Making Tools and Making Sense: Complex, Intentional Behaviour in Human Evolution

8/12/2019 Making Tools and Making Sense: Complex, Intentional Behaviour in Human Evolution

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Cambridge Archaeological Journalhttp://journals.cambridge.org/CAJ

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Making Tools and Making Sense: Complex, Intentional Behaviour in HumanEvolution

Dietrich Stout and Thierry Chaminade

Cambridge Archaeological Journal / Volume 19 / Issue 01 / February 2009, pp 85 - 96DOI: 10.1017/S0959774309000055, Published online: 10 February 2009

Link to this article: http://journals.cambridge.org/abstract_S0959774309000055

How to cite this article:

Dietrich Stout and Thierry Chaminade (2009). Making Tools and Making Sense: Complex, Intentional Behaviour in Human Evolution.Cambridge Archaeological Journal, 19, pp 85-96 doi:10.1017/S0959774309000055

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Steps to a Neuroarchaeology of Mind, part 2

Dietrich Stout & Thierry Chaminade

Making Tools and Making Sense:

Complex, Intentional Behaviour in Human Evolution

2001; Deacon 1997; Gibson & Ingold 1993; Mithen 1996;Noble & Davidson 1996; Roux & Bril 2005; Wynn 2002;Wynn & McGrew 1989).

Of particular interest have been possible relationsbetween language, gesture and tool-use in humanevolution. Such relations have received renewed aen-tion in recent years as a result of research into motorresonance, the tendency for neural structures involved

in action execution also to be recruited during actionobservation. Resonance is thought to provide a directmechanism for understanding the intentions and goalsof others through what amounts to a kind of motorempathy. Mirror-neurons found in a putative Brocasarea homologue (ventral premotor area F5) in mon-keys (Rizzolai & Craighero 2004) provide a specicdemonstration of resonance at the cellular level andhave inspired recent hypotheses for the evolutionof articulate language through intermediate stagesof imitation, pantomime and protosign language(Arbib 2005; Rizzolai & Arbib 1998). Many othersuch scenarios and mechanisms for the co-evolution

of language and tool-making have been proposedover the years.A number of researchers have argued that lan-

guage and tool-making share underlying cognitiveand neural requirements for hierarchically structuredaction sequencing (Greeneld 1991; Holloway 1969).This is consistent with broader motor hypotheses oflanguage origins, which derive key language proper-ties from the multi-level motor coordination required

Stone tool-making is an ancient and prototypically human skill characterized by multiplelevels of intentional organization. In a formal sense, it displays surprising similarities tothe multi-level organization of human language. Recent functional brain imaging studies ofstone tool-making similarly demonstrate overlap with neural circuits involved in language

processing. These observations are consistent with the hypothesis that language and tool-making share key requirements for the construction of hierarchically structured actionsequences and evolved together in a mutually reinforcing way.

Although it may appear esoteric in the modern world,stone tool-making has been practised in one form oranother by virtually every human society for the past2.5 million years. 2.6-million-year-old stone artefactsfrom Ethiopia (Semaw et al.1997; 2003) provide theearliest evidence of uniquely hominin tool-makingcapabilities and exemplify a basic human technologythat remained widespread until the recent past, and

in some cases endured into the modern era (Roux etal.1995; Skertchly 1984; Stout 2002; Weedman 2000).In fact, stone tool-making is a prototypical humanskill integrating demands for planning, problem-solving and perceptual-motor coordination withina pragmatic, collaborative context. Together withits likely evolutionary importance, this makes stonetool-making an important object of study for cognitiveneuroscience as well as archaeology.

The Early Stone Age (ESA) alone encompassesroughly 90 per cent (2.60.25 Ma) of human prehistoryand charts a technological progression from simple Old-owan stone chips to large, skilfully shaped Acheulean

cuing tools. During this period hominin brain sizenearly tripled, from the high end of the chimpanzeerange to the low end of the modern human range. It isreasonable to conjecture that many distinctive aspectsof modern human brain structure and function evolvedduring this period of massive brain expansion. Morecontroversial is the role which changing lithic technolo-gies may have played as cause, consequence or correlateto hominin brain and cognitive evolution (e.g. Ambrose

Cambridge Archaeological Journal19:1, 8596 2009 McDonald Institute for Archaeological Researchdoi:10.1017/S0959774309000055 Printed in the United Kingdom.


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in speech (Liberman & Whalen 2000; MacNeilage 1998;Studdert-Kennedy & Goldstein 2003) and/or cerebralasymmetries in manual and vocal control (Corballis2003; MacNeilage et al. 1984). Another, less direct,strand of argumentation has been to emphasize theselective pressures for intentional communication aris-ing from collaborative technological activities (Engels2003; Reynolds 1993) and pedagogy (Greeneld 1998;Stout 2002).

