Late Quaternary palaeoenvironments and Middle-Late Stone Age habitat preferences in the Nakuru-Naivasha Basin, Kenya: phytolith-based evidence from the site of Prospect Farm
The Central Rift Valley of East Africa has proven to be a key region for our understanding the emergence and diversification of our species. Genetic, fossil, archaeological, and palaeoclimatic evidence suggest that mosaic refugia may have existed here during the late Quaternary period. Such putative refugia are posited to have been centered around highland lake basins that could have buffered human populations, plant, and faunal communities from the most severe deteriorations of climate during the last glacial period. On a sub-regional scale, East Africa’s heterogeneous topography and localised expressions of regional climate is hypothesised to have created a ‘push-pull’ system in which repeated fragmentation, isolation, and expansion of populations occurred, allowing diversity to arise. Competing Middle Stone Age (MSA) behavioural-ecological models disagree in respect to their conceptualization of the nature of past human adaptations to xeric savanna, mesic savanna, and forest/wooded habitats, as well as how MSA groups responded to various palaeodemographic pressures. Consequently, these competing models propose contrasting changes in hunter-gatherer mobility and contact/isolation between MSA groups in reaction to the same underlying palaeoenvironmetal and palaeodemographic factors. Local late Quaternary palaeoenvironmental records from archaeological sites in highland areas of East Africa are at present temporally and spatially fragmentary. As such, the relative ecological stability of the Kenyan Central Rift (KCR) and its capacity to have acted as a refugium remain poorly understood, as are the various logistic and adaptive challenges that inhabitants of different ecological zones of East Africa faced. In order to evaluate which behavioural-ecological model(s) best explains patterns in the archaeological record in these settings, this thesis reports the results of phytolith-based vegetation reconstructions and geoarchaeological investigations from renewed excavations at the open-air site of Prospect Farm, Mt. Eburru, Nakuru-Naivasha Basin, Kenya, as part of the In-Africa project (INAP). The Prospect Farm Formation preserves a stratified sequence covering the last glacial period and Holocene, in which four main archaeological phases covering the MSA to Late Stone Age (LSA) transition were previously identified. The site is one of the few which preserve this transition in East Africa, that is associated with major social and technological reorganisation between ~60-20 ka. Consequently, it provides a unique opportunity to establish the environmental context of the MSA-LSA transition in the KCR. Sedimentological, stratigraphic, and elemental analysis (by ICP-OES) of the pyroclastic, colluvial and palaeosol deposits that form the Prospect Farm Formation were conducted to elucidate site formation processes alongside new archaeological investigations of the site. Phytolith analysis (samples [n = 68]) was used to determine temporal and spatial palaeovegetation changes at the site. Phytolith-based palaeovegetation reconstructions were supported by published reference material and a new modern reference collection for the Nakuru-Naivasha Basin. For this new reference set, phytoliths were extracted from little-studied non-grass plants (mainly ligneous and herbaceous dicotyledons [n = 87]) identified as indicator species of different vegetation communities within the study area. Highly diagnostic phytoliths, variation and redundancy in morphology, phytolith production rates across plant families, and dissolution potential were recorded. Multivariate statistical analysis (unconstrained ordination) was used to compare fossil phytolith samples to modern East African soil phytolith assemblages. Phytolith analysis from the six palaeosols studied indicates limited past spatial heterogeneity in plant communities across sampling locations. Results do however demonstrate marked changes in past vegetation composition and human habitat preferences, changing from open xeric C4 grassland to closed canopy Afromontane forest, during the two earliest phases of MSA activity at the site prior to ~50 ka. Environmental variability associated with MSA-LSA ‘transitional’ assemblages was found to be nominal. Habitat reconstructions dispute the long-held view that MSA populations exclusively tracked the ecotonal boundary between montane forest and savannah. This is taken to indicate higher levels of behavioural diversity within MSA groups in this area than had previously been imagined. A new Ar40/Ar39 based chronology and tephrostratigraphy for the site is ongoing; but the environmental variation at Prospect Farm agrees with local lacustrine records and suggests that glacial-interglacial cycles had a significant effect on the basin’s vegetation history and climate. Provisional comparisons with other East African palaeoenvironmental records indicate that the Nakuru-Naivasha Basin remained relatively ecologically stable compared to adjacent lowland areas. While further archaeological analysis is necessary to more fully test competing behavioural-ecological models and the site’s role as a refugium, comparisons of shifts in local ecological conditions to changes in raw material procurement strategies at Prospect Farm indicate that environments with higher arboreal cover tend to be associated with reduced mobility. However, findings also suggest that palaeoclimatic factors are most likely to have played a secondary or mediating role in techno-cultural and mobility changes that occurred during the MSA and across the MSA-LSA transition at Prospect Farm, rather than being a primary driver of these changes.
Human Evolution, Middle Stone Age, Palaeoecology, Refugia, Refugium Network, late Quaternary Palaeoenvironments, East Africa, Phytoliths, Palaeovegetation, Vegetation dynamics
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