A faltering origin for the Acheulean? Technological and cognitive implications from FLK West (Olduvai Gorge, Tanzania)

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A faltering origin for the Acheulean? Technological and cognitive implications from FLK West (Olduvai Gorge, Tanzania) F. Diez-Martína,∗, T. Wynnb, P. Sánchez-Yustosa, J. Duquea, C. Frailea, S. de Franciscoa, D. Uribelarreac, A. Mabullad, E. Baquedanoe,f, M. Domínguez-Rodrigof,g a

Department of Prehistory and Archaeology, University of Valladolid, Plaza del Campus s/n, 47011, Valladolid, Spain Department of Anthropology, Center for Cognitive Archaeology, University of Colorado at Colorado Springs, 1420 Austin Bluffs Parkway, Colorado Springs, CO, 80918, USA c Department of Geodynamics, Complutense University, José Antonio Novais 12, 28040, Madrid, Spain d Archaeology Unit, University of Dar es Salaam, P. O. Box 35050, Da res Salaam, Tanzania e Museo Arqueológico Regional, Plaza de las Bernardas s/n, 28801, Alcalá de Henares, Spain f Institute of Evolution in Africa (IDEA), Calle Covarrubias 36, 28010, Madrid, Spain g Department of Prehistory, Complutense University, Prof. Aranguren s/n, 28040, Madrid, Spain b

A R T I C LE I N FO

A B S T R A C T

Keywords: Earliest Acheulean Africa Olduvai Handaxe Tool conceptualization Cognition

The discovery in FLK West lowermost level 6 (Olduvai Gorge, Tanzania) of a massive and symmetrical handaxe, at odds both with the other LCT specimens so far unearthed from this site and with the technological traits commonly reported for the earliest Acheulean ca. 1.7 Ma, challenges the hypothesis of a gradual development of technological and cognitive qualities during the Acheulean. This thought-provoking specimen points to a less sharp difference between the first and later phases of the Early Acheulean (i.e. between ca. 1.7 and 1.2 Ma) and supports a somewhat punctuated flourishing of the handaxe technical and cognitive conceptualization. In this regard, this paper provides a detailed descriptive study of the large olivine basalt handaxe, in the framework of both the FLK West L6 lithic collection and the LCT shaping strategies. This analysis includes a morphometric description of the specimen and a comprehensive analysis of the knapping and shaping sequencing, through a diacritical study. It also includes a techno-functional assessment of the final design and symmetry. Finally, this paper examines some of the most significant implications of this large tool for our current views on temporal divisions within the African Acheulean and the cognitive abilities of the individual that made it.

1. Introduction The set of innovations commonly linked to the emergence of the Acheulean represent a significant turning point in human evolution (Semaw et al., 2009). The innovative technological trend towards the production of large flakes (Isaac, 1969) and their subsequent transformation into large-sized shaped tools (including a number of new tool concepts such as picks, knives, handaxes, and cleavers) has been understood in the framework of the adaptive responses (i.e., biological, behavioral, social, economic, ecological and cognitive) to the environmental challenges that framed the emergence and/or development of the species Homo erectus/ergaster (Antón, 2003; Antón et al., 2014; Cachel and Harris, 1998; Domínguez-Rodrigo et al., 2001; Gowlett et al., 2012; Maslin et al., 2015). In clear concomitance with the presence of proto-type constructions and cultural norms responsible for the recurrent production of specific tool forms (Gowlett, 2006; Herzlinger ∗

et al., 2017), it has been argued that the Acheulean knapping strategies imply more complex neurophysiological skills than those observed during the precedent Oldowan techno-complex. Acheulean cognitive demands would imply somewhat enhanced mental capacities, including a higher degree of planning and strategic judgments, hierarchical organization of the technical actions, intentional shaping and advanced spatial and geometric competence, particularly through the imposition of bilateral symmetry (Hodgson, 2015; Stout et al., 2015; Wynn, 1989, 2000, 2002; Wynn and Gowlett, 2018). A number of authors support the idea that a gradualist diachronic formal progression does exist within the African Acheulean and that this incremental change could also imply an increasing tendency in cognitive complexity (Shipton, 2018; Stout, 2011; Stout et al., 2014). From a traditional typological perspective, some authors proposed a temporal division of the Acheulean, based primarily on stylistic variations and the growing trend towards handaxe refinement (Clark, 1994;

Corresponding author. E-mail address: [email protected] (F. Diez-Martín).

https://doi.org/10.1016/j.quaint.2019.09.023 Received 8 April 2019; Received in revised form 21 June 2019; Accepted 17 September 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: F. Diez-Martín, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2019.09.023

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(from gravels in the lowermost L6 and L5 to sands and silts towards the top of the sequence, L4-L1). The densest accumulations of fauna and lithics occur in L6 and L5 (Diez-Martin et al., 2015; Sánchez-Yustos et al., 2018; Yravedra et al., 2017) (Table 1). Table 2 shows the distribution of large-sized lithic categories sorted by level at FLK W. Most of these specimens have been retrieved from L6 (n = 104, 87.4% of all large pieces recovered from FLK West). Thus, large-sized specimens only contribute significantly to the L6 lithic sample (representing 9.1% of the total L6 lithic collection). A collection of 65 LCTs have been retrieved so far from FLK West, most of them (n = 61) from basal L6. Concerning raw material type, the bulk (n = 60) has been produced on Naibor Soit quartzite. Regarding blank type, 38 LCTs have been shaped on blocks or slabs and 26 on large flakes (including four igneous rock specimens). At FLK West it is extremely difficult to systematize the range of LCT shaping strategies undertaken. In previous works (Diez-Martín et al., 2015; Sánchez-Yustos et al., 2017, 2018) we have chosen a technofunctional perspective to describe the large tool collection, combining both tool design and functional potentiality (Boëda, 2001, 2013). As a general rule, LCT blanks have been partially and superficially transformed through marginal and abrupt or semi-abrupt retouch, within a volumetric treatment that very rarely shows bifacial or invasive concerns. Simple shaping processes produce two basic morphological features, pointed areas (trihedrals) or broad cutting edges (dihedrals), which can operate independently or combined in the same tool (DiezMartín et al., 2015; Sánchez-Yustos et al., 2017) (Fig. 2).