Palaeolithic stone artefacts provide a uniquesource of behavioural, chronological and contextualevidence that may be used to constrain such evolu-tionary hypotheses (Wynn 2002). The availability ofthis evidence is largely due to the durability of stoneartefacts; however putative links between stone toolsand language are more than just a maer of conven-ience for archaeologists. The hand and mouth are thetwo most complex and exible eectors of the human

body and are regulated by neighbouring or even par-tially overlapping neural circuits. This is particularlynotable in the case of object manipulation (Hamzeiet al. 2003). These cortical circuits include regionsof inferior frontal and parieto-temporal associationcortex connected by the arcuate fasciculus (Fig. 1),all of which have been disproportionately expanded(Rilling 2006; Rilling et al.2008) in the massive corticalgrowth that began soon aer the appearance of therst stone tools.

Bipedal locomotion, another distinctive humancharacteristic, has been hypothetically linked tothe evolution of basal ganglia circuits important to

language (Lieberman 2002) but does not appear tooverlap with cortical language circuits (Santi et al.2003). Furthermore, bipedalism predates signicantbrain expansion by at least 1.5 million years, wasassociated with an ape-like vocal tract includinglaryngeal air-sacs (Alemseged et al.2006), and showslile of the higher-level intentional organization seenin both language and tool-making. Even if bipedalismdid promote early preadaptations for speech, it isunlikely to be directly relevant to major subsequentdevelopments. Other putative hominin behaviourssuch as hunting or social strategizing are complex andcognitively demanding, but do not share demands for

the rapid sequencing of complex motor gestures seenin language and tool-making. Lithic and other lessarchaeologically visible technologies (e.g. Mazza et al.2006; Thieme 1997) involving manual object manipula-tion thus continue to provide the closest correspond-ence with language, and are the most likely candidatesfor hypothetical co-evolutionary relationships.

Until recently, however, empirical evidence of thespecic neural and cognitive substrates of particular

ancient technologies has been lacking. In an aemptto address this problem, we have recently undertakena series of functional brain imaging investigations ofESA tool-making (Stout & Chaminade 2007; Stout et al.2000; 2008). By comparing tool-making with a baselinemanipulative task we sought to isolate the distinctiveneural demands of ESA technologies. Among otherthings, this provided evidence of specic points ofoverlap with cortical language circuits. Here wepresent a more extended discussion of the theoreticaland evolutionary implications of these ndings.

Neurobehavioural foundations of articulatelanguage and manual action

Quite some time ago, Holloway (1969) pointed outthat any motor act can be described as a hierarchi-cally structured sequence of behavioural units. For

Holloway, the more interesting question was whetherthere is any meaningful correspondence betweenspecic units of speech and stone tool-making. Or,as Arbib (2006) more recently asked, a sentence is tospeech as what is to action?. Functional brain imagingresults suggest that meaningful correspondences doexist between language and ESA tool-making, andfurthermore that that these correspondences are to befound at increasingly higher levels of organization inmore sophisticated stone technologies.

Articulate language involves the combina-tion of units on at least three nested levels, looselydescribed as sound combinations (phonology), word

combinations (syntax), and conceptual combinations(semantics). Those of an anthropological bent mightwish to distinguish a fourth level of more extendedconceptual combinations in discourse semantics(Rose 2006). These levels correspond to intervals onincreasing scales of temporal duration and hierarchi-cal abstraction.

Paerns of brain activation associated with par-ticular linguistic tasks similarly vary according to scale.This probably reects general organizational propertiesof the cerebral cortex (Deacon 1997; Fuster 2001) ratherthan anything special to language. For example, corti-cal elds with relatively direct connections to the rest

of the body (e.g. primary and secondary sensorimotorareas) generally tend to be involved in rapid process-ing whereas those increasingly dominated by intra-cortical connections (association areas) are involved inlarger-scale integration across time. There is also goodevidence of hemispheric dierences in the scale ofprocessing, with the le hemisphere (LH) apparentlyspecializing in small-scale rapid processing and theright hemisphere (RH) specializing in larger scale and


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longer duration processing (Deacon 1997). This trendis evident across linguistic (Bookheimer 2002; Poeppel2003), perceptual (Gazzaniga 2000), and cognitive (Gaz-zaniga 2000; Goel et al.2007) domains.

