Hum Nat (2015) 26:331–345 DOI 10.1007/s12110-015-9233-8
Before Cumulative Culture The Evolutionary Origins of Overimitation and Shared Intentionality Ceri Shipton 1,2 & Mark Nielsen 3,4
Published online: 9 July 2015 # Springer Science+Business Media New York 2015
Abstract In the 7 million years or so since humans shared a common ancestor with chimpanzees we have colonized more of the planet’s terrestrial habitat than any other mammalian species and come to account for more biomass than all other terrestrial vertebrates combined. Chimpanzees, in contrast to and under pressure from ourselves, have veered toward extinction. There are multiple reasons for the stark evolutionary trajectories humans and chimpanzees have taken. Recent theoretical and empirical interest has focused on the emergence of cumulative culture whereby technological innovations are progressively incorporated into a population’s stock of skills and knowledge, generating ever more sophisticated repertoires. Here we look at the role of high-fidelity imitation and intention-reading in the establishment of cumulative culture. By focusing on the lithic record, we aim to identify when in our evolutionary history these skills became part of our ancestors’ behavioral repertoire. We argue that evidence of cooperative construction in stone tool manufacture, along with speculation regarding changes to the mirror neurone system, hint at the foundations of overimitation and shared intentionality around 2 million years ago. However, these are not the only ingredients of cumulative culture, which is why we do not see convincing evidence for it until slightly more than a million years later. Keywords Overimitation . Shared intentionality . Acheulean . Biface * Ceri Shipton
[email protected] * Mark Nielsen
[email protected] 1
McDonald Institute for Archaeological Research, University of Cambridge, Cambridge, UK
2
British Institute in Eastern Africa, Nairobi, Kenya
3
Early Cognitive Development Centre, School of Psychology, University of Queensland, Brisbane, Australia
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Faculty of Humanities, University of Johannesburg, Johannesburg, South Africa
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These words are being written at the edge of the Kalahari Desert on a laptop that has enough music stored on it to play continuously for more than 5 days without repeating a single song. Less than 50 years ago, doing the same would have required a great many vinyl records and an amplifier system large enough to be considered furniture in its own right—and unlikely to pass as carry-on luggage for any airline. The technological advances required to go from masses of vinyl records to small digital files can be easily taken for granted, yet nothing any other animal, living or extinct, has done comes remotely close to this achievement. Multiple social and cognitive processes are necessary for such cumulative culture—where innovations are progressively incorporated into a population’s stock of skills and knowledge, generating ever-moresophisticated repertoires (Boyd and Richerson 1996; Tomasello 1999). Among the most fundamental of these processes are a species-specific proclivity for high-fidelity imitation, so useful innovations, once gained, are easily transmitted (Wasielewski 2014), and a capacity for cooperation, so ideas are shared between individuals (Tennie et al. 2009). Here we examine when in our evolutionary past the twin abilities for high-fidelity imitation and shared intentionality may have emerged.
