Stone Age South Africans in the Western Cape made the superglue of their day by burning yellowwood leaves.
Now scientists from Germany and the University of Cape Town have recreated the method used at Diepkloof rock shelter in a scene reminiscent of the popular YouTube channel Primitive Technology.
The tar they made turned out to be significantly stronger than glues and resins collected from several plants by early Homo sapiens occupants of Diepkloof — a shelter occupied continuously between 130,000 and 45,000 years ago.
Molecular analysis of adhesive on stone tools from a 3.6m-deep trench detected resins from the Breede River yellowwood (Podocarpus elongatus), or plum pine. The discovery intrigued Patrick Schmidt and Tabea Koch from Tübingen University in Germany and Edmund February from UCT.
In a paper published yesterday in the prestigious Proceedings of the National Academy of Sciences, they said this was because yellowwoods do not ooze resin like many other plants.
“It was not straightforward for Middle Stone Age foragers to identify Podocarpus as a plant of potential interest for adhesive making,” they said. “They must have used a ... transformative process.”
The scientists believed this could shed light on the development of human intelligence. “Podocarpus tar production is an excellent proxy for recognising complex cognition in Middle Stone Age archaeological records.”
To investigate further they travelled to Diepkloof, about 17km from Elands Bay on the west coast, and set out to recreate its ancient inhabitants’ glue-making exploits using only materials that would have been available in the area at the time.
First, they fashioned clay into a double distillation chamber. Yellowwood leaves in the upper vessel were heated by an external fire and the tar that emerged fell into a limpet shell. When tested, however, its adhesive properties were no stronger than the sap from a bitter aloe — another local glue used in the Stone Age.
The second, simpler experiment produced the superglue. The scientists collected two rocks from a beach and placed them in a fire built from dry branches. Then they held burning yellowwood leaves in the flames and later scraped tar deposits off the stones.
They used “lap shear” adhesive tests to measure the strength of the tar and other Diepkloof glues harvested from plants such as the yellow milk bush, Cape fig, rockwood, dune currant rhus, mountain cypress and acacia.
The scientists said: “Their strengths and fracture behaviours vary, providing a wide range of properties. There are not necessarily better or worse adhesives. Strong adhesive may be most appropriate for some tasks, whereas others may be more suitable under different circumstances.
“Adhesives with low overall strength and those that abruptly break by catastrophic failure may be suitable for hafting of projectiles, where the tips are expected to detach from the haft.”
The yellowwood tar's strong bond was probably vital for hand-held spears or tools used for scraping and cutting, they said.
“Residues described on Diepkloof artefacts were mainly found on unspecific flakes and blades. This suggests that Podocarpus tar was part of the tools used in hand-held repetitive motion. Its exceptional properties would have been a clear advantage over other substances in this situation.”
Schmidt and his colleagues said the “condensation method” that produced the superglue could have been discovered accidentally, “for example, in situations in which a fire is lit with Podocarpus leaves that are then discarded beside stones while still burning.
“The shiny aspect of the tar deposited on these stones may be recognised and the processes understood.
“The invention of Podocarpus tar and its production, though requiring a fire-related transformative technique, are well within the capabilities of Middle Stone Age foragers.
“What our study shows is that they understood the benefit of this transformative process for obtaining materials with new properties. Podocarpus tar must be produced instead of being simply collected in nature.
“Because the first adhesives made by Homo sapiens in Africa are Podocarpus tars, they are likely good markers for complex cognitive processes. In this sense, Middle Stone Age adhesive making, like stone heat treatment, is one of our best proxies that allows pinpointing the advent of new behaviours during this period.”
Schmidt told TimesLIVE Premium a video of the yellowwood experiment will be uploaded in a few weeks to Tübingen University’s fledgling Experimental Archaeology YouTube channel.
The pioneering Stone Age glue-makers were emulated by another South African in the 1960s when George Pratley developed his eponymous epoxy putty. It was the first of its sort, stronger than anything else on the market, and was used on the Ranger spacecraft that went to the moon in the mid-1960s.
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