The neural bases of manual action reect many ofthese same scale-related organizing principles, as wellas some points of more specic overlap with systemsinvolved in articulated language. This is particularlyevident in inferior lateral frontal cortex, which displaysa clear posterior to anterior gradient of increasingprocessing scale and abstraction (Fiebach & Schubotz2006; Hagoort 2005; Koechlin & Jubault 2006). Thisregion includes (from back to front) orofacial andmanual motor cortex, ventral premotor cortex, anddistinct posterior and anterior potions of Brocasarea (Fig. 1). All of these appear to play an importantrole in both articulate language and manual action.

Ventral premotor cortex: phonology and prehension

Many hypotheses of language origins have empha-sized the importance of enhanced human articulatorycontrol (e.g. Deacon 1997; Liberman & Whalen 2000;Lieberman 2002; MacNeilage 1995; Studdert-Kennedy& Goldstein 2003). This control would appear toarise from increased cortical regulation of the vocalapparatus, and particularly from the contributions ofa le hemisphere circuit linking the superior temporalgyrus (Hickok et al. 2000) with portions of frontallobe ventral premotor cortex (PMv) and the adjoiningorofacial motor cortex. PMv likely plays an important

role in phonological processing (Bookheimer 2002;Hagoort 2005) by combining phonological elementsfrom the superior temporal gyrus into intonationalphases.

PMv is thought to play a closely analogous rolein the combination of manual grasp elements (e.g.exion, rotation) provided by parietal lobe associa-tion cortex during object prehension (Fagg & Arbib1998). In fact, PMv participates in multiple neuralcircuits supporting sensorimotor transformations foraction across a variety of modalities. Macaque PMv,for example, displays overlapping responsiveness tovisual, tactile and auditory stimuli (Graziano et al.

1999), and has recently been found to co-activate withtemporal lobe auditory cortex (specically a putativeWernickes area homologue) in the perception ofspecies-specic calls (Gil-da-Costa et al. 2006). Inhumans, PMv is divided into inferior and superiorelds which are responsive to auditory and visualstimuli respectively (Schubotz & von Cramon 2003).This division, which mirrors the superior/inferiororganization of hand and orofacial regions in adjacent

primary motor cortex, is also evident during actionobservation, with the superior PMv eld respondingto observed hand actions and the inferior portion toobserved mouth actions (Buccino et al. 2001). Kin-ematic studies reveal that grasping movements withthe hand aect concurrent movements of the mouth,with larger manual target objects being associated

with wider, faster opening of the mouth and withincreased power of the voice spectrum during syllablepronunciation (Gentilucci et al.2001). PMv has thusbeen characterized as producing an action vocabularyacross a wide array of dierent behaviours, reectinga more general role in processing sequentially struc-tured events (Schubotz & von Cramon 2004).

PMv activity has been described as characteristi-cally goal-oriented (Rizzolai et al.1988; Schubotz &von Cramon 2004; van Schie et al.2006). PMv neuronsare tuned to specic goals like grasping, placing orholding an object and are less responsive to eitherdiscrete action elements (e.g. nger exion) or larger

action composites (e.g. eating a peanut). IndividualPMv neurons also display a temporal tuning to par-ticular portions of these goal-directed actions, such ashand opening or closure. These response characteristicsindicate the role of PMv in combining discrete gesturalelements into simple goal directed actions, much assyllables are produced from coordinated paerns ofarticulatory gestures by the tongue, lips, velum andso forth (Studdert-Kennedy & Goldstein 2003).

Figure 1.Regions (Brodmann Areas) of frontal, parietaland temporal cortex discussed in the text. Superimposedis the connectivity between frontal and posterior areas viathe arcuate fasciculus. (Aer Rilling et al. 2008.)


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Overlapping PMv contributions to phonologicalprocessing and object manipulation provide evidenceof a specific neurobehavioural correspondencebetween language and manual action involving thisregion. In particular, this correspondence is found atthe level where discrete articulatory and prehensileelements are assembled into short goal directed actionunits, such as grasping an object or pronouncing anintonational phrase.

Posterior Brocas area: syntax and action sequencingAnterior to PMv in inferior prefrontal cortex is Bro-cas area, classically associated with uent speechproduction. A more current view sees Brocas areaas multifunctional, involved in both linguistic andnon-linguistic behaviours, and encompassing aposterior-anterior gradient from phonology to syntaxto semantics (Hagoort 2005). The more posterior

portion of Brocas area (Brodmann Area [BA] 44) ispreferentially recruited during syntactical process-ing (Bookheimer 2002) and appears to support thecombination of lexical elements at the sentence level.Loosely, this level of syntactical organization reectsaspects of word order and phrase structure that mayor may not aect meaning, as in The woman lit thematch that started the re vs The match that thewoman lit started the re.

