Objectifying processes: The use of geometric morphometrics and multivariate analyses on Acheulean tools

Objectifying processes: The use of geometric morphometrics and multivariate analyses on Acheulean tools Paula García-Medrano 1, Elías Maldonado-Garrido 1, Nick Ashton 1, Andreu Ollé 2,3 1. Dept. Britain, Europe and Prehistory, British Museum, Frank House, 56 Orsman Road N1 5QJ, London, U.K. Email: Garcia-Medrano: ; Maldonado-Garrido: ; Ashton: 2. Institut Català de Paleoecologia Humana i Evolució Social (IPHES), Zona educacional 4, Campus, Sescelades URV (Edifici W3), 43007 Tarragona, Spain. Email: 3. Àrea de Prehistòria, Universitat Rovira i Virgili (URV), Fac. Lletres, Av. Catalunya, 35, 43002 Tarragona, Spain. Abstract: Nowadays, the fruitful discussion regarding the morphological variability of handaxes during the Middle Pleistocene has reached a decisive moment with the use of more accurate statistical methods, such as geometric morphometrics (GM) and multivariate analyses (MA). This paper presents a preliminary methodological approach for checking the utility of these new approaches on the analysis of the tools‟ shape. It goes beyond the simple description of morphology and isolates the variables which define the final morphology of a tool. We compared two Middle Pleistocene sites, Boxgrove and Swanscombe, which are morphologically very different. Then, we applied the GM analysis on 1) 2D images, with two semi-landmark distributions: 28 semi-landmarks, specially concentrated on the tip and butt, and 60 equally spaced points; and 2) on 3D models using a new software (AGMT3-D Software) including 5000 semi-landmarks. The more points used to define the tool‟s outline, the more accurate will be the interpretation of the variables affecting shape. On the other hand, if the semi-landmarks are localized on specific sectors of the tool, a bias is created, by concentrating on those sectors, rather than the general tool shape. The 3D models offer a new dimension on the shape analysis, as their results mean the combination of plan-shape, profileshape and the tool‟s topography. Keywords: Middle Pleistocene; Acheulean; handaxes; geometric morphometrics; multivariate analyses; Boxgrove; Swanscombe Journal of Lithic Studies (2020) vol. 7, nr. 1, p. xx-xx DOI: https://doi.org/10.2218/jls.4327 Published by the School of History, Classics and Archaeology, University of Edinburgh ISSN: 2055-0472. URL: http://journals.ed.ac.uk/lithicstudies/ Except where otherwise noted, this work is licensed under a CC BY 4.0 licence. 2 García-Medrano et al. 1. Introduction Since the early stages of prehistoric research, there have been persistent attempts to develop a common set of technological features and measures in which the morphology of different lithic assemblages could be classified and compared. These classical methods for the study of stone artefacts have led us to distinguish between different technological modes, or cultural technological groups (Clark 1996). The differences are mainly based on the combination of presence or absence of some technological markers (cores and flake systems, Large Cutting Tools or the Levallois technique) with the chronological boundaries of each site. For a long time, the aim of the study of these technological groups was the characterization of the operative chains and the determination of the final tools‟ typology (Boëda et al. 1990; Bordes 1961; Carbonell et al. 1992; Newcomer 1971; Wenban-Smith 1989). Nowadays, there is a focus on the morphological variability of tools and their possible origin. Traditionally, this analysis has been made using metrical approaches, quantifying the morphology by a set of discrete metric measures, and characterizing an assemblage by the distribution of these measures (Bordes 1961; Roe 1968). Nevertheless, the morphology of tools is a much more complex issue than the superficial description of measures and indices, and it should refer to the entire shape, or to global properties of the artefact, more than to local measures (Grosman et al. 2008). GM analysis is a powerful tool for the quantitative description of shape variability within and between groups. Nevertheless, it has had two significant handicaps: the limited computing power and 3D scanning possibilities (Lycett et al. 2006; Lycett 2009; Rovner 1995); and the problems and difficulties entailed in the application of landmark or semilandmark-based methods to material cultural objects (Archer & Braun 2010; Bretzke & Conard 2012; Costa 2010; Eren & Lycett 2012; Herzlinger & Grosman 2018; Lycett & Chauhan 2010; Lycett & von Cramon-Taubadel 2013; McNabb 2007). The main problem, among others, has been the lack of readily identifiable, homologous landmarks on artefacts, (Lycett & Chauhan 2010). The lack of protocols requires a comparison of methods to test which is the most appropriate to analyse the morphology of tools. Here, we have compared the results between the use of 2D images or 3D models. In addition, it is also necessary to define how many semi-landmarks are required to define accurately the tool‟s outline. In the context of the Western European Acheulean Project (WEAP, Marie Skłodowska Curie IF-EF-ST Fellowship, Project ID: 748316), we have defined a common method of analysis for the study of handaxes and cleavers from several sites in the UK, France and Spain, based on a selection of technological attributes from the main traditions of lithic analysis (typological, technological, morphometrical and sequential) that are considered to be particularly significant (Garcia-Medrano in prep.). Besides, combined with this technological characterization, WEAP has used GM on 2D images and 3D models to analyse both plan and profile shapes of handaxes and cleavers. The aim of this work is to make a preliminary methodological approach comparing the use of 28 (concentrated on tip and butt) and 60 (equally distributed) semi-landmarks, both on 2D images; and 5000 points, on 3D models. The results have been interpreted using MA. This is increasingly used to discern patterns of variability in lithic assemblages and help with their interpretation. One of the most frequently used is Principal Component Analysis (PCA). IN this case, we have applied this analysis on two of the main Middle Pleistocene sites in Britain: Swanscombe and Boxgrove. 2. Archaeological context WEAP includes the analysis of handaxes and cleavers from several sites in the UK, France and Spain (La Noira, Brandon Fields, Cagny La Garenne, Elveden, Swanscombe, Journal of Lithic Studies (2020) vol. 7, nr. 1, p. xx-xx DOI: https://doi.org/10.2218/jls.4327 Garcia-Medrano et al. 3 Saint Pierre-lès-Elbeuf, Menez-Dregan and Galería and TD10.1 sublevel of Gran Dolina, Atapuerca). For this paper, we have chosen two of the key British Middle Pleistocene sites: Boxgrove and Swanscombe (Figure 1). The clear association between fauna and very rich lithic assemblages, together with hominin remains attributed to Homo heidelbergensis and early Neanderthals makes them two of the key sites for interpreting the A (...truncated)