Component glue was already used by the Neanderthals?

Bitumen and ocher

The Neanderthals already made a composite adhesive from bitumen (dark) and ocher powder. © P. Schmidt

Adhesives are considered one of the earliest signs of human cultural evolution. A study now suggests that the Neanderthals already produced complex adhesives. Researchers have found traces of an adhesive made from bitumen and ocher on stone tools from a site in France that are more than 40,000 years old. Neanderthals probably used the mixture to add handles to their stone tools. The ocher could have ensured that the finished handle did not stick to the hands.

Our early human ancestors probably created the first stone tools around 2.6 million years ago. However, it is unclear when the production of such stone hammers and blades began consciously and purposefully. A clear sign of a cultural technological process, however, is the use of glue, for example to provide tools with handles. Neanderthals are known to have used birch pitch, which they probably produced specifically 80,000 years ago.

Glue residue
Glue residue on a stone tool from Le Moustier. © State Museums in Berlin, Museum of Prehistory and Early History/ Photo: Gunther Möller, drawing: Daniela Greinert

Found in the museum

Adhesives, which are made up of several ingredients, go one step further. “African Homo sapiens is known to have produced composite adhesives from naturally occurring sticky substances and ochre, an engineering behavior that is thought to mark the emergence of sophisticated cognitive processes in our species,” explains a team led by Patrick Schmidt of the University of Tübingen. “In Europe, on the other hand, before the beginning of the Upper Paleolithic around 40,000 years ago, no ocher-containing adhesives were known, neither from Neanderthals nor from the earliest European Homo sapiens.” The Upper Paleolithic is the youngest, last epoch of the Paleolithic, which began with the arrival of Homo sapiens began in Europe and ended with the end of the Ice Age.

However, the team has now discovered older evidence of composite adhesives in several museum specimens that had not previously been examined in detail. The artifacts come from the Le Moustier site in what is now France, where Neanderthals lived around 120,000 to 40,000 years ago. “The collection items were individually packaged and untouched since the 1960s. As a result, the adhering remains of organic substances were very well preserved,” reports Schmidt’s colleague Ewa Dutkiewicz. It was precisely these organic remains that aroused the researchers' interest. Chemical and spectroscopic analyzes showed that it was a mixture of ocher and the sticky raw material bitumen. Bitumen, a component of asphalt, can be made from petroleum, but also occurs naturally, sometimes as a viscous liquid and sometimes as an air-dried solid mass.

Homemade Stone Age adhesives

To their surprise, the researchers found that the ocher content of the mixture was more than 50 percent. “Such a high proportion of ocher impairs the adhesive performance for classic applications in which stone tools are glued to wooden handles, for example,” explain Schmidt and his colleagues. However, it was already known from previous finds that Neanderthals often used the sticky materials themselves as a handle instead of using them to connect other materials.

In order to find out to what extent different mixtures of bitumen and ocher are suitable for this purpose, the researchers themselves experimented with the two raw materials and tested the mixtures for adhesive strength and other properties. To do this, they used naturally occurring bitumen from France, both in air-dried form and as a liquid mass. To make the liquid bitumen more usable, they boiled it before mixing it with varying proportions of ochre.

Useful mixture

As expected, the air-dried bitumen stuck the strongest and increasingly lost its adhesive strength when ocher was added. However, it had a serious disadvantage: When Schmidt and his colleagues tried to use it to form a handle for stone tools, parts of the material always stuck to their hands. In addition, the handle was not tight enough and therefore unusable. “It was different when we used liquid bitumen, which is actually not suitable for bonding on its own,” says Schmidt. “If you add 55 percent ocher, you get a malleable mass.” This is just sticky enough to hold the stone tool in place while keeping your hands clean. The ocher also gives the material strength.

Microscopic analyzes of the wear marks on the tools showed that the Stone Age handles were probably made and used in exactly this way. “Such technological developments and the understanding of material properties can be viewed as the first expression of comprehensive human cognitive processes that correspond to our current way of thinking about industrial processes,” says Schmidt. Early humans probably had to collect ocher and bitumen from places far apart, which involved a lot of effort, required a lot of planning and suggested a targeted approach.

Not just with Homo sapiens

“Taking into account the context of the find, we assume that the elaborately produced adhesive material was made by Neanderthals,” says Dutkiewicz. This suggests that Neanderthals were capable of similar cognitive abilities to early Homo sapiens in Africa. “Both invested time and effort in producing composite adhesives and had the necessary cognitive skills,” the researchers write. “This ability and willingness to invest in tools that required a great deal of effort document the complexity of hominin behavior in the late Middle Paleolithic.”

Source: Patrick Schmidt (University of Tübingen) et al., Science Advances, doi: 10.1126/sciadv.adl0822

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