Howell and Clark, 1964; Leakey, 1951). Although challenged by Mary Leakey's work at Olduvai Gorge (1971), this cultural evolutionary perception of the Acheulean has received support from different perspectives (Bar-Yosef, 2006; Chazan, 2015; Clark, 2001; Gallotti and Mussi, 2017; Hodgson, 2015; Isaac, 1989; Roe, 1994; Sahnouni et al., 2013; Schick and Toth, 2017; Shipton, 2018; de la Torre and Mora, 2014). The work by Beyene et al. (2013), for instance, analyzing the evolution of the Acheulean at Konso (Ethiopia) between ~1.7 and 0.8 Ma, epitomizes forcefully the way in which this progressive change can be understood within an evolutionary scheme. While the earliest phase of the Acheulean (in accordance with a rather abrupt onset of the new knapping/shaping principles) would be best defined by irregularity and roughness, the plan form control, thinning, volumetric reduction and three-dimensional symmetry characteristic of the Later Acheulean handaxes (Edwards, 2001) would indicate a clear mastery of technique, social learning and transmission. By this later phase of the Acheulean, different prepared core techniques would also become widespread (Shipton et al., 2013 and references therein), reinforcing the idea of operational and mental progression. Therefore, technological and stylistic differences seen between the Early and Late Acheulean would account for a trend of cumulative technological knowledge over time and the verification that the whole pack of cognitive improvements and/or social transmission skills usually assumed for the Acheulean would find their complete outline during the final phase of the technocomplex (Hodgson, 2015; Schick, 1998; Wynn, 1979, 1985, 1995; 2002). This interpretation would favor a scenario of gradualism for the modulation of the developments related to the Acheulean technocomplex (but see, for instance, McNabb and Cole, 2015; Li et al., 2015; Saragusti et al., 1998; Saragusti et al., 2005). The discovery at FLK West lowermost level 6 of a massive and symmetrical basalt handaxe (Diez-Martín et al., 2015; Sánchez-Yustos et al., 2017, 2018), at odds both with the other LCT specimens so far unearthed from this site and with the technological traits commonly reported for the Earliest Acheulean (EtA), challenges the hypothesis of a gradual development of technological and cognitive qualities during the Acheulean, particularly within its initial phase. This thought-provoking atypical specimen could point to a less sharp differentiation between the emergence and later phases of the Early Acheulean (i.e. within the temporal framework between ca. 1.7 and 1.2 Ma) and could support a somewhat punctuated flourishing of the same handaxe technical and cognitive conceptualization that would be mastered much later in Africa and elsewhere (Crompton and Gowlett, 1993; GorenInbar et al., 2018; García-Medrano et al., 2019). In this regard, this paper provides a detailed morphometric and technological description of the specimen, including a comprehensive analysis of the shaping sequencing, in the framework of the FLK West LCT collection. It also examines the cognitive implications linked to the manufacture of this exceptional handaxe.

3. The FLK west large handaxe The handaxe studied in this work was unearthed during the 2015 field season in lowermost L6, a 20 cm thick conglomerate of gravels (including cobbles and blocks) in a coarse sand matrix (Figs. 1 and 3). This basal unit represents the most erosive and the highest energy event within the water course. Besides, L6 is a high density patch of lithics and fossil bones showing relatively moderate taphonomic impact due to water action (Diez-Martín et al., 2015; Yravedra et al., 2017). Particularly, regarding the lithic collection, the small fraction (≤25 mm) represents 46% of the total, while lithic specimens with a variable degree of polishing sum 99 pieces (8.6%) (Sánchez-Yustos et al., 2018). The handaxe shows no sign of alteration (polishing, abrasion) due to fluvial and/or mechanical traction. Ridges, edges, surfaces and the tip region have been preserved in mint condition (Fig. 4) (Appendix A). 3.1. Metrics and raw material The metric values of the handaxe are the following (Bordes, 1961; McPherron, 2003; Roe, 1964): Length (L) = 300.203 mm; Breadth (B) = 138.94 mm; Thickness (T) = 93.07 mm; Base length (L1) = 107.13 mm; Tip length (L2) = 193.07 mm; Tip width at 4/5 (B1) = 75.14 mm; Base width at 1/5 (B2) = 124.35 mm; Mid width (B3) = 127.49; Tip thickness (T1) = 22.12 (Fig. 5). Mass is 3660 g. Taking into consideration these measurements, the handaxe can be described, following Roe's calculation ratios (1968), as a thick and elongated point specimen: L1/L (planform) = 0.35; T/B (refinement) = 0.66; B/L (elongation) = 0.46. The handaxe has been made in a fine-grain greyish basalt with small-sized plagioclases. The blank shows large (up to 5 mm) olivine crystals and smaller pyroxene crystals. Semi-quantitative analyses undertaken with a portable fluorescence equipment (Thermo Niton XL3t 950 He, X-ray tube head with a 50 kV Ag anode and SDD Si detector) show low content of silica (< 50–55%) and K20 (< 3%). After combining these illustrative geochemical analyses with macroscopic observations, we can say that the large handaxe is made in a basaltic rock with olivine phenocrystals and pyroxene. In other words, we define the rock type as an olivine basalt. This type of rock comes from the volcano Lemagrut, to the south of the Olduvai basin (Hay, 1976: table 7). Regarding the

2. The FLK west lithic record FLK West is part of the Frida Leakey Korongo site complex and is located 100 m to the west of the main iconic FLK site. It is named after Leakey (1971: Table 2) and corresponds to the locality in which a hominin tooth (OH26) was found in 1969. The archaeological site is constituted by sediments, rich in fossil bones and stone tools, that infill a fluvial channel excavated in clay deposits near the base of Bed II (Fig. 1). The 40 m wide and 1.2 m deep channel, flowing on the alluvial plain in a SE-NW direction, has been bracketed radiometrically between two local marker horizons, Tuffs FLKWa and FLKWb, dated to 1.698 Ma and 1.664 Ma, respectively, directly above Tuff IIA (Diez-Martin et al., 2015; Uribelarrea et al., 2017). The FLK West channel is part of a complex fluvial environment that formed over a wide area of the Gorge in the Lower Augitic Sands (LAS) of Lower Bed II (Uribelarrea et al., 2017, 2019). The channel is infilled with six litho-stratigraphic units (L6 to L1) that correspond to a fining-upward sequence of fluvial events 2

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Fig. 1. Horizontal and vertical association of archaeological remains in the 2013–2015 excavation area at FLK West. The large handaxe is identified in black within the archaeological accumulation.

Table 1 Number of archaeological materials (lithics and bones) and density (items per m2) in FLK West, sorted by archaeological level. Total area opened is 12 m2. Average depth per archaeo-unit (see Diez-Martín et al., 2015) is provided in cm. Archaeo-unit

Avg. Depth

Lithics

Fossil bones

Density

L1 L2 L3 L4 L5 L6

20 15 25 20 25 20

26 89 97 286 1154 1138

76 31 37 210 355 332

8.5 10 11.1 41.33 125.7 122.5

Total

125

2790

1041

319.2

Table 2 Number of lithic specimens within large-size lithic categories sorted by archaeological level. Percentage numbers per archaeo-unit refers to the following overall lithic counts: L4 n = 286, L5 n = 1,154, L6 n = 1138 (Sánchez-Yustos et al., 2018: Table 1).

