Exactly when did human beings start tipping their weapons with poison to hunt prey? This is a question at the forefront of recent archaeological research.

In southern Africa San (or Bushman) hunter-gatherer groups, such as the /Xam of the Western Cape and the Ju/wasi and Hei//om of Namibia, used poisoned arrows for hunting during the 19th and 20th centuries. The origins of this technology, though, may be far older than we thought.

Recently, traces of the poison ricin were found on a 24 000 year-old wooden poison applicator at Border Cave in South Africa’s Lebombo mountains. If this identification is correct it would mean that people in southern Africa were among the first in the world to harness the potential of plant-based poisons.

South Africa has provided plenty of evidence of behaviours that could be attributed to cognitively complex Homo sapiens. This includes early evidence of hafted projectile technology, the selection of aromatic plants for bedding materials), and the use of ochre as an insect repellent.

The early use of poison is one more indicator of an advanced repertoire of behavioural and technological traits that have characterised our species from the earliest times. The problem is that it’s not easy to identify the remnants of ancient poisons. Organic molecules, including those that make up different poisons, degrade over time and seldom resemble their parent compound. For this reason it is often very difficult to accurately identify ancient organic residues.

Now a team of archaeologists and organic chemists from the Universities of the Witwatersrand, Pretoria and Johannesburg has published details of a method that can – with reasonable accuracy – identify plant-based toxins and other unique chemical markers present on archaeological artefacts .

This may allow scientists to infer the presence of toxic plant ingredients applied to ancient weapons. It adds to our growing appreciation of the full complexity of early human populations – in southern Africa as well as in the world.

Testing the method

Anyone who’s watched BBC nature documentaries will recall scenes of small groups of Bushman hunting antelope with their delicate little bows and arrows. This flimsy equipment was able to bring down large game because of poison.

The most well known source of arrow poison in southern Africa is a beetle larva known as Diamphidia. The Diamphidia grub is still used today by traditional hunters living in the Kalahari. The grub is eviscerated between the hunter’s fingers and its entrails applied directly to an arrowhead’s base. The poison, known as diamphotoxin, can bring down a fully-grown giraffe.

But historical records indicate that many other, different plant ingredients were used. The particular ingredients and recipes used to make arrow poison differed between groups and locations.

A recent archaeological discovery at Border Cave (on South Africa’s border with Swaziland), revealed trace amounts of a substance still adhering to a 24 000 year-old wooden poison applicator. This substance was identified as by-products of the poison ricin. Ricin is produced by the castor bean plant, from which castor oil originates. This discovery, though not without its detractors, sparked renewed interest in identifying poison ingredients on archaeological artefacts in various parts of the world.

This is where our research comes in.

Our paper presented the results of a pilot study designed to accurately detect minute amounts of organic compounds from poisonous plants found on archaeological artefacts. We used an analytical technique known as ultra performance liquid chromatography – mass spectrometry (UPLC-MS) – to characterise the organic compounds present in 11 species of poisonous plant found in southern Africa.

To test the reliability of our detection technique and our ability to accurately identify the most likely plant source of identified compounds, we conducted a blind test. Three plant extracts were prepared following a known poison recipe and applied to a modern arrowhead. The plants used in this recipe were known to only one of the authors. Once the poison coating on the arrowhead had dried, a small amount was scraped off and analysed using UPLC-MS.

We were able to identify two of the three plants used in the poison recipe; identification of the third, belonging to the euphorbia taxa, was not definitive.

Finally, a 90-year-old poisoned arrowhead from Namibia was analysed following the same protocol. The results showed that our method can be used tentatively to identify toxins based on comparative overlays with fresh plant material. Furthermore, the method is able to identify non-toxic compounds that may be unique to specific species of plants. This means the plant in question could be identified even in the absence of known toxins.

Opening new doors

Our study’s importance lies in the ability to recognise organic components of ancient plant-based poisons that may be hundreds – or even thousands – of years old. This is particularly impressive in instances where several ingredients were mixed together to prepare an arrow poison and where only minute amounts of this poison survive on the implement.

No historical information exists on the variety of plants used (nor, indeed, the recipes) for arrow poisons in the eastern half of southern Africa. Also, apart from the single discovery at Border Cave, we have no idea when people started using poisons to assist in hunting. Hopefully this new method can help to address both of these issues and build on existing scholarship of Africa’s indigenous knowledge systems.

Written by Justin Bradfield

Postdoctoral Research Fellow (Evolutionary Studies Institute), University of the Witwatersrand

The Conversation