Discriminating between the end products of the earlier Middle Stone Age sub-stages at Klasies River using biplot methodology

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Abstract

The difference between the culture-stratigraphic entities, the MSA I and MSA II, in the Klasies River sequence is explored by statistical analysis of the end products. Technological analysis of the cores, end products and waste products suggested that the MSA I and MSA II represent distinct technological traditions aimed at producing different end products. To quantify the difference between the end products, points and blades, extensive univariate and multivariate statistical analyses of continuous variables have been undertaken. Biplot methodology is adopted for enhancing the statistical analysis. Canonical variate analysis biplots are constructed and alpha-bags added for visual displays of the overlap and separation among the different groups. It is demonstrated that the platform thickness relative to length is useful in discriminating between the end products of the MSA I and MSA II. Statistical analyses support a clear distinction between the MSA I and MSA II.

Introduction

The African Middle Stone Age has become of increased interest because under the Out-of-Africa hypothesis it was during this stage in the Late Pleistocene that modern humans may have migrated to Eurasia. Sub-Saharan Africa may have been the biogeographic centre for the evolution and dispersal of modern humans [26] and this contention has support in various molecular biological studies [15]. The evidence from Middle Stone Age archaeological sites is thus relevant to the study of the emergence of modern humans. In the main, it is sites dating to the first half of the Late Pleistocene that are well represented, notably in southern Africa, and this time was probably a time of continent-wide population expansion [10]. This time range also provides some of the earliest evidence for regional distributions of typologically distinctive artefact types in horizon markers, like the Howiesons Poort, Lupemban and the Aterian [8], [32], suggesting patterning different from the Earlier Stone Age.

For the most part, Middle Stone Age artefact assemblages include few distinctive retouched artefact types which makes their typological description less informative. Assemblages give the impression of belonging to a culture stratigraphic stage of long duration with little change in the artefacts produced. In the classic South African sequence established by reference to the Klasies River main site [39], five culture-stratigraphic sub-stages have been recognised as subdividing the Middle Stone Age. In stratigraphic order from oldest to youngest, Singer and Wymer labelled these sub-stages MSA I, MSA II, Howiesons Poort, MSA III and MSA IV. An extension of this scheme proposed by Volman [49], [50] recognises an older sub-stage as MSA I and labels the original MSA I and MSA II, 2a and 2b to indicate that these two sub-stages are very similar to each other. The sub-stages of the Middle Stone Age, with the exception of the Howiesons Poort, have not been viewed as discrete industries [3], [39], [50], [42], [43]. The reasons posited include the lack of clear-cut technological and typological divisions, and continuities in metric parameters between the sub-stages. These assessments are mainly based on typological considerations, which for such assemblages may not be an entirely appropriate approach. This is because the end products are predetermined by preparation of the core rather than through retouch and therefore, technological analyses are more informative.

Technological analyses of the Middle Stone Age at Klasies Rivers has led to the conclusion that the MSA I and MSA II are more than entities of convenience and that they represent culture-stratigraphic divisions at the same level as the Howiesons Poort [54], [55]. In this paper, quantitative measurements of the end products, points and blades, from the MSA I and MSA II sub-stages have been subjected to extensive univariate and multivariate statistical analyses in order to explore this proposition further. The MSA II sub-stage has been divided into two classes, MSA II lower (MSA II L) and MSA II upper (MSA II U) to investigate the degree to which the end products of the two sub-stages differ in terms of seven continuous variables. A further goal has been to determine whether the differences between MSA I and MSA II are of the same order as the differences between the two MSA II groups. Ultimately then, the purpose is to develop a methodology that would not only explore the statistical basis of the sub-stage divisions in the Klasies River sequence but also allow comparisons to be drawn with other Middle Stone Age assemblages in southern Africa and elsewhere.

