The Gunk on Old Teeth Could Help Scientists Map Ancient Migrations

The DNA in dental plaque contains a gold mine of information about the past—and may even help demystify the settlement of Polynesia.

Left undisturbed by brushing and flossing, the bacteria in your mouth will form a sticky film called plaque. Left further undisturbed, plaque will turn hard and yellow, calcifying on teeth as dental calculus, also known as tartar.

At this point, the tartar is very durable. Just ask these Neanderthals, whose 40,000-year-old tartar scientists recently analyzed to figure out the real paleo diet. Tartar grows in layers—almost like tree rings—entombing DNA from tiny bits of food as well as bacteria in the mouth. Forty thousand years later, scientists can analyze that DNA to reconstruct what was going on in the mouths of long-dead Neanderthals.

Having traveled so far back in time using ancient tartar, some of the same scientists have embarked on a more ambitious project: using the DNA from the bacteria in tartar to figure out how humans settled the 10 million square miles of Polynesia.

Polynesia has confounded the traditional ways of tracing human migration—archaeology, linguistic analyses, even human DNA—because large parts of it were settled so fast. Humans first reached the Society Islands, in the center of the Polynesian Triangle, perhaps around 1,000 AD. Then in the span of just a couple hundred years, they took canoes across vast tracts of open ocean to find specks of inhabitable rock as far-flung as Hawaii, New Zealand, and Easter Island. How Polynesians navigated these waters in the 11th century is a subject of considerable fascination. But even more basically, archaeologists are not sure exactly when the islands were settled and in what order. That’s where the tartar comes in.

Because bacteria are constantly dividing, they accumulate frequent mutations in their DNA. By comparing the mutations in tartar found in one island and the next, the researchers hope to figure out whether early settlers systematically hopscotched across Polynesia or skipped certain islands. “The traditional means of looking at human migrations might be too coarse. Hopefully, the rapid rate of evolution in that bacteria will allow us to answer some of the questions,” says Raphael Eisenhofer, a Ph.D. candidate at the University of Adelaide, and coauthor of a recent paper proposing the use of tartar to track human migrations.

He and a group of DNA researchers, as well archaeologists, have since begun sampling teeth from Polynesia. Removing ancient tartar is not that different from modern dentistry, says Laura Weyrich, an ancient-DNA researcher at the University of Adelaide and Eisenhofer’s adviser. She pops it off with a dental pick. The group has collected hundreds of samples from collections of the Natural History Museum in London and other museums.

It’s much easier, Weyrich says, to convince museums to give up some tartar than to sacrifice a bone for human DNA analysis. In fact, museums traditionally cleaned the gunk from teeth—to better analyze their shape and to make them look better on display. “From my perspective, it’s incredibly frustrating. It’s like, ‘Noooo, what did they do?’” says Weyrich. More often than not, the most famous specimens have been cleaned. “We’re looking for the skulls that were left in the closet and still dusty and nobody cleaned,” she says.

Back in Adelaide, Eisenhofer is sequencing the samples in a lab built specifically for ancient DNA. Working with ancient DNA is tricky because samples are usually small and degraded, easily swamped by the modern DNA shed by bacteria living in and on our bodies. To prevent contamination, he wears a suit, a mask, and three pairs of gloves.

The team has picked out dozens of strains of common mouth bacteria that bind to the tooth surface and are commonly found in plaque. And they hope to sequence specific genes and track mutations in them over time. Scientist have analyzed specific pathogens—such as Helicobacter pylori in the gut—as proxies for human migration before, but Weyrich and Eisenhofer hope to extract more information by studying the overall community of mouth bacteria. There could be a lot of information hidden in the gunk that museums once threw away.

When Weyrich samples tartar from teeth, she’s careful to pick jaws that have more than one tooth and to always leave some tartar behind. You don’t want to destroy it all, she explains, because you never know what techniques might come along in the future.