A nuisance to dentists is now a boon for archaeologists. Researchers have successfully sequenced DNA from fossilized plaque on 700-year-old teeth.
Solidified plaque—called calculus, tartar, or that chalky stuff the dentist scrapes off—contains a whopping 25 times more DNA than ancient tooth or bone. And, in a paper published Wednesday in the American Journal of Physical Anthropology, Christina Warinner and colleagues detail how they‘ve used plaque in research, a process that could catch on as a way to gather otherwise unobtainable information about the ancient world.
Calculus is the only part of your body to fossilize while you're still alive. It begins as plaque, which forms when bacteria and bits of food meet saliva. If it’s not washed away by brushing and flossing, the calcium phosphate in saliva solidifies, depositing layer after layer of calculus where the teeth meet the gums. Bacteria, food, human DNA, and proteins are all trapped in the calculus like bugs in amber.
Warinner removed calculus from six Oneota Native American skeletons buried about 700 years ago at a site called Norris Farms in central Illinois. Since calculus is mostly made of bacteria, less than one percent of the ancient DNA was human. But there was still enough human DNA for Warinner’s team to sequence the whole mitochondrial genome of each of the six individuals. That took just 20 milligrams of calculus, which would be about the size of a grain of rice. The results matched a previous study of the genomes done on three of the individuals.
Most archaeological DNA research involves pulverizing a tiny bit of bone and then extracting DNA from the resulting powder. Because it’s not human tissue, calculus may offer an avenue for studying Native Americans and other groups who don’t want those chips of bone or teeth taken from their ancestors.
Mitochondrial genomes can reveal clues about how populations migrated and changed over time, and whether cultures were matrilocal—meaning husbands moved to their wives’ villages. This would fill significant gaps in our understanding. “We don’t have very much information about North American genetic diversity in prehistory,” says Jennifer Raff at the University of Kansas, who was not part of the research.
“I think it’s really exciting,” says Anne Katzenberg at University of Calgary, who was also not involved in the study. “There’s a lot of promise.”
Getting DNA out of calculus was a back-burner project for Warinner. “People told me it won't work, there's no DNA in it, why would you even look at it?” she says. The first time she tried to measure whether she had isolated any DNA from the calculus, she got an error message from her equipment. But it turned out there was too much DNA for the instrument to measure accurately.
“This is the richest source of ancient DNA that's ever been described,” Warinner says. “Then I was doing a happy dance.”
Warinner is involved in negotiations with native groups in Oklahoma and Alaska to see if they would consent to calculus removal for study of their ancestors.
“Anywhere in the world where there’s a known cultural or biological connection with ancient human remains, you have to take into account and respect the wishes of the descendant communities. They get to decide,” Raff says. “It's really important that we do this with sensitivity.”