Posterior Brocas area is also active duringthe observation and execution of object-directedactions (Hamzei et al. 2003), and is specicallyinvolved in the processing of simple action chunks.

Koechlin & Jubault (2006) were able to characterizethis functional contribution of Brocas area using ahierarchically structured sequential buon pressingtask. This task included simple chunks consistingof pre-learned buon press sequences as well assuperordinate chunks in which the rulesgoverningbuon selection changed according to a pre-learnedsequence. Results showed a clear posterior-anteriorgradient, with PMv active between single acts, pos-terior Brocas area active during transitions betweensimple chunks, and anterior Brocas area activebetween superordinate chunks. This led Koechlin& Jubault (2006, 968) to conclude that posterior

[Brocas area] regions are involved in selecting andinhibiting simple action chunks in response to exter-nal signals or as successive components of ongoingsuperordinate actions. Applied to lexical elements,this would also be a reasonable description of therole of posterior Brocas area in language process-ing, suggesting a further correspondence betweenlanguage and manual action at the level of syntaxand action sequencing.

Anterior Brocas area: semantics and meaningful actionAnterior Brocas area (here including BA 45 and 47) isinvolved in semantic unication (Hagoort et al.2004).An example of such unication is the clarication ofambiguous word meanings in relation to broaderworld knowledge and sentence context. This requiresincreased hierarchical abstraction of the kind arib-uted to anterior Brocas area by Koechlin & Jubault(2006) as well as the ability to integrate informationover a relatively extended temporal frame. Both char-acteristics t well with the general trend toward moreabstract, integrative, longer-duration and modality-independent processing in more anterior prefrontalcortex (Deacon 1997; Petrides 2005). The selectiveinvolvement of anterior Brocas area in the semanticprocessing of words and sentences is well aested inthe neuroimaging literature (Bookheimer 2002; Martin& Chao 2001), and likely reects an underlying role

in the retrieval and selection between semantic repre-sentations (Kan et al.2006; Martin 2003).

Anterior Brocas area is similarly involved innon-linguistic action semantics. For example, themore caudal portion (BA 45) is preferentially recruitedduring the observation of meaningful actions (likeopening a bole or hammering a nail) as opposed tomeaningless hand motions (Decety et al.1997). Themore rostral BA 47 has been implicated in numeroushigher-order aspects of action organization, includingtask switching, reversal learning and the selection,comparison and evaluation of stimuli held in memory(Ramnani & Owen 2004), which are critical to the

production of complex, exible and goal-orientedbehaviours. This indicates a third correspondencebetween language and manual action at the level ofsemantics and meaningful action.

Right cerebral hemisphere: discourse and complex,multi-step actionFurther increases in temporal scale to the level ofdiscourse are associated with increased involvementof the right cerebral hemisphere. Although the lehemisphere (LH) has classically been viewed as thelanguage hemisphere it is now clear that this is anoversimplication. In fact, both hemispheres are com-

monly activated in speech perception (Poeppel 2003),and the right hemisphere (RH) is known to play animportant role in larger-scale elements of language,including metaphor, gurative language, connotativemeaning, prosody, and discourse comprehension(Bookheimer 2002; Xu et al.2005).

Bilateral activation is also present in Brocasarea and its RH homologue during hierarchicallystructured manual action (Koechlin & Jubault 2006).


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Although there is strong evidence of a le-lateralizedsystem involved in planning single actions with eve-ryday tools (Johnson-Frey et al.2005), the actual useof tools tends to produce a less pronounced LH domi-nance (Lewis 2006). When it comes to assembling moreprotracted action sequences, RH plays a key role. Forexample, Frey & Gerry (2006) found that RH and notLH activation was correlated with success in imitatingthe specic sequence of actions used in assembling atinkertoy. Hartmann and colleagues (2005) similarlyfound that, although patients suering RH brain dam-age are typically unimpaired in the single-action useof familiar tools, they are impaired when it comes toexecuting multi-step, naturalistic actions like makinga cup of coee. This suggests a nal correspondencebetween language and manual action at the level ofpragmatic discourse and complex, multi-step action.

Making sense: meaning in language and tool-making

The preceding survey highlights the facts that (1)language is not a single thing but rather a complexphenomenon involving multiple interacting systemsand levels of organization, and (2) the particular neuralsubstrates of language reect more general trends ofbrain organization and overlap extensively with otherforms of intentional action. This is consistent with theview that language is a complex learned behaviourmapped onto relevant brain systems, rather than theproduct of invariant, genetically specied languagecircuits(Deacon 1997). Similarities in the hierarchical

processing requirements of language and manualaction result in neural overlap on a variety of levels.