flaking qualities of this olivine basalt, Jones (1994: 258) states that a number of the finer olivine basalt flake extremely well. This is the case of this large handaxe: it is a fine-grained rock with optimal response to conchoidal fracture that, however and due to its toughness, shows abundant step fractures in the knapping process. The blank cannot be precisely identified. Although the cross-section is plano-convex (i.e., the interaction of a flat upper surface –hereafter Face A- and a convex lower surface –hereafter Face B-), Face A has been intensely transformed through invasive detachments and no clear signs of a potential ventral surface can be recognized. Thus, the original blank could have been either an extremely large and thick flake detached from a massive boulder, a hemi-spherical preform, or a large and

Large-size category

L4

L5

L6

Large cores Large flakes LCTs

LCT fragments

– 1 – – – –

6 3 3 1 – 1

7 30 35 25 1 6

Total large-sized tools (n) % of the whole lithic sample

1 0.34

14 1.21

104 9.13

Slab/block Flake Undetermined

relatively flat or hemi-spherical cobble. Taking into account the typometric values of volcanic cobbles (both unmodified specimens and complete hammerstones) retrieved from FLK West L6 (Diez-Martín et al., 2015, supplementary material; Sánchez-Yustos et al., 2018), significantly smaller than the massive blank needed for the production of this large handaxe, we estimate that the original blank was not procured in the river channel itself or other paleo-channels located in surrounding area of the site but might have been quarried in outcrops close to the primary source, presumably on the slopes of Lemagrut volcano.

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3.2. Diacritical analysis A diacritical analysis aimed at reconstructing the sequence of operational processes and reduction strategies carried out for the handaxe manufacture has been undertaken for this study. Diacritical analysis is the outcome of a macroscopic interpretation of the last technical gestures prior to discard, reproducing the different reduction and shaping phases (Inizan et al., 1995). Scar directionality has been interpreted through the topographic analysis of negative scars and the observation of waves and grooves. Negative scar chronology has been determined through negative scar superposition and the interpretation of attributes such as breakages of the scar morphology and/or the shape of side or distal grooves (Baena and Cuartero, 2006; Callahan, 1988). The diacritical scheme of the knapping and trimming sequencing can be established as follows: 3.2.1. First volumetric treatment and reduction (Face A, S1–S3) (Fig. 6) On Face A (a flat area, probably corresponding to a ventral surface), a first series of seven simple (< 55°) detachments arranged orthogonally can be observed (S1). Negative scars correspond to invasive small and medium-sized flakes (the largest negative scar of the series is 65 × 48 mm) that cover the entire surface and have been detached from the periphery (presumably following a centripetal rotation of the original core) in order to undertake a first reduction and regularization of the natural volume. The area identified as #1 (29 × 22 mm) is the topographically highest, and therefore, oldest, surface of the series. Delineation tends to flat or slightly convex. It might correspond to the remains of a ventral surface, although surface morphology cannot be properly assessed. Due to its dimensional significance, a large negative scar (103 × 113 mm) has been individualized as the second series (S2). This simple (< 55°) large flake has been obliquely struck from the upper right side of Face A and shows a prominent step distal end (creating an abrupt step, 21 mm thick), representative of step termination and fracture. This noticeable accident, the most conspicuous example in the handaxe of the breakage responses to the tenacity of this olivine basalt raw material, could be seen as part of the same initial centripetal reduction sequence that represents the first series (as it fits with the same directional pattern). However, this large detachment would most likely represent an intentional knapping strategy aimed at a massive thinning of the distal region of the blank, constituting therefore an independent shaping stage within the reduction sequence. Next, on the basal right area, three abrupt, short (#2, 22 × 90 mm) or mediumsized (#3, 40 × 31 mm) flakes showing prominent step accidents have been detached (S3). All in all, these three first series represent peripheral, invasive and centripetal work that pursued comprehensive transformation and regularization of the flat volume of the hemispherical blank (angular fragment/cobble or very large flake). 3.2.2. General point shaping (Face B, S4–S7) (Figs. 6 and 7) Once the previous work had been accomplished, the knapper turned over the blank and started the transformation of Face B, initially a convex and cortical surface. The oldest series on this face is represented by the remains of three detachments struck from the left side following a lineal or unipolar directional pattern (S4). On top of this, a new series of three lineal, semi-abrupt, and not completely invasive detachments (the largest negative scar is #2, 50 × 87 mm) started the process of shaping the point of the handaxe (S5). This work affected the whole perimeter of this surface and was organized counter clockwise, continuing with a series of six small (the largest is #3, 31 × 47 mm) step and abrupt detachments that shaped the base of the handaxe (S6), and 11 semi-abrupt and abrupt detachments on the right side (S7), the largest being #8 (47 × 82 mm). Once this work was finished, the general pointed design of the tool was already accomplished. In fact, the distal tip obtained its characteristic chisel-like form during this phase. Due to morphological constraints of Face B (a thick and convex cortical surface), the knapper was unable to detach deeper and more

Fig. 2. Naibor Soit quartzite LCTs from FLK West L6: 1. Pointed tip shaped on a flat slab by trifacial series of marginal detachments; 2. Trihedral pick on a block; 3. Specimen showing bifacial, marginal and convergent detachments on a flat slab; 4. Handaxe/knive type on a large flake with invasive reduction on the dorsal surface.

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Fig. 3. The large handaxe in situ during the process of excavation at FLK West L6 in 2015.

detachments that finished the characteristic globular shape of the basal area of the handaxe. Retouching continued with a series of 17 virtually unifacial, simple and semi-abrupt small removals on the whole left edge (S10) (the largest negative scar is #5, 23 × 42 mm). Then it turned to the mid-section of the right edge, with a series of 7 simple and semiabrupt, unifacial detachments (S11). Finally, a last series of 14 bifacial removals (S12) on Face A and Face B provided the final form of the tip. It is important to remark that no knapping fractures or accidents have been identified in the process of tip configuration.

invasive flakes, in order to thin out this massive volume. Therefore, a 169 mm long by 72 mm wide cortical strip is retained along the central area of this surface. 3.2.3. Final configuration (Faces A and B, S8–S12) (Figs. 7 and 8) A second phase of work is constituted by a series of small and marginal trimming detachments covering almost the whole perimeter of the piece, aimed at shaping the edges and the final enhancement and thinning of the distal tip. Most of this work was undertaken on Face A. It started with two series of seven (S8) and six (S9) small and abrupt

Fig. 4. The FLK West large olivine basalt handaxe (photograph by Mario Torquemada/Museo Arqueológico Regional de Madrid). 5

