Papers by Shelby Putt
Nature
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Interaction Studies
We present a new road map for research on “How the Brain Got Language” that adopts an EvoDevoSoci... more We present a new road map for research on “How the Brain Got Language” that adopts an EvoDevoSocio perspective and highlights comparative neuroprimatology – the comparative study of brain, behavior and communication in extant monkeys and great apes – as providing a key grounding for hypotheses on the last common ancestor of humans and monkeys (LCA-m) and chimpanzees (LCA-c) and the processes which guided the evolution LCA-m → LCA-c → protohumans → H. sapiens. Such research constrains and is constrained by analysis of the subsequent, primarily cultural, evolution of H. sapiens which yielded cultures involving the rich use of language.
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Journal of Archaeological Method and Theory, 2019
Inter- and intra-rater reliability studies in experimental archaeology promote consistency and re... more Inter- and intra-rater reliability studies in experimental archaeology promote consistency and replicability in the lithic analysis methods that are applied to interpretations of the archaeological record. Replication attempts to classify a knapper’s hand preference post-hoc using published methodologies that focus on right- and left-oriented flake features, have been largely unsuccessful. We tested the validity of flake feature categories described in three s tudies to be useful for determining a knapper’s hand preference (Bargalló and Mosquera, Laterality,19(1), 37–63,2014; Dominguez-Ballesteros and Arrizabalaga, Journal of Archaeological Science: Reports, 3,313–320,2015; Rugg and Mullane, Laterality, 6(3),247–259,2001). Five experienced lithic analysts independently made blind predictions of knapper hand preference on an experimental assemblage of mode I flakes produced by 18 knappers (9 left-handed), which included 344 complete flakes from 43 knapped cores. Inter- rater reliability measures (using Fleiss’ Kappa) showed significant agreement between raters for only one of the features (eraillure scar), with fair agreement for impact point, and poor agreement for the other features (cone of percussion, hackles, ripples, extraction axis, and platform inclination); poor agreement was found even within raters. Chi squared tests and correspondence analyses show that raters fail to perform significantly better than chance at predicting hand preference. These results suggest not only that these flake features are unreliable predictors of a knapper’s hand preference, but also that most of these features do not represent objective categories. We therefore urge caution in applying these methods to archaeological assemblages pending further independent replication.
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NeuroImage, 2019
Trends toward encephalization and technological complexity ∼1.8 million years ago may signify cog... more Trends toward encephalization and technological complexity ∼1.8 million years ago may signify cognitive development in the genus Homo. Using functional near-infrared spectroscopy, we measured relative brain activity of 33 human subjects at three different points as they learned to make replicative Oldowan and Acheulian Early Stone Age tools. Here we show that the more complex early Acheulian industry recruits left dorsolateral prefrontal cortex when skills related to this task are first being learned. Individuals with increased activity in this area are the most proficient at the Acheulian task. The Oldowan task, on the other hand, transitions to automatic processing in less than 4 h of training. Individuals with increased sensorimotor activity demonstrate the most skill at this task. We argue that enhanced working memory abilities received positive selection in response to technological needs during the early Pleistocene, setting Homo on the path to becoming human.
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Squeezing Minds from Stones: Cognitive Archaeology and the Evolution of the Human Mind, 2019
Language origins remain shrouded in mystery. With little remaining from our earliest ancestors, l... more Language origins remain shrouded in mystery. With little remaining from our earliest ancestors, language evolution researchers have turned to stone tools to learn about ancestral language capacities. Because inferior frontal areas of the brain, once thought specific to language, are now known to participate during manual motor tasks as well, technological-origin hypotheses propose that tool-making was a potential cause or contributor to the evolution of language. Cutting-edge neuroimaging techniques to monitor regional brain activation patterns associated with tool-making processes are helping to investigate the potential evolutionary relationship between language and tool-making. These experiments have identified one area in the left dorsal pars opercularis portion of Broca’s area where language and stone tool-making functions rely on similar cognitive operations. A more general motor origin for language seems likely in other inferior frontal areas of the brain. Clearly, stone tools have stories to tell if we know how to listen.
