Innovations in Acheulean Biface Production at La Noira (Centre France): Shift or Drift Between 700 and 450 Ka in Western Europe?

The archaeological sequence of la Noira, in the Middle Loire Basin (Centre region, France) yielded two phases of occupation: ca 700 ka (stratum a) and ca 450 ka (stratum c). No site between these two dates has yet been discovered in the area, and this chronological period has thus been interpreted as a gap in settlement from MIS 16 to MIS 12. Here, we compare these two levels and track technological innovations or/and inventions during this time, based on the technological and morphometric comparison of the Large Cutting Tools (LCTs), applying a new method of analysis. We explore inter and intra-level variability in order to address the hypotheses of (1) the liation over time of populations settling sporadically in the region or (2) the arrival of new populations from other European or extra-European areas. Stratum a presents mainly short shaping sequences on local millstone, taking advantage of natural slab geometry, and with special attention to tips, but with clear management of tool volume. Stratum c differs in that both local millstone and ints from distant sources show longer shaping sequences, the use of soft hammers for several series of removals, combined with nal regularizing retouch on entire edges. The morphometric approach shows a morphological transition from oval to teardrop shapes for the thinnest tools. A technological liation between strata a and c and between la Noira populations from MIS 16 and MIS 12 in this area has to be considered. in stratum a, due to the lesser degree of edge shaping standardization, most of the angles are between 45° and 80° along the whole edge, and only some tips extend beyond this range. In stratum c, where a predominant use of soft hammers is associated with longer sequences, we observe a signicant change in angles. The angles of the cutting edges are more acute, homogeneous and differ between the distal and the proximal sectors of the tool. Tip angles are between 30° and 45°, mid part edge angles between 40° and 70° and butt angles between 60° and 80°.


Introduction
At the end of the Middle Pleistocene Transition (MPT) in Western Europe, after the Brunhes-Matuyama reversal (780 ka), climatic cycles changed, leading to signi cant variations in resources. These cycles must have affected the extinction/relocation of small groups of hominins, and subsequent recolonizations, between the two long MIS 16 and MIS 12 cold events (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). MIS 12 was followed by a long interglacial stage (MIS 11) which marks the beginning of new behaviours and the advent of the Neanderthal technical world (13). After the MIS 12 glaciation, considered as a major crisis for hominins, archaeological records show a high quantity and variety of occupations, new subsistence behaviours and important technical innovations (e.g., Levallois technology, increase in light-duty tools and use of re), and evidence of an early regionalization of traditions (13)(14)(15). These behavioural changes suggest increased cognition with new skills and social interactions (16)(17)(18)(19).
Among other kinds of behavioural innovations, the Acheulean has been traditionally de ned by the emergence of a new tool with bifacial management of volume: the handaxe (20,21), which has been considered as evidence of new skills and changes in human cognition (22,23). Due to the effectiveness and the versatility of this type of instrument, handaxes persisted for more than 1 Ma over a vast geographical area (24). This tool rst appeared in East Africa around 1.75 Ma, but is not present in Western Europe before 900 ka and especially from 700 ka onwards (ref). In spite of apparent technological stability, this kind of instrument encompasses huge variability in terms of production, as well as in terms of the morphological outcome of shaping processes. Strata a and c of the la Noira site, located in the centre of France, and the large corpus of handaxes are appropriate case studies for tracking technological behaviours common to both levels and identifying innovations/inventions over time. We aim to discuss two crucial phases of hominin settlement in Western Europe but also to contribute to hypotheses positing either a liation between European populations over time or arrivals of new populations introducing new skills after MIS 12. By inventions, we mean evidence of technological breaks with previous tradition(s), while innovations refer to internal behavioural evolution rooted in the past and sometimes motivated by external or internal changes. Common features between the two phases of occupation and existence of innovations rooted in the past would point to a possible liation over time of populations between the MIS 16 and MIS 12 glacial periods and would imply that these populations were able to return to abandoned areas when the climate was favourable, aided perhaps by more complex behaviours due to internal evolution and increase of skills (25).
The site of la Noira is located in the Middle Loire Basin (Centre region, France), on the western slope of the Cher River Valley (26) (Fig. 1).
