Sticky stuff that connected stone blades to spears: Around 100,000 years ago, people in southern Africa used a tar-like substance with particularly good adhesive properties, researchers report. The special thing about it is that the adhesive was not naturally available in the environment. Instead, experiments show that it had to be specifically obtained from plant material using a manufacturing process. The glue thus bears witness to the cultural evolution of humans in Africa, say the scientists.
Other creatures are mostly dependent on physical performance - we, on the other hand, use our brains: The success of the human species is based on the ability to understand connections, to cooperate and to use knowledge and objects in a complex way. In the course of human development, our ancestors learned to use and manufacture tools made of wood and stone in increasingly sophisticated ways. In this context, it is known from finds that modern man in southern Africa around 100,000 years ago combined stone and wood technology in an advanced way: Traces of adhesives have been discovered at several sites that helped to attach stone tools to handles or spears to fix.
Surprising source of raw materials
Chemical analyzes of the material revealed: The adhesive consists of a substance that comes from holm yew plants. These are evergreen shrubs and trees of the genus Podocarpus that grow in the vicinity of the sites. This finding was astonishing, because these plants are at least not obvious producers of glue: "Yew trees do not excrete any appreciable amounts of tree resin or any other sticky substance," explains Patrick Schmidt from the University of Tübingen. Together with his colleague Tabea Koch and Edmund February from the University of Cape Town, he has now investigated how people around 100,000 years ago could have gotten the tar-like glue from the Podocarpus plants.
As they explain, it was obvious that the substance was due to the resin that is contained in small amounts in the leaves of the plants. In order to obtain it, it apparently had to have been distilled out, the scientists say. In order to track down possible processes, the team used methods from experimental archaeology: The researchers investigated how the adhesive could have been produced using means that were available to Stone Age people at the time.
Hot generated super glue
The team discovered two possible manufacturing processes: It was shown that the tar substance is already formed in the form of a condensate if Podocarpus leaves are burned right next to flat stones. The sticky material can then be scraped off the surfaces. "People could have discovered this process by accident," says Schmidt. However, they may also have used a more complex process that proved to be effective in the researchers' experiments: bundles of leaves are covered with clay and heated by a fire burning above them for several hours. Tar then forms in this small "distillation system" and drips into a collection container. It remains unclear to what extent people extracted the glue from the stone yew in a rather simple or more sophisticated way. Nevertheless, it had to be produced in principle, the researchers emphasize.
This raised the further question, why did people use these glue suppliers? “They could have just collected tree sap. In several species that occurred in their area, it flows noticeably from the trunk. For example, some plants release sticky latex when the leaves break off,” says Koch. The scientists then discovered the background to the preference for the stone yew glue by means of special laboratory tests, such as are typically used in the glue industry to record the characteristics of test substances. "Our tar distilled from stone yew trees had particularly good mechanical properties and proved to be stronger than all other naturally occurring adhesive substances from the Stone Age in South Africa - it was able to hold significantly greater loads," says Schmidt, summing up the results of the comparative tests.
According to the scientists, their results now have a special meaning for anthropology: The fact that people in southern Africa deliberately produced particularly good adhesives around 100,000 years ago sheds light on the innovative thinking and cultural development of that time. "People no longer chose materials based solely on their properties, but changed the existing material," says Schmidt.
Source: University of Tübingen, specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2209592119