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No signs of potential use accidents, fractures, scars or rounding have been observed in the distal tip area (Fig. 10). On the contrary, the edges are in mint condition, confirming that from a macroscopic inspection, the tip is intact and no evidence of tool use can be detected. 3.4. Symmetry Plan view from Face A shows a clear teardrop shape with remarkable bilateral symmetry around the longitudinal axis. A total of 48 measurements of the distance from the main longitudinal axis to the right and left edges at the different vertical planes (including 1/5, 2/5, 3/5, 4/5, mid width and maximum width) were taken (Fig. 9A) (Appendix B) (Lycett, 2008). A Levene test, to evaluate the equality of the variances of distances in both sides was applied. The results show a remarkable homogeneity, confirming that both sides are statistically symmetrical [Pr (> F) = 0.8905]. Regarding the cross-section view, the same statistical test was applied to the same number of measurements taken from the main axis to both sides (Fig. 9B). Although with significantly less strength than in the case of the frontal view, the results also confirm cross-section symmetry [Pr (> F) = 0.3079]. 4. The FLK west large handaxe in context In order to present a comparative framework that can shed light on the exceptional nature of the large olivine basalt handaxe within the FLK West LCT collection, this study will take into consideration the 23 most representative LCTs recovered from the site for comparison (Appendix C). All of them were made on Naibor Soit quartzite and are illustrative of the set of shaping and volumetric strategies undertaken at FLK West. This collection includes: a) clearly defined pointed/trihedral specimens (n = 8); b) tools characterized by broad cutting edges, mostly lateral and occasionally convergent (n = 10). Although difficult to classify, some of these specimens could be seen as crude forms of handaxes and/or knives. c) More convincing handaxes, in which invasive bifacial series can be recognized (n = 3); d) diverse (large, short and thick) specimens that could be formally interpreted as heavy duty scrapers (n = 2). Table 3 shows the set of variables considered for this comparative study. Together with typometric, mass and area information, total number of scars counted (including trimming negatives), various indexes have been considered: Elongation (EI), Refinement (RI), Scar Density (SDI), Reduction Intensity (RII), and Symmetry (SI) (Li et al., 2015; McPherron, 1995; Roe, 1968; Sánchez-Yustos et al., 2017; Schick and Toth, 2017). Regarding dimension, boxplots in Fig. 11 and Table 4 show measurements for the dimensional variables length, breadth and thickness of the 24 LCTs included in this comparative study. The outlier within the maximum length corresponds to the basalt handaxe. In this regard, it is illustrative to note that if we take into consideration the maximum length figures available for the whole collection of 84 large flakes and LCTs recovered so far from FLK West (mean = 131.4881 mm; sd = 44.55307, Sánchez-Yustos et al., 2017), the probability of finding a specimen in the rank of 290–300 mm maximum length within a normal distribution constitutes a meager 0.01093%. A Pearson correlation analysis between pairs of variables assessed the strength of linkage among them. For this purpose, Cohen's standard was used, where correlation coefficients (rp) below 0.29 denote a small effect size, between 0.30 and 0.49 a moderate effect size, and above 0.50 a strong relationship between variables (Online Resource 4). The results show (Table 5) a significant positive correlation and a large effect size between length and thickness, length and weight, length and SA, length and RII, width and weight width and SA, width and EI, width and RII, thickness and weight, thickness and SA, thickness and RI, thickness and RII, weight and SA, weight and RII, SA and RII, SDI and RII. There was significant positive correlation with moderate effect between length and RI, length and SDI, width and SDI, width and SI, weight and SDI, SA and SDI, SDI and SI, RII and SI. There was also a

Fig. 5. Graphic representation of the measurements taken for the handaxe (based on Roe, 1964, 1968): L = Maximum length; L1 = Distance from the base to the point of maximum width; L2 = Distance from the point of maximum width to the tip; B = maximum width; B1= Width at tip, or 4/5 of the tool; B2= Width at base, or at 1/5 of the tool; B3= Width at midpoint; T = maximum thickness; T1 = thickness at tip, or 4/5 of the tool. Other measurements taken include: width at 2/5 (120 mm), 3/5 (180 mm).

3.3. Morpho-functional and macroscopic analyses The total perimeter length is 735 mm. In this specimen, the edges, which were shaped by bifacial trimming, tend to be sinuous, particularly on the left side of the tool (Fig. 9A). Mid width approximately divides the handaxe into two morpho-potential units (Boëda, 2001, 2013), according to the potential capability of the edges (i.e. edge angle) to penetrate in soft tissues or materials. A photogrammetric 3D scan (using Agisoft Photoscan Professional software v. 1.2.3) was used to measure edge angles. The obtained 3D model was then processed with Blender software (v. 2.72). From its orthogonal view (persp/ ortho), the scan was divided into different cross-sections using Blender's ruler/protractor tool. Each cross-section was saved as an individual image in the add-on intersection blender file and then cross-section edge angles were measured using the ruler/protractor tool. The proximal half of the handaxe can be defined as a passive zone, as abrupt (8196°) and semi-abrupt (72°) edge angles predominate along 385 mm, supporting a prehensile function for the globular base of the tool (Key et al., 2016). The distal and pointed half of the handaxe is more suited to represent an active or transformative zone, formed by 350 mm of more continuous semi-abrupt (58-75°) and, towards the tip, simple (37°) edge angles. The distal tip is a 24 mm thin chisel-like sharp edge (with a 41° sharp angle). According to the general principle that “form organizes action” (Boëda, 2001: 8), the specific design of this handaxe would have conditioned the set of feasible actions and potential use of the tool. A macroscopic functional analysis has been undertaken on the distal tip area of the handaxe. This type of analysis focuses on the identification of a variety of edge damage, such as micro-fractures, scarring or rounding (Odell and Odell-Vereecken, 1980; Tringham et al., 1974) and has been already used for preliminary functional analyses of similar large tools in other contexts (Méndes-Quintas et al., 2018). Macroscopic inspection has been carried out with a Leica MS binocular magnifying glass with a connected MC170 HD Leica camera. Due to the low depth of field, we have used Helicon Focus software for photograph editing. 6

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Fig. 6. Diacritical scheme and shaping sequence of the large olivine basalt handaxe showing: a) the first volumetric arrangement and reduction on Face A (Series S1–S3); b) the process of point shaping on Face B (S4–S6).

Clarkson, 2015). Our analysis shows that in the case of FLK West a higher reduction is related to higher scores in frontal symmetry. The symmetrical basalt handaxe perfectly epitomizes this correlation, as with a total number of 82 negative scars and a surface area of 599.7 cm2, it presents the highest SDI (0.14) and RII (1) in the whole collection. Interestingly, surface area also shows a significant positive correlation with SDI (rp = 0.44, p = .020). In sum, the basalt handaxe is an extraordinarily large specimen, in particular when compared to the LCT collection retrieved from FLK West: it is extremely large, significantly wide and thick, and a very heavy specimen. Mean metric and mass values for the rest of the large tools are 178 × 96 × 64 mm and 1318 gr. In accordance with the mentioned figures, its surface area is the highest (mean surface area of the rest of the collection is 243 cm2, interval 166–399). It also differs significantly in terms of reduction intensity. At FLK West, LCTs show a moderate reduction, expressed in the number of negative scars per tool (mean 15, sd = 7.08, interval 4–28), the SDI (mean ratio 0.06, sd = 0.03, interval 0.02–0.1), and the RII (mean = 0.3, sd = 0.24, interval 0.06–0.9). Regarding elongation, the handaxe is included within the group of the more elongated specimens, although other pieces show a higher elongation index (particularly five rectangular specimens transformed into trihedral picks with an EI between 0.42 and 0.34). In general, most tools in the collection fall within an EI ≥ 0.51 (62.5% of