Five successive sediments strata can be observed at the site (from bottom to top): a coarse slope deposit (stratum a), covered by two sequences of sandy alluvial layers (stratum b), diamictons of pebbles with frost shattered debris and coarse colluvia (stratum c) and the a washed sandy-silty soil (stratum d). This paper focuses on the oldest archaeological level (henceforth referred to as the 'lower level'), located in stratum a, while the younger upper level is located at the top of stratum c.
The lower layer (stratum a) was deposited on the limestone bedrock at the beginning of a glacial stage after river incision. The slope deposits contained local lacustrine millstone slabs, some of which were selected by hominins for knapping and shaping. Occupations were located on the river bank. The age of uvial formation was determined using the ESR method applied to optically bleached sedimentary quartz grains. The mean ESR age value obtained for the sandy formations of stratum b is 655 ± 55 ka. Tests with cosmogenic nuclide dating provide a similar value of 730 ± 210 ka, but with an excessively high margin of error (27). The average age of the human occupation is thus around 700 ka (26). The hominin occupation occurred between the end of river incision and the uvial deposits, suggesting that hominins were present during the beginning of the MIS 16 glacial stage, just before the pleniglacial uvial depositions. They left the area during the early glacial MIS 16 at around 670 − 650 ka, when cold conditions became too rigorous (14,28).
The petrographic composition of the sediments of stratum c (composed of three sub-levels) is identical to that of the lower stratum a, composed of sands and quartz gravels with endogenous pebbles (granite and quartz) and sedimentary siliceous rocks. The ESR date of 449 ± 45 ka indicates that diamicrons of stratum c could have been deposited at the end of the MIS 12 glacial (frozen cracks soil and frost debris) and mainly during the MIS 11 interglacial (eroded soil and deep channels). The remarkable preservation of the artefacts could indicate that they were not affected by frost as they were overlain by thick colluvial deposits (29)(30)(31)(32). While local millstone was the only raw material used during the early occupations, the stratum c assemblage is composed of 30% of int and silici cations transported from long-distance Jurassic and Cretaceous outcrops, between 50 and 100 km from the site (33).
In recent decades, new dating and excavations have been conducted to identify gaps in human occupations in Western Europe (14). The Centre region of France is one of the areas where two gaps have been documented, one between 1 Ma and 700 ka and a second one between 700 and 450 ka (26), possibly explained by climatic factors due to the location of the area beyond the 45th parallel. In Western Europe, few sites can be used to investigate such gaps in human occupation. La Noira is one such example (14,28). The technological analysis of all the lithic material from stratum a at la Noira has already enabled us to explore (13,14,28) the onset of the Acheulean at 700 ka in Western Europe and the technological skills of these hominins. A speci c morphometric analysis of handaxe symmetry in strata a and c (32) has shown that human groups mastered tool symmetry from 700 ka onwards, despite lower shaping intensity at that time. In order to continue to track the technological drift between ca 700 and ca 450 ka, we applied the WEAP method (34) on the whole corpus of handaxes of this site, combining for the rst time a technological analysis with a broader morphometric approach using AGMT3-D software (35,36).

Results
The studied corpus is composed of 31 handaxes for stratum a and 47 handaxes and cleaver-like tools (bifacial tool with a round or transverse extremity) for stratum c. They were collected in situ, and come from recent excavations and systematic surveys carried out for the three last decades in the quarry. All the handaxes from the lower level (stratum a) are made on local millstone slabs, and in nearly 65% of cases, slabs are only used for shaping. In the upper level (stratum c), our corpus of tools is shaped on comparable proportions of millstone slabs (48.94%) and int nodules (51.06%, Fig. 2). For 34% of the series, it was impossible to identify the type of blank, due to invasive shaping. indicates that technological differences exist between the two levels ( Fig. 1). This PCA accounts for more than 66% of the variability of the series. PC1 (43.11%) divides the samples according to level, whatever the raw material (millstone or int) or type of blank (slab, ake or nodule).