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Interaction Studies, Aug 2018
We used optical neuroimaging to explore the extent of functional overlap between working memory (... more We used optical neuroimaging to explore the extent of functional overlap between working memory (WM) networks involved in language and Early Stone Age toolmaking behaviors. Oldowan tool production activates two verbal WM areas, but the functions of these areas are indistinguishable from general auditory WM, suggesting that the first hominin toolmakers relied on early precursors of verbal WM to make simple flake tools. Early Acheulian toolmaking elicits activity in a region bordering on Broca's area that is involved in both visual and verbal WM tasks. The sensorimotor and mirror neurons in this area, along with enhancement of general WM capabilities around 1.8 million years ago, may have provided the scaffolding upon which a WM network dedicated to processing exclusively linguistic information could evolve. In the road map going forward, neuro-archaeologists should investigate the trajectory of WM over the course of human evolution to better understand its contribution to language origins.
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Nature Human Behaviour, May 2017
After 800,000 years of making simple Oldowan tools, early humans began manufacturing Acheulian ha... more After 800,000 years of making simple Oldowan tools, early humans began manufacturing Acheulian handaxes around 1.75 million years ago. This advance is hypothesized to reflect an evolutionary change in hominin cognition and language abilities. We used a neuroarchaeology approach to investigate this hypothesis, recording brain activity using functional near-infrared spectroscopy as modern human participants learned to make Oldowan and Acheulian stone tools in either a verbal or nonverbal training context. Here we show that Acheulian tool production requires the integration of visual, auditory and sensorimotor information in the middle and superior temporal cortex, the guidance of visual working memory representations in the ventral precentral gyrus, and higher-order action planning via the supplementary motor area, activating a brain network that is also involved in modern piano playing. The right analogue to Broca’s area—which has linked tool manufacture and language in prior work1,2—was only engaged during verbal training. Acheulian toolmaking, therefore, may have more evolutionary ties to playing Mozart than quoting Shakespeare.
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Journal of Archaeological Science: Reports, Jun 2015
A reassessment of many of the archaeological assemblages older than two million years has resulte... more A reassessment of many of the archaeological assemblages older than two million years has resulted in a general consensus that the earliest Oldowan artifacts were made by skilled toolmakers who had a clear understanding of the fracturing mechanics of different toolstone materials. This has led several researchers to propose a simpler lithic reduction stage that occurred prior to 2.6 Ma. Three lithic reduction techniques that are within the behavioral repertoire of our closest living relatives in the genus Pan are proposed as potential intermediate stages between the percussion behaviors of the last common ancestor of chimpanzees and humans and the skilled knapping of the Oldowan toolmakers. These include direct and indirect projectile percussion and bipolar flaking techniques. Measures of productivity, expediency, and efficiency were obtained and compared between these three reduction techniques and novice freehand knapping in order to better understand some of the factors that influenced how early hominins with little to no understanding of lithic fracturing mechanics achieved sharp flake tools. The provisional results of this proof-of-concept experiment indicate that, of these four conditions, dropping or throwing a large hammer stone on a brittle core is the most efficient way to exploit a core, while bipolar flaking is the most expedient method; however, novice freehand knapping creates the most productive flakes with large, sharp cutting edges. Thus, the transition to knapping in the late Pliocene may have been due to a shifting emphasis on productive toolmaking over expediency or efficiency.
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Lithic Technology, Jun 2014
Many researchers have hypothesized an analogous, and possibly evolutionary, relationship between ... more Many researchers have hypothesized an analogous, and possibly evolutionary, relationship between Paleolithic stone tool manufacture and language. This study uses a unique design to investigate how spoken language may affect the transmission of learning to make stone tools and comes to surprising results that may have important implications for our views of this relationship. We conducted an experiment to test the effect of verbal communication on large core biface manufacture during the earliest stages of learning. Previously untrained flintknappers were assigned to two different communication conditions, one with and one without spoken language, and were instructed to replicate the bifaces produced by the same instructor. The attempted bifaces (total = 334) from the two groups were compared using an Elliptical Fourier analysis, the Flip Test, and a rating scale by an independent lithicist. We found no significant difference in the overall shape, symmetry, or other measures of skill among the two groups, using all three of these methods. Analysis of the 18,149 debitage elements from the experiment, however, revealed that the two groups set up their striking platforms in fundamentally different ways. The nonverbal group produced more efficient flakes than the verbal group, as evidenced by the significantly higher ratios of platform width to platform thickness and size to mass of the nonverbal subjects’ flakes. These results indicate that verbal interaction is not a necessary component of the transmission of the overall shape, form, and symmetry of a biface in modern human novice subjects, and it can hinder the progress of verbal learners because of their tendency to over-imitate actions of the instructor that exceed their current skill set.
