Neanderthals were ancient, compared to us. They lived long before civilisation, before even the most prehistoric dentists began experimenting with ways to tackle tooth decay.

So if you were to guess at what kind of teeth they had, you might expect the worst: a mouth full of rotting and missing teeth.

It is becoming clearer that this was far from the case. One recent study actually suggests that Neanderthals lost fewer teeth than humans with equivalent diets. What's more, another new analysis offers a hint that they used toothpicks to keep their teeth clean.

Neanderthals lived long before modern humans walked the Earth. Estimates suggest they first appeared between 300,000 and 250,000 years ago, and died out about 32,000 years ago.

Until recently, researchers studying ancient teeth simply scrubbed off the calculus

It was once believed that they were predominantly meat-eaters, hunting large game in the forested environments where they lived.

Their carnivorous habits seem to have included eating each other. The bones of 12 or 13 Neanderthals, found in El Sidrón cave in northern Spain, are covered in cut marks associated with butchery. Their skulls appear to have been split open so that others could get to the marrow inside.

If meat was all Neanderthals ate, it has been argued, then they were at a significant disadvantage to modern humans, who exploited many other food sources.

This view is quickly changing. While they certainly had a meat-rich diet, there was much more on their menu. We now know they were plant-eaters too.

We know this because scientists can analyse food remnants left on their teeth.

If you do not brush your teeth, plaque builds up and transforms into a hardened substance called dental calculus. This accumulates into a little hollow between your teeth and gums. The same was true of Neanderthals.

Until recently, researchers studying ancient teeth simply scrubbed off the calculus. "They thought it was just a waste product," says Karen Hardy, ICREA research professor at the Autonomous University of Barcelona in Spain.

It's not really surprising that Neanderthals would have been self-medicating

However, this calculus has revealed unexpected surprises. In the last 10 years, Hardy and others have shown that it contains micro-fossils of ancient plants.

These tell us in great detail what our close relatives ate. For instance, we have evidence that they ate edible grass, nuts and legumes.

On top of that, Neanderthals were eating other strange things. In 2012, a team led by Hardy discovered that the Neanderthals from El Sidrón cave were self-medicating with medicinal plants.

The team looked at chemical traces on their teeth and found that they had been eating two plants with no nutritional value: camomile and yarrow. The latter has historical medicinal uses such as restricting the flow of blood, inducing sweating and even easing toothache, while camomile is known to calm an upset stomach.

"There was no other reason at all for Neanderthals to be eating them," says Hardy. "If you look at the animal kingdom, [most] animals self-medicate. It's not really surprising that Neanderthals would have been self-medicating."

This behaviour reveals that Neanderthals had a detailed knowledge of their environment. Eating plants with no nutritional value came at considerable risk: they first had to separate the harmless from the poisonous.

If this wood had no nutritional benefits, why were Neanderthals putting it in their mouths?

A genetic study published in 2009 offers a clue to how they did this. An independent team found evidence of a gene important for bitter taste perception.

"That's really important, because when you eat plants you have to be able to distinguish between plants that are poisonous and not," says Hardy. As toxins often taste bitter, it makes sense to avoid bitter food.

This gene may have been important for Neanderthals. It suggests that they could have exploited a wide range of plants without poisoning themselves in the process.

In 2016, Hardy and colleagues took another look at some 50,000-year-old teeth and found another surprise. In research published in the journal Antiquity, they discovered traces of conifer wood.

"Some parts of the tree you can eat, but this came from a part of the tree that is not edible," she says. If this wood had no nutritional benefits, why were Neanderthals putting it in their mouths?

Hardy proposes that Neanderthals were using their teeth as a "third hand" to hold onto objects. The dental wear patterns suggest they were using their teeth for more than just eating.

Conifer resin is known to have antibacterial properties

Women appear to have done so more than men, based on additional wear on their teeth. This points to "a gendered division of labour among individuals from the same group," the team says.

The Neanderthals could also have been using wooden toothpicks to pick or rub their teeth, as some apes and monkeys do today. The use of toothpicks dates back to long before the Neanderthals: 1.8-million-year-old fossils from Georgia reveal that a Homo erectus with gum disease was using a toothpick.

Alternatively, maybe the conifer wood was another medicine: conifer resin is known to have antibacterial properties.

As well as hinting at their intelligence and resourcefulness, Neanderthals' teeth might even tell us something about their attitudes towards each other. If so the teeth, not the eyes, are the windows of the soul.

Cassandra Gilmore and Tim Weaver of the University of California, Davis compared Neanderthal teeth to those of human hunter-gatherers with equivalent diets, as well as dozens of orangutan, chimpanzee and baboon teeth.

It has been suggested that other Neanderthals ground up their food for them

The research, published in the Journal of Human Evolution, found that modern humans actually had worse teeth. The Neanderthals kept theirs for longer and had fewer cavities.

"We realised nobody had directly compared Neanderthal [teeth loss] to modern humans, so we didn't realise Neanderthals had [slightly less] tooth loss," says Weaver.

This flies in the face of previous studies, which suggested that several Neanderthals lived long after losing all, or nearly all, their teeth.

But bizarrely, the finding that Neanderthals apparently had healthy teeth actually suggests something rather negative about them.

"Teeth are quite an important component in the way your body breaks down food," says Weaver. "If you lose your teeth you cannot process it."

The Neanderthals could also have been using wooden toothpicks to pick or rub their teeth

So it has been suggested that other Neanderthals ground up their food for them, and that finding Neanderthals without teeth is evidence that these disabled individuals were cared for. In other words, toothless Neanderthals have been proposed to be evidence of compassion.

Gilmore and Weaver's study calls that into question. There are just not enough cases of pre-death tooth loss, they argue, to support the idea that Neanderthals were compassionate individuals who cared for their sick.

The argument also looks weak when you consider that there is plenty of evidence that Neanderthals ate softer plant food and seafood, so they could have survived without meat.

This does not mean that Neanderthals were not caring for their sick, simply that teeth cannot be used as an argument that they did so, agrees Bence Viola of the University of Toronto in Canada.

All in all it's amazing what you can figure out from a few teeth.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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