Stratum a
All the handaxes are made with hard hammers (58.06%) or with a combination of hard and soft (32.26%) hammers, especially on tool extremities (Fig. 3). The tips appear much more worked and retouched than the cortical butts (more than 83%). Tool edges are mainly sinuous (48.39%) and the pro le is non-symmetric (80.65%). The high variability of the corpus is mainly due to the type of façonnage of the tips. For more than 50% of tools, we observe one or two face by face or alternate series of removals. Less than 50% bear nal retouch, and retouch is absent from lateral and proximal cutting edges. Removals affect the edges either marginally (51.61%), producing regular edges, or more intensely (48.39%), generating more denticulate and irregular plan-shape pro les. When only one series of removal exists, it is noninvasive over the tool surface. When there are several series of removals, shaping is more invasive, and can extend up to the midpart of the tool surface. In nearly 40% of cases, there is a combination of an invasive rst series of removals and a second series along the tool edges.
Butts retain 40-90% of the original cortex for more than 45% of tools. When removals are present, they are concentrated on butt edges.
Finally, the corpus of Large Cutting Tools (LCTs) from stratum a presents high shaping variability with a signi cant difference in the management of tips and butts. Tips present more careful treatment, sometimes with nal retouch while butts remain mainly cortical (Suppl. Inform., Fig. 1, lower part).
The only exception to this high variability concerns pieces on unknown blanks (13% of tools). The PCA shows how this category of tools is clearly affected by the PC2 (23.14%) (Supp. Inform. Figure 2). They differ in that they are characterized by longer operative chains with two series of removals, the rst one invasive and the second one short on both the tip and the upper part of the tool, followed by nal retouch only on the tip. In both cases, hard and soft hammers are used. The butt is less cortical (40%) and shaped by only one invasive series of removals by hard hammer percussion.

Stratum c
The corpus from stratum c includes handaxes and some 'cleaver-like' handaxes (handaxes with wider convexity on tips, generating a sort of transverse end). The tools mainly show higher standardization with longer operative chains and signi cant blank reduction (Fig. 4).
Shaping extensively affects the entire tools with evidence of the use of hard and soft hammers on the whole piece. The presence of cortex is limited to the butts or part of the lower surface and the tips have no cortex (89%). 23% of tools bear no cortex. The tools present mainly non-symmetric pro les but the proportion of symmetric tools increases (up to 20%) with rectilinear edges (54%). The use of soft hammers (around 60%) is clearly visible on all the sectors of the tools (tip, mid and butts). For 49%, dense nal retouch obliterates the last removals.
For 27.66% of the tools, the tip is shaped by two series of removals, combining invasive and non-invasive scars. Final retouch can extend to the midpart of the tool surface or can be limited to the edges. Finally, for 17% of cases, we also documented a coup de tranchet removal with a non-retouched distal edge.
The midpart of tools is above all worked by two series of alternate removals (63.87%) with nal retouch and without cortex (28%). Like in stratum a, this type of shaping profoundly modi es edges. Nevertheless, for this level, we observe a change in shaping strategies mainly for tools on millstone slabs. The edges are more regular, with a combination of a rst invasive series of removals, followed by a non-invasive second series and nally, marginal retouch con ned to the edges. Butts are non-cortical or with small patches of cortex. In 92% of cases, there is only one series of removals and marginal use of a soft hammer, mainly on tools shaped on akes.

Stratum a vs stratum c LCTs
The Principal Component Analysis de nes the existence of two clear groups of tools: strata a and c (Fig. 5). The differences are independent of the type of raw material (millstone and int) and the type of blank used for shaping (slabs, akes or nodules). The distance between strata a and c shows rather a technological origin, possibly related to a change in shaping strategies. The rst main difference between these groups is that sequences are more diversi ed and shorter for stratum a tools, and longer and more standardized for stratum c tools. In addition, out of the whole set of technological features considered here, the presence of original cortex (Fig. 5A) and the different combinations of series of removals (Fig. 5B) have a major effect on the distance between these two assemblages (Suppl. Inform. Figure 1 and Fig. 2), which is also visible by Cluster analysis (Fig. 5C). Handaxes in stratum a present cortex on 50% of tools (butt and mid parts), and sometimes covers the whole instrument. In stratum c, there is an increase in the ratio of non-cortical tools, as well as in the use of nal retouch, independently of the type of blank used. PCA also points to a clear differentiation of tools from stratum a, which present longer shaping sequences and unknown or indeterminate blanks (Unkn). They are clearly apart on the PCA graph and are represented as an independent branch of the Cluster. Tools from stratum c show a different pattern, re ecting a certain association between raw material and blank type. Millstone is mainly associated with what slabs, and int types present the same technological features as handaxes made on unknown blanks. Flakes appear as an independent group, regardless of raw materials.
