A new experimental methodology for assessing adhesive properties shows that Neandertals used the most suitable material available.

The use of adhesives for hafting stone tools at least 191 ka was a major technological development. Stone tools could be more securely attached to handles, thus improving their efficiency and practicality. To produce functional adhesives required forethought and planning, as well as expertise and knowledge of the resources available in the landscape. This makes adhesives important in discussions about Neandertal and early modern human technological and mental capabilities. However, we currently know very little about how these early adhesive materials behaved under different circumstances, or why certain materials were used and others were not. Here we present the results of controlled laboratory bulk property tests (hardness, rheology and thermogravimetric analysis) on replica Paleolithic adhesives. We conclude that birch tar is more versatile, has better working properties, and is more reusable than pine resin, the most likely alternative material. Neandertals may therefore have invested more time and resources to produce birch tar because it was the best material available, both functionally and economically, throughout the majority of Europe during the Middle to Late Pleistocene. Our results further demonstrate that Neandertals had high levels of technological expertise and knowledge of the natural resources available to them in their environment.