Adhesives are an incredibly important part of every day life. They help hold together everything from shoes and mobile phones to satellites in space. But we didn't invent adhesives: Neanderthals did, to make handles for stone tools over 191,000 years ago. Leiden researchers now found that Neanderthals purposefully made birch bark tar because it has better properties than other naturally available glues.
There are many different natural materials that can be used as a simple glue. Resins from trees, beeswax, and even a naturally occurring form of petroleum called bitumen. Neanderthals knew about these materials, and used them when they were available. But they also did something more. They made an entirely new substance. By heating birch bark under specific conditions they produced tar. The tar was then used to connect stone tools to wooden handles. An example of this is the recent finding of a flint tool with traces of birch tar on its surface.
Birch tar is stronger
Researchers from Leiden University and the Delft University of Technology tested replica adhesives, measuring the effect heating has on the flow and hardness of the materials, to determine precisely what makes a good Stone Age glue. They found that birch tar, a material known to have been used by Neanderthals, is superior to many other natural adhesives. This shows that Neanderthals likely chose to make tar, despite its costly production, because it was the best material for the task at hand.
More effort for better glue
The results provide new insight into the lives of Neanderthals, shedding light on the material and technological choices they were making. In earlier studies, the Leiden archaeologists replicated the way Neanderthals made glue - using only tools and techniques from the Paleolithic - to try and uncover more about this ancient technology. 'Previously we conducted experiments to show how Neanderthals made birch bark tar' says Paul Kozowyk, who is conducting the research for is PhD. 'But now we wanted to know why they were going through the trouble of synthesizing an entirely new material, when other, simpler alternatives already exist in nature.' By weighing the more difficult production of tar against its better adhesive properties, the researchers conclude that Neanderthals were able to make technological choices based on a clear understanding of adhesive material properties.
The research was conducted in the Delft Aerospace Structures and Materials Laboratory (DASML) and was published on 29 October 2019 in the Journal of Human Evolution.
Photo above: replicas of flint tools, shafted with birch tar in wooden handles.
Text: Marieke Epping
Images: Paul Kozowyk
