Parsimony, homology and the continuity of stone tool use in the hominin clade

Whether the last common ancestor (LCA) of Homo and Pan (ca. 6-7 Ma) used stone tools is a foundational question in palaeoanthropology. However, it is often treated as known rather than as a hypothesis requiring substantiation. Lacking fossil evidence for the LCA, this assumption is based on data from extant primates and the available hominin fossil record. Phylogenetic approaches invoking parsimony argue that the presence of stone tool use in both Pan and Homo reflects homology rather than convergent evolution. While methodologically legitimate, we argue that this framework has been applied without sufficient attention to the specific criteria for this to be true. For homology to hold, stone tool use must have been continuously expressed across generations of both lineages from the LCA to the present. We evaluate this claim against three lines of evidence: behavioural data from extant primates, showing that stone tool use is heterogeneous within Pan; the hominin fossil and archaeological record, which leaves a substantial evidential gap between the LCA and the earliest known documented technologies; and comparative data from phylogenetically distant tool using animals, which show that similar behaviours can arise independently under comparable ecological conditions. Together, these lines of evidence favour a model of multiple, context-dependent origins of stone tool use over unbroken inheritance from a tool using LCA.