PALEOENVIRONMENTAL CONTEXT FOR MODERN HUMAN ORIGINS IN EQUATORIAL AFRICA: INTRODUCING THE PLEISTOCENE WASIRIYA BEDS OF RUSINGA ISLAND (KENYA)

Western Kenya is best known for its abundant Miocene fossils, particularly those of early hominoids. However, as a result of recent fieldwork investigating the Wasiriya Beds of Rusinga Island, Kenya, we have identified a Pleistocene sedimentary archive that contains stone tools and abundant, well-preserved fossil fauna. The co-occurrence of Middle Stone Age artifacts and fossils is rare in eastern Africa, particularly in the region bounding Lake Victoria. The surface and in situ Middle Stone Age artifacts from the Wasiriya Beds are typologically similar to those found with the earliest known fossils attributed to Homo sapiens, and co-occur with a diverse fossil fauna that includes numerous ungulate taxa, abundant micromammals, and gastropods. The fossil ungulates suggest a predominantly arid grassland setting that differs substantially from the modern environment, where local climate is strongly affected by moisture availability from Lake Victoria. In particular, the presence of oryx (Oryx gazella) and Grevy’s zebra (Equus grevyi) suggest an expansion of arid grasslands at this time. The suggested arid condition is also supported by the presence of a number of specialized extinct grazers (Pelorovis antiquus, Megalotragus sp., and a small alcelaphine) that are unknown from Holocene contexts in East Africa. Artifacts and fossils occur in a complex cut-and-fill sequence of fluvial sediments that includes intercalated trachytic fallout and reworked distal tephra deposits. Electron microprobe geochemical analyses demonstrate that these tephra deposits can be used for local stratigraphic correlations among sites on Rusinga Island and suggest that these likely derive from Quaternary volcanoes in the Central Kenyan Rift Valley. Further, they provide the potential for precise radiometric dating. The combination of artifacts, a rich fossil fauna, and volcaniclastic sediments makes the Wasiriya Beds a key site for understanding the paleoenvironmental context for modern human origins in Equatorial Africa.