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Lambda Alpha Journal, Jan 1, 2009
One of the prominent questions in paleoanthropological studies is the origin of bipedalism.There ... more One of the prominent questions in paleoanthropological studies is the origin of bipedalism.There have been several hypotheses presented on the ancestral type of locomotion that predated bipedalism. These hypotheses include a terrestrial knuckle-walking quadrupedal ape, a brachiating hylobatid-like ancestor, a palmigrade terrestrial ape, and a climbing arboreal ape.Thorpe et al. present an extension to the climbing hypothesis with the data that they gathered from wild orangutans of the Gunung Leuser National Park in Sumatra, Indonesia (2006, 2007).They present the hypothesis that bipedalism originated with an arboreal ape similar to extant orangutans, and they provide that the orangutan locomotor data that they gathered supports this claim. This paper includes locomotor data that I collected from captive orangutans in the Fort Wayne Children's Zoo. The two data sets are not correlative, but my observations of assisted and unassisted bipedalism in the captive orangutans lend some support to the climbing hypothesis. When considering the functional anatomy of the wrist and ankle of extant primates and extinct hominins, the answer still remains inconclusive.
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Dissertation by Shelby Putt
Some of the biggest questions in human evolution are why we have such large brains and how our an... more Some of the biggest questions in human evolution are why we have such large brains and how our ancestors acquired language and exceptional intelligence. Our extreme reliance on technology has set us humans and our ancestors apart from other primates for more than three million years. It is widely thought that tools from the distant past may hold the clues to answering these questions because they represent all that is left of ancient minds at work. This study addresses these questions by using brain imaging technology to determine which areas of the brain of modern-day humans become most active as they make two types of tools from the past, one from as early as 2.6 million years ago (Ma) known as the Oldowan industry, and the other from 1.75 Ma known as the early Acheulian industry. Because it remains unclear whether early humans possessed language this far back in the past, an instructor taught half of the participants in this study to make stone tools with language, while the other half learned by nonverbal imitation. The analysis of the resulting brain imaging data revealed that Acheulian toolmaking requires higher-order conceptualization than Oldowan toolmaking. Selection for individuals who could store and manipulate more information and therefore make the most productive Acheulian tools may have been the prime reason for the evolution of large brain size in humans. The complex cognition that evolved as a result of such technology likely provided the framework on which language could build.
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International Presentations by Shelby Putt
Europe has a rich archaeological record extending from the simple flake and pebble tools of the L... more Europe has a rich archaeological record extending from the simple flake and pebble tools of the Lower Paleolithic to the intensely creative and diverse material cultures of the Upper Paleolithic. There is little doubt among scientists that the first H. sapiens migrants out of Africa were cognitively modern; however, there is still the question of which cognitive abilities Neandertals and their predecessors possessed or lacked in comparison to the newly arriving humans to Europe. Lithic artifacts, as the most enduring evidence of hominin occupation throughout the Paleolithic, are also the best markers for ancient hominin cognition in action. The field of neuro-archaeology investigates which neural networks become activated when replicating the operational sequences necessary to recreate stone tools from the past. To better understand the minimum cognitive abilities of European hominins in relation to stone tool manufacture, I use a case study approach, applying the latest experimental findings from the field of neuro-archaeology to prehistoric sites in different regions of Europe that represent important developments in technological behaviors over time. Notable developments include the introduction of the first lithic technology (Mode 1) and large core bifaces (Mode 2) to the continent at 1.78 and 0.7 million years ago (Ma), respectively, and the transition to prepared core technology (Mode 3) around 0.3 Ma. Multiple studies indicate that Mode 1 toolmakers were capable of coordinating visual attention and motor control for the successful removal of simple flakes from a core. Because of the increased complexity of bifacial toolmaking, the hominins at Mode 2 bearing sites in South and West Europe integrated higher-order motor planning, working memory, and auditory feedback mechanisms as they attended to the multitude of goals associated with this task. Research is currently underway to reveal the neural correlates of Mode 3 tool manufacture, and presumably, it would require a similar or enhanced level of planning, working memory, and problem-solving ability relative to Mode 2 activities. Because of the nature of neuro-archaeological evidence, it is impossible to conclude that hominins at any of the featured sites lacked specific cognitive features. The results do, however, suggest a cognitive evolutionary course among European hominin populations that parallels that occurring among the ancestors to modern humans in Africa.