The particular points of overlap described abovesupport and rene the early work of Holloway (1969)who proposed that stone tool-making, like language,involves phonemic, grammatical and semanticlevels of organization. Holloway suggested that thephonemic level might include actions like striking aake or rotating the core, that the grammar is a con-catenation of these elements, and that the semanticsare to be found in the intended purpose of individualactions and nished tools. The more recent evidencediscussed above suggests that the phonemic units

should probably be smaller (e.g. hand congurations,arm movements) than those envisioned by Holloway,with relatively straight-forward knock-on eects onthe scale of the rest of the scheme. One key point thatdoes require further consideration, however, is theconcept of tool-making semantics.

The issue of meaning is central to Hollowaysbroader argument about the uniqueness of humanculture, which he identies with the imposition of

arbitrary form on the environment. Ingold (1996)has similarly argued that human uniqueness residesin the self-conscious authorship of design and theprojection of symbolic meaning onto the environment.Symbolic meaning is obviously a key feature of humanlanguage and culture, but in what sense might it beevident in stone tool-making?

As Deacon (1997; 2003) has explained, symbolicreference is itself constructed in a hierarchical fashionfrom underlying iconic and indexical relationships.Iconicity, rooted in physical similarity, is the basis forrecognizing dierent instances of a behaviour (e.g.vocal uerances, hand postures) as the same thing.Indexicality is then based on correlation (causal orarbitrary) between iconically related classes of stimuli,for example smoke and re, food and a bell, or theword apple and an apple. Symbolic reference lieson the next level of the hierarchy as a relationship

between indices. More precisely, the referential powerof a symbol comes from its position within a struc-tured set of indexical relationships among symboltokens (Deacon 2003, 122). For example, the symbolicmeaning of apple comes from a superordinate systemof indexical associations with other words like fruit,sweet or computer as well as from immediatecontext. This is why dictionaries dene word mean-ing by providing systematic mappings of each wordonto others.

In stone tool-making, one might similarly saythat the meaning of an action comes from its possibleassociations with other actions in a superordinate sys-

tem of technological rules. The closest correspondencewith word meaning is probably at the level of relationsbetween individual goal-oriented actions, as in strik-ing a ake as part of a planned reduction sequence.The meaning of this action is constructed by recogniz-ing iconic stimulus classes (grips, gestures, outcomes),their indexical (cause and eect) relation to eachother, and nally the possible relationship of theseindexical relationships to each other in achieving atechnological goal. The hierarchical structure involvedis thus formally similar to that used in constructingsymbolic reference. But is meaning in the sense of thegoal-oriented intention of an action really comparable

to symbolic meaning? This depends on what it actu-ally means for a tool-making act to be intentional.

Intentional is a word used in multiple ways. Ineveryday speech it typically means deliberate or onpurpose. For philosophers it has come to have a morespecialized meaning, denoting aboutness (Denne& Haugeland 1987) or the property of referring to orbeing directed at something (the intentional object). Inboth colloquial and technical senses intention is about


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meaning, be it the meaning of an action (Did you intendto do that?) or the meaning of a symbol (Do you meanan apple, the appleof my eye, or AppleComputers?).In each case meaning is dependant on context andexists on multiple levels (Searle 1992). For example,a stone tool-maker might simultaneously intend tomove his arm, to detach a ake, to modify a bifacialedge, to shape a handaxe, to disarticulate a carcass,and to increase his prestige through a display of skill,depending on the level of description (Kezer 1998;Stout 2002). Similarly, it is part of the power of languagethat I hear you can be a statement of fact, an indicationof agreement, and a gesture of support, all at the sametime. The true meaning of this statement has to do withthe level of discourse to which one aends.

This highlights the fact that speech is itself a formof action (Austin 1975) used to enact relations andconstrue experience (Rose 2006). Internal cogitation,

oen including sub-vocal speech, is also typicallyaimed at construing experience in one way or another.Even such apparently abstract intentional phenomenaas hopes and beliefs may be seen as concrete reactionsto the world, with meanings dened in relation toour goals (e.g. seeking gratication, understandingour surroundings). On this view, intentionality is aproperty of goal-directed interactions between agentand environment. From this it follows that the levelat which we nd intentionality is closely related to thebehavioural competence of the agent:

