Paleoenvironmental context of the Middle Stone Age record from Karungu, Lake Victoria Basin, Kenya, and its implications for human and faunal dispersals in East Africa

Abstract

The opening and closing of the equatorial East African forest belt during the Quaternary is thought to have influenced the biogeographic histories of early modern humans and fauna, although precise details are scarce due to a lack of archaeological and paleontological records associated with paleoenvironmental data. With this in mind, we provide a description and paleoenvironmental reconstruction of the Late Pleistocene Middle Stone Age (MSA) artifact- and fossil-bearing sediments from Karungu, located along the shores of Lake Victoria in western Kenya. Artifacts recovered from surveys and controlled excavations are typologically MSA and include points, blades, and Levallois flakes and cores, as well as obsidian flakes similar in geochemical composition to documented sources near Lake Naivasha (250 km east). A combination of sedimentological, paleontological, and stable isotopic evidence indicates a semi-arid environment characterized by seasonal precipitation and the dominance of C4 grasslands, likely associated with a substantial reduction in Lake Victoria. The well-preserved fossil assemblage indicates that these conditions are associated with the convergence of historically allopatric ungulates from north and south of the equator, in agreement with predictions from genetic observations. Analysis of the East African MSA record reveals previously unrecognized north-south variation in assemblage composition that is consistent with episodes of population fragmentation during phases of limited dispersal potential. The grassland-associated MSA assemblages from Karungu and nearby Rusinga Island are characterized by a combination of artifact types that is more typical of northern sites. This may reflect the dispersal of behavioral repertoires-and perhaps human populations-during a paleoenvironmental phase dominated by grasslands.

Reconstruction of Late Pleistocene Paleoenvironments Using Bulk Geochemistry of Paleosols from the Lake Victoria Region

“…Artifacts from excavations at Nyamita and Wakondo on Rusinga Island in particular appear associated with formerly stable land surfaces, including weakly developed paleosols (Jenkins et al, 2017;Blegen et al, in press). Artifact density and the presence of non-local raw material types from excavated areas and surface collections are generally consistent with production by highly mobile populations of foragers (Tryon et al, 2010;Faith et al, 2015;Garrett et al, 2015;Blegen et al, in press).…”

“…More recently, this research has expanded to include the deposits around the region known as Karungu ∼40 km to the south near the town of Sori (Figure 1; Beverly et al, 2015a,b;Blegen et al, 2015;Faith et al, 2015). Abundant fossils and MSA artifacts have long been reported from Karungu and Rusinga and Mfangano Islands (Pickford, 1984;Owen, unpublished manuscript), but until recently only limited geological mapping and reconstructions of the paleoclimate or paleoenvironment have been conducted.…”

“…Vegetation models (Cowling et al, 2008) and phylogeographic observations derived from savannah ungulates (Lorenzen et al, 2012) predict that more open vegetation would have facilitated dispersals of mammal species, including humans, across equatorial east Africa, and the reduction in size of Lake Victoria, which would have removed a significant geographic barrier, is likely to have further enhanced dispersal potential. Importantly, many of the mammalian taxa that co-occur in fossil assemblages from Rusinga, Mfangano, and Karungu are allopatric today and historically, forming what Faith et al (2015Faith et al ( , 2016 refer to as "non-analog supercommunities, " comprised of taxa for which Lake Victoria currently serves as a major barrier. This includes, for example, the co-occurrence of white rhinoceros (Ceratotherium simum; historically northwest of the lake), Grevy's zebra (northeast of the lake), and southern reedbuck (Redunca arundinum: south of the lake).…”

“…These sites sample an approximately 55 km-long north to south transect along the eastern margin of the modern lake: Rusinga Island (n = 2), Mfangano Island (n = 1), and Karungu (n = 4) (Figure 1; Tryon et al, 2014Tryon et al, , 2016Beverly et al, 2015a,b;Blegen et al, 2015;Faith et al, 2015;Garrett et al, 2015). These deposits are all part a sequence of late Pleistocene deposits around the northeastern margin of Lake Victoria and the stratigraphy is dominated by paleosols, freshwater tufa, fluvial deposits, and volcaniclastic deposits (tuffs) that can be geochemically correlated between outcrops (Tryon et al, 2010Van Plantinga, 2011;Beverly et al, 2015a,b;Blegen et al, 2015Blegen et al, , 2016Faith et al, 2015). The objectives of this study are to: (1) use field and micromorphological observations of paleosols and bulk geochemical proxies for paleoenvironment and paleoclimate to reconstruct a series of paleo-CZs across the northeastern margin of Lake Victoria, (2) provide additional context for fossil fauna and MSA sites, and (3) integrate observations and MAP estimates into other Late Pleistocene reconstructions of regional paleoclimate and paleoenvironment to better understand their impact on human evolution and dispersal.…”

Distal tephras of the eastern Lake Victoria basin, equatorial East Africa: correlations, chronology and a context for early modern humans

“…At 66,400 km 2 , Lake Victoria is the largest lake in Africa by surface area (Adams, 1996). The habitats surrounding this lake have undergone substantial climate-driven changes throughout the Quaternary (Nicholson, 1998;Bootsma and Hecky, 2003), likely with profound impacts on human and other animal communities (e.g., Faith et al, 2011Faith et al, , 2015Tryon et al, 2010Tryon et al, , 2012. However, until recently, an understanding of environmental variation prior to the Last Glacial Maximum has been poorly constrained, and the nature of spatial variation in environments and human behavior obscured.…”