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Teaching is uniquely developed in humans and was likely critical to the emergence of cumulative c... more Teaching is uniquely developed in humans and was likely critical to the emergence of cumulative culture. However, the importance of various forms of teaching, including the use of language, in transmitting Paleolithic skills like stone knapping is less understood. Here we examine the knapping behaviors of 17 subjects who learned to make Oldowan and Acheulian stone tools from watching video demonstrations either with verbal instruction or without sound. Despite intriguing differences in brain activity (measured with fNIRS) between these groups, the mode of social transmission has yet to reveal significant effects on handaxe morphology, suggesting that verbal teaching may not have been necessary for the social reproduction of early stone toolmaking skill. We coded video-recordings of subject performance using BORIS event-logging software and a knapping ethogram to test for possible behavioral differences between conditions that might explain observed differences in brain activity and indicate influences of teaching on early-stage skill acquisition that are difficult to detect from artifacts.These data allow us to test for differences in the frequency and sequential structure of knapping actions across conditions and individuals, potentially guiding further artifact analyses and providing a more detailed picture of the influence of verbal teaching on knapping skill acquisition.
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The enhanced working memory hypothesis proposes that modern cognition evolved when a recent genet... more The enhanced working memory hypothesis proposes that modern cognition evolved when a recent genetic mutation led to enhanced working memory capabilities in Homo sapiens. According to this model, stone tool artifacts produced prior to this event would have required procedural memory and not working memory. Because both human and nonhuman primates have been shown to recruit working memory brain areas during the early stages of learning a new motor task, we predicted that stone toolmaking should also recruit a working memory network when first being learned. Over the course of a 7-hour-long training program, we used image-based functional near-infrared spectroscopy to record the functional brain activity of 33 right-handed, adult, human participants at three different points as they learned to make replicative Oldowan and Acheulian stone tools. The optic signals from the two knapping tasks were processed to produce functional images of the brain that were analyzed using a multifactorial ANOVA. The knapping tasks were then compared to a motor baseline task with Wilcoxon signed-rank tests to exclude any general motor areas that are involved in stone knapping. Results revealed that only the Acheulian technology recruits the left dorsolateral prefrontal cortex, an area critical to working memory function. We conclude that working memory was the most likely cognitive strategy that pre-sapiens Homo utilized when learning to make Acheulian and other complex tools. We should therefore consider that “modern” cognition emerged in a piecemeal fashion over the course of human evolution rather than suddenly occurring with the appearance of H. sapiens.
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This presentation uses an optical neuroimaging technique to explore the extent of functional over... more This presentation uses an optical neuroimaging technique to explore the extent of functional overlap between the working memory (WM) systems involved in language and Early Stone Age toolmaking behaviors. Oldowan stone tool production activates two verbal WM areas, but the functions of these areas are indistinguishable from general auditory WM, suggesting that the earliest hominin toolmakers relied on early precursors to verbal WM to make simple flake tools. Acheulian stone tool production elicits activity in a region bordering on Broca’s area that is involved in both visual and verbal WM tasks. The sensorimotor and mirror functions of this area, along with enhancement of general WM capabilities around 1.8 million years ago, may have provided the scaffolding upon which a WM system dedicated to processing exclusively linguistic information could evolve.
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After more than 800,000 years of making simple Oldowan flake and pebble tools, the adoption of Ac... more After more than 800,000 years of making simple Oldowan flake and pebble tools, the adoption of Acheulian handaxes by early Homo in the Early Stone Age is thought to represent an evolutionary change in hominin cognition. The present study uses a neuroarchaeological approach to test the hypothesis that Acheulian toolmaking recruits higher-order cognition areas of the brain to a greater extent than Oldowan toolmaking. Functional near-infrared spectroscopy (fNIRS) was used to measure the brain activity of 31 right-handed, adult, human participants as they made replicative Oldowan and Acheulian stone tools, after receiving seven hours of training. The optic signals for the two knapping tasks were processed to produce functional images of the brain that were analyzed using ANOVA. The knapping tasks were then compared to a motor baseline task with Wilcoxon signed-rank tests to exclude any general motor areas that are involved in stone knapping. Results reveal that Acheulian tool manufacture requires the integration of multimodal information and the guidance of visual working memory representations to accomplish higher-order motor planning than what is necessary for Oldowan tool manufacture. Interestingly, this Acheulian cognitive network is nearly identical to one that comes online when trained pianists play the piano, which is consistent with this network being critical for audiomotor integration when monitoring one’s performance during a complex task. Selection for this cognitive network around 1.8 Ma marked a turning point in the evolution of the hominin brain, potentially leading to larger brain size and to the appearance of more complex, human-like behaviors.