Early Stone Age tool-making and language evolution

ESA knapping methods include Mode I (Clark 1961)ake production and Mode II bifacial shaping. ModeI aking is characteristic of the Oldowan IndustrialComplex (Isaac 1976), which is currently known from2.6c. 1.5 Ma (although examples of this simplestof knapping methods may be found throughoutprehistory). The method involves the production ofsharp-edged akes by striking one stone (the core)with another (the hammerstone). Mode II knapping,which requires the deliberate shaping of the core toachieve a pre-determined form, rst appears aerc.1.7 Ma and characterizes the Acheulean IndustrialComplex (Clark 1994). The prototypical Acheuleanartefact is the so-called handaxe, a more-or-lesssymmetrical, teardrop-shaped tool systematicallyworked on both faces and well-suited for butchery and

other heavy-duty cuing tasks (Schick & Toth 1993).The archaeologically documented progression fromMode I to Mode II tool-making provides direct evi-dence of increasing hierarchical complexity in at leastone sphere of early hominin behaviour, as well asindirect evidence of increasingly eective mechanismsfor the social reproduction of technological skills.

Mode I tool-making and PMv expansionEective Mode I ake detachment requires visuomotorcoordination and evaluation of three-dimensionalcore morphology (Stout & Chaminade 2007) so thatforceful blows may reliably be directed to appropriate

targets. In addition to sophisticated visual perception,this requires speed and accuracy (Stout 2002) of theright1 hand in delivering blows and the use of eec-tive grips (Marzke et al.1998) by both hands so thatthe stones may be properly stabilized and orientedwith points of impact exposed. Detailed evidence ofthe complex and demanding motor synergies of theright limb involved in eective percussion comesfrom experimental studies of traditional stone beadknappers in India (Biryukova et al.2005; Roux et al.1995). Modern knappers engaged in freehand percus-sion (the most likely posture for Oldowan knapping)further report that the ngertips of the le hand,

positioned directly below the point of impact, canserve as a proprioceptive guide to percussion (Stout2002) and exert pressure that may help to guide akedetachment (Jones 1994).

Brain activation during Mode I knapping (Stout& Chaminade 2007; Stout et al.2008) reects thesevisuomotor challenges, including visual shape per-ception and manual grip coordination. Of particularinterest is activation of an object prehension circuit

Thus, for example, the beginning skier may requirean intention to put the weight on the downhill ski,

an intermediate skier has the skill that enables himto have the intention to turn le, a really expertskier may simply have the intention to ski thisslopeSimilarly, when I am speaking English, I donot have the intention to match singular nouns withsingular verbs or plural nouns with plural verbs Ijust talk. (Searle 1992, 195)

Depending on the social and linguistic aptitude ofthe speaker, just talking might similarly contributeto a range of higher-order intentions. For example,the exaggerated intonational contours and repetitiontypical of infant directed speech (motherese) mayserve to facilitate word segmentation and phoneme

recognition during language acquisition (Falk 2004).Similarly, in stone tool-making the intentional exag-geration of certain actions for the benet of observersmight serve a pedagogical purpose. Together withother forms of intentional instruction and collabora-tive technological action, such demonstration wouldprovide a context for the transition from imitation tointentional communication without the need for anintermediate protosign stage (contraArbib 2005).


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linking the anterior intraparietal sulcus (IPS) andventral premotor cortex (PMv). As described above,this circuit is thought to support the combination ofgrasp elements into an action vocabulary, much as aparallel circuit connecting superior temporal gyrus toPMv is thought to support the combination of phono-logical elements into intonational phases.

This is broadly consistent with the evolutionary-developmental hypothesis of Greeneld (1991), whoproposed that discrete manual and linguistic circuitsin the posterior inferior frontal lobe emerge throughpostnatal developmental dierentiation of a commonneural substrate. Such dierentiation is thought toresult from the exuberant growth and subsequent prun-ing of synaptic connections, driven by neuronal compe-tition, intrinsic gene expression gradients, and extrinsicstimuli. This is a plastic process, even into adulthood(Hihara et al.2006), and it is quite plausible that PMv

expansion related to selection for manipulative func-tions could have provided an exaptive foundation forthe evolution of neighbouring articulatory circuits.

Evidence of preferential PMv recruitment dur-ing Mode I tool-making by modern humans (Stout &Chaminade 2007; Stout et al.2008) strongly suggeststhat comparable tool-making by Oldowan homininsalso involved similarly distinctive demands on theprimitive (Rizzolai et al. 1998) manipulative func-tions of this region. Expansion of PMv would be onepossible outcome of selection acting on these functionsand, in fact, premotor cortex as a whole does seemto be expanded in humans (Blinkov & Glezer 1968),

though not to the same extent as prefrontal and tem-poro-parietal association cortices (Deacon 1997; Rilling2006). The appearance of Oldowan tools 2.6 millionyears ago provides evidence of novel demands andcapabilities for action organization in this region, andmay have contributed to the evolution of the neuralsubstrates of articulate language. This scenario diersfrom Greenelds (1991) proposal in that it deals witharticulatory and manual coordination in PMv ratherthan word and object combination in Brocas area, andby virtue of its specic link to the archaeological record.However the proposals are not mutually exclusive andthe underlying evolutionary logic is the same.

