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The origin of the Middle Stone Age (MSA) marks the transition from a highly persistent mode of stone toolmaking, the Acheulean, to a period of increasing technological innovation and cultural indicators associated with the evolution of Homo sapiens. We used argon-40/argon-39 and uranium-series dating to calibrate the chronology of Acheulean and early MSA artifact–rich sedimentary deposits in the Olorgesailie basin, southern Kenya rift. We determined the age of late Acheulean tool assemblages from 615,000 to 499,000 years ago, after which a large technological and faunal transition occurred, with a definitive MSA lacking Acheulean elements beginning most likely by ~320,000 years ago, but at least by 305,000 years ago. These results establish the oldest repository of MSA artifacts in eastern Africa. [ABSTRACT FROM AUTHOR]

Previous research suggests that the complex symbolic, technological, and socioeconomic behaviors that typify Homo sapiens had roots in the middle Pleistocene <200,000 years ago, but data bearing on human behavioral origins are limited.We present a series of excavated Middle Stone Age sites from the Olorgesailie basin, southern Kenya, dating from ≥295,000 to ~320,000 years ago by argon-40/argon-39 and uranium-series methods. Hominins at these sites made prepared cores and points, exploited iron-rich rocks to obtain red pigment, and procured stone tool materials from ≥25- to 50-kilometer distances. Associated fauna suggests a broad resource strategy that included large and small prey. These practices imply notable changes in how individuals and groups related to the landscape and to one another and provide documentation relevant to human social and cognitive evolution. [ABSTRACT FROM AUTHOR]

Olorgesailie, Kariandusi, and Isinya are archeological sites with Acheulean artifacts in or on the flanks of the southern Rift Valley of Kenya. 40 Ar/ 39 Ar ages on feldspar from tuffs in the Olorgesailie Formation of 992 ± 39 ka (Member 1), 974 ± 7 ka (Member 5), and 747 ± 6 ka (Member 9), 662 ± 4 ka (Member 10), and 601 ± 3 ka (Member 11) bracket most of the Acheulean sites there. A 40 Ar/ 39 age on a tuff in the Kariandusi sequence is 977 ± 10 ka, and both sites are associated with thick diatomite deposits. No age control has been available for the site of Isinya. Electron microprobe analyses of glass from volcanic ash layers from these sites allow tephrostratigraphic correlations between them. Here we report that samples from Kariandusi below the archeological levels correlate with samples in Member 4 and Member 2 at Olorgesailie (992 ka–974 ka). Samples from a volcanic ash at Isinya above the archeological site correlate with an ash layer in Olorgesailie Member 4, showing that these artifacts are >974 ka, providing the first age control for the artifacts at Isinya. The correlation from Olorgesailie to Kariandusi shows that the diatomites at those sites were deposited at the same time ∼120 km apart but now differ in elevation by ∼700 m. Chemical similarity with obsidians from localities in the Naivasha region suggest that some of the ash layers may originate from that area. [ABSTRACT FROM AUTHOR]

Abstract: The Pleistocene archeological record in East Africa has revealed unusual accumulations of Acheulean handaxes at prehistoric sites. In particular, there has been intensive debate concerning whether the artifact accumulation at the Middle Pleistocene Olorgesailie (Southern Kenya Rift) and Kariandusi (Central Kenya Rift) sites were a result of fluvial reworking or of in situ deposition by hominids. We used a two-step approach to test the hypothesis of fluvial reworking. Firstly, the behavior of handaxes in water currents was investigated in a current flume and the flow threshold required to reorientate the handaxes was determined. The results of these experiments suggested that, in relatively high energy and non-steady flow conditions, handaxes will reorientate themselves perpendicular to the current direction. Secondly, an automated image analysis routine was developed and applied to archeological plans from three Acheulean sites, two at Olorgesailie and one at Kariandusi, in order to determine the orientations of the handaxes. A Rayleigh test was then applied to the orientation data to test for a preferred orientation. The results revealed that the handaxes at the Upper Kariandusi Site and the Olorgesailie Main Site Mid Trench had a preferential orientation, suggesting reworking by a paleocurrent. The handaxes from the Olorgesailie Main Site H/6A, however, appeared to be randomly oriented and in situ deposition by the producers therefore remains a possibility. [Copyright &y& Elsevier]

Abstract: The Olorgesailie Formation (1.2–0.49Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin (Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8–9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200–16,000μScm−1 and pH of 7.5–9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution. [Copyright &y& Elsevier]

