These skeletons might be evidence of the oldest known mercury poisonings

If you lived in antiquity and you wanted to paint something, then your colors probably came from squeezing a natural material into powdered pigment. If you wanted red, you had a few options: ochre, for instance, or madder, which comes from plant roots. But those pigments cannot produce nearly as vivid a red as vermillion, which comes from a scarlet-colored rock, cinnabar—prized for its vibrancy throughout the ancient world.

Vermillion has one slight downside. Chemists might know cinnabar by another name: mercury(II) sulfide. Ingesting that mercury, as you might imagine, is toxic. 

In fact, researchers recently found mercury’s fingerprints in the archaeological record in Spain and Portugal. Testing mercury concentrations in bones from ancient Iberia, researchers singled out a period in the region’s history when its denizens used a great deal of cinnabar—and may have suffered the brunt of its toxic effects. If so, this would be the world’s oldest known cases of mercury poisoning.

“These people, who had no written language, were using this [cinnabar], and their bones are telling us something about their lives now,” says Steven D. Emslie, a biologist at the University of North Carolina Wilmington. Emslie and his colleagues published their findings in the International Journal of Osteoarchaeology on October 13.

In rock form, cinnabar isn’t toxic; the mercury is tightly bound to the sulfur. But when cinnabar is crushed into a powder, it becomes more dangerous. If you breathe cinnabar powder in, or if it makes skin contact—perhaps, by wearing vermillion-dyed clothes—it will enter your bloodstream.

The world’s richest known cinnabar deposits can be found at Almadén in central Spain. Almadén, a UNESCO World Heritage Site, reached its height in the 16th and 17th centuries, when liquid mercury extracted from cinnabar could be used to refine gold and silver ore that Spanish colonizers shipped back from the Americas. As Almadén’s importance grew, so did its infamy as a toxic place where prisoners and slaves withered away from mercury poisoning.

But the area churned out cinnabar long before Columbus—in fact, long before the Romans ruled Iberia. As early as 5000 BCE, its ancient denizens were mining and trading it.

This ancient time intrigued the study researchers. Archaeologists in Spain and Portugal sampled bone from 370 skeletons, found at 23 different sites across the Iberian peninsula. Emslie, who typically studies the bones of seabirds, offered to examine them.

[Related: Indigo, vermillion, and other ancient colors that have decorated the world for millennia]

The study authors weren’t looking for mercury at first, but a pattern quickly became clear. Bones dating from between roughly 2900 and 2300 BCE—part of a time period in Iberia that archaeologists call the Copper Age—had astoundingly high mercury concentrations.

Emslie and his colleagues found concentrations of mercury as high as 400 parts per million (ppm) in some Copper Age bones. Poisoning symptoms begin above 10 ppm in hair, another material used to measure mercury exposure, according to the World Health Organization. 

It’s hard to compare bone to the biomarkers typically used to measure mercury poisoning in humans today, which haven’t lasted those millennia. Hair, blood, and urine, for instance, tend to accumulate mercury quickly, and mercury will deposit in the liver, kidneys, and other organs. Bone will accumulate mercury much more slowly, and mercury there may indicate higher exposures elsewhere in the body.

“I don’t think this individual should be alive at those kinds of levels in bones, because the levels in the brain or the kidney or the liver would be significantly higher than that,” says Michael Aschner, a toxicologist at Albert Einstein College of Medicine in New York, who was not involved with the study.

 At the same time, bone “provides a record of exposure. That’s the important thing: it provides a historical record that would otherwise be lost to time,” says Paul Wax, executive director of the American College of Medical Toxicology, who was also not involved with the study.

The researchers wondered whether mercury seeped into the bone from the outside, but there wasn’t enough mercury in the surrounding soil to seep in the bones at such high levels. So the researchers settled on another culprit: cinnabar.

The mercury-laced bones mostly came from Copper Age tombs, elaborate affairs housing the resting places of multiple people—one tomb, for instance, belonged to seven women who may have been priestesses. They were buried with a wealth of artifacts that archaeologists presume were grave offerings.

Some of those tombs are also resplendent in cinnabar. It’s found across the tombs’ rocks and sprinkled on the bodies. “There was a period of time when cinnabar was really important in this community, in these populations,” Emslie says.

Aschner wonders if, because of the high mercury levels in the bone, “either there was some ritual after [death], or the mercury somehow got into the bone after the person was buried.”

But not all the tombs with mercury-poisoned skeletons contained cinnabar. Those ancient people, then, may have encountered cinnabar in their life. They may have ingested it by accident, but archaeologists have raised another theory: that the Iberians knew perfectly well what they were playing with. 

“They must have known that it was toxic for the amount of time it was being used, and they might have actually taken it as a drug, for ritual, because of the effect it gave them,” says Emslie. “We don’t know for sure. We’ll never know.”

Toward the end of the Copper Age, mercury levels in the bones drop off. Archaeologists aren’t entirely sure why. Perhaps new people migrated there, bringing new rituals. What they do know, however, is that the Copper Age’s elaborate mass burials gave way to simpler, smaller tombs, and that cinnabar faded away. It would not be until Roman times, several millennia later, that vermillion would again color the walls of Iberia.

Archaeologists don’t know whether Iberia’s access to the mines at Almadén made it a special nexus of cinnabar. Cinnabar appears in volcanic regions around the globe. People in both the Old and New Worlds used it everywhere from Mesoamerica to China, where it was traded along the fabled Silk Road. People in pre-Columbian South America, in fact, may have known of cinnabar’s toxicity and sprinkled it in tombs to ward off grave robbers. 

“There’s just so many other places where this could be investigated,” Emslie says. “It’ll be interesting to see if similar high values show up in other parts of the world.”