The Ontogeny of High-Fidelity Imitation The past decade has seen the identification and charting of a rather remarkable behavior: overimitation. In their seminal work, Horner and Whiten (2005) had an adult demonstrator show 3- to 4-year-old human children and wild-born chimpanzees how to obtain a reward from a novel box. A bolt on the top of the box was first removed, revealing a hole into which a stick was poked. A door located on the front of the box was then opened, and the stick was used to extract the reward. Because the box was opaque, the participants could not see how the actions occurring inside the box were causally related to the outcome. When given their own turn with the box, both chimpanzees and children copied all of the demonstrated actions. When a transparent box was substituted for the opaque box, the effect of the internal actions could be identified, making it obvious that when the stick was inserted into the top hole it struck a barrier and made no contact with that part of the apparatus from which the reward could be retrieved. That is, the action involving the top hole had no causal relation to the outcome. When the same actions that had been demonstrated on the opaque box were demonstrated on the transparent box, the chimpanzees now ignored the first action and instead copied only the model’s insertion of the tool into the front hole. They ignored the initial action, which was now visibly, causally irrelevant. In contrast, the children replicated the model’s entire sequence of actions, including the obviously irrelevant insertion of the stick into the top hole. Exploring this phenomenon in young infants, Nielsen (2006) presented 12-, 18-, and 24-month-olds with an adult demonstrating how to retrieve a toy from a series of closed boxes by disengaging a latch located on the front of each. Although the boxes could easily be opened by hand, the adult complicated the demonstration by using a miscellaneous object to operate the latch. The older infants attempted to open every box using the modeled method, not only attempting to use the object as the adult had demonstrated, but persisting in their overimitation to such an extent they often failed to open the boxes. In stark contrast, 12-month-olds ignored the demonstrated method and
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attempted to open the boxes using the more causally efficacious hand approach. The 12-month-olds thus opened as many boxes as did the 24-month-olds, despite the comparatively immature motor skills of the former. The 18-month-olds performed in between the other two age groups. This research establishes the second year as the period of emergence for overimitation, with the proclivity to overimitate increasing with age from this point (Marsh et al. 2014; McGuigan et al. 2007). At first glance this behavior appears maladaptive given the potential for wasting time by copying redundant processes and the possibility that they constitute deviant approaches to task solutions. However, when viewed in the context of the unrelenting need for children to acquire multiple, contrasting object-related skills, its adaptive value becomes evident. That is, all human environments, including those of other species in our genus, feature tools and artifacts that commonly lack ready perceptual information about their key functions and modes of operation. This cognitive opacity makes it challenging for novices to identify which actions or behaviors are appropriate for each artifact and which are not (Gergely and Csibra 2006). Directly and comprehensively copying others may thus afford the rapid acquisition of a vast array of essential skills that have been developed and accumulated through multiple past generations. Though subject to much debate (e.g., Bonnie et al. 2007; Tennie et al. 2012), there is sufficient evidence to support the notion that chimpanzees can imitate to a degree—but they do not overimitate (Horner and Whiten 2005).
The Ontogeny of Intention Reading Critically, children do not blindly copy everything they see shown to them. In order for overimitation to be exhibited, actions must be perceived to have been performed intentionally (Lyons et al. 2007). Actions appearing to have been done accidentally are not adopted. This tendency to read intentions into the actions of others emerges relatively early in life. At 6 months of age, infants expect others to be consistent in their interactions with objects, at least over a short timespan (Woodward 1999), and at 9 months they understand that people have goals driving their behavior and will persist until these goals are reached (Gergely et al. 1995). By the middle of their second year, infants interpret others’ actions in terms of what they are trying to achieve, rather than what is actually achieved, and will adjust their own behavior accordingly. For example, Meltzoff (1995) presented 18-month-old infants with an adult model who grasped both ends of a small dumbbell-like object and acted as if she were trying to pull one end off, but failed in her efforts. When given the object, infants pulled the object apart and did so at rates equivalent to infants who actually saw the completed operation. When the same actions were done by a mechanical device, the failed attempts were not turned into successful ones by the infants, suggesting their responses were driven by an interpretation of the intention of the adult model. However, to participate fully in human cultural life something more is needed. According to Tomasello and colleagues (2005; Tomasello and Moll 2010) this “something” is shared intentionality: collaborative interactions in which participants have a collective goal and coordinated action roles for pursuing that goal. Shared intentionality arises when individuals who understand one another as intentional agents interact socially. The capacity for shared intentionality emerges in humans in their second year
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of life and is evident in the kinds of triadic engagements young children begin to share with their parents regarding objects in their environment (Scaife and Bruner 1975). Declarative pointing also emerges at this time, when pointing is used in a cooperative communicative act underpinned by an intention to help by informing (Liszkowski et al. 2004). Although chimpanzees show a capacity for understanding intention (Buttelmann et al. 2008; Call et al. 2004), evidence for shared intentionality is at best fleeting (Carpenter and Tomasello 1995). Thus, the origins of shared intentionality and overimitation must be in the 7-million-odd years since we diverged from our last common ancestor with chimpanzees.
