A recent study

Several Middle Stone Age sites in South Africa exhibit adhesives, mainly in the form of scraper or stone blade remnants that had been adhered to handles or spears. Chemical testing had revealed that this glue was frequently obtained from yellowwoods. This is unusual because yellowwoods don't exude tree resin or any other sticky substance, and the limited amounts of resin found in their leaves must be extracted through distillation.

The substance can only be created using a elaborate technique as the final compound has outstanding adhesive characteristics. The research team claims that the lack of use of more accessible adhesives by early modern people tens of thousands of years ago is evidence of their creativity and ingenuity.

The group identified two processes for producing the adhesive. Directly adjacent to flat stones is a very simple place to burn the leaves. The second choice is more time- and labor-intensive. In order for the tar to drip into a container, the leaves must be heated for a number of hours in a sort of underground distillery. It is unknown what approach was taken.

In any case, it was astounding that early modern humans at the time did not use any other plants as sources of glue besides yellowwoods. Simply collecting tree resin was an option for people. It flows clearly from the trunk in a number of species that were present in their habitat. And after the leaves fall off, certain plants exude sticky latex.

Standard laboratory tests, such as those used in the adhesives industry, helped the team come up with an explanation. 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 could also support noticeably greater loads.