The second option is more difficult and time consuming. In it, the leaves have to be heated in a kind of underground distillery for several hours, so that the tar drips into a container. It is not known which method was used.
Either way, says Schmidt, it was astonishing that early modern humans at that time did not use any plants other than yellowwoods as sources of glue. "People could have simply collected tree resin. In several species that occurred in their environment, it flows visibly from the trunk. And some plants release sticky latex when the leaves break off," says Tabea Koch. The team found the explanation with the help of standard laboratory tests, such as those used in the adhesives industry: "Our tar distilled from yellowwoods had particularly good mechanical properties and proved to be stronger than all other naturally occurring adhesive substances of the Stone Age in South Africa; it was able to hold significantly larger loads," says Schmidt.
The fact that modern humans in southern Africa purposefully produced particularly good adhesives around 100,000 years ago was a turning point in the development of our direct ancestors, Schmidt says. "People weren’t selecting materials based on their properties, they were modifying the existing material." Such new engineering technology required higher cognitive abilities and innovative thinking, he says.
Patrick Schmidt, Tabea J. Koch, Edmund February: Archaeological adhesives made from Podocar-pus document innovative potential in the African Middle Stone Age. PNAS - Proceedings of the National Academy of Sciences, https://doi.org/10.1073/pnas.220959211
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