Symbolic behaviour - such as language, account keeping, music, art, and narrative - constitutes a milestone in human cognitive evolution. But how, where and when did these complex practices evolve? This question is very challenging to address; human cognitive processes do not fossilize, making it very difficult to study the mental life of our Stone Age ancestors. However, in a new study published in the Proceedings of the National Academy of Sciences journal PNAS, an interdisciplinary team of cognitive scientists and archaeologists from Denmark, South Africa and Australia takes up the challenge. They used engravings on ochre nodules and ostrich eggshells made between about 109 000 and 52 000 years ago in a series of five cognitive science experiments to investigate their potential symbolic function.

The engravings originate from the South African Middle Stone Age sites of Blombos Cave and Diepkloof Rock Shelter, and are considered among the earliest examples of human symbolic behaviour. They were found in different layers of the cave sediments, which has made it possible to reconstruct the approximate time and order in which they were produced. Lead scientist Kristian Tylén, Associate Professor at the Department of Linguistics, Cognitive Science and Semiotics and at the Interacting Minds Centre, Aarhus University, Denmark, explains:

"It is remarkable that we have a record of a practice of making engravings spanning more than 40 000 years. This allows us to observe how the engraved patterns have been developed and refined incrementally over time to become better symbols - that is - tools for the human mind, similar to the way instrumental technologies, such as stone tools, are honed over time to do their job more efficiently".

In the experiments, participants were shown the engraved patterns while the researchers measured their responses in terms of visual attention, recognition, memory, reaction times, and discrimination of patterns belonging to different points in time. The experiments suggest that over the period of more than 40 000 years, the engravings evolved to more effectively catch human visual attention, they became easier to recognize as human-made, easier to remember and reproduce, and they evolved elements of group-specific style. However, they did not become easier to discriminate from each other within or between each of the two sites.

Several previous studies have presented speculations on the possible symbolic function of the Blombos and Diepkloof engravings. Some have suggested that they should be regarded as fully-developed symbols pointing to distinct meanings, more or less like written glyphs. This suggestion is, however, not supported by the present study:

"It is difficult to make well-grounded interpretations of these ancient human behaviours", says archaeologist and co-author Niels N. Johannsen, Associate Professor at the Department of Archaeology and Heritage Studies and at the Interacting Minds Centre, Aarhus University, "and we have been missing a more systematic, scientific approach. The main advantage of our experimental procedure is that we work directly with the archaeological evidence, measuring cognitive consequences of the changes that these engravings have undergone through time - and from these data, we argue, we are in a better position to understand the possible function of the engravings made by our ancestors tens of thousands of years ago."

The experimental findings suggest that the engravings from Blombos and Diepkloof were created and refined over time to serve an aesthetic purpose, for instance as decorations. However, they also evolved elements of style that could have worked to mark the identity of the group, that is, they could be recognized as coming from a particular group.

The experiments make use of contemporary participants and concerns could be raised that the measurements say little about cognitive processes unfolding in the minds of stone age humans 100 000 years ago.

Kristian Tylén explains:

"Previous investigations have relied exclusively on studies of archaeological artefacts, the size and shape of cranial casts, or the mapping of genes. These are very indirect measures of human cognitive processes. While our experimental approach is also indirect in the sense that we cannot travel back in time and directly record the cognitive processes of our Stone Age ancestors, it is, on the other hand, dealing directly with those basic cognitive processes critically involved in human symbolic behaviour."

The study can thus inform foundational discussions of the early evolution of human symbolic behaviour. Not unlike manual tools, the findings suggest that the engravings were incrementally refined over a period of more than 40 000 years to become more effective 'tools for the mind' as their producers became more skilled symbol makers and users. In the challenging pursuit of understanding human cognitive evolution, the approach and findings provide novel insights into the minds of our Stone Age ancestors that cannot be achieved through the traditional methods of archaeology and genetics, or by theoretical work alone.

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