Archaeologists in South Africa have made a noteworthy discovery, reports Science Alert.

Excavating a cave in South Africa in 1983, they had discovered an unusual femur bone. It belonged to an unspecified antelope and was found to be 7,000 years old. X-rays revealed that three modified bone arrowheads had been placed into the marrow cavity.

At the conclusion of the 1983 excavation the bone, together with other artifacts recovered from the cave, was placed in the University of the Witwatersrand's Archaeology Department storerooms. It lay there until 2022. That's when new archaeological investigations began at the site where the femur had been discovered: Kruger Cave, in the western Magaliesberg mountains, about 1.5 hours' drive from Johannesburg. This renewed interest prompted scientists to take a fresh look at Kruger Cave's treasures.

Research has revealed that the femur's contents are arguably the oldest multi-component arrow poison in the world. It's a complex recipe combining at least two toxic plant ingredients. There's also evidence of a third toxin

The X-ray images taken in the 1980s were of relatively poor quality. So researchers decided to re-image the femur using micro-CT. This process essentially uses thousands of X-rays to reconstruct artifacts in three dimensions, at very high resolution. And the results revealed that the sediment-like matrix filling the marrow cavity into which the arrowheads had been placed was not regular archaeological sediment. It was clearly foreign matter, explained the study’s first author, Justin Bradfield.

A small sample of the material was taken and its chemical constituents were analyzed. The chemistry results revealed the presence of two toxic cardiac glycosides (which disrupt the functioning of the heart muscle): digitoxin and strophanthidin. Both are known to have been used historically in some poisons associated with bow hunting.

Also, the researchers found ricinoleic acid, which can occur as a result of the oxidative breakdown of the toxic lectin ricin. These organic compounds, and others they identified, do not occur in the same plants. This indicates that several plant ingredients must have been combined to create a poisonous recipe.

Ultimately, the researchers concluded that the application of poison to hunting weapons is thought to have originated about 60,000 to 70,000 years ago.