The results of the geometric morphometric analyses of tools from strata a and c of la Noira indicate the extent of intra-group shape variability, expressed as the mean multidimensional Euclidean distance of all items of a group from its group centroid. Overall, the groups considered are fairly similar (Fig. 6) but tools from stratum a present higher variability. The most homogeneous group is composed of millstone tools from stratum c. The distribution of the total standardized coe cients across the three dimensions X, Y and Z shows differences in relative width, length and thickness respectively ( Table 2). In the archaeological assemblages, most of the variability corresponds to differences in relative thickness, mainly in stratum c and speci cally for millstone tools. On the other hand, the tools from stratum a show higher variability in width and length.   Tool size and thickness decrease from stratum a to stratum c (Suppl. Inform, Table 5). Millstone and int tools from stratum c present nearly the same values, indicating common strategies, regardless of the stones and their natural geometry. In addition, we must point out signi cant variation in distal vs proximal length. In stratum a, proximal length is higher, while in stratum c, distal length is higher (Fig. 7A). This is consistent with the geometric morphometric analysis and the contrast between oval shapes in stratum a, with longer bases, and more pointed shapes, with longer distal parts in stratum c.
Through the analysis of six angles measured along each edge, we document more acute angles on the mid-distal part, and wider angles on the mid-proximal part, in both strata (Fig. 7B, Table 4). However, in stratum a, due to the lesser degree of edge shaping standardization, most of the angles are between 45° and 80° along the whole edge, and only some tips extend beyond this range. In stratum c, where a predominant use of soft hammers is associated with longer sequences, we observe a signi cant change in angles. The angles of the cutting edges are more acute, homogeneous and differ between the distal and the proximal sectors of the tool. Tip angles are between 30°a nd 45°, mid part edge angles between 40° and 70° and butt angles between 60° and 80°. The Scar Density Index (SDI) in relation to tool volume is coherent with the technological and morphometric analysis ( Table 5). This ratio is higher for tools from stratum c, as well as for tools made on an unknown blank in stratum a. Therefore, the longer the shaping process, the higher the ratio between SDI and volume. But this also implies that the higher variability and lower standardization of the handaxes from stratum a has a clear effect on this result ( Table 2). For raw materials, we can see the same pattern, between int tools, which present the highest ratio, and millstone handaxes with the lowest ratios. The statistical analysis of the degree of symmetry of tools shows that the main differences are between millstone handaxes from stratum a and stratum c. We note an increase in bilateral symmetry with an average of 25% (Table 6). Wilcoxon rank sum tests con rm that this difference is statistically signi cant (n1 = 32, n2 = 18, ranksum = 916, p = 0.04). In terms of bifacial symmetry, there is an increase of nearly 35% throughout the sequence, which is statistically signi cant (n1 = 32, n2 = 18, ranksum = 968, p= 0.01). The edge irregularity test shows that, in all cases, both edges of the same tool are always different. Nevertheless, as bilateral and bifacial symmetry show higher diversity for millstone tools from stratum a, int tools present more regular edges. As mentioned previously, the main difference between the tools from the two strata is the combination of several series of removals (duration of shaping processes). In the case of int, there is often a third series, and nal non-invasive retouch on the cutting edges (Fig. 4D). This has a clear impact on the regularity of the edges (pro le symmetry). Nevertheless, the main difference between the millstone handaxes in the two strata is the massive use of at least two series of removals on the midparts and butts of the tools from stratum c, which dramatically reduces tool thickness. Bilateral and bifacial symmetry (plan shape symmetry) is thus affected.  (2,4,45,46), led to more extreme conditions which could have profoundly impacted human populations and dispersions, and may explain possible successive depopulations or extinctions of small groups of hominins during cold events in the north, necessitating re-colonization from the south during warmer events (47). The second climatic transition (Mid-Brunhes Event-MBE) between MIS 13 and 11, with more marked glacial-interglacial cycles might explain in part the wider diffusion of the Acheulean through Western Europe during warmer interglacials and the extension of the mammoth steppe in the northwest from 500 ka (48,49). For north-western Europe, evidence suggests occasional dispersals, which would account for the diversity of strategies due to regular introductions of new behaviours and populations. Gaps are also recorded in southern Europe. At Atapuerca, for example, an occupation hiatus is observed between 800 and 500 ka (6,50), which is similar to the hiatuses observed in the centre of France, between 1 Ma and 700 ka and 700 to 500 ka (26).
