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The first study focused on the bacterium that causes toothaches, known as Streptococcus mutans. This bacterium, which lives naturally in the mouth, metabolizes the sugars from food and excretes lactic acid, which wears away at the enamel covering the teeth. This process causes tooth decay and dental caries (Latin for “rot”), better known as cavities. In the new study, scientists analyzed S. mutans DNA extracted from teeth dating back to the Bronze Age. They found that the bacterium has been mutating rapidly throughout the course of human history, and becoming more diverse as the human population grows.

Researchers hope that by further understanding the development of S. mutans, they can figure out what factors cause it to grow, and how to fight it. As Marc Simón of the Universitat Autònoma in Barcelona, Spain, said to LiveScience of his team’s research: “Hopefully, it will allow us to predict how it will react under certain circumstances, and predicting its behavior might help us to fight it off in the future.”

Simón and his colleagues extracted the first S. mutans DNA from an ancient skeleton in 2007. In the current study, published this week in the journal Proceedings of the Royal Society B, they examined DNA extracted from 10 human skeletons, both European and American, dating from the Bronze Age (3200-600 B.C.) to the 1900s. The researchers analyzed a single fragment of the bacterial genome–the part that was known to give S. mutans the ability to cause disease–and found that it evolved neutrally, due to random genetic mutations that didn’t give the organism any special advantages. The results suggested that specific events (particularly the European invasion of North America) most likely did not cause S. mutans to evolve in any way, but that the bacterium did become more diverse as the human population grew. With so many more mouths to occupy, it seems, it was able to try on different mutations and still survive.

Simón’s team now plans to expand its study beyond a single gene fragment, delving further into the DNA analysis as well as examining the moment of the first European-North American contact more closely. They also plan to go even further back in time, to the Neolithic Period, to investigate whether dietary changes then caused any selection in the S. mutans gene.

In the other new cavity-related study, published last week in the open-source journal PLOS ONE, researchers analyzed bits of plaque on the teeth of ancient skeletons found in a cemetery in central Sudan. The results suggested that a plant known as purple nut sedge (Cyperus rotundus)–today regarded as a pesky weed–was an important part of the ancient humans’ diet. Study author Karen Hardy, also of the Universitat Autònoma, told the Washington Post that recent studies have indicated that the plant can fight the growth of Streptococcus mutans. Her team’s research supported that conclusion, as the skeletons they examined in Sudan had a surprisingly low number of cavities.

Used over a period of at least 7,000 years, the Sudanese cemetery spanned pre-agricultural periods, when humans relied on hunting and gathering for food, to the beginning of the agricultural age. In general, according to the fossil record, cavities increased with the shift from hunting to agriculture, particularly in Europe and North America, as fruits and vegetables are high in the carbohydrates, or sugars, that help S. mutans cause tooth decay. The prevalence of purple nut sedge in the Sudanese diet even after the advent of agriculture, the new study suggests, may indicate that our ancestors recognized the plant’s medicinal properties, as well as its usefulness as a food and energy source. According to Hardy, “These small biographical details add to the growing evidence that prehistoric people had a detailed understanding of plants long before the development of agriculture.”

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