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 the DNA can survive within the calcified plaque, known as 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 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.
The arrival of farming in Europe about 8000 years ago and the industrial revolution in the 1800s 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.'' Having established that DNA can survive within the tartar for more than 8000 years, scientists now have an untapped source of information stretching back thousands of years. 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 drop, 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. ''They seem to be exploiting that change.''