The Emergence of Overimitation in the Paleolithic At around 2.6 million years ago we see the emergence of the Oldowan Industrial Complex, which comprises mostly sharp-edged flakes and the cores from which they were struck (Toth 1985). Being deliberately flaked through percussive blows, these tools represent a new stage in hominin tool manufacture. Because the sharpness of the end product—a stone flake—is not readily apparent from the raw material—an unworked stone nodule—Oldowan knapping appears to have required a social transmission mechanism in which both actions and goal are encoded: in other words, imitation (Caruana et al. 2013). When chimpanzees modify plant stems to create a brush, it might also be argued that the affordances of the brush are not apparent in the original plant stem (Sanz et al. 2009). Critical to stone knapping, however, are precise hammer blows and the positioning of the stone nodule being struck (Rein et al. 2013; Roux and Bril 2005). Oldowan knappers demonstrate good understandings of these principles of stone knapping from the outset (de la Torre 2004; Roche et al. 1999). Early Oldowan knapped artifacts are comparable to those produced by modern expert knappers in most variables relevant to knapping skill, suggesting a steep learning curve in hominin ontogenetic acquisition of these skills (Stout et al. 2009). Battering on stone cores shows unsuccessful attempts to remove flakes and may reflect the extent of trial and error learning as opposed to social transmission. The rate of battering on early Oldowan cores is low relative to that of two bonobos who were taught to knap; however, it is still significantly less than that produced by a modern expert knapper (Roche et al. 1999; Toth et al. 2006). Notwithstanding confounding variables such as hand morphology, the evidence tentatively suggests Oldowan social transmission was stronger than that of chimpanzees. If overimitation was responsible for the transmission of Oldowan knapping skills, evidence of stylistic traditions could be expected when chance variations became canalized, yet there are no clear traditions within the Oldowan prior to 2 million years ago (Stout et al. 2010). The Oldowan may instead be characterized as constituting leasteffort strategies for the production of sharp flakes that vary in response to the varieties of stone clasts (Toth 1985), strategies that could be maintained by lower-level forms of social learning than overimitation (Nielsen 2012; Tennie et al. 2015). Beginning around 1.75 million years ago in East Africa our ancestors used the abilities to strike large stone flakes and bifacially shape stone tools to create the characteristic artifacts of the Acheulean industry: handaxes and cleavers (Beyene et al. 2013; Lepre et al. 2011). Handaxes are large teardrop-shaped bifaces, usually
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with pointed tips and a cutting edge extending around much of their perimeter (Fig. 1a). Cleavers are U-shaped bifaces with a broad bit as their principal cutting edge (Fig. 1b). Several facets of Acheulean hominin behavior suggest that the species who manufactured this industry were characterized by a propensity for overimitation and shared intentionality. Most striking is the unparalleled homogeneity of the Acheulean: the industry persisted for around 1.5 million years (Beyene et al. 2013; Shipton et al. 2013) and spread as far afield as South Africa and North Wales, and from Morocco to Nepal. The Acheulean even transcends species boundaries, being manufactured by various hominins, including Homo erectus and Homo heidelbergensis. Some question whether the Acheulean is a genuine tradition maintained by social transmission or simply equifinality of generic artifact forms (Davidson 2002; Lycett and Gowlett 2008; Tennie et al. 2015). The latter interpretation seems unlikely. The symmetry and size of handaxes have been shown to be more homogenous than would be expected under conditions of random variation, indicating cultural constraints on these factors (Kempe et al. 2012; Lycett 2008). Whereas handaxes are a somewhat generic tool, with similar forms occurring at various times and places in later prehistory (e.g., Moore 2003), cleavers, with their distinctive bits, are a very specific tool belonging exclusively to the Acheulean (Tixier and Inizian 1983). At the ca. 750,000-year-old site of Gesher Benot Ya’aqov in the Levant, 16 layers of Acheulean occupation were uncovered, spanning a 50,000-year period (Sharon et al. 2011). The remarkable technological homogeneity in biface production throughout the sequence shows consistency in raw material selection, flake blank creation, retouch strategies, and the size of the finished piece despite considerable environmental variation. This intrasite consistency on a timescale of tens of thousands of years and the recurrence of the distinctive cleaver
Fig. 1 Acheulean bifaces from the Middle Son Valley, India. Stippling denotes cortex, the original outer skin of the stone before flaking. a = handaxe made on a chert slab parallel to the bedding plane; b = cleaver made on a large quartzite flake struck sub-parallel to the bedding plane
