The idea took root with the offer of an unusual gift—a hefty chunk of ancient dental tartar. In 1996, while a postdoc at the University of Oxford, biological anthropologist Alan Cooper visited archaeologist Keith Dobney at York University. They had already discussed several ideas, including how they might use ancient tartar—otherwise known as dental calculus—when Dobney pulled from his desk drawer a matchbox-size brownish-grey block of the stuff, taken from a 1,000-year-old skeleton. “I was amazed,” recalls Cooper.

Dobney, now at the University of Aberdeen in Scotland, had been using scanning electron microscopy to examine the calculus for traces of food when he noticed an abundance of fossilized bacteria. He suggested it might also be a source of extractable bacterial DNA—a prize that could offer a glimpse into the evolution of the human microbiota and of human disease. Cooper was keen to try, but knew that even the most sterile lab consumables—from plasticware to enzymes—contained traces of bacterial DNA, so he was never confident that his early results were free from contamination.

Almost 2 decades later, armed with ultrasterile consumables and an ultraclean lab at the University of Adelaide’s Australian Centre for Ancient DNA—where he is now director—Cooper and his colleagues have used DNA extracted from dental tartar to trace how the bacterial communities that live in our mouths have changed over the past 8,000 years (Nature Genetics, 45:450-55, 2013).

Dental calculus develops when plaque, a dense bacterial biofilm that accumulates at the base of teeth, is mineralized when calcium and phosphorus in saliva combine to form calcium phosphate. That hardening process locks bacteria into a crystalline matrix similar to bone, entombing and preserving dead microbes for thousands of years.

“It’s clear that the dental plaques and associated calculus of living individuals contain valuable biological and medical information,” says Christina Warinner, an archaeological geneticist at the University of Oklahoma who was not involved in the study. “It’s also clear that dental calculus deposits preserve over very long periods of time in the archaeological record. So at this point, the question really is, does the information contained in dental calculus also survive that long?”