The emergence of modern human behavior during the Late Middle Stone Age in the Kenya Rift Valley
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Archaeological evidence suggests modern human behavior patterns emerged during the late Middle Stone Age (MSA) and early Later Stone Age (LSA) in Africa between 50 and 100 ka. Sites of this age are scarce and their chronologies are ambiguous. We report on excavations at new archaeological sites in the central and southern Kenya Rift Valley that contain late MSA and early LSA occurrences with stratiﬁed volcanic ashes (tephra) that are being dated by the 40Ar/39Ar technique and chemically ﬁngerprinted for regional tephrostratigraphic correlation. Obsidian artifacts are being sourced to study mobility and interaction patterns.Marmonet Drift (GtJi15) is located in the Naivasha-Nakuru basin close to the main obsidian sources. Four main MSA horizons and twelve tephra are stratiﬁed in a 21 m paleosol sequence. Assemblages from the three earliest horizons contain radial cores and faceted platform ﬂakes. The youngest horizon contains retouched points and has the most distant obsidian sources.Sites on the western margin of the southern Rift are 60-90 km from the major central Rift obsidian sources. Ntuka River 4 (Norikiushin, GvJh12) contains a 2.5 m sequence with large obsidian backed ‘‘microliths’’, blades with faceted platforms and points from radial cores, stratiﬁed above and between two tephra. Ntuka River 3 (Ntumot, GvJh11) contains a 9 m sequence with two stratiﬁed tephra. Obsidian bifacial points and narrow backed microliths are stratiﬁed 1-3 m below the lowest tephra. Three major LSA horizons lie 3-5 m above this tephra. The youngest LSA is dated 29,975 bp. In the central and southern Rift the highest frequencies of non-local lithic raw materials occur in the youngest MSA and in MSA/LSA occurrences. Lithic source distance data indicate increased range size and/or intensiﬁcation of regional exchange networks. Dramatic changes in socio-territorial organization may have accompanied the MSA/LSA transition. Supported by grants from the National Science Foundation (BCS-0113565) and the L.S. B. Leakey Foundation.