetic information about the bacteria within the samples, enabling us to investigate the impact of past diet changes on oral health. We wanted to obtain genetic information so that we could identify which bacterial species were present in the calculus deposits.

We collected dental calculus samples from European human remains. This included skeletons dating from before the introduction of farming 7500 years ago to the medieval period 400 years ago (Fig. 2). The samples were from individuals with different diets, including hunter-gatherers and agriculturists.
Adler2Figure 2. The right lower jaw of a medieval individual. The molar on the left has a deposit of dental calculus covering much of the enamel surface.

Sourcing the dental calculus samples involved visits to museums that housed collections of skeletons, such as at Aberdeen University in the United Kingdom. We also removed calculus during the excavation of skeletal remains at archaeological dig sites. To remove dental calculus from skeletons we used a tooth scalar much like your dentist would to clean your teeth.

From the collected dental calculus samples we extracted DNA. The genetic analysis was done in a specialised laboratory because the DNA molecules in samples that are thousands of years old are often very degraded. Hence, we needed to take special precautions and be very careful about not contaminating our samples with genetic material from other sources, such as ourselves.

DNA extracted from ancient dental calculus samples provides us with information about the bacteria present in the mouth of a person who lived thousands of years ago. To identify the bacteria present within the ancient dental calculus, we compared the microbial DNA sequences obtained from these samples with DNA from modern oral samples. This revealed that, at a broad level, the oral microbial make-up of humans today was similar to humans in the past. In other words, the same large groupings of closely related bacteria, or phyla, were present in both the ancient and modern oral samples.

The dominant group of bacteria in the ancient dental calculus samples belonged to the Firmicutes phyla (Fig. 3). If we were to genetically analyse a person’s oral environment today, most of their bacteria would also belong to the Firmicutes grouping.
Adler 3Figure 3. The average frequency of microbial phyla found in 34 samples of ancient dental calculus.

Impact of Past Diet Change on Oral Health

While at a broad level we found that the groupings of bacteria in our mouths have been relatively constant over time, there have been lots of changes in the composition of oral bacteria at a finer level as diets have altered. By this I am referring to changes in individual species of bacteria rather than groupings of these species.

We examined the impact of two diet changes on the composition of oral bacteria, which included the switch from a hunter-gatherer to agricultural diet and the transition from an agricultural to industrial (or modern) diet.

We found that the hunter-gatherers we examined had an oral environment that was not prone to developing either dental decay or gum disease. The bacteria present in the dental calculus from hunter-gatherers were primarily associated with a healthy mouth.

We found that the switch from a hunter-gatherer to agricultural diet did have a big impact on the composition of oral bacteria in human mouths. In comparison to the hunter-gatherers, the early agriculturists we examined had an oral environment that was more likely to develop cavities and gum disease. This finding was indicated by the increased frequency of bacteria associated with cavities, such as taxa from the bacterial group Veillonellaceae.

We also found an increased frequency of bacteria associated with gum disease among the individuals living an agricultural lifestyle, such as the species Porphyromonas gingivalis (Fig. 4). These findings are supported by the increased frequency of these diseases in the skeletal record of agricultural populations compared with hunter-gatherers.
Adler 4Figure 4. The frequency of bacteria associated with caries and gum disease over the past 6500 years. BP = before present; S. mutans = Streptococcus mutans; P. gingivalis = Porphyromonas gingivalis

The change from an agricultural to modern industrial diet has had an even larger impact on the oral bacterial composition of our mouths than the initial transition to agriculture. When we compared the oral bacteria present in people’s mouths today to people with an agricultural diet (before the Industrial Revolution), we found that people now have an oral environment that is much more susceptible to the development of caries, and diseases in general, compared with at any time before. People today have far higher frequencies of decay-associated bacteria, such as Streptococcus mutans (Fig. 4), than during the agricultural or hunter-gatherer periods.

How Can Information About Our Past Oral Health Help Us Improve Health Today?

By looking into the past to a time when oral disease were rare, we can draw conclusions about how we can get our oral health back to this state. Our research indicates that eating a diet low in carbohydrates, and in particular low in refined sugar, would help to reduce the current high levels of decay and gum disease.

Furthermore, to improve the composition of oral bacteria in our mouths and promote health we may want to think about developing probiotic treatments to replace harmful bacteria with good oral bacteria.

Christina Adler is an Associate Lecturer in the Faculty of Dentistry at the University of Sydney.