Simulating symbolic evolution at Blombos and Diepkloof: implications for using transmission chains to study early symbolic behaviour in humans

Engraved ochres and ostrich eggshells from the South African Blombos Cave and Diepkloof Rock Shelter are among the earliest expressions of human symbolic behaviour. They appear to document a continuous practice of non-figurative mark-making across ~40.000 years. During this time, the engraved markings change from simpler unstructured patterns to more complex, ordered and symmetric markings such as cross-hatchings. To inform discussions of the possible function of the engravings, we conducted a two-part experimental study. Based on the assumption that artifact use will motivate incremental adaptive refinements, we used a transmission chain experiment to reconstruct the cultural evolutionary trajectory of the engraved marking. Each transmission chain was seeded with drawings derived from the early Blombos and Diepkloof engravings. We then conducted five experiments to assess the cognitive implications of the changes to the drawings and compared these results to a similar analysis of the original engravings. The drawings became increasingly easier to discriminate and increased in their perceived intentionality over the experimental generations. There was no evidence of a systematic change in drawing saliency or memorability, and the drawings did not acquire stylistic properties over the experimental generations. Although we observe interesting qualitative similarities between the original engravings and the transmission chain drawings, our findings suggest that cognitive biases and working memory constraints are not sufficient to generate the patterns observed in the archaeological record, highlight the significance of social and functional contexts in shaping early symbolic artifacts. By integrating archaeological and experimental research we can better inform inferences on sparse records of early symbolic behaviour. Our study thus leads to a broader consideration of the role, strengths and potential limitations of the transmission chain approach in analysing trajectories of early symbolic behaviour.