Earliest Evidence of Mercury Poisoning in Humans Found in 5,000-Year-Old Bones

Researchers discovered the toxic element in remains buried across the Iberian Peninsula between the Neolithic period and antiquity

Early humans were likely exposed to mercury through cinnabar, a sulfide mineral that produces a bright red powder when pulverized. H. Zell via Wikimedia Commons under CC BY-SA 3.0

A chance find at a vineyard in Portugal has led to the discovery of the earliest evidence of mercury poisoning in humans. Researchers found moderate to high concentrations of the lethal element in the bones of 120 people buried on the Iberian Peninsula between the Neolithic era and antiquity—a period spanning some 5,000 years, reports David Bressan for Forbes. The highest levels of mercury appeared in bones dated to the early Copper Age (roughly 2900 to 2600 B.C.E.).

As the team writes in the International Journal of Osteoarchaeology, the individuals were likely exposed to mercury through cinnabar, a toxic mercury sulfide mineral that yields a bright red powder when pulverized. During the period studied, cinnabar was used to decorate megalithic chambers, figurines and the bodies of the dead. Some scholars posit that people intentionally ingested cinnabar as a drug during religious ceremonies, but this hypothesis remains unproven.

For the study, the researchers analyzed 370 individuals interred across 23 archaeological sites in Portugal and Spain—“the largest sampling ever undertaken on contamination of human bone through archaeological evidence,” according to the study. Not all of the bones showed high levels of mercury, but the rates recorded were higher than expected.

The team analyzed the bones of 370 individuals buried across 23 archaeological sites in Portugal and Spain over a 5,000-year period. Rosa Barroso-Bermejo

Lead author Steven Emslie, a biologist at the University of North Carolina Wilmington, tells Smithsonian magazine that he discovered the unexpected trend largely by accident. In 2012, he visited a vineyard in southern Portugal that happened to have an archaeology museum on its grounds. (The venue’s owners had unearthed a large ceremonial complex dated to the late Neolithic and Copper Age while plowing for a new vineyard in 1996; they built the museum to house artifacts and human remains found at the site.) Intrigued, Emslie offered to analyze the bones’ stable isotopes in hopes of gaining insights on the prehistoric people’s diets.

“This project arose from my love of wine and a chance analysis,” says Emslie. He adds that he was surprised to discover high levels of mercury in the bones, as “mercury pollution was not the problem it is today [during the Copper Age], and only natural background mercury would be preserved in tissues at very low values.”

The World Health Organization (WHO) considers mercury levels of 1 or 2 parts per million (ppm) normal for human hair. (People who eat fish every day may have levels closer to 10 ppm, notes the WHO.) Per a statement, some of the samples included in the study had levels in excess of 400 ppm. A total of 31 individuals had levels higher than 10 ppm.

Dig Site
The priestesses buried in the Montelirio tomb in southern Spain were covered with powdered cinnabar. Álvaro Fernández Flores

Spain was once home to the largest mercury mine in the world. Now a Unesco World Heritage Site, Almadén’s cinnabar trove was first exploited during the Neolithic period, around 7,000 years ago, reports Yasemin Saplakoglu for Live Science. Mining continued at the site through modern times.

In a statement provided to Smithsonian, the research team says:

The heavy use of cinnabar throughout the Copper Age is indicated by its lavish application in tombs with high status burials at some of largest sites of this age in Iberia. For example, at the Valencina mega-site [in] southern Spain, the lavishly decorated Montelirio tomb had a thick cinnabar coating laid over the large stone slabs that line the passageway and chambers of the tomb. All the individuals (mostly females) buried in that tomb were covered with powdered cinnabar and were accompanied with an incredible assortment of artifacts, many of them finely crafted and made in exotic raw materials such as amber, rock crystal, flint, ivory or gold.

The use (and abuse) of cinnabar dropped dramatically during the Bronze Age, says Emslie, but surged again under the ancient Romans, who used the mineral in mural paintings.

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