Determining the presence of secular change using geometric morphometrics: an analysis of the craniofacial morphology in South African European males of the Raymond A. Dart and Pretoria Skeletal Collections
Fu, Carissa Angela
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The use of geometric morphometrics (GM) in physical anthropology has increased markedly over the recent years. In current studies of secular change, anthropologists have more frequently turned to this technique as it provides scientists with a powerful tool for shape analysis. Secular change is defined as changes in the skeletal biology, usually seen in a population, resulting from shifts in living standards or exposure to a new environmental factor over a short timeframe (Jantz and Meadows Jantz 2000; Weisensee and Jantz 2011). Studies conducted in Europe, Asia, and the United States have shown significant signs of secular change in craniofacial morphology. This thesis will utilize GM analyses of 57 craniofacial landmarks from 313 individuals to determine secular change in the European male populations of the Raymond A. Dart and Pretoria Skeletal Collections located in Johannesburg and Pretoria, South Africa, respectively, with birth years ranging from 1850 to 1956. Craniofacial data points were collected using a 3D Microscribe digitizer, upon which the Generalized Procrustes Analysis (GPA) was used to align all landmarks into one coordinate reference plane. In order to determine the presence of shape change, a Principal Components Analysis (PCA) was run on the Procrustes coordinates of all individuals. Then, a multivariate regression of shape score on year of birth was conducted to determine the magnitude of change as explained over time. Following the multivariate regression, various Canonical Variates Analyses (CVA) were performed to determine whether secular change was occurring. In addition to collecting metric data, ultimate and proximate causes are explored to provide a more holistic understanding of the potential reasons for the changing or unchanging nature of the crania in the ancestrally European South African population. This study hypothesizes these collections will exhibit craniofacial secular change resulting from greater exposure to increased nutrition over time, greater access to healthcare, and socioeconomic and political stability. Additionally, genetic factors could be affecting the development of the crania through time. As many studies use the Dart and Pretoria collections in tandem to understand population-specific traits of modern South Africans, the presence of secular change will greatly affect the way researchers utilize samples for their studies. Forensic anthropologists study collections to create better estimations for elements of the biological profile such as stature, age, and sex. However, failure to take into account secular change would provide erroneous results. This study provides answers regarding the need to account for secular change if necessary. This research indicates that there are some changes occurring in the craniofacial morphology as see by the PCA, but the results of the CVA indicate that this is not necessarily due to secular change. The results do not clearly indicate the presence of secular change. There are many possibilities dictating potentially why. The first possibility is that there are small changes occurring in the craniofacial morphology; however, this is not caused by secular change. There are other variables, potentially genetic, that are influencing these slight changes that we see. Despite other nations with similar economic development trajectories experiencing a definite presence of secular change, the unique history and population structure of European South Africans could be contributing to the lack of secular change present. Another possible reason is the lack of passage of time from the industrialization of the nation. Furthermore, there is potentially not enough data tested to warrant a reliable conclusion that secular change is or is not occurring. With the cranium, the possibility exists that a minimum threshold of specimens is needed in order to have a reliable conclusion.