OUR mouths are now in ''a permanent state of disease'' because the refined modern diet has dramatically decreased the diversity of oral bacteria.
After studying 34 prehistoric human skeletons from northern Europe, an international team of researchers has established for the first time that DNA can survive within calcified plaque, tartar, for more than 8000 years.
That DNA trapped in the tartar reveals that the meat-dominated, grain-free diet of the hunter gatherers gave our ancestors much healthier mouths.
Published in Nature Genetics, the research shows that declining oral health can be pegged to major changes in the way humans lived and ate, with the start of farming in the Neolithic age and the industrial revolution being key turning points in human health.
The arrival of farming in Europe around 8000 years ago and the industrial revolution in the 18th and 19th centuries each increased the amount of refined carbohydrates and sugars humans consumed, which led to our mouths being dominated by cavity-causing bacteria.
''That's when you see a really big drop in diversity and a really significant rise in bacteria associated with dental caries, which cause holes in the teeth,'' said Alan Cooper, director of the Australian centre for ancient DNA at the University of Adelaide.
Professor Cooper, who led the study, said the natural human bacterial ecosystem found in people's mouths benefited from diversity. ''We have been raised in Western medicine to think that bacteria is bad,'' he said. ''But a diverse bacterial ecosystem is a healthy and resilient one. It's hard to upset.''
Professor Cooper said there had typically been a reduction of between 30 to 40 per cent in the rate of diversity of bacteria since the Stone Age. Within that there was much greater representation of disease-causing bacteria.
''So as the diversity comes down, the proportion of the bacteria that is disease-causing goes up,'' he said.
Christina Adler, an associate lecturer in dentistry at Sydney University, said the change in mouth health had wider implications, as oral bacteria was associated with a range of other diseases such as diabetes, heart disease and some cancers.
''We're in a permanent disease state now,'' Professor Cooper said.