Earliest evidence of mercury poisoning identified in 5,000-year-old bones

The researchers attribute the incidence of mercury poisoning during the Chalcolithic period to the increased exploitation of cinnabar, a mercury sulfide mineral

A recent study of human skeletal remains buried at archaeological sites along the Iberian Peninsula has revealed the earliest known evidence of mercury poisoning, shedding new light on the ritual, ceremonial, and artistic uses of mercury in ancient societies.

As discussed in an article recently published in the International Journal of Osteoarchaeology, a team of 14 researchers associated with the Universidad de Sevilla carried out the largest ever study on the presence of mercury within ancient human skeletal remains.

This study involved analysing mercury levels within samples of cortical bone extracted from the humeri of 370 individuals excavated across 23 archaeological sites in Spain and Portugal spanning from the Middle Neolithic period to Antiquity.

Image showing cinnabar crystals in rock extracted from the Almadén mine in central Spain. Size: 7.7 x 5.6 x 2.5 cm. Image: Wikimedia Commons /Robert Lavinsky/ iRocks.com.

According to the report, human bones dating back 5,000 years provided the earliest evidence of mercury poisoning. Their results also revealed that the highest levels of mercury exposure occurred during the Early Chalcolithic period (c. 2900 – 2600 BC).

The authors attribute the incidence of mercury poisoning during this period to the increased exploitation of cinnabar, a mercury sulfide mineral that becomes a bright red powder when pulverised. Its pigment was used to paint stelae and megalithic monuments, to dye cloth and decorate figurines.

Cinnabar was also imbued with sacred and esoteric significance, as indicated by its lavish application in high-status burials along the Iberian Peninsula. Its special characteristics made it a valuable commodity and therefore a signifier of social status for the region’s emerging local elites.

At the Chalcolithic site of Valencina in southern Spain, for example, the stone passageways of a megalithic structure known as the tomb of Montelirio were thickly coated in cinnabar, and the deceased interred within had been sprayed with cinnabar powder.

Cinnabar powder was sprayed over the 20 individuals interred at the tomb of Montelirio near Sevilla, Spain. Image: Leonardo García Sanjuán, courtesy of the Universidad de Sevilla.

Mercury levels of up to 400 parts per million (ppm) were recorded in some skeletal samples during the study. As the World Health Organisation (WHO) considers normal mercury levels to be no higher than one or two ppm, the authors suggest that cinnabar may have been deliberately ingested – or burned and inhaled as a vapour – on account of beliefs in its medicinal and esoteric properties.

The slabs of the tomb of Montelirio were coated in cinnabar pigment. Image: Leonardo García Sanjuán, courtesy of the Universidad de Sevilla.

These extreme levels of intoxication will have undoubtedly led to severe health problems.

The skeletal remains analysed also reflected the decline in the use of cinnabar towards the end of the third millennium BC as funerary rituals changed.

The largest cinnabar mine in the world is located in Almadén in central Spain. Exploitation first began there around 7,000 years ago.

Situated on one of the main tributaries of the Guadiana River, an important trade route throughout history, the Almadén mine was likely the main source of cinnabar for other major Chalcolithic sites in Spain and Portugal. Since closing in 2002, the mine has been declared a World Heritage Site.