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This study aims to shed light on how and when mechanisms of the human brain evolved to support complex cognition and language. The field of evolutionary cognitive archaeology asserts that prehistoric technologies, as products of past cognition in action, are informative of the minimum cognitive and linguistic abilities that hominins needed to possess for their production. Previous researchers attempted to reconstruct the neural correlates of two Early Stone Age (ESA) tool industries, the 2.6 million-year-old Oldowan industry and the 0.7 million-year-old late Acheulian industry, by using positron emission tomography (PET) to observe the functional activation occurring in the brains of trained and expert stone knappers after making these different tool types. Because of evidence for overlap between the knapping and language circuits of the brain and increased anterior frontal activity during Acheulian tool production, these researchers argued that their results 1) indicate increased cognitive demands for late Acheulian tool production relative to Oldowan tool production and 2) support a technological origin for language, meaning that certain language functions co-opted the neural substrate and functions that were already established for toolmaking and tool use. Because of the motion limiting aspects of PET, however, these studies were unable to record the hemodynamic response of naturalistic stone knapping in real-time. They also were unable to observe the functional activation associated with the earliest stage of learning, which is likely to differ from late stage learning or expertise. Furthermore, any conclusion regarding a technological origin for language is problematic if it relies on data obtained from participants who learned to knap with verbal instruction.
To test these two claims, this dissertation utilized a neuroimaging technique called functional near-infrared spectroscopy (fNIRS) to explore the neural correlates of real-time, naturalistic Oldowan and Acheulian stone knapping at three different points in learning. Participants in the study were separated into two groups to learn ESA knapping skills. Both groups watched the same video tutorials that depicted an expert’s hands as he made stone tools, but those in the verbal group heard spoken instructions, while those in the nonverbal group watched a version with the sound turned off. Functional brain images were reconstructed from the digitized landmarks of each participant’s head and from the optical data. An analysis of variance (ANOVA) revealed a clearer distinction between the neural processes of Oldowan and Acheulian tool manufacturing tasks than has previously been demonstrated. Only the Acheulian task recruited a frontotemporal working memory network. Selection for individuals with increased working memory capacities, which would have allowed them to make increasingly complex tools to gain access to novel dietary items, may have spurred the evolution of larger brain size in the genus Homo during the early Pleistocene. The results also demonstrated that the presence or absence of language during training dictated which higher-order cognitive areas of the brain become engaged and at what point in training. Thus, the results of previous neuroarchaeological studies reflect a very specific condition of stone knapping skill acquisition that involves linguistic instruction, which may not be analogous to how skills were transmitted during the ESA. Finally, evidence of overlap between left hemisphere language and stone knapping circuits among the participants in the nonverbal group lends additional support for the technological origin for language hypothesis.