Understanding preservation and identification biases of ancient adhesives through experimentation

Adhesive production is one of the earliest forms of transformative technology, predating ceramics and metallurgy by over 150,000 years. The study of the adhesives used by Neandertals and early modern humans currently plays a significant role in debates about human technological and cognitive evolution. Depending on the type of adhesive used, different production sequences were required. These can vary in complexity and would have needed different knowledge, expertise, and resources to manufacture. However, our knowledge of this important technological development is severely hampered by poorly understood taphonomic processes, which affects the preservation and identification of adhesive materials and leads to a research bias. Here we present the results from a 3-year field preservation experiment. Flint flakes hafted and non-hafted with replica adhesives were left to weather naturally on and below the surface at two locations with different soils and climatic conditions. Differential preservation was recorded on a variety of natural adhesives by digitally measuring the surface area of each residue before and after the elapsed time. Residues were further assessed and photographed using metallographic optical microscopy. Results show that certain adhesives preserve to a significantly higher degree than others, while some materials may be more easily overlooked or visually misdiagnosed. We must therefore be aware of both taphonomic and identification biases when discussing ancient adhesive technology. This research provides a first look that will help us understand the disparities between which adhesives were used in the past and what we find in the archaeological record today.