significant negative correlation with large effect size between length and EI, EI and RI. In all, metric variables such as length and width strongly impact over the rest of variables (Fig. 12), while indexes tend to have a moderate effect in pair relationships. However, it is worth taking into account the role played by bilateral symmetry within the collection. In order to establish a comparative framework for the LCT sample studied here, a symmetry index (SI) has been calculated following the procedure used by Schick and Toth (2017). The SI of the large basalt handaxe is a remarkable 0.93, while for the LCT sample studied here, mean SI has a figure of 0.71 (sd = 0.13) (Fig. 13). The exceptional frontal symmetry of the basalt handaxe is better understood if we consider that the probability of finding an SI between 0.9 and 1 within a normal distribution in the LCT collection from FLK West is only 2.6%. The histogram in Fig. 14 illustrates the frequency of SI intervals. All in all, specimens with mainly asymmetrical frontal view represent the bulk of the sample. However, it is remarkable to note that the SI interval between 0.81 and 0.89 is the most representative of the collection. Within this group we include the three tools showing bifacial and invasive detachments, as well as other specimens with associations of broad cutting edges and/or distal tips. Pearson test has shown that symmetry positively correlates with width (rp = 0.47, p = .020) (Fig. 15) and SDI (rp = 0.44, p = .032) (Fig. 16). SDI is a good proxy for evaluating reduction intensity (Shipton and 7

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Fig. 7. Diacritical scheme and shaping sequence of the large olivine basalt handaxe showing: a) the final series of the process of point shaping on Face B (S7); b) final configuration and trimming on Face A (S8–S10).

cognitive neuroscience (Wynn and Coolidge, 2016), these clues reveal significant developments in several cognitive abilities when compared to the products of earlier hominin stone knappers. These include developments in cognitive control, spatial cognition, aesthetic cognition, and category formation (Fig. 18). Cognitive control is a key component of executive reasoning ability and working memory (Baddeley, 2007; Levy and Wagner, 2011; Rouault and Koechlin, 2018). In studies of modern humans, measures of cognitive control always correlate highly with measures of general intelligence (Alloway and Alloway, 2012; Unsworth et al., 2014), and the evolution of cognitive control is thus of considerable interest when documenting hominin cognitive evolution. Generally speaking, cognitive control encompasses an individual's ability to focus and maintain attention on task-relevant information. Experimental psychologists have isolated and tested several components of cognitive control, two of which – response inhibition and task switching – may be recognizable in the decision chains deployed in stone knapping (Faisal and Stout, 2010). Response inhibition is the ability to suppress a prepotent (automatic) response in order to focus on a different goal, and task

the sample), in agreement with the preferential use of blocks and slabs as blanks for the production of large tools. With respect to refinement, the basalt handaxe is included within the group of the thickest specimens, again in agreement with its massive dimensional and mass figures. However, the LCT collection from FLK West is largely made of thick tools with a RI ≥ 0.5 (83%). The four specimens showing lower ratios (0.48–0.4) were shaped on either thin slab or thin flake blanks (Fig. 17).

5. Cognitive implications The large handaxe from FLK West carries with it a number of implications about the cognitive abilities of the individual who made it, and by extension the community of hominins active at Olduvai ca. 1.7 Ma. Several features of the handaxe provide clues to the thinking of the knapper: the overdetermination of form in the direction of symmetry (Fig. 9), the organization of sequential trimming into discontinuous series (Figs. 6–8), and the exaggerated size compared to other FLK West LCTs (Fig. 13). Viewed from the perspective of modern 8

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Fig. 8. Diacritical scheme and shaping sequence of the large olivine basalt handaxe showing: a) the final configuration on Face A (S9) and bifacial tip trimming (S12); b) complete reconstruction of diacritical scheme.

mirroring one side with the other. He or she maintained these goals through several separate, non-contiguous series of trimming, and three flips of the artifact, interrupting the rhythmic knapping gesture several times. Shipton has reached a similar assessment in his discussion on handaxe thinning in general: “The easiest flake removals must often be ignored in order to maintain adequate edge angles and control the overall form of the piece” (Shipton, 2018: 112). The cognitive control mechanisms in play here were response inhibition and task switching. Compared to simpler core and flake knapping, the FLK West knapper needed to attend to more overarching goals (curved edge, symmetry, thinning), and had to switch back and forth between them. In sum, greater cognitive control in the guise of response inhibition and task switching was required for this artifact than for any known Oldowan artifact. The imposition of symmetry on this handaxe was unlikely to have been an accident of the knapping procedure. Recently, Moore and Perston (2016) has demonstrated experimentally that modern knappers who have been instructed to select only the best available striking platform on a core while producing flakes will often inadvertently produce an oblong core with a single plane of detachment, a result that resembles the so-called “proto-bifaces” of Mary Leakey, (1971). This effect may well help explain the advent of biface technology, especially

switching is the ability to move back and forth between tasks or goals without losing task-relevant information. In neuroimaging studies of modern stone knappers (fMRI), Stout and colleagues identified a neural locality generally associated with cognitive control (pars triangularis of the inferior right frontal lobe) when knappers visualized handaxe production (Hecht et al., 2014; Stout et al., 2014, 2015). This region was not activated for core and flake production. There is thus strong a priori reason to expect that the knapper of the FLK West large handaxe relied on a level of cognitive control not evident for Oldowan knappers. The diacritical analysis clearly bears this out. The knapper began trimming on one quadrant of face A, shifted to the opposite side, then flipped to face B, worked down one side sequentially, and then down that side again. He or she then continued around the margin and trimmed sequentially to the tip, in the process roughly mirroring one lateral side with the other (Figs. 6 and 7, S1 through S7). Next, the knapper flipped back to trim face A (Fig. 7, S8 and S9), working to modify the line of the butt, followed by trimming the left lateral edge to the tip (Fig. 7, S10), then shifting to the right lateral edge (S11), refining the symmetry in the process. Finally, the knapper trimmed both sides of the tip (Fig. 8, S12). The knapper clearly tracked three goals: 1) artifact thinning, especially early in the sequence; 2) production of two gradually curving bifacial edges that converged to the tip; and 3) 9

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Fig. 9. A. Distance (in mm) from the maximum axis to right and left edges in 24 vertical planes and angles taken at the intersection of B, B1, B2, B3, 2/5 and 3/5 planes with the edge. B. The same measurements taken from the cross-section view.

Table 3 Variables considered for the analysis of the LCT collection: L (Length), W (Width), T (Thickness), M (Mass), A (Surface area in m2), SC (Scar count), EI (Elongation Index), RI (Refinement Index), SDI (Scar Density Index), RII (Reduction Intensity Index), SI (Symmetry Index). Measurements for the olivine basalt handaxe are colored in grey.

