Archaeologically speaking, it's a difficult question to answer. Cavemen dentists were notoriously poor record keepers. And while bones can survive the march of time, biological material like chewed food isn't as hearty. That makes it hard to know, say, what a cavewoman ate for dinner on a chilly night in northern Spain, or whether she preferred Colgate or Crest.
Karen Hardy may have cracked the mystery, literally, by breaking down calcified plaque from some of the oldest human remains in Europe.
“The dental plaque is a film that covers your teeth and that's why you have to brush your teeth every day,” she told The Washington Post. “If not, it hardens and becomes calcified. Within about 10 days, it’s attached onto your tooth as this extremely hard material that you can’t get off unless you go to the dentist.”
If you can't make it to the dentist, you could also have an archaeologist chisel some off your teeth a million years from now.
That's what Hardy did with a fossil from the Sima del Elefante archaeological site in Atapuerca, a mountainous region in northern Spain.
The site “contains a rich fossil record of the earliest human beings in Europe,” according to UNESCO. The bones provide “an invaluable reserve of information about the physical nature and the way of life of the earlier human communities.”
Researchers also found painted and engraved panels on the cave walls, complete with hunting scenes and animal figures.
But Hardy had a specific goal: She wanted to know what those early humans put in their mouths. Modern technology and a million-year-old mandible helped her find out.
She scraped off some of the calcified plaque, then broke it down to find microscopic evidence of what was preserved inside.
Turns out, a lot. She was able to discern that they ate grass, seeds, other plants and meat — all raw, indicating they didn't yet use fire to cook. She also found spores, tiny insect fragments and pollen grains — things they inhaled because they likely lived in a forest.
But the most compelling thing were pieces of indigestible wood fibers. Hardy believes they're from small sticks early humans would jam in their teeth to clean them.
“We all get stuff stuck between our teeth,” she said. “I haven’t done the experiment of eating raw meat, but if you think about all the fibers and the tendons in meat, it would probably be worse with a raw diet.”
Researchers like Hardy have spent a lot of time exploring how people kept their teeth clean throughout history.
People who lived in Sudan 2,000 years ago, for example, chewed purple nutsedge, a bitter weed whose antibacterial properties warded away cavity-causing bacteria, according to National Geographic.
Our oldest ancestors had great teeth, despite the lack of toothbrushes, toothpaste and lies to dentists about daily flossing. But as humans transitioned from hunting and gathering to farming, tooth-decaying bacteria that feast on carbohydrates proliferated in human mouths, according to NPR. The industrial revolution made things worse, pumping sugar and processed flour into our diets.
Our teeth are whiter and straighter than our ancestors' but also more likely to develop cavities. We've also replaced sticks with dental floss, although an Associated Press study recently cast serious doubt on the practice.
Researchers have long suspected that early humans wedged sticks into their teeth to clean them, Hardy said.
Chimpanzees, which are connected to humans via a common ancestor, use sticks and pieces of grass to clean between their teeth.
And ancient fossils of teeth have tiny holes on the sides, called interproximal grooves, that are likely caused by repeated cleanings with sticks.
Hardy insists her findings are from a small data set — the plaque from one fossil.
But her research is able to be replicated because of the hearty nature of the film on our teeth.
“Once it's there it stays there,” she said. “It's kind of like a tattoo of biological information — a personal time capsule.”