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form strongly suggest that the Acheulean is indeed a single technological tradition united by common descent from an East African ancestor. Would it be possible for the Acheulean tradition to have been maintained using weaker forms of social transmission? A key feature of overimitation is the focus on the actions used over the outcomes achieved. For example, in a recent study by Nielsen and colleagues (2015), preschool children watched an adult experimenter model redundant actions on a box (e.g., tapping the side of it with a tool) after the box had been opened. When given the box and tool, children reproduced the redundant action despite there being no causal value in doing so (the box was open at the time the actions were produced and the toy that had been hidden inside was accessible). This behavior fits with arguments that overimitation emerges from a need for social affiliation expressed by doing just as others do (Nielsen and Blank 2011; Over and Carpenter 2012), and that it is a normative act emerging through interpreting actions as essential parts of a larger conventional, generic activity (Kenward 2012; Keupp et al. 2013). With regard to the Acheulean, several authors have noted that bifaces are overly elaborate for their utilitarian needs (e.g., Kohn and Mithen 1999). The form is often a radical departure from the initial clast shape, and making the form is presumably not always the easiest way to create a bifacial cutting edge. They are deliberately shaped to be symmetrical, often in two planes (Wynn 2002), yet butchery experiments suggest this symmetry does not greatly improve their utilitarian value (Machin et al. 2007). It may be that overimitation of an approximately symmetrical form maintained the symmetry and overall morphology of these objects across multiple generations (Lycett 2008). Further evidence for an overimitative approach to social learning in the Acheulean is provided by cleaver manufacture at two early Acheulean sites in India. At the 1.21million-year-old site of Isampur Quarry, the entire manufacturing sequence from clast procurement to finished tool is preserved with spatial integrity for handaxes and cleavers (Shipton 2013). Handaxes were made by reducing thin slabs of limestone to leave the teardrop handaxe shape parallel to the bedding plane of the slab. Cleavers, by contrast, were made by setting up platforms on thick slabs of limestone from which large flakes could be struck obliquely to the bedding plane, which were then retouched into the cleaver shape (Petraglia et al. 1999; Shipton et al. 2009). The cleaver manufacturing sequence involves several hierarchically organized stages, and to a novice it would not be obvious how some of the earlier stages relate to the finished cleaver (Shipton 2013). Such preparatory steps include removing thick flakes from the slab perimeter to set up suitable platforms from which to strike the large flakes that will then be made into cleavers. To understand why this platform setup step is necessary requires some experience of knapping to know the angles and surfaces that are good for striking large flakes. At Chirki-on-Pravara, a site more than 780,000 years old (Sangode et al. 2007), many cleavers were produced according to a standardized hierarchical method (Corvinus 1983). Flakes were first removed from the side of a block of basalt to create a strong striking platform. From this platform, flakes were then struck to flatten the base of the block and create the future cleaver bit. A particularly forceful blow was then aimed at a point where two of the platform scars intersected—a dihedral platform (Fig. 2). Aiming the blow at such a platform creates expanding flakes that are usually much wider than they are long. The resultant large flake was then reshaped along its lateral margins, particularly where the bulb of percussion protruded on one
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side (Fig. 2). The cleaver bit was left unworked where the lateral edge of the large flake meets the preparatory scars that were struck on the base of the core block (Corvinus 1983) (Fig. 2). Overimitation would ensure all aspects of hierarchically organized manufacturing sequences such as these were replicated, even if the purpose of earlier stages was not initially apparent. In the manufacture and the maintenance of form characteristic of the Acheulean, we thus see converging lines of evidence to indicate it was in this period that a propensity for overimitation emerged. Given the strength and dexterity required to produce large stone tools, it is likely that these skills were not acquired by hominins until adolescence or adulthood. Social learning in modern human adults, just as with children, is characterized by overimitation, but only when the demonstrator is perceived as an expert (Flynn and Smith 2012). If overimitation was a characteristic of Acheulean hominin behavior, this further suggests an ability to recognize knapping skill in others. We have argued that overimitation was necessary for the transmission of some Acheulean knapping sequences because the reasons for earlier steps would not be apparent to a novice. It may even be that experts did not need to fully understand the reasons behind their actions, as long as the fidelity of transmission was sufficient (Boyd et al. 2011). One of the fascinating things about the Acheulean is that handaxes and cleavers occur in very different rock types, with substantial variability in their granularity, toughness, and the natural form of the clasts. Experiments with human children show that imitatively learnt actions are applied in novel situations, without full causal understanding but with sensitivity to context
Fig. 2 Acheulean cleavers from Chirki-on-Pravara, India. In the upper artifact the dihedral platform originally on a side of the core is preserved. In the lower artifact this platform has been retouched away. Both cleavers show the large preparatory scar that would have been struck across the base of the core prior to the detachment of the cleaver, in order to create the bit (the cutting edge)
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relevance (Bushnell et al. 2006; Yang et al. 2010, 2013). Critically, children will overimitate even when the causal irrelevance of redundant actions included in a demonstrated sequence is emphasized (Nielsen et al. 2012, 2015). The parsing of knapping actions during imitation may thus have enabled Acheulean hominins to transfer knapping skills between rock types requiring very different reduction sequences.
