The best teeth that Christina Warinner sees in her office are the ones with chunks of plaque as big as lima beans fixed to their enamel. Warinner is not a dentist, although she uses some of the same tools. She is an anthropologist at the University of Oklahoma, and she performs centuries-delayed dental cleanings on the likes of Vikings and Stone Age farmers to scrape up details about how humans once lived.
Fresh, sticky plaque picks up everything in the mouth, and when it hardens, it can entomb bits of plants, pollen, bacteria, starch, meat, charcoal, textile fibers, and more. As scientists have recently learned, fossilized plaque is the richest source of DNA in the archaeological record. “One of the biggest challenges with ancient DNA is not having enough material to work with,” Warinner says. Plaque eliminates that problem: it carries between 100 and 1,000 times more nucleic acids per milligram than any other known source.
A top priority in Warinner's laboratory is to compile a DNA inventory built from the plaque found on corpses in museum collections and archaeological sites from around the world. The hunt now commences for increasingly ancient and diverse patients to find out how human health and eating habits have changed throughout history.
Three Plaque Projects
Without regular brushing and flossing, plaque fossilizes on teeth and serves as a time capsule for otherwise ephemeral bits of a person's diet. Long after a milk carton finds its way into the trash bin, for example, an extremely long-lasting and abundant milk protein called beta-lactoglobulin may remain on the drinker's teeth. Warinner's lab looks for this protein in plaque on ancient bodies to understand why many groups of humans can consume fresh milk without getting sick, even though all other mammals lack the ability to digest it after infancy. A tolerance for dairy shows up in several different cultures, but scientists debate when it first evolved in our species. Plaque analysis can reveal exactly who was drinking milk at an archaeological site and from which animal—be it a cow, or a sheep, or a camel—the liquid originated. This approach eliminates some of the ambiguity of other archaeological methods, such as searching for milk fats on ancient pottery. Last year Warinner's team found the first direct evidence for milk drinking by sequencing beta-lactoglobulin proteins in plaque dating as far back as the Bronze Age in parts of Europe and southwestern Asia. Now the researchers are examining plaque samples from the Neolithic period, when humans first domesticated animals.
Regardless of one's commitment to dental hygiene, every person harbors hundreds of species of bacteria on the surface of his or her teeth. Last year Warinner and her colleagues found that skeletons from a medieval cemetery in Germany had remarkably modern oral microbiomes despite changes in hygiene and diet over the past 1,000 years. The medieval plaque had bacteria associated with periodontitis, a common gum disease that causes teeth to fall out and occurs in nearly 50 percent of American adults today. To better understand when humans became susceptible to such dental diseases—and how they might be linked to factors such as diet, environment and culture—Warinner's lab is now scraping plaque from chompers dating all the way to the Stone Age and from humanity's closest living relatives, chimpanzees.
Filling the Holes
Bones buried in frozen ground can yield extremely well preserved genetic material. The oldest full genome sequenced so far (700,000 years old) came from a horse leg bone uncovered in permafrost in northwestern Canada. The oldest human genome (45,000 years old) to be sequenced was extracted from a femur found in Siberia. But not all biological samples are found in natural freezers. Densely mineralized plaque may be an ideal source for intact DNA both in and out of frozen areas because it is hardier than porous bones. To date, Warinner's team has reliably extracted DNA and proteins from plaque samples up to 10,000 years old. Now the scientists are working to collect genetic material from even further back in time. The oldest example of preserved plaque that Warinner knows of is more than eight million years old and was taken from a fossil of an orangutan ancestor.