Fig. 10. Edge magnification (10×) of different areas of the distal tip showing the mint condition of the cutting edge.

when combined with Gowlett's ergonomic design criteria (Gowlett, 2006), but it cannot account for the high degree of symmetry clearly evident on the FLK West large handaxe. Metrical data (Fig. 9), and the placement of trimming series (Figs. 6–8), indicate that the knapper strove to achieve bilateral symmetry. This required significant developments in visuo-spatial processing (Hodgson, 2009; Wynn, 1989, 2002). There are two major neural pathways in visuo-spatial

Ref.

L

W

T

M

A

SC

EI

RI

SDI

RII

SI

4 1969 3023 3024 3027 3030 3035 3043 3041 3044 3047 3052 3058 3373 3444 3530 3535 3548 3572 3616 4027 4028 4029 4030

170 149 156 174 184 178 155 184 235 230 208 186 189 156 200 300 203 120 145 200 171 146 204 151

116 90 87 68 109 92 119 79 81 81 131 97 89 92 121 139 100 94 78 94 104 92 78 106

62 44 39 60 48 55 48 60 80 77 95 62 83 53 86 97 81 53 71 47 64 51 75 76

1408 786 603 870 1273 1185 1326 1075 1758 1758 2738 1283 1110 656 2350 3660 1750 924 1138 1406 1154 682 1631 1461

252.44 192.92 186.72 188.48 279.36 237.06 279.96 166.26 276.64 276.64 398.66 274.2 220.5 182.1 361.94 599.7 264.9 185.78 175.5 323.76 229.3 192.28 229.26 225.24

23 14 10 4 21 24 11 11 13 13 23 28 22 7 11 82 26 8 5 18 15 6 17 18

.68 .6 .55 .39 .59 .51 .76 .42 .34 .35 .62 .52 .47 .58 .6 .46 .49 .78 .53 .47 .6 .63 .38 .7

.53 .48 .44 .88 .44 .59 .4 .65 .98 .95 .72 .63 .93 .61 .71 .69 .81 .56 .91 .5 .6 .55 .96 .71

.09 .07 .03 .02 .08 .10 .04 .06 .04 .04 .05 .10 .09 .04 .03 .14 .09 .04 .02 .05 .06 .03 .07 .07

.45 .37 .06 .10 .20 .46 .21 .25 .34 .34 .82 .67 .43 .04 .26 1 .9 .25 .15 .25 .39 0 .37 .26

.89 .85 .68 .81 .71 .58 .8 .74 .62 .62 .87 .82 .81 .57 .58 .93 .76 .89 .48 .75 .83 .47 .49 .88

processing: a dorsal stream in which information passes from the primary visual cortex (occipital lobe) to the parietal lobes, and a ventral stream in which information passes from the primary visual cortex through the temporal lobes (Hodgson, 2000). The dorsal stream processes spatial information, and the ventral stream processes shape 10

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Table 5 Pearson correlation matrix among L (Length), W (Width), T (Thickness), M (Mass), SA (Surface Area), EI (Elongation Index), RI (Refinement Index), SDI (Scar Density Index), RII (Reduction Intensity Index), SI (Symmetry Index). Critical values are 0.4, 0.52, and 0.63 for significance levels 0.05, 0.01, and 0.001.

1. L 2. W 3. T 4. M 5. SA 6. EI 7. RI 8. SDI 9. RII 10. SI

1

2

3

4

5

6

7

8

9

10

– .32 .67 0.82 .81 -.63 .41 .47 .59 .009

– .36 .68 .76 .51 -.39 .42 .52 .47

– .8 .59 -.31 .7 .29 .62 .10

– .93 -.18 .28 .42 .69 .25

– -.10 .02 .47 .62 .32

– -.67 -.08 -.12 .33

– -.06 .18 -.28

– .73 .44

– .44

–

and the level of symmetry are exceptional among the FLK West LCTs, and exceptionalism suggests that someone made an appraisal of the handaxe's appearance. Finally, the makers of the FLK West LCTs almost certainly possessed a tool concept of some kind. By tool concept we mean an epistemological category that encompasses members of one domain (tools in this case) but excludes others (non-tool objects). Experimental psychology has established convincingly that people (and other primates) establish categories through proto-types (Carey, 2009; Palmeri, 2014; Smith, 2014). A specific item warrants inclusion in a category through its resemblance to a proto-type, not because it meets a list of definitional criteria. Categories emerge “… when attention is focused repeatedly on the same kind of thing in the world, by utilizing associative mechanisms among modalities, which, in turn, might permit re-enactment and simulation” (Pezzulo et al., 2011: 6). The bilateral symmetry, the extent of secondary trimming, and the extreme size and weight (peak-shift) of the FLK West large handaxe are overdetermined qualities. The knapper very clearly thought about this handaxe, and its visual impact. The tool itself was a goal, not just a step in a task to be performed. An FLK West knapper almost certainly considered the tool that could emerge from the clast, not just the work it could do. Moreover, the basic teardrop shape of handaxes appears to have been a proto-type, one that had remarkable staying power but whose ontological status is still open to debate (Corbey et al., 2016; Wynn and Gowlett, 2018). At a deeper cognitive level, proto-type construction relies on attention and association (Carey, 2009; Palmeri, 2014; Pezzulo et al., 2011; Smith, 2014), thus corroborating the increase in cognitive control documented through the diacritical analysis. Note that it is not necessary that a handaxe proto-type be a visual image (the ill-defined ‘mental template’). It could have been, indeed probably was, based upon neuromuscular, ergonomic and visual experiences (Barsalou, 2008; Malafouris, 2013; Pezzulo et al., 2011). Was the knapper of this FLK West handaxe an evolutionarily precocious individual who was atypical of the hominins of his or her residential group and also atypical of other hominins living in East Africa 1.7 million years ago? If so, then he or she was an interesting curiousity rather than a representative of the typical cognitive profile, and thus of minimal evolutionary significance. There are three reasons for rejecting this conservative interpretation. First, in conceiving of and executing

Fig. 11. Boxplots of dimensions of the FLK West LCT collection studied. The outlier in the variable length corresponds to the large handaxe.