The Emergence of Shared Intentionality in the Paleolithic As previously alluded, imitation goes hand-in-hand with shared intentionality. Therefore if our tendency for overimitation characterizes the Acheulean, we should also see evidence for shared intentionality in this period. Demonstrating cooperative shared intention activities in the Paleolithic is extremely difficult, but there some pieces of evidence are suggestive. Chimpanzees are known to hunt small game collaboratively (Boesch 2002), but although each chimpanzee appears to take into account the position of the others, there is no active coordination of roles (Tomasello et al. 2012). For the Acheulean hominins, carcass procurement often involves very large species such as elephants, which would likely require coordinated activity between individuals (Rabinovich et al. 2008; Stiner et al. 2009). A number of high-integrity Acheulean sites, such as the elephant butchery sites at Notachirico in Italy (Piperno and Tagliacozzo 2001) and Aridos in Iberia (Villa 1990), and the horse butchery locale at Boxgrove in Britain (Pitts and Roberts 1997), show multiple handaxes being used to butcher a single large carcass. Whether they were groups of hominins cooperating in the task of butchery or hominins acting individually to get their own meat is open to interpretation. In layer II-6 level 1 at Gesher Benot Ya’aqov, nine handaxes were found around the carcass of an elephant (Goren-Inbar et al. 1994). The skull of the elephant had been turned upside down, with the aid of a wooden branch as a lever, to access the brains. The detachment of the head and turning it over in this way would likely have necessitated several hominins working together to deal with the weight and awkward shape of the head. Elsewhere in Gesher Benot Ya’aqov different activities, such as nut cracking, stone knapping, and the processing of shellfish, were carried out in discrete areas structured around hearths (Alperson-Afil et al. 2009). Alperson-Afil and colleagues argue that this horizontal spatial organization, as opposed to different activities being carried out in succession on the same spot, represents contemporary activities, with the proceeds from each being shared between the individuals present. While this is suggestive of cooperation, it is possible that the different activities were carried out contemporaneously but not necessarily shared. For evidence of different activities carried out as part of the same overarching task we return to Isampur Quarry. Here the cleaver manufacturing sequence was spatially partitioned such that the production of large flake blanks was carried out in certain knapping clusters while the retouching into finished forms was carried out in adjacent clusters (Shipton 2013). If a single hominin was conducting the entire manufacturing sequence we would expect to find all stages within a single cluster. The production of large flake blanks requires powerful yet accurate blows, and it is unlikely that all individuals in a hominin group would have been capable of delivering such blows. The task of producing the blanks may thus have fallen to a few strong and skillful individuals while others finished off the tools. Large, heavy basalt hammerstones for
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striking large flakes were procured from a few kilometers away, and smaller quartzite hammers for retouching blanks were also transported to the site from a distant source. This convergence of resources from different locations for use in a single task further suggests multiple individuals cooperating in the production of cleavers. Perhaps the best evidence for cooperation at Isampur Quarry comes from a giant slab core that weighs about 65 kg (Shipton 2013). A series of flake scars struck from the side of the core on which it was lying when it was found indicates that the core had been turned over by hominins. It must also have been pried up from the bedrock before the flake scars were removed, when it would have been even heavier. Although one individual could have moved the core, the task would have been much more easily accomplished by two people working together. To strike a large flake blank off a giant core it is necessary to stabilize the core so the force travels through the core; if the core is shaking, the force exits too early. Any experienced knapper will recognize the importance of the non-dominant hand in stabilizing the core during free-hand percussion (e.g., Stout et al. 2011). In the case of these giant cores, which must be struck with large hammers held in two hands, the role of the non-dominant hand must be played by another person. Experimental evidence shows that stabilizing the core could be achieved in one of two ways: either one person could lift the core while another put dirt underneath to cushion it and prop it up, or the second person could hold the core directly while the first person strikes it (Shipton 2013). Either way, producing large flake blanks from these giant cores was apparently a two-person job. This is unlike the collaboration in chimpanzee hunting because it requires explicit role differentiation rather than individual assessment of an unfolding situation. We thus have converging lines of evidence to suggest that the complementary capacities for overimitation and shared intentionality were present in the Acheulean, something that may have been underscored by a change in the cognitive architecture of our ancestors.