Mode II tool-making and anterior Brocas areaAlthough initially quite crude, by the later ESA (< 0.5million years ago) many handaxes achieved a level ofrenement indicative of advanced tool-making skills(Edwards 2001) and possibly of aesthetic concernsbeyond the purely utilitarian (Machin et al.2007). Incomparison to Mode I tool-making, the production ofsuch tools involves increased demands for hierarchical

action organization and ne bimanual coordination. Inthe brain, this is associated with additional activationof the RH homolog of anterior Brocas area (Stout etal.2008).

As described elsewhere (Stout et al. 2006; 2008),the additional demands of Mode II tool-making havea lot to do with the need to strike highly invasivethinning akes that travel at least half way acrossthe surface of the piece. This is accomplished throughcareful platform preparation, oen using a dierenthammerstone and following very dierent technicalrules from primary ake detachment. Various dif-ferent percussors and techniques may also be usedto accomplish sub-goals such as bifacial edging,thinning and shaping. The production of a well-formed handaxe thus requires individual actions tobe coordinated, not only with respect to the ultimategoal, but also in terms of various superordinate rule

systems pertaining to dierent technical operations.This is analogous to the superordinate sequencingtask of Koechlin & Jubault (2006), which also elicited(bilateral) anterior Brocas area activation. The activa-tion of the RH Brocas homologue specically by ModeII tool-making likely reects the particularly criticalrole of the le hand in manipulating, orienting andsupporting the core (Stout et al.2008), and may alsohave to do with putative specializations of this regionfor response inhibition and task-set switching (Aronet al.2004).

Anterior Brocas area and RH contributionsto higher-level aspects of language and multi-step

action organization are described above. Activationof this region during Mode II tool-making by modernhumans (Stout et al.2008) provides direct evidence ofthe increased hierarchical complexity of the activity,and implies similar demands in comparable prehis-toric (i.e. late Acheulean) technologies. While handaxemaking does not provide direct evidence of languagecapacities, it does reect the presence of hierarchicalprocessing capabilities important to both activities.These may have been in place at an earlier date, butare rst aested in the archaeological record by theappearance and subsequent renement of Mode IItool-making. As was the case with PMv and Mode

I tool-making, this neurobehavioural overlap is alsoconsistent with the hypothetical co-evolution of lin-guistic and tool-making capabilities.

Conclusion

It is nothing new to propose an evolutionary linkbetween language and tool-making. In 1871, Darwin(2004, 69) himself argued that To chip a int into the


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rudest tooldemands the use of a perfect handthestructure of the hand in this respect may be comparedwith that of the vocal organs. In more recent years,however, many archaeologists have instead stressedthe dissimilarities between language and stone tool-making (e.g. Chase 1991; Graves 1994; Mithen 1996;Noble & Davidson 1996; Wynn 1995). Brain-imagingstudies of ESA tool-making (Stout & Chaminade 2007;Stout et al. 2008) provide important new empiricalsupport for the early intuitions of Darwin, as well asfor more recent proposals regarding the co-evolutionof language and technology.

The unique contribution of the imaging studies isto establish direct links between archaeologically vis-ible behaviours and neural substrates. Results to datehighlight the demands of manual grasp coordinationin PMv during both Mode I and Mode II tool-makingand the increased prefrontal (anterior Brocas area)

and RH demands of Mode II tool-making. The increas-ing sophistication of ESA tool-making seen throughtime thus documents the expression to increasinglycomplex capacities for hierarchical action organiza-tion. Furthermore, this progression closely parallelsascending phonetic, syntactical, semantic and dis-course levels of language processing in terms of bothformal structure and neural organization. While stonetools still do not provide direct evidence of linguisticabilities, they do indicate the presence of analogouscapacities in the realm of manual action organization(Holloway 1969). This is consistent with the hypoth-esis that selection acting on tool-making ability could

have contributed to the evolution of language-relevantneural circuits (and vice versa) through a process ofdevelopmental displacement (Greeneld 1991).

