Ancient Glue Suggests Neanderthals and Early Humans Had Similar Thought Patterns

A new study of stone tools from the Middle Palaeolithic period – between 120,000 and 40,000 years ago – suggests that Neanderthals might have had a higher level of cognition than previously thought.

Looking at the traces of ancient bitumen-based adhesives left behind on the tools, researchers found surprisingly high levels of the naturally occurring earth pigment ochre.

This addition of ochre to the adhesive would make it less sticky to the skin, the researchers say, allowing it to be molded to form a handle. At the same time, the amount of ochre used would still leave the adhesive with enough gluing power to hold the tool head firmly in place.

Making such a complex adhesive suggests that Neanderthals may have been capable of more advanced cognition than previously estimated. This new study, published in Science Advances, is also thought to be the earliest evidence of complex adhesive use in Europe.

Putting museum archives under the microscope

The stone tools examined in this study were re-discovered during an internal review of the collection at Berlin’s Museum of Prehistory and Early History. They were all found at Le Moustier, an archeological site in France that was discovered in the early 20^th century. The tools had previously been assessed, but not examined in detail.

“The items had been individually wrapped and untouched since the 1960s,” said study author Ewa Dutkiewicz, a research associate at the National Museums of Berlin. “As a result, the adhering remains of organic substances were very well preserved.”

The researchers recognized the potential scientific value of these well-preserved pieces and, upon noticing flecks of red and yellow colorants on their surface, decided to examine the pieces more closely.

Using photographic and scanning electron microscope images, the researchers determined that these color stains were indicative of ancient adhesive use. Powder samples and a small protuberance scraped from the surface of two of the tool samples were then analyzed using energy-dispersive X-ray spectroscopy (EDX), transmission infrared spectroscopy, and micro-computed tomography (microCT) to get a better chemical and structural understanding of the artifacts.

This analysis confirmed the presence of a bitumen-based adhesive that also contained significant amounts of ochre.

“We were surprised that the ochre content was more than 50 percent,” said Patrick Schmidt, an adjunct professor at the University of Tübingen. “This is because air-dried bitumen can be used unaltered as an adhesive, but loses its adhesive properties when such large proportions of ochre are added.”

Why would Neanderthals need a composite glue?

To understand why such a large amount of ochre might be mixed with the bitumen, the researchers decided to conduct a range of mechanical tests – including a tensile strength test – on their own recreations of this recipe.

They found that raw liquid bitumen alone was too runny and sticky to be useful. But the addition of 55 weight percent (wt%) ochre transformed the adhesive into a material that could be molded to form a grip for a basic stone tool.

A recreation of a typical hand-held composite tool, consisting of a stone tool and grip made from liquid bitumen and 55 wt% ochre. Credit: Patrick Schmidt.

The microscopic analysis performed by the researchers on the ancient tools also backs up the theory that the composite adhesive could have been molded into a functional handle.

“The tools showed two kinds of microscopic wear: one is the typical polish on the sharp edges that is generally caused by working other materials,” explained paleoanthropologist Radu Iovita, an associate professor at New York University’s Center for the Study of Human Origins, who conducted this part of the analysis. “The other is a bright polish distributed all over the presumed hand-held part, but not elsewhere, which we interpreted as the results of abrasion from the ochre due to movement of the tool within the grip.”

What do these tools mean for human evolution?

The use of multi-component adhesives is a behavior that has been seen among early modern humans, most notably the Homo sapiens in Africa. However, this type of behavior had not previously been observed from earlier Neanderthals in Europe.

The tools discovered at Le Moustier could be a sign that European Neanderthals in this time period were more cognitively and culturally advanced than previously thought; it would take significant understanding to create such an adhesive and to travel the distances needed to access the closest known bitumen outcrop.

“These astonishingly well-preserved tools showcase a technical solution broadly similar to examples of tools made by early modern humans in Africa, but the exact recipe reflects a Neanderthal ‘spin,’ which is the production of grips for handheld tools,” Iovita said.

“Compound adhesives are considered to be among the first expressions of the modern cognitive processes that are still active today,” added Schmidt.

While there is no way to be completely sure that these tools were developed by Neanderthals and not some other contemporaneous group of Middle Paleolithic Homo sapiens, the identification of these adhesives is still an important finding. If early modern humans had made the tools, it would mean this knowledge persisted long throughout the Out of Africa migration – a remarkably long example of technological continuity. However, the team believes the Neanderthals are the more likely toolsmiths.

“Taking into account the overall context of the finds, we assume that this adhesive material was made by Neanderthals,” said Dutkiewicz.

“What our study shows is that early Homo sapiens in Africa and Neanderthals in Europe had similar thought patterns,” Schmidt concluded. “Their adhesive technologies have the same significance for our understanding of human evolution.”

Reference: Schmidt P, Iovita R, Charrié-Duhaut A, Möller G, Namen A, Dutkiewicz E. Ochre-based compound adhesives at the Mousterian type-site document complex cognition and high investment. Sci Adv. doi:10.1126/sciadv.adl0822