Neanderthals started using tar to make tools and weapons at least 200,000 years ago. The ability to stick pieces of bone or stone to a handle marked a key moment in their evolution, showing they had the cognitive and technological capabilities to advance as a species. How they learned to make the glue, is, however, a mystery.

In a bid to understand the process Neanderthals used to make the glue, researchers in the Netherlands have now recreated a technique that could have resulted in the level of tar production seen in the archaeological record of the time.

Understanding how this came about is important to the debate relating to Neanderthal intelligence and capabilities. If scientists know how they were producing the adhesive, they will have a better idea of their mental abilities, and how their technologies evolved over time.

However, there are many ways to make tar. Later, during the Neolithic Period, ceramic pottery vessels were used. Without ceramics, early humans and Neanderthals would have had to somehow heat birch bark to a high temperature without air. Previous experiments to replicate tar production yielded far too little of the adhesive, much less than the amount we know Neanderthals produced.

In their study, published in Scientific Reports, the team led by Paul Kozowyk from Leiden University, created a framework for the manufacture of tar without ceramics, but so the quantities yielded are similar to what is seen in the archaeological records.

The researchers tried out three methods that could have been used in Palaeolithic tar production. These ranged from simple through complex approaches—from an ash mound on the ground, through to a raised structure. They recorded how much time, fuel, temperatures and materials were needed to yield tar then comparing them with Palaeolithic records.

“Our results indicate that it is possible to obtain useful amounts of tar by combining materials and technology already in use by Neandertals,” they wrote. Experiments indicate that Neanderthals likely began producing tar by rolling bark in hot ashes. Repeating this process simultaneously could produce the quantities of tar seen in archaeological records.

The authors suggest Neanderthals may have recognized small traces of tar in birch bark within partially burned rolls, then worked out how to produce it on a larger scale over time. “Neanderthals must have been able to recognize certain material properties, such as adhesive tack and viscosity,” they wrote.

“Considering that birch bark was available in Europe during the Pleistocene, and that Neanderthals are known to have used wood resources and fire, it is now clear that Neanderthals could have invented the transformative technology simply by recombining knowledge they already had.

"Such an invention must have been driven by curiosity and interest in properties like the tack and viscosity of the newly discovered material. Moreover, in order for tar production to become a perennial innovation, Neanderthals must have been able to maintain the process of dry distillation as a useful technique for producing adhesives.”