Other recent hypotheses of language originshave focused on three key features: parity, recursionand symbolic reference. Parity is the requirementthat sender and receiver aribute the same meaningto same elements of communicative behaviour. Inother words, an iconic relationship must be estab-lished between actions which are the same whetherproduced by the self or perceived in another. In therealm of manual action this is thought to be achievedthrough the phenomenon of motor resonance, in

which the same neural substrates are recruited duringaction observation as during action execution. Suchresonant properties are evident in human premotorcortex, including but not limited to Brocas area. Mir-ror neurons found in the ventral premotor cortex (areaF5) of monkeys oer one specic example of resonanceand, although homologous neurons have not beenobserved in humans, have inspired hypotheticalscenarios of language evolution in which manual

imitation led to protosign language and eventuallyprotospeech (Arbib 2005; Rizzolai & Arbib 1998).

Preferential recruitment of PMv and the RHhomolog of Brocas area during stone tool-makingdirectly grounds such hypotheses in the known activi-ties of early hominins. The close correspondence inthe multi-level intentional structure of tool-makingand articulate language may even obviate the needfor a transitional protosign stage, which has previ-ously been proposed as a necessary bridge betweenimitation and intentional communication (Arbib 2005).Technological pedagogy and joint action provideequally plausible, archaeologically aested contextsfor this transition without the need to posit an inter-mediate evolutionary stage with no modern analogue.A similar proposal was actually made by FriedrichEngels 125 years ago: the development of labournecessarily helped to bring the members of society

closer together by increasing cases of mutual supportand joint activity, and by making clear the advantageof joint activity to each individual. In short, men in themaking arrived at the point where they had somethingto say to each other (2003, 73).

Parity may be a basic requirement for any com-municative system, but it does not explain the greatcomplexity and power of human language. Thispower is oen aributed to the property of recursionor discrete innity which allows discrete units to becombined and re-combined in a (theoretically) inniteseries of nested layers. Recursion has been singledout by some as thecore element of language unique

to humans (Hauser et al.2002), but it is not clear thatlinguistic recursion is really so dierent from the hier-archy of behavioural chunks seen in stone tool-makingor any other motor behaviour (Holloway 1969). Whatappears important is the level of abstraction at whichrecursion is employed. Linguistic recursion is theoreti-cally innite, but in practice the use of more than a fewembedded phrases tends to be very confusing for thelistener. Similarly, one might imagine an arbitrarilycomplex series of nested operations in tool-making,although this does not occur in practice. Indeed, theobserved neural overlap between Mode II tool-makingand semantic language processing suggests that the

degree of recursion involved may not be that dierent,at least in terms of how it is handled by the brain.

Much the same may be said of the relation ofstone tool-making to symbolic reference. In both casesmeaning is constructed in a hierarchical fashion fromiconic and indexical relationships. The real question iswhether the specic units of construction are in anyway comparable. It has been argued that comparisonsbetween symbolic reference and tool-making are


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misleading (Chase 1991; Graves 1994; Noble & Dav-idson 1996; Wynn 1995), however the brain imagingevidence suggests that important overlap does exist.In the end, we may come to see language and tool-making as alternate expressions of an underlyinghuman capacity to make sense of the world in increas-ingly complex ways.

Notes

1. For simplicity, and because all subjects in the experi-ments discussed here were right-handed, right handand le hemisphere will be used to refer to the domi-nant side. This also avoids some misleading termino-logy, in that hands and hemispheres are best seen asspecialized for dierent tasks rather than simply beingdominant or non-dominant.

Acknowledgements

Dietrich Stouts contribution was supported in part by theCommission of the European Communities Research Direc-torate-General Specic Targeted Project number 029065,Hand to Mouth: a framework for understanding the archae-ological and fossil records of human cognitive evolution.

Dietrich StoutInstitute of Archaeology

University College London3134 Gordon Square

LondonWC1H 0PY

UKEmail: [email protected]

Thierry ChaminadeMediterranean Institute for Cognitive Neuroscience

INCM, UMR6193CNRS - Aix-Marseille Universit

31 Chemin Joseph Aiguier13402 Marseille Cedex 20

FranceEmail: [email protected]

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Author biographies

Dietrich Stoutis a Lecturer in the Archaeology of HumanEvolution at the University College London, Institute ofArchaeology. His research has included ethnographic

observations of stone tool-making in highland New Guinea,archaeological eldwork at the site of Gona in Ethiopia, andexperimental investigations of stone tool-making actionorganization and brain activation.

Thierry Chaminade is a CNRS research scientist at theMediterranean Institute for Cognitive Neuroscience. Heobtained his PhD in Cognitive Neuropsychology in 2003for his contribution to the understanding of the neuralmechanisms of human imitation. He continues to workon the overlap between action execution and perception,addressing scientic questions ranging from human evolu-tion to humanoid robotics.

Making Tools and Making Sense: Complex, Intentional Behaviour in Human Evolution

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