This study was funded by the Leakey Foundation, Wenner-Gren Foundation (#8968), Sigma Xi, the Scientific Research Society, and the University of Iowa. The first author held an American Fellowship from AAUW.
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This study aims to shed light on how and when mechanisms of the human brain evolved to support co... more 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 early humans needed to possess for their production. Previous researchers attempted to reconstruct the neural correlates of two Early Stone Age (ESA) tool industries, including the 2.6 million-year-old Oldowan industry and the 0.5 million-year-old late Acheulian industry. These studies used positron emission tomography (PET) to observe the functional activation occurring in the brains of human participants after they replicated the production (knapping) process of 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, they 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. 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 study utilized functional near-infrared spectroscopy to explore the neural correlates of real-time, naturalistic Oldowan and Acheulian stone knapping among 31 trained, right-handed, adult participants, who were separated into two training groups. Both groups trained by watching the same video tutorials showing hands making 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 digitized optode locations and 10-20 landmarks of each participant’s head and from the optical data (see Wijeakumar et al., 2016). A two-way analysis of variance revealed that only the Acheulian task recruited a frontotemporal cognitive control network, and the presence or absence of language during training dictated which higher-order cognitive areas of the brain become engaged. Selection for prehistoric individuals with increased working memory capacities, which would have allowed them to make increasingly complex tools to gain access to novel diets, may have spurred the evolution of larger brain size in the genus Homo during the ESA. Furthermore, this study demonstrates that the results of previous experiments 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.
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Language is considered a defining human feature, yet there is still little known about how this u... more Language is considered a defining human feature, yet there is still little known about how this unique communication system evolved. Previous research has emphasized a potential co-evolutionary relationship between stone toolmaking and language; however, participants in earlier brain imaging studies on stone knapping received linguistic training, which may be a poor analogue for extinct human species. We test the validity of the language and technology co-evolution hypothesis using a cutting-edge brain imaging technology (fNIRS) to directly observe the effect of language instruction on neural activation during naturalistic stone knapping. Thirty-three right-handed, adult subjects participated in the study by attending seven knapping practice sessions, during which they received verbal (sound enabled) or nonverbal (no language input) video instruction. Participants completed three neuroimaging sessions at different stages in their learning. fNIRS data were acquired at 25 Hz with a 24-channel TechEn CW6 system. The optic signals were processed to produce functional images that were analyzed using ANOVA and subsequent Wilcoxon signed-rank tests. The nonverbal group had significantly greater centers of activation in spatial cognition areas than the verbal group, but both groups had similar levels of activation in Broca’s area during knapping and language tasks. These results support a co-evolutionary relationship between language and technology but indicate that the inferior parietal and prefrontal areas that participate in spatial working memory networks may have received stronger selection, especially if Early Stone Age hominins did not possess language. This could help explain why these regions underwent major expansion in the course of human evolution.
This study was funded by the Wenner-Gren Foundation, Leakey Foundation, Sigma Xi, the Scientific Research Society, and University of Iowa. The first author held an American Fellowship from AAUW.
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Homo sapiens is the only primate species that currently displays a population level preference fo... more Homo sapiens is the only primate species that currently displays a population level preference for right hand dominance. Previous studies have attempted to establish methodologies to determine handedness from stone tool debris because of the link between handedness and brain lateralization of the classic language centers, and its implications for the evolution of language. However, these experimental studies have produced varied results, and it is questionable whether handedness can be accurately determined from flake features. We conducted an experiment in which 9 left-handed and 9 right-handed novice flintknappers were video recorded as they made simple stone tools similar to the Oldowan industry. We used this footage to code their flintknapping gestures, such as the angle of percussion and positioning of the core on the support, in order to test whether left- and right-handers differ in the flintknapping gestures they use to produce flakes. A Chi-Square test revealed that right-handed individuals are more likely to utilize a horizontal position of the core than left-handed individuals (p < 0.001). We explore the possibility that these flintknapping gestures, rather than the dominant, percussing hand, are responsible for the flake features that previous studies have attributed to handedness.
