Size matters. An evaluation of descriptive and metric criteria for identifying cut marks made by unmodified rocks during butchery

One of the key concerns in human evolution studies is tracing the development of stone tool use by early hominins to acquire meat. It has been suggested that the earliest tools used for this purpose might have been unmodified, naturally sharp rocks. However, it has proven challenging to distinguish marks on bones made by hominins using humanly unmodified rocks (HURs) for butchery, from marks made by natural processes. Here we present the results of a study aimed at comparing marks made by HURs during butchery, versus marks made by the same HURs through simulated natural processes, specifically, the fluvial tumbling of bones with naturally sharp rocks (replicated here using a rock tumbler). The results of this study, in which the lithological effector is held constant while the actor is varied, confirm earlier studies suggesting that many existing categorical attributes do not effectively distinguish between marks made by HURs versus those made by other tools or trampling. However, we also present a novel way of measuring mark depths which shows that marks made by the human actor are much deeper and longer than those made by natural processes. The size of marks, therefore, matters. This knowledge may help us assess the likelihood that marks on bone surfaces may have been produced by natural forces, as opposed to by humans using unmodified rocks for butchery.