In Early Acheulean African assemblages, for example, Olduvai, Bed II (1. are also recorded over time in the assemblages. Stratum a of la Noira attests to the mastery of biface production, with the management of volume and tool symmetry, assisted in some cases by the use of soft hammers, mainly on distal parts (14). These features justify the hypothesis of a well-established Acheulean tradition at c. 700 ka in Western Europe (13,14,28). Coupled with a geometric morphometric analysis using 3D models, this technological study of the LCTs tracks for the rst time innovations v. common features and the degree of liation between strata a and c for the heavy-duty component. Our method enables us to determine a large set of similar and different tool features related to shaping modes and nal morphometry.
Data clearly distinguish the two different technological assemblages (stratum a and stratum c), regardless of raw material or blank types.
In stratum a (13,14,28), a single local raw material is used, millstone slabs. Handaxe shaping retains large cortical surfaces and exploits stone geometry with one or two series of removals, mainly with hard hammers. This behaviour generates a few standardized assemblages.
Nevertheless, technological control of the tips is also observed, using both hard and soft hammer percussion. We can also mention evidence of longer shaping chains on some tools (group of unknown blanks). On the other hand, stratum c is characterized by the use of diverse stones and the introduction of raw materials from long-distance areas. The use of local stones in stratum c indicates an increase in the size of the procurement zone, suggesting higher mobility for hominin groups at the end of MIS 12 and the beginning of MIS 11. The From a morphometric point of view, there is a transition between the two levels from oval (globular) shapes and few standardized tools, with the maximum width of the tool at mid-length, to 'teardrop' shapes in a more homogeneous assemblage, with the maximum width of the tool at the base (Fig. 8). There is also a transition from short distal parts, with wider convex tips, to longer convergent edges with more pointed tips, opposed to wider bases. In addition, intense technological work on the tools of stratum c results in reduced tool thickness. As Iovita et al. (2007) stated, Acheulean toolmakers had the technical abilities and skills to produce symmetric tools from 700 ka onwards. Nevertheless, there is an increase in this tool symmetry in stratum c. Iovita et al. (2007) concluded that this symmetry was dependent on the degree of reduction and the raw material. The use of 3D models in the geometric morphometric analyses led us to go further and clarify this conclusion. Bilateral and bifacial symmetry increase on average by 25% and 35% respectively. Plan symmetry is mainly affected by the façonnage strategy, by more than one series of removals on the whole perimeter of the tool, reducing the thickness and modifying the original geometry of the blank whatever the raw material. Nevertheless, edge regularity depends on nal edge retouch.
How should these differences between the two occupation phases be interpreted? Do they stem from local or on a broader scale innovations rooted in the past motivated by external or internal changes or do they represent a shift, inventions as a result of a break in populations with new dispersals?
At la Noira, our analysis highlights two main features. First, in stratum a, we do not observe any differences in shaping or in morphological results for different blank types, even if slabs predominate. Only some tools with longer sequences stand out from the rest of the corpus (n = 4, 12.9%) due to more intense shaping, making it impossible to identify the type of blank. The tools present a combination of soft and hard hammer percussion and more intensive nal retouch. Consequently, the hominins of stratum a were able to develop complex and versatile operative chains.
Should we consider that this ability at 700 ka is evidence of a technological liation between populations from MIS 16 to MIS 12? If we look at the results of the geometric morphometric analysis, we do not observe real morphological breaks between the two corpuses.
