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Neanderthal adhesives were made through a complex synthesis process

Birch bark was heated in underground chambers to create a tougher adhesive.

Ars Technica

Neanderthal tools might look relatively simple, but new research shows that Homo neanderthalensis devised a method of generating a glue derived from birch tar to hold them together about  200,000 years ago—and it was tough. This ancient superglue made bone and stone adhere to wood, was waterproof, and didn’t decompose. The tar was also used a hundred thousand years before modern humans came up with anything synthetic. After studying ancient tools that carry residue from this glue, a team of researchers from the Eberhard Karls University of Tübingen and other institutions in Germany found evidence that this glue wasn’t just the original tar; it had been transformed in some way. This raises the question of what was involved in that transformation. To see how Neanderthals could have converted birch tar into glue, the research team tried several different processing methods. Any suspicion that the tar came directly from birch trees didn’t hold up because birch trees do not secrete anything that worked as an adhesive. So what kind of processing was needed? Each technique that was tested used only materials that Neanderthals would have been able to access. Condensation methods, which involve burning birch bark on cobblestones so the tar can condense on the stones, were the simplest techniques used—allowing bark to burn above ground doesn’t really involve much thought beyond lighting a fire. The other methods involved a recipe where the bark was not actually burned but heated after being placed underground. Two of these methods involved burying rolls of bark in embers that would heat them and produce tar. The third method would distill the tar. Because there were no ceramics during the Stone Age, sediment was shaped into upper and lower structures to hold the bark, which was then heated by fire. Distilled tar would slowly drip from the upper structure into the lower one. The resulting tars were all put through chemical and molecular analysis, as well as micro-CT scans, to determine which came closest to the residue on actual Neanderthal tools. Tars synthesized underground were closest to the residue on the original artifacts. “[Neanderthals] distilled tar in an intentionally created underground environment that restricted oxygen flow and remained invisible during the process,” the researchers wrote. “This degree of complexity is unlikely to have been invented spontaneously.”