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Papers by Shelby Putt
Dissertation by Shelby Putt
International Presentations by Shelby Putt
This study was funded by the Leakey Foundation, Wenner-Gren Foundation (#8968), Sigma Xi, the Scientific Research Society, and the University of Iowa. The first author held an American Fellowship from AAUW.
To test these two claims, this study utilized functional near-infrared spectroscopy to explore the neural correlates of real-time, naturalistic Oldowan and Acheulian stone knapping among 31 trained, right-handed, adult participants, who were separated into two training groups. Both groups trained by watching the same video tutorials showing hands making 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 digitized optode locations and 10-20 landmarks of each participant’s head and from the optical data (see Wijeakumar et al., 2016). A two-way analysis of variance revealed that only the Acheulian task recruited a frontotemporal cognitive control network, and the presence or absence of language during training dictated which higher-order cognitive areas of the brain become engaged. Selection for prehistoric individuals with increased working memory capacities, which would have allowed them to make increasingly complex tools to gain access to novel diets, may have spurred the evolution of larger brain size in the genus Homo during the ESA. Furthermore, this study demonstrates that the results of previous experiments 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.
This study was funded by the Wenner-Gren Foundation, Leakey Foundation, Sigma Xi, the Scientific Research Society, and University of Iowa. The first author held an American Fellowship from AAUW.
This study was funded by the Leakey Foundation, Wenner-Gren Foundation (#8968), Sigma Xi, the Scientific Research Society, and the University of Iowa. The first author held an American Fellowship from AAUW.
To test these two claims, this study utilized functional near-infrared spectroscopy to explore the neural correlates of real-time, naturalistic Oldowan and Acheulian stone knapping among 31 trained, right-handed, adult participants, who were separated into two training groups. Both groups trained by watching the same video tutorials showing hands making 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 digitized optode locations and 10-20 landmarks of each participant’s head and from the optical data (see Wijeakumar et al., 2016). A two-way analysis of variance revealed that only the Acheulian task recruited a frontotemporal cognitive control network, and the presence or absence of language during training dictated which higher-order cognitive areas of the brain become engaged. Selection for prehistoric individuals with increased working memory capacities, which would have allowed them to make increasingly complex tools to gain access to novel diets, may have spurred the evolution of larger brain size in the genus Homo during the ESA. Furthermore, this study demonstrates that the results of previous experiments 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.
This study was funded by the Wenner-Gren Foundation, Leakey Foundation, Sigma Xi, the Scientific Research Society, and University of Iowa. The first author held an American Fellowship from AAUW.
consensus that the earliest Oldowan artifacts were made by skilled toolmakers who had a clear understanding of the
fracturing mechanics of different toolstone materials. This has led several researchers to propose a simpler lithic reduction
stage that occurred prior to 2.6 Ma. Three lithic reduction techniques that are within the behavioral repertoire of our
closest living relatives in the genus Pan are proposed as potential intermediate stages between the percussion behaviors of the last common ancestor of chimpanzees and humans and the skilled knapping of the Oldowan toolmakers. These include direct and indirect projectile percussion and bipolar flaking techniques. Measures of productivity, expediency, and efficiency were obtained and compared between these three reduction techniques and novice freehand knapping in order to better understand some of the factors that influenced how early hominins with little to no understanding of lithic fracturing mechanics achieved sharp flake tools. The results of this experiment indicate that, of these four conditions, dropping or throwing a large hammer stone on a brittle core is the most efficient way to exploit a core, while bipolar flaking is the most expedient method; however, novice freehand knapping creates the most productive flakes with large, sharp cutting edges. Thus, the transition to knapping in the late Pliocene may have been due to a shifting emphasis on
productive toolmaking over expediency or efficiency.
skill via an independent assessor. It was expected that the verbal group would produce qualitatively better handaxes than the nonverbal group, implying that spoken language increases the ease of transmission from teacher to novice in this task, but this was not the result. Implications for these findings will be discussed.