The Role of a Changing Mirror Neuron System In the first psychology textbook, William James (1890) wrote: “every mental representation of a movement awakens to some degree the actual movement which is its object.” One hundred years later this speculation received clear empirical support in the discovery of mirror neurons in the monkey ventral premotor cortex (Gallese et al. 1996). Identified through single-cell recording, these neurons were discovered to fire when a monkey performed an action and when it observed the same action being performed by another monkey. Ethically prohibitive, single-cell recording in humans has only occurred in epileptic patients undergoing surgery, with recordings being taken only from wherever electrodes were placed for pre-surgery planning (Mukamel et al. 2010). Extracellular activity was recorded from 1177 cells in the medial frontal and temporal cortices while patients executed or observed hand-grasping actions and facial emotional expressions. Consistent with the presence of a mirror system, a significant proportion of neurons in the supplementary motor area, as well as the hippocampus and environs, responded to both observation and execution of these actions. This work adds to a corpus of studies using multiple techniques (e.g., TMS, fMRI) reporting activation in the human brain (the inferior parietal cortex, the inferior frontal gyrus, and the superior temporal sulcus) in areas that correspond to those containing mirror neurons in monkeys (Molenberghs et al. 2012).
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Of prime relevance here is that the mirror system is thought to be the neural substrate for imitation and other aspects of social cognition (Iacoboni 2009; Iacoboni and Dapretto 2006; Iacoboni et al. 1999). Mirror neurons fire when observing another’s actions as though the observer is mentally rehearsing the same action (Rizzolatti et al. 1996). Equivalent mirroring neurons for internalizing emotional states witnessed in others have also been identified (Gallese et al. 2004). Through this internal simulation one individual may achieve motor, emotional, and intentional attunement with another and thus understand others as being goal-directed agents with emotional motivations (Gallese et al. 2004). Critically, unlike those of monkeys, human mirror neurons fire when observing miming and intransitive actions, suggesting that the default mode of operation in our mirror neuron system is to interpret actions as intentional (Gallese et al. 2004). Internal simulation of observed actions in the mirror system could enable an observer to understand the intention of the action in a bottom-up process termed “motor resonance” (Gallese et al. 2009; Iacoboni et al. 2005). However, mirror neurons are sensitive to the different intentions of actions even when they are kinetically similar (Fogassi et al. 2005). The interpretation of goals and intentions thus relies on more than associating specific movements with particular outcomes. In experimental studies of observing stone knapping, Stout et al. (2011) found that experts employed a combination of sensorimotor matching in the posterior parietal cortex and top-down mentalizing in the medial prefrontal cortex. Naive subjects, in contrast, relied on bottom-up kinematic simulation in the pars opercularis (Brodmann Area 44) to understand unfamiliar intentions. The pars opercularis is an integral component of the mirror system (Rizzolatti and Craighero 2004) and is responsible for generating kinematic models to execute or simulate actions (Kilner et al. 2007). Learning novel motor actions and intentions may thus rely on the motor resonance of the mirror neurone system, whereas expertise may use alternative top-down pathways. The left posterior cerebellum has also been shown to be active in the imitation of novel motor actions but not familiar ones (Grezes et al. 1998; Leslie et al. 2004). Endocranial evidence indicates that KNM-ER 1470, an early Homo cranium, possesses a left third inferior frontal convolution or Broca’s area (the region of the brain containing the pars opercularis) similar to that of Homo sapiens (Falk 1987; Holloway 1983; Tobias 1987). The KNM-ER 1470 specimen is dated to 1.88 mya (Fitch, Miller, and Mitchell 1996), just prior to the appearance of the earliest