Abstract: The Olorgesailie Basin in the southern Kenya Rift Valley contains a variety of lacustrine, wetland and terrestrial facies that were laid down during the last 1.2 million years. This study integrates diatom, ichnofossil, and rhizolith (silica and carbonate) evidence in order to identify and characterize shallow wetland deposition at the top (Member 14; ca. 493 ka) of the Olorgesailie Formation. A key feature of these deposits is the presence of facies with aquatic indicators (diatoms) that are overprinted by ichnofossils and rhizoliths that suggest drier conditions. Diatom floras include several fresh water Aulacoseira spp. and saline indicators such as Thalassiosira rudolfi, T. faurii, Cyclotella meneghiniana and Anomoeoneis sphaerophora. The main indicators of shallow fresh to mildly saline waters include Epithemia argus, Rhopalodia gibberula, Encyonema muelleri, and Synedra spp. Three main suites of ichnofossils indicate varying water table depths during their formation. These consist of 1) horizontal tunnels that suggest saturated substrates; 2) vertical burrows formed in substrates with relatively lower water tables; and 3) termite, hymenopteran, and beetle ichnofossils that indicate dry substrates. Member 14 of the Olorgesailie Formation also contains carbonate and siliceous rhizoliths, which occur both together and separately. Four possible models are suggested to explain the silica and carbonate rhizolith formation: 1) a complex of spring-fed marshes and dry areas; 2) ephemeral wetlands and floodplains; 3) wetlands with abundant organic decay, and 4) initial carbonate rhizoliths that were replaced by silica. Preservation of cellular structures and the reworking and incorporation of rhizoliths in overlying younger strata suggest that they formed early during diagenesis. [Copyright &y& Elsevier]

Abstract: Norton et al. (2006) compared "handaxes" from Korea and two basins with Acheulean assemblages (Olorgesailie, Kenya and Hunsgi-Baichbal, India). The authors found significant morphological variance between Eastern and Western handaxes, leading them to conclude that East Asian tool forms were not morphologically similar to typical Acheulean implements. We test this finding using a larger array of localities, and find some metrical overlaps between handaxes and cleavers in the West and East. We indicate the role of convergence in lithic assemblage formation, but we also raise the possibility that handaxes and cleavers in the Luonan Basin (China) may represent evidence for Acheulean stone tool manufacturing methods. [Copyright &y& Elsevier]

The article discusses research on the use of tools by Middle Stone Age (MSA) humans in the Rift Valley of Africa. Topics include the dating of stone tools, the butchering of animal meat by Homo erectus at the Olorgesailie site, and the use of evidence about pigment use to show the existence of social networks.

Abstract: We welcome the reply by Trauth and Maslin (Comment on “Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley” by R.B. Owen, R. Potts, A.K. Behrensmeyer and P. Ditchfield [Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 17–37], Palaeogeography, Palaeoclimatology, Palaeoecology, 2009-this issue) to Owen et al. (2008, Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley, Palaeogeography, Palaeoclimatology, Palaeoecology, 269, 17–37) and accept the clarifications made concerning their earlier work. However, we also note some of the differences between their broad characterization of the Olorgesailie record and our more detailed approach. [Copyright &y& Elsevier]

Abstract: Owen et al. [Owen, R.B., Potts, R., Behrensmeyer, A.K., Ditchfield, P., 2008, Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley. Palaeogeography, Palaeoclimatology, Palaeoecology, 269, 17–37], Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift, Palaeogeography Palaeoclimatology Palaeoecology, 269, 17–37) argued that diatom assemblage variations in the Olorgesailie Formation indicate considerable environmental instability with both wetter and drier periods, contradicting the proposed period of lake stability and wet climatic conditions between ca. 1.1 and 0.9million years ago as proposed by Trauth et al. [Trauth, M.H., Maslin, M.A., Deino, A., Strecker, M.R., 2005. Late Cenozoic moisture history of East Africa. Science 309, 2051–2053., Trauth, M.H., Maslin, M.A., Deino, A., Bergner, A.G.N., Dühnforth, M., Strecker, M.R., 2007. High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution. Journal of Human Evolution 53, 475–486]. Contrary to the interpretation of our work by Owen et al. [Owen, R.B., Potts, R., Behrensmeyer, A.K., Ditchfield, P., 2008, we never said that the proposed periods of large lakes were characterized by stable conditions. [Copyright &y& Elsevier]