information. When one imagines, rather than perceives, visuo-spatial phenomena, one uses the same resources (Kosslyn, 1994; Kosslyn et al., 1997). Stone knapping is primarily a spatially guided motor task (Stout et al., 2000). Knapping the FLK West handaxe also required the knapper to coordinate spatial features (topological features such adjacency, for example) with shape recognition features (symmetry, gradually curving edge). For modern humans, this coordination appears to occur in the anterior supra-marginal gyrus of the parietal lobes (Orban and Caruana, 2014). The coordination of spatial and shape information does not require that the knapper have an internal visual image of the goal, only that he or she coordinate the different varieties of visuo-spatial information. The knapper of the FLK W large handaxe exploited several neuroaesthetic effects. Symmetry itself is a ‘good’ Gestalt form, and the vertebrate visual system includes cells in the primary visual cortex that are especially sensitive to symmetry (bilateral and radial) (Hodgson, 2015). These evolved for their survival value long before the advent of hominins (symmetrical patterns are often living organisms). In addition, as Derek Hodgson has discussed at length (Hodgson, 2000, 2010, 2011; 2015), perception of symmetry elicits a pleasurable response downstream in visual processing, activating the same cell groups as any pleasurable activity. The knapper also exploited the well-known neuroaesthetic effect of peak-shift (Chatterjee, 2014; Leder and Nadal, 2014; Ramachandran and Hirstein, 1999). If a perceptual pattern elicits pleasure (or displeasure), an exaggerated version elicits an even greater response. This effect is common in modern visual art, perhaps most obviously in political cartoons and shock cinema. When the FLK West knapper produced a large symmetrical handaxe he or she made an artifact with a visual impact. To qualify as aesthetic experience there must also have been an appraisal of some sort (“I like it” or “it is attractive”). Without knowledge of the context of use it is difficult to make a case for appraisal at FLK West, but the extent of modification Table 4 Descriptive dimensional and mass figures for the FLK West LCT collection studied.

Length Breadth Thickness Mass

Mean

SD

IQR

0%

25%

50%

75%

100%

183.08 97.37 65.29 1416.04

37.48 17.84 16.47 698.42

45.00 21.25 25.25 623.50

120 68 39 603

155.75 85.5 52.5 1037.25

181 106.75 77.75 1660.75

200.75 106.75 77.75 1660.75

300 139 97 3660

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Fig. 12. Scatterplot of length by thickness. Pearson test shows a significant positive correlation between both variables with a large effect size (rp = 0.67, p= < .001). The black dot indicates the position of the olivine basalt handaxe within the LCT sample.

techno-functional characteristics of the LCTs recovered from Kokiselei 4 fit within a pattern of invention. Texier (2018) considers that, despite those knappers being able to display a technological control of threedimensional space, their LCT production reflects an elementary and limited sequence of removals, which does not include more than 12 negative scars in the most elaborate examples. The inter-assemblage variability reported by Beyene et al. (2013) at Konso strongly supports a remarkable progressive scenario that departs from very crude, marginally modified and asymmetric pointed forms. At Gona, LCTs are also defined by their crudeness and simplicity (Semaw et al., 2009, 2018). However, an elementary departure towards structural bilateral symmetry and over-determination (Wynn and Gowlett, 2018) can already be observed in some of the early specimens retrieved from Kokiselei 4 and KGA6-A1 in Konso (Beyene et al., 2013; Chevrier, 2012). However, a progressive management of volumes and the obtainment of symmetry have been documented in other pene-contemporaneous or slightly younger sites. In Garba IVD (Melka Kunture, Ethiopia) at ~1.6 Ma, although large flake blanks were marginally retouched and transformed (Gallotti, 2013), significant technical innovations have been observed in LCT production through such traits as striking platform preparation, recurrence in reduction, management of convexities, and hierarchical relationship among surfaces (Gallotti and Mussi, 2017, 2018). In Olduvai Gorge, SHK, TK and BK, in Middle and Upper Bed II, have also provided specimens that meet these technical and volumetric principles (Diez-Martín et al., 2017; Rubio-Jara et al., 2017; Santonja et al., 2014; de la Torre and Mora, 2005, 2014). Among them, SHK, in Middle Bed II, is the oldest in the sequence. This archaeological site underlies Tuff IIC (Diez-Martín et al., 2014, 2017), a marker tuff that has never been directly dated by radiometric means, and that is tentatively located at 1.5 Ma (Domínguez-Rodrigo et al., 2012). TK and BK are both stratigraphically related to Tuff IID, recently dated to 1.35 Ma (Domínguez-Rodrigo et al., 2013). Interestingly, TK and BK are chronologically close to the Acheulean site of ST-69, in the Moinik Formation of Peninj and stratigraphically dated to about 1.2 Ma (Diez-Martín et al., 2009, 2018), where some skilled and rather elaborate handaxes have been retrieved.

this technical task, the knapper relied on several distinct cognitive abilities. It strikes as very unlikely that a single individual could have been unique in all of them. Second, the aesthetic component suggests a role for observers other than the knapper, observers capable of understanding the handaxe and evaluating the maker. Third, the atypical components in evidence here became ubiquitous in later Acheulean assemblages. The FLK knapper, if unique, possessed an unlikely ability to anticipate specific future developments. A more parsimonious conclusion is that the knapper of this exceptional artifact was an adept individual who fell within the normal range of variation. 6. Discussion The FLK West large handaxe hardly matches the formal crudeness reported for the LCTs recovered from EtA sites (including its FLK West counterparts), that exhibit: a) absence of invasive shaping, b) minor transformation of blanks through the detachment of few negative scars, c) scarce, crude and discontinuous secondary retouch, d) scarce symmetry concerns, e) the main purpose of rough and sinuous edge modification (Beyene et al., 2013; Chevrier, 2012; Diez-Martín et al., 2015; Sánchez-Yustos et al., 2017; Texier, 2018). On the contrary, and regarding this elaborate specimen: a) the blank selected is uncommon for its context; b) its dimensions and mass clearly exceed the LCT average at FLK West; b) knapping includes volumetric control thorough a number of series of invasive reduction aimed at blank thinning and conditioning; c) shaping strategies embrace intensive edge and prehensile area transformation and precise form imposition through continuous secondary trimming; and d) as a consequence of the shape treatment, the knapper had in mind clear symmetry goals, particularly in her or his search of bilateral symmetry. Until now, on the occasions in which the LCT technology in African Early Acheulean assemblages (i.e., c.1.7–1.0 Ma, Sahnouni et al., 2013) has been scrutinized in some detail, scholars have generally agreed that this phase represents an initial stage, in which operational knowledge applied to new tool concepts are only rudimentary shown (Clark, 1994; Schick, 1998). For the EtA, for instance, Chevrier (2012) argues that the 12

