Researchers who reverse-engineered an ancient superglue have found that Stone Age people were smarter than we thought.

Making the glue, originally used on 70,000-year-old composite tools, clearly required high-level cognitive powers. Anthropologists usually use symbolic art as the benchmark for modern cognition, but making the glue was an equally profound accomplishment.

“These artisans were exceedingly skilled; they understood the properties of their adhesive ingredients, and they were able to manipulate them knowingly,” wrote University of Witwatersrand archaeologists in a paper published Monday in the Proceedings of the National Academy of Sciences.

The archaeologists took design cues from stone tools found during a decade of excavation at South Africa’s Sibudu Cave site. The stones were still covered with traces of an iron-rich red pigment and acacia gum, a natural adhesive found in the bark of acacia trees.

Acacia gum was almost certainly used to attach the stones to wooden shafts, but researchers have debated the pigment’s role. Some suggested that it was decoration. The Witersrand team suspected a more functional use.

Indeed, when they used Stone Age toolmaking techniques to attach stones to wooden shafts with nothing but acacia gum, the tools soon fell apart. When they added the pigment, the tools stuck together. But making the glue required much more than simple mixing. It demanded careful and sustained attention.

Keeping the fire at the right temperature required certain types of wood, with a certain degree of moisture content. If glues were mixed too close to the fire, they contained air bubbles. If too dry, they weren’t cohesive; if too wet, they were weak. The Sibudu Cave’s Stone Age inhabitants, wrote the researchers, were “competent chemists, alchemists and pyrotechnologists.”

The Sibudu tools were about as old as the first possible evidence of symbolic art, also found in South Africa. But some archaeologists say that art, consisting of cross-hatched engravings on stone, may represent absent-minded doodles rather than a cognitive leap. The glues are a more convincing indication of modern intelligence.

“The glue maker needs to pay careful attention to the condition of ingredients before and during the procedure and must be able to switch attention between aspects of the methodology,” wrote the Witwatersrand team. “To hold many courses of action in the mind involves multitasking. This is one trait of modern human minds, notwithstanding that even today, some people find multilevel operations difficult.”

Brandon Keim