Ancient teeth rarely have a cavity-causing bacteria commonly seen today. A new study reveals why

(CNN) — Two teeth from a man who lived approximately 4,000 years ago have been discovered to have an abundance of bacteria that primarily cause tooth decay and gum disease. The rare find could help scientists further understand how changes in the human diet have led to the prevalence of cavities today.

Uncovered during two excavations from 1993 and 1996, the teeth were among several human teeth and other remains found within a limestone cave in County Limerick in Ireland. The two molars sampled, dating between 2280 and 2140 BC, were both from the same person who lived during the Bronze Age, according to the paper published Wednesday in the journal Molecular Biology and Evolution.

One tooth had a surprising abundance of Streptococcus mutans (S. mutans), an oral bacteria that causes cavities. The bacteria is rare in the ancient genomic record, likely because it does not preserve well due to its acid-producing nature that causes decay and DNA degradation within teeth, said Lara Cassidy, senior author of the paper and an assistant professor in the department of genetics at Trinity College Dublin.

Researchers also believe the bacteria is not as commonly found within ancient teeth because the human diet included less refined sugar and fewer processed foods than are consumed today, Cassidy said. A significant dietary shift was seen with the start of agriculture around 10,000 years ago, but the past few hundreds of years have seen major changes with the popularization of sugar, she added.

A connection between dietary changes and tooth decay

It is unclear why the bacteria on the newly discovered tooth were extremely well preserved, but Cassidy said the cool and dry conditions of the cave were likely factors.

While cavities have been observed on other ancient teeth finds, S. mutans has only been discovered in very low amounts in a handful of remains, such as an older, Neolithic tooth from southwestern France (dating between 3400 and 2900 BC) or a chewed pitch from the Scandinavian Mesolithic (dating between 9890 and 9540 BC). Observations of cavities from other ancient teeth become more frequent following the adoption of cereal agriculture, the farming of grains such as wheat and barley, according to the paper.

By analyzing the bacteria found on the Bronze Age teeth and comparing it to modern samples, researchers found the ancient S. mutans evolutionary tree to be more complex than originally thought — and had found the ancient bacteria’s traits, such as the virulence (ability to cause damage), to have evolved alongside changes in human diet, including the popularization of sugar and cereal grains, Cassidy said.

“The past few hundred years has seen a crazy amount of change (in the human diet), so particularly understanding how that has impacted the microbiome (the microorganisms, such as bacteria, that naturally live on and inside human bodies), not just the oral microbiome, the gut microbiome as well, might just help us understand a bit about why certain diseases have become so prevalent in Western populations or westernized populations in the past few centuries,” she added.

Oral health of the Bronze Age

No signs of tooth decay were found on the Bronze Age teeth, but if the adult male they belonged to had lived a little longer, the abundance of bacteria present suggests he would’ve soon developed cavities, Cassidy said.

The two teeth also contained DNA evidence of Tannerella forsythia (T. forsythia), a bacteria involved in gum disease that is more commonly found within the ancient genome record. But the researchers had found two distinct strains of the bacteria within the teeth — today, only one strain of the bacteria is commonly seen, implying that the ancient microbiomes were much more diverse than modern microbiomes. The biodiversity loss is of concern as it can have negative impacts on human health, according to a news release from Trinity College Dublin.

The several other teeth found within the cave showed signs of tooth decay, but it is unknown whether those remains are from the same person or other members of the community as they were found disarticulated, separated from other skeletal remains, Cassidy said. “It’s possible that other teeth from his mouth did have cavities on them, or other members of his community were suffering from dental disease.”

Analysis of ancient S. mutans suggests the bacteria has become more prevalent in recent centuries due to the consumption of sugar, which has created a favorable habitat for the species within human mouths, Cassidy added. By understanding the lineages of the modern cavity-causing bacteria, it further helps scientists understand how dietary change can impact oral health today, she said.

The analysis of the ancient S. mutans in comparison to the modern S. mutans “revealed a major change within the last few hundred years linked to increased consumption of sugar” and supports previous research that had found higher rates of cavities after refined sugar became widely available in the 19th century, said Louise Humphrey, research leader at the Centre for Human Evolution Research at the Natural History Museum, London, who was not involved with the study.

“The oral microbiome has implications for many areas of human health and disease. … ancient teeth can help us understand how the human oral microbiota (range of microorganisms) has evolved over time and the impact of these changes on human health in the past and today,” Humphrey said in an email.

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