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Certainly, a more contentious and provocative issue is constituted by the meaning of this large handaxe. Although the functional meaning of Acheulean large tools is far for being determined, researchers agree that LCTs could have been multi-purpose tools, involved in an increasing number of tasks and economic/extractive interests. Among these tasks, cutting and chopping organic tissues, digging, hammering, throwing or even producing useable flakes have been cited in the literature (Diez-Martín and Eren, 2012; Goren-Inbar et al., 2002; Key and Lycett, 2017; Sahnouni et al., 2013; Toth and Schick, 2009; Whittaker and McCall, 2001). Archaeological and experimental research has provided evidence that LCTs were efficient for large animal butchery (Galán and Domínguez-Rodrigo, 2014; Jones, 1980; Keely, 1980; Machin et al., 2005, 2007; Rabinovich et al., 2008; Schick and Toth, 1993; Solodenko et al., 2015) and suitable for woodworking (Clark, 1975; Domínguez-Rodrigo et al., 2001; Jones, 1994). A number of researchers have explored the relationship between form/design and usability of the Acheulean large tools (Chevrier, 2012; Key et al., 2016; Sánchez-Yustos et al., 2017; Viallet, 2019). Techno-functional approaches (Airvaux, 1987; Boëda, 2001, 2013) can provide alternative and valuable information to more conventional functional studies. Although this tool was designed to produce a refined and functionally active distal tip, no evidence of use has been identified through macroscopic inspection. Detailed edge analysis has clearly shown absence of edge damage in form of micro-flakes, scarring or edge polishing (Méndez-Quintas et al., 2018). A mint distal edge is unexpected for a tool that, due to its massive dimensions and weight, would have been very difficult to manoeuver in tasks not bearing a heavy-duty component. In this regard, the mass of this extremely heavy tool (3.6 kg) might have conditioned its gripping capabilities (Key et al., 2018) and/ or limited its functional options. This specimen exceeds average LCT weight ranges in Africa and elsewhere (Wynn and Gowlett, 2018), a factor that might have influenced both ergonomics and manipulation, imposing intensive manual pressure on to potential actions (WilliamsHatala et al., 2018). Due to its mass, the use of this specific tool could have implied a more complex set of kinematic solutions when performing a number of different tasks. However, as has been experimentally demonstrated (Key et al., 2016), the design of this specimen (including both clear passive and transformative units) fits well within a scenario of ergonomic features enhancement and use efficiency facilitation. The way in which mass relates to tool use proficiency is a key factor in the functional interpretation of this large and heavy specimen. This aspect needs further study, although there are examples in which mass variability in handaxes has been purportedly related to specific functional goals (Samson, 2006). It is striking that such a costly investment in the production of this large handaxe seems to go beyond functional parameters. Furthermore, raw material exoticism with respect to the blank morphology of the igneous rocks predominant in the site (rounded basalt cobbles) and the preferential use of Naibor Soit quartzite for the production of LCTs, adds extra value to the effort invested in the tool's design target. As already mentioned in the previous section, symmetry imposition in the production of this extraordinarily large, and by no means functional, handaxe made on an exotic and heavy clast embodies aesthetic implications and a significant visual impact. The imbalance between aesthetic value and functional value observed in the case of the FLK West large handaxe can fuel (adding now the EtA to the discussion) the controversial debate on the role of these exceptional tools within the expanding social structures, social codes and social cognition of the Acheulean large tool knappers (Cole, 2014, 2015; Gamble et al., 2011; Hodgson, 2010; Kohn and Mithen, 1999; Machin, 2008; Nowell and Chang, 2009; Shipton, 2010).

Fig. 13. Frontal outline superposition of the 24 LCTs compared for this work.

It is apparent, through the African Acheulean lithic record bracketed between 1.5 and 1.2 Ma, that these knappers already showed a defined precision of execution in volume management and large bifacial tools. The FLK West large handaxe fits much better within this chronological interval than within its EtA contemporaries. In a recent paper, Chazan (2015) proposed a local technological structure for the Acheulean of Wonderwerk Cave in South Africa. Throughout the archaeological sequence, he identifies three Acheulean phases, based on specific technological trends. The characteristics of the FLK West handaxe would fit within Chazan's more recent Acheulean phase (characterized by the use of invasive removals, secondary trimming, and prehensile zone transformation). If the knappers from FLK West were able to display such mastery in the production of an individual tool, we should then assume that they possessed the technical and formal competence for its replication. Why they did not do it regularly? Why does the large FLK West handaxe stand today as a unicum within the EtA and even slightly younger sites? Did these hominins, who possessed a proper conceptual and technical definition of the handaxe concept, lack the ability or the concern of tool standardization? Or, despite showing these skills in the production of an individual handaxe, were their large tool manufacture choices primarily driven by functional goals? In a framework of LCT design defined by high efficiency and less effort parameters, the outstanding metric and technical characteristics of the large olivine basalt handaxe do not seem to be related to a stochastic phenomenon or an accidental result of random variability. Instead, the particular oddity of this tool denotes the intentionality implied in its manufacture.

7. Conclusions The formal and technological characteristics of the large olivine basalt handaxe unearthed from FLK West lowermost level 6 at Olduvai 13

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Fig. 14. Histogram showing the frequency of LCTs included in different SI intervals. The black bar indicates the position of the olivine basalt handaxe within the histogram.

Fig. 15. Scatterplot of width by SI. Pearson test shows a significant positive correlation between both variables with a moderate effect size (rp = 0.47, p = .020). Black dot indicates the position of the olivine basalt handaxe within the LCT sample. 14

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Fig. 16. Scatterplot of SDI by SI. Pearson test shows a significant positive correlation between both variables with a moderate effect size (rp = 0.44, p = .032). The black dot indicates the position of the olivine basalt handaxe within the LCT sample.

Fig. 17. Scatterplot of EI by RI. Pearson test shows a significant negative correlation between both variables with a large effect size (rp = −0.67, p= < .001). The black dot indicates the position of the olivine basalt handaxe within the LCT sample.

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Fig. 18. Diagram showing the different cognitive developments implicit in the FLK West handaxe.

Gorge. We also appreciate major funding provided by the Spanish Ministry of Economy and Competitiveness for funding this research through the National Plan I + D + i (HAR2013-45246-C3-3-P). We are grateful to Andoni Tarriño for his valuable help with the petrological identification.

Gorge (Tanzania), one of the most significant examples of the EtA, is not in agreement with the rest of LCTs so far recovered from this site and, in a broader context, with the hard evidence provided by the EtA phase. This is an extremely large specimen, made on an unusual blank. The thick and heavy hemi-spherical blank support, probably a massive flake or a hemispherical preform, has hindered intense thinning and the total removal of cortical areas. However, the handaxe bears other exceptional progressive traits that clearly distance it from its spatial and temporal context. A diacritical analysis of this tool has shown a determinate design strategy that followed three consecutive steps, namely: 1. Volume transformation, mass control and thinning adjustment through knapping series of invasive detachments; 2. Form imposition through series aimed at defining the pointed character of the tool (forward extension) and at individualizing both the transformative and the prehensile zones; 3. Hard-hammer secondary trimming in the whole perimeter of the piece aimed at adjusting both final design and bilateral symmetry. The precise sequencing of technological and spatial decision-making shows a surprisingly developed definition of the handaxe tool concept. It also suggests the concurrence of a number of associated cognitive developments, such as cognitive control, spatial cognition, category formation and aesthetic cognition. Macroscopic analyses of the potentially active distal pointed edge have shown no sign of edge damage related to tool use. The imbalance in terms of costbenefit between the high investment in tool design and the low functional output entails over-determination.

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Declaration of competing interest None. Acknowledgements We wish to thank the Tanzanian Commission for Science and Technology (COSTECH), the Department of Antiquities and Ngorongoro Conservation Area Authority in the Ministry of Natural Resources and Tourism for permission to conduct research at Olduvai 16

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