Now, it's no surprise that Neanderthals didn't brush their teeth. Nor did they go to the dentist.

That means bits of food and the microbes in their mouths just stayed stuck to their teeth. While not so good for dental hygiene, these dental plaques are a great resource for scientists interested in understanding more about Neanderthal diet and lifestyle.

Luckily for researchers, there is an abundance of Neanderthal teeth in the fossil record. "We have complete jaws with teeth, we have upper jaws with skulls with teeth intact, isolated teeth," says Keith Dobney, an archaeologist at the University of Liverpool.

He and his colleagues have been studying Neanderthal dental plaques — or rather, the hardened version of plaque, tartar, or what scientists call dental calculus. They scraped off some of the calculus and analyzed the DNA that was preserved in it for clues to what the Neanderthals ate.

They looked at plaques from the teeth of three Neanderthals living in Europe about 50,000 years ago. One individual was from a cave in Spy, Belgium, and the other two were from El Sidrón cave in Spain.

As they report in a study published in this week's Nature, the Belgian individual ate mostly meat. "We found evidence of woolly rhino. We found the DNA of wild sheep," says Dobney.

The researchers also found evidence of mushrooms, but this was certainly a meat lover. This isn't that surprising to scientists who study Neanderthal diets. After all, the butchered bones of woolly rhinos, mammoths, horses and reindeer had been found in the Spy cave and other sites, suggesting a meat-heavy diet.

There had also been other indirect sources of evidence of carnivory, like high levels of a certain nitrogen isotopes, which suggested meat- and/or mushroom-heavy diets.

"Most Neanderthals that had been analyzed [before] were really heavy meat eaters," says Laura Weyrich, at the Australian Center for Ancient DNA at the University of Adelaide, Australia, and the lead author on the new study. She says those previous studies had suggested that "Neanderthals were as carnivorous as polar bears."

And this is where the new study offered a big surprise. According to the DNA in dental plaques, the Neanderthals in Spain ate no meat at all.

"We find things like pine nuts, moss, tree barks and even mushrooms as well," says Weyrich. "It is very indicative of a vegetarian diet, probably the true Paleo diet." (Not all of the region's Neanderthals were necessarily vegetarians: The El Sidrón cave also contained grisly evidence of cannibalism.)

She says the difference in diets reflects the fact that the two groups lived in two very different environments.

Northern Europe, including Belgium, had wide open spaces with grasslands and many mammals. "It would have been very grassy, and kind of mountainous," says Weyrich. "You can imagine a big woolly rhino wandering through the grass there." Perhaps tracked by hungry Neanderthals looking for dinner.

But farther south in Spain, the Neanderthals lived in dense forests. "It's hard to imagine a big woolly rhino trying to wedge themselves between the trees," says Weyrich. And so, she says the Neanderthals there feasted on all kinds of plants and mushrooms. "They're very opportunistic, trying to find anything that's edible in their environment."

"Perhaps we shouldn't be surprised that Neanderthals are adapting to local conditions and varying their diets," says Chris Stringer at the Natural History Museum in London. He studies human origins, but wasn't involved in the new study.

For example, Neanderthals living on the coast of Gibraltar "were collecting molluscs and baking them," he says. "They were butchering at least one seal. There [was] dolphin material at the site. That may have been stranded dolphin that they scavenged."

Stringer says it was the Neanderthals' adaptability that allowed them to thrive for tens of thousands of years across Europe and Asia.

"They were very evolved humans," he says. "They lived over a range of very different environments. They lived in different climatic conditions."

But Stringer cautions that the new study's findings probably don't reflect everything about the diets of these Neanderthals. "Not everything that you eat has an equal chance of getting incorporated into the calculus," he says. "And not everything has a chance of being preserved long term."

Perhaps more surprising than the clues about diets is what the DNA revealed about other aspects of Neanderthal life. The scientists uncovered the identities of more than 200 different species of microbes that lived in the mouths of these Neanderthals. It also gave clues to some diseases that might have ailed them.

One of the individuals in Spain seems to have had a painful tooth abscess and was suffering from a stomach bug. "We saw that he also had Microsporidia, which is a gastrointestinal pathogen," says Weyrich.

That means he probably had diarrhea and was throwing up. "He was a sick individual," says Weyrich. "He was a young adolescent male. He was mostly with ... females. So we like to think of him as this sick boy that the females were dragging along with them."

But what's more remarkable, she says, is that DNA in his dental plaque suggests he was self-medicating by eating the bark of poplar trees. "And poplar bark contains salicylic acid, one of the natural sources of what we call aspirin," she says.

Even more surprising was that they also found evidence of Penicillium in his plaque. That's the mold that makes the antibiotic penicillin. "It's pretty phenomenal that these guys were so in tune with their environment and to know what was going on and how to treat things," says Weyrich.

But the surprises didn't end there. Weyrich and her colleagues also identified the DNA of a microbe that causes gum disease in humans today. "We were able to track back that this particular microorganism was actually obtained from humans, likely about 120,000 years ago." Weyrich and her colleagues don't believe the microbe caused disease in Neanderthals, but they think it tells a fascinating story about how the two species — our ancestors and Neanderthals — interacted.

Genetics has shown that the two interbred and swapped genes. "A lot of these breeding interactions had been thought to be rough interactions, something that wouldn't be sensual or enjoyable," says Weyrich. But if they were swapping microbes in their mouths, that suggests a different story: "It suggests that there's kissing — or at least food sharing — going on between these two groups. So we really think that those interactions were probably more friendly, and much more intimate, than what anyone ever imagined before."

Weyrich and her colleagues think that in the years to come, we will learn a lot more about Neanderthals and other human ancestors, just by studying the ancient DNA trapped in their dental plaques.

"It's a very exciting paper," says Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology, who was not involved in the new study. "It opens a new window into the past, a new way to investigate [the] life and behavior of Neanderthals."

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