Irish Bronze Age teeth reveal human diet evolution

Teeth from 4,000 years ago reveal bacteria which cause gum disease and tooth decay.

It’s no surprise that, without modern dental care, ancient oral health isn’t quite up to today’s standards. This latest study, published in the journal Molecular Biology and Evolution, helps archaeologists track the development of dental health over thousands of years.

Both teeth analysed belong to the same ancient man. They were found in a limestone cave in Ireland and date to the Bronze Age in the British Isles (4,500–2,800 years ago).

A cave in ireland woods
Killuragh Cave, County Limerick, Ireland. Credit: Sam Moore and Marion Dowd/Molecular Biology and Evolution.

Because genetic material is so well preserved in teeth, the oral cavity is one of the best studied aspects of the ancient human body. But getting a full genome of bacteria found in human mouths from before the Medieval era has proven challenging.

The study analyses the microbiome – the collection of microorganisms that live in our bodies – found in the ancient teeth.

DNA extracted from the teeth includes the first high-quality ancient genome from Streptococcus mutans – the bacterium which is a major cause of tooth decay.

The lack of S. mutans in ancient mouths could suggest less favourable conditions for the bacterium in the oral cavities of ancient humans. Previous studies have shown a marked increase of S. mutans in dental remains following the beginning of cereal agriculture, but cavities become much more common after about 1500 CE coinciding with higher sugar consumption.

“We were very surprised to see such a large abundance of mutans in this 4,000-year-old tooth,” says senior author Lara Cassidy, an assistant professor at Trinity College Dublin. “It is a remarkably rare find and suggests this man was at high risk of developing cavities right before his death.”

Another reason S. mutans could be rare in the ancient record is because it produces acid which can break DNA down. Because the cave at Killuragh, County Limerick provides a cool, dry and alkaline environment, it may have helped preserve S. mutans.

But other teeth found at the site don’t have the same prevalence of the tooth decay-causing bacterium. This suggests dysbiosis in the microbiome of this individual – S. mutans had outcompeted other bacterial species leading to a pre-disease state.

This supports a “disappearing microbiome” theory which suggests that the microbiomes of our ancestors were more diverse than today.

Also extracted from the Irish teeth were 2 genomes which show highly divergent strains of a bacterium involved in gum disease, Tannerella forsythia. Over the last 750 years, one strain of T. forsythia has become dominant globally.

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