Researchers from the United States, their graduate students, and colleagues from the National Museums of Kenya are conducting archaeological and geological research on the islands of Rusinga and Mfangano, near the Kenyan shores of Lake Victoria. Genetic, fossil, and archaeological evidence suggest that the origins of living humans can be traced to early populations of Homo sapiens from the Late Pleistocene of eastern Africa, roughly 130,000-40,000 years ago. Research on the sites of Rusinga and Mfangano provides an important opportunity to examine the biological and behavioral diversity among these eastern Africa source populations, and the social and environmental contexts that may have contributed to this diversity and ultimately, the evolutionary success of our species. Located in equatorial eastern Africa, Lake Victoria is the largest lake on the continent and today supports diverse ecosystems and dense human populations. Past lake size has periodically dramatically fluctuated between completely drying up and nearly doubling in size, and these events have likely had major impacts on the composition, size, and distribution of surrounding environments and human populations. Using archaeological survey and excavation and a variety of field geological and laboratory-based geochemical and geophysical methods, past human diversity is investigated through the study of Middle Stone Age artifacts, site age is determined by correlating and dating associated volcanic ash deposits, and ancient environments are reconstructed through the analysis of sediments and fossils. Some of these fossils include extinct species, as well as extant taxa, that are indicative of arid grassland settings, substantially differing from today's regional environment. Integration of these data with detailed studies of neighboring lake basins contributes to our understanding of past and present patterns of climate change, particularly the timing and periodicity of pan-African intervals of increased aridity.
This study provides an improved understanding of Late Pleistocene paleoenvironments of eastern Africa, which has clear implications for attempts to understand and moderate the effects of global climate change today. This two-year project is providing field and laboratory training, thesis research opportunities, and professional experience for US undergraduate and graduate students. Additionally, the project is fostering collaborations between US universities and contributes to the National Museums of Kenya and other local cultural heritage museums on Rusinga and Mfangano.
Fossil and genetic evidence indicate that modern humans (Homo sapiens) originated in eastern Africa ~150-200 thousand years ago (ka) and by ~50-100 ka early human populations began to intermittently disperse within and out of Africa. This period of time during the Late Pleistocene also represents a key interval for understanding the origins of modern human behavior and diversity. It has often been argued that these significant milestones in human behavioral evolution are linked to changes in climate. However, there are relatively few records of environmental change during the Late Pleistocene making it difficult to determine the influence of environmental change on human evolution and dispersal. Our project aimed to address this issue by reconstructing the ancient environment of the Lake Victoria region in equatorial Africa through the examination of Late Pleistocene fossil- and artifact-bearing deposits on Rusinga Island, Mfangano Island, and mainland Kenya near Karungu. Our project used an interdisciplinary, multi-proxy approach to reconstruct the environment using methods in archeology, paleontology, geology, paleopedology, and isotope geochemistry. Geologic evidence from the sediments on Rusinga, Mfangano, and Karungu indicate that all of the deposits are roughly contemporaneous and were deposited between 100 and 30 ka. Detailed analyses of the sediments and fossil soils indicate the climate was dry and seasonal and the landscape was relatively short-lived and unstable with ephemeral streams and springs present. We recovered Middle Stone Age (MSA) artifacts, which are considered a direct indicator of Homo sapiens, at all of the sites and evidence of cut-marked bones at sites on Rusinga. The artifact style from all sites is most similar to tool technology common in open habitat sites in the Rift Valley. The tools are made of a variety of raw material types, including obsidian, of which the closest source is >200 km away in the Rift Valley. The archeological evidence is consistent with the western movement of human populations or human behavioral adaptations during to the Lake Victoria region during the Late Pleistocene. The fossil mammal assemblages are dominated by extinct taxa and mammals adapted for dry, open environments. The fauna from Rusinga, Mfangano, and Karungu are most similar to open grassland communities in equatorial Africa. Species occurrences in the fauna indicate major range expansions of open adapted taxa from across Africa into the equatorial Lake Victoria region. For example, Grevy’s zebra and oryx, which occur at all sites, are far west and south of their current home range indicating a dramatic home range expansion. The fauna suggests a westward expansion of grassland communities into the Lake Victoria region and that the Lake Victoria region was an area of faunal mixing and interchange during the Late Pleistocene. Isotopic analyses of the sediments on Rusinga and Mfangano Islands indicate that the local environment was likely a riparian woodland. Analyses of the fossil teeth indicate that the mammals were predominately eating a diet of C4 grasses and that the climate was relatively dry. Taken together, the isotopic analyses indicate that the deposits on Rusinga, Mfangano, and by extension Karungu, sample fairly closed and wet stream- and spring-side habitats within a large, open, dry, C4-dominated grassland ecosystem. Our paleoclimate reconstructions suggest that the sediments on Rusinga, Mfangano, and Karungu were deposited during an interval when Lake Victoria was much smaller than at present, and perhaps dried up completely. The absence of the lake would have removed a major barrier to large herbivores, and may explain the dispersal of some of the taxa seen at Rusinga, Mfangano, and Karungu, as well as other examples of arid-adapted taxa found further to the wast in central Africa. The absence of Lake Victoria and the spread of grasslands may have also facilitated the dispersal of early modern humans from eastern to central Africa. Our work demonstrates that during the Late Pleistocene the Lake Victoria region was considerably more arid than at present and was dominated by a large, widespread grassland ecosystem that may have allowed the dispersal of humans and animals from eastern and central Africa. These results provide a key datum point to help understand the influence of environment on human behavioral evolution and dispersal during this time period. This project supported the research and training of several graduate and undergraduate students. The results of this work have been presented by students and the PIs at conferences and disseminated in peer-reviewed journal articles. Researchers on this project collaborated with researchers from the National Museums of Kenya and worked with members of the Abaduba Community Peace Museum on Mfangano Island. Additionally, three exhibits on the significance of the geology, archeology, and paleontology of Mfangano Island that incorporate our research findings are on display at the Abasuba Museum.