Graphical representations of results to accompany any statistical analysis are strongly recommended by Chambers et al. [7] in the statement “there is no single statistical tool that is as powerful as a well-chosen graph”. In particular, this is true for multivariate statistical analysis, where by nature multidimensional data are explored. Biplot methodology is innovatively used in the present study, not only to provide graphical representations of multidimensional data accompanying well-established multivariate statistical methods, but also to direct the multivariate statistical analysis itself. The biplot method is of particular importance firstly, as it summarises the multivariate analyses, secondly, because it is an exploratory tool and thirdly, because it provides a single informative quantitative display of all observations from the different sub-stages together with all the variables at a glance. Gardner [19] provides a detailed exposition of biplot methodology for discriminant analysis. She proposed several novel extensions in biplot methodology for describing and quantifying multidimensional separation and overlap of predefined groups. In the present paper some of these proposals are implemented for displaying the observations from the different sub-stages together with all variables simultaneously. The biplot techniques can be extended to cluster analysis situations where groups are not predefined as they are here. This extension of the technique may have application in biological taxonomy and archaeology.

Section snippets

Site and sample

Klasies River main site is situated on the southern coast of South Africa (33.06°S, 23.24°E). Main site is a single depository against a cliff face with several openings into the cliff, described as caves 1, 1B, 1A and 2 (Fig. 1). The deposits, which were continuous between the caves before truncation by erosion, have a thickness of more than 18 m. They consist of numerous layers of hearths, ash, stone and bone intercalated by sandy layers. This suggests intermittent occupation by groups of

Statistical analysis of the end products

Refitting and experimentation have indicated that end products can be recognised by their symmetry, prepared platforms and a regular disposition of dorsal scars [46]. These characteristics were used to select the end products of the MSA I and MSA II. In this analysis end products were labelled blades (the term flake-blade is used by some researchers) and points (termed pointed flake-blades by Singer and Wymer [39] and convergent flake-blades by Thackeray and Kelly [44]). In the process of

Blades

The properties of the blades data are summarised in the boxplots appearing in Fig. 2. (In the electronic version of this paper, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 appear in colour. This substantially enhances the interpretative value of the graphical displays). The boxplots (Fig. 2) display a comparison of the observations from the three groups in the two sub-stages in terms of the symmetry/skewness, variation, location, range and outlying values for each variable recorded. Each box

Relationship between the statistical results and technological behaviour

From the statistical procedures carried out it is concluded that the MSA I differs from the two MSA II groups to a greater degree than the two MSA II groups differ from each other. The procedures show that the ratio of length to platform thickness is the variable that discriminates most clearly between the MSA I and MSA II blades and points. The MSA I has larger ratio of length to platform thickness values while the MSA II U and MSA II L are very similar in terms of this ratio. Further, it is

Classification using CVA biplot classification procedure

The above discriminant analyses not only provide a description of the overlap and separation among the various groups, but offer the potential for blades and points of uncertain provenance to be classified asbelonging to one or other of the sub-stages. In order to evaluate the results obtained by the CVA biplot classification procedure a resubstitution method (cf. [17]) was used. Using the data above, it is found that this classification procedure does not classify the end products of the MSA

Concluding discussion

This paper has applied a suite of multivariate statistical techniques to an archaeological problem, the validity of the separation of two proposed sub-stages of the Late Pleistocene Middle Stone Age in the stratified sequence at Klasies River main site. The focus has been on end products of core reduction, in this case regular blades and point blanks that are rarely retouched. The form of the end products is predetermined by the shaping of the core and in the process flaking debris is produced

Acknowledgements

The Katholieke University of Leuven, Belgium and the University of Stellenbosch, are thanked for supporting this study through post-doctoral research fellowships. This paper is based on D.Phil and Ph.D studies for which financial assistance was provided by the NRF (Centre for Scientific Development). The Struwig Trust facilitated research in the field and the Iziko Museums of Cape Town made collections available for study. Hannes van der Merwe is thanked for valuable comments and Liesl van

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