Handaxes from stratum a are not homogeneous and short shaping sequences are correlated with greater tool thickness. Morphometrically, the complexity of this biface production at 700 ka is observed in the ability to manage tool plan shape for oval shapes, placing the centre of the mass at the midpart of tools. The stratum c corpus is characterized by a higher standardization of shapes, creating wider bases opposed to thinner and more pointed tips. The use of several series of removals reduces tool thickness. We observe total control of volume and edge morphology, which become more regular. A liation between the populations at la Noira is thus possible, suggesting that the long interglacial MIS 11 in Western Europe was not really a threshold and enabled local and European populations to re-occupy abandoned areas when the climate became more favourable, facilitated by demographic expansion and the implementation of more complex strategies.

Methods
We applied the WEAP Method to 78 handaxes and cleaver-like tools from stratum a and stratum c as part of a detailed technological and morphometric study aiming to analyse nal tool variability. The WEAP method was developed in the context of a Marie Skłodowska Curie IF-EF-ST Fellowship (IP: 748316) devoted to investigating the variability of Acheulean industries in Europe (34). This method considers each Large Cutting Tool (LCT) from two points of view. 1) As a single unit, including aspects such as raw material type, blank type, facial shaping, cortex presence, edge delineation, pro le symmetry, number of scars). 2) As the sum of the different parts, each of which are analysed independently, de ning the type of hammer used, number of removal series, depth of scars on edges, invasiveness of each removal series, and type of shaping (Fig. 9). Combining all these features, a Multivariate Analysis (PCA) identi es the differences and similarities of LCTs from both levels, comparing raw materials and types of blank.
To complete the technological analysis, we also applied the Geometric Morphometric analysis to describe tool shape with 3D models. All the tools were scanned using a laser scan (DLP projector) and Flexscan software (LMI technologies), transferred from the Fragmented Heritage Project (University of Bradford). All models are available for scienti c and academic purposes at ZENODO (35). The 3D models were processed using the AGMT3-D software (36,37). This consists of a data-acquisition procedure for automatically positioning 3D models in space and tting them with grids of 3D semi-landmarks. In fact, each point of the grid consists of two semi-landmarks, one placed on each face of the artefact, so that a 50×50 grid provides 5,000 landmarks (Fig. 10A). The top and bottom latitudes capture the exact 3D outline of the artefact's distal and proximal ends. Therefore, this protocol provides a list of landmarks that accurately express the artefact's volumetric con guration. It also provides a number of analytical tools and procedures that enable data processing and statistical analysis (36). For this paper, data obtained with 3D models are presented.
The multivariate outline data were projected into two dimensions so that the underlying shape variables could be qualitatively examined and compared. In order to interpret the Principal Component Analysis (PCA) results from a morphological perspective, Procrustes superimposed shape data were examined using thin-plate splines to facilitate the visualization of shape changes from the group mean along relative warp (i.e., principal component) axes (60). By examining the morphological deformations and XY plots of specimens from the PCA scatters, it was possible to interpret shape variation by itself, without the size effect, and compare the different tools within a site or between different sites. In addition, the derived principal component scores also allowed for the application of other quantitative tests of multivariate equality of means between the groups (36,37,61).
The latest version of this software also offers different quantitative approaches to the analysis of speci c variations in shape. Firstly, we will use the surface analysis (in 2 ) and volume (in 3 ) data to apply a quantitative approach to reduction intensity. The Scar Density Index (SDI, 62-65) has been de ned as the number of ake scars (greater than 10 mm in maximum dimension) divided by the surface area. As García-Medrano et al. (2019) noted, a loss of information during the knapping process, contrasting this value with volume information, could establish a useful relationship between the number of scars and tool size.
Lastly, the landmark data were used to calculate the degree of deviation from perfect bilateral (Fig. 10B) and bifacial symmetries (Fig. 10C), as well as the edge section regularity (Fig. 10D) of each item in the sample (37). For bilateral symmetry, this was conducted by measuring the mean 3D Euclidean distance between a mirror re ection of the landmarks placed on one lateral half of each object and the corresponding landmarks on the other half. The same procedure was performed for bifacial symmetry, but on the two opposing faces. In a perfect bilaterally or bifacially symmetrical object, the value of these indices will be 0, with increasing values indicating less symmetrical objects.