SAS Research Support Award - John Murray

How did Middle Stone Age humans heat-treat silcrete over time in South Africa?

John Murray, Institute of Human Origins, Arizona State University

I received an SAS Student and ECR Research Support Award to support my dissertation research that focuses on better understanding the emergence of lithic heat treatment technology during the Middle Stone Age (MSA) of South Africa. Specifically, my research aims to address the question: how did MSA humans heat-treat silcrete on the south coast of South Africa over time? Currently, the heat treatment of silcrete at Pinnacle Point 13B (PP13B) in South Africa ~162,000 years ago (ka) represents the earliest evidence for humans using fire to improve the quality of lithic tool- stone. Researchers have argued that heat treatment requires analogical reasoning, which is a feature of complex cognition, but this is debated. The relevance of heat treatment to arguments of cognition are dependent on which method of heat treatment was utilized by MSA humans, however, we currently lack well-established approaches to identify this in the archaeological record.

There are multiple working hypotheses surrounding the method used to heat-treat silcrete in the MSA which include: (1) the direct method where silcrete nodules are directly placed in the fire; (2) the ember method where the nodule is placed near the fire under a pile of coals, and (3) the sand-bath method in which silcrete nodules are buried underneath the fire and heated indirectly through the sand. To test these hypotheses, I have experimentally heat-treated silcrete using the sand bath, ember, and direct heating method to create experimental reference assemblages for each method of heating (Figure 1). These experimental assemblages have been used to establish a multi- proxy approach that includes attribute criteria and novel quantitative methods with empirical support for identifying each method of heat treatment in the archaeological record. The novel quantitative methods include the application of two non-destructive methods that I developed to detect heat-treated silcrete in the archaeological record – the surface roughness method and the quantitative color method. These methods have been expanded to not only determine if a silcrete artifact was heated, but also how it was heated. The attribute criteria and methods developed with the experimental assemblage are now being used to analyze three MSA archaeological sites – PP13B, PP 5-6, and Boomplaas Cave – to determine how the method of heat treatment varied from ~165 ka to 30 ka and across diverse paleoenvironmental, paleoecological, and technology contexts.

Figure 1. Experiments demonstrating the three methods of lithic heat treatment technology being tested (Photo courtesy of John Murray).

This award supported the analysis of silcrete artifacts from PP13B and PP5-6 during the 2023 field season (Figure 2). It allowed me to purchase silicone peeling compound to replicate the surface roughness of lithic artifacts at 1 nanometer resolution. Although preliminary, the results suggest that the ember method was primarily used to heat-treat silcrete at Pinnacle Point, but a significant portion of artifacts were heat-treated using the sand-bath method. This suggests that multiple strategies of heat treatment were utilized over time, including within the same layers. This research has implications for understanding when and in what context lithic heat treatment technology arose which can help shed light on debates regarding the emergence of advanced cognition.

Figure 2. The analysis of the silcrete artifacts from Pinnacle Point during the summer of 2023 (Photo courtesy of John Murray).