Acheulean. KNM-ER 1470 is succeeded by hominins who display similar configurations of Broca’s area, including KNM-ER 3733, dated to 1.78 mya (Holloway et al. 2004), and KNM-WT 15,000, dated to 1.5 mya (Walker and Leakey 1993). Owing to poor preservation, negative evidence for Broca’s area in earlier hominins is equivocal (Holloway et al. 2004). However, the appearance of this derived brain region at approximately the same time as the Developed Oldowan and Acheulean traditions indicates that the transmission of these complex technologies may have been underpinned by the evolution of key components of the mirror system. That is, the mirror system likely evolved in the context of precise technological action transmission in the Oldowan, with selection for increasing fidelity resulting in the overimitation characteristic of our own species by the time of the Acheulean (see also Fuhrmann et al. 2014). Interestingly, KNM-ER 1470 also exhibits triangular cerebellar lobes like those of Homo sapiens but distinct from the more globular form evident in the hominin most closely associated with the Oldowan, Homo habilis (Holloway et al. 2004).
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Conclusion: Not Yet Cumulative Culture In Acheulean lithic artifacts we see evidence of the emergence of a propensity for overimitation and engagement in activities that relied on shared intentionality, a change that was possibly supported by a change in the hominin mirror system. The question arises as to why such behaviors emerged at this time. Tomasello et al. (2012) argue that these traits emerged in the context of collaborative foraging, such as the procurement of medium-sized to large game carcasses, when working together provided greater total yields than individual foraging efforts would. Oldowan sites indicate hominin procurement of small to medium-sized bovid carcasses, with the stone flakes being used to butcher these carcasses, among other functions (Ferraro et al. 2013; Lemorini et al. 2014). Social transmission and cooperation may have been selected for in this context of collaborative carcass procurement and technologically assisted butchery. Such trends culminate in the Acheulean, when hominins were procuring very large carcasses such as elephants, and butchering them with often elaborately made handaxes. These are the cornerstones (pun intended) of the remarkable technological advances characteristic of our species. But are they enough? Key features of human culture and the broad success of our species are not only that skills are readily transmitted from one generation to the next, but that such skills are modified and improved on, sometimes at a remarkable pace. Yet for more than a million years during the Acheulean, innovations were at best sporadic (Goren-Inbar 2011; Hopkinson et al. 2013). Cumulative culture thus likely gained a foothold in the Acheulean but didn’t emerge until later (Whiten et al. 2003). Identifying the selective pressure that prompted the transition from overimitation to cumulative culture is beyond the scope of this article, but Tomasello et al. (2012) have suggested intergroup competition as a key factor. Cumulative culture was likely also contingent on the establishment of childhood as a major life history stage (Nielsen 2012), and its expression appears highly sensitive to demographic conditions (Kempe and Mesoudi 2014). To return to the theme of the opening sentence: that a member of a remote Bushman community in the Kalahari Desert can be introduced to the glory of Motörhead on the same laptop I am using to write this paper is remarkable. It likely only became possible because a long time ago our ancestors started to understand each other’s intentions, to share them, and to be motivated to copy each other with fine attention to detail. They were a long way from having a man in a loincloth puzzle over exactly what Lemmy meant when screaming, “If you like to gamble, I’m your man.” But it was a start. Acknowledgments We thank Professors K. Paddayya, Sheila Mishra, and J. N. Pal and the Archaeological Survey of India for access to the archaeological collections discussed and illustrated in this paper. We thank three anonymous reviewers for their comments on an earlier draft of this paper.
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