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56 Abstract Obsidian occurs in the eastern Rift Valley region of Ethiopia, Eritrea, Kenya, and Tanzania, and in a restricted region of West Africa, in Cameroon. Source locations and chemical compositions are best documented in Kenya and Tanzania, and significant advances are now being made in Ethiopia. Several sites have been dated by obsidian hydration, one by fission tracks and four by radiogenic argon isotope ratios. Hydration dating appears to be limited to sites younger than 120,000 years. Although the most advanced methods of hydration dating have not been applied in this region, radiometric dating demonstrates that the Middle to Later Stone Age transition in eastern Africa is substantially older than the Middle to Upper Paleolithic transition in western Eurasia. Obsidian was first used for flaked stone-tool manufacture during the Early Stone Age (Oldowan), more than 1.7 million years ago. Rare instances of long-distance transport of obsidian occurred during the Oldowan at Olduvai Gorge and during the early Acheulean in Kenya and Ethiopia. However, substantial amounts of obsidian were not transported long distances until the second half of the Middle Stone Age (MSA). The increase in long-distance transport of obsidian during the late MSA reflects the African origin and development of large-scale regional social networks, which is an important milestone in the evolution of modern human behavior. Patterns of obsidian use in Kenya and Tanzania during the Holocene may reflect adaptations to changing environments by hunter-gatherers, and exchange patterns and territorial boundaries of Neolithic pastoralists. Obsidian in Egyptian sites as early as the Predynastic era reflect long-distance contacts with Ethiopia and Eritrea. 1. Introduction The primary purpose of this paper is to review studies of African obsidian source chemical composition and hydration, fission track, and radiogenic argon methods of dating of archaeological sites. Changes in obsidian source exploitation patterns throughout the 2.6 million–year-long African archaeological record are also reviewed. Table 5.1 provides a brief summary of the chronology and terminology of African Stone Age industrial complexes. Obsidian chemical characterization and dating research in Africa has lagged far behind that of other regions, but has demonstrated great potential for addressing outstanding issues in paleoanthropology. The results of these studies have important implications for the evolution of modern human behavior during the late Quaternary in Africa, for Holocene huntergatherer ecology, and for Neolithic pastoral territorial boundaries and trade patterns. Chronometric dating of the long African archaeological record with obsidian hydration remains problematic because hydration layers on exposed artifact surfaces rarely survive more than 120,000 years (Michels et al. 1983). Temperature histories must be known in order to calculate accurate hydration ages. Late Quaternary temperatures were often lower Chapter 5 Obsidian Dating and Source Exploitation Studies in Africa Implications for the Evolution of Human Behavior Stanley H. Ambrose Obsidian Dating and Source Exploitation Studies in Africa | 57 during the late Pleistocene (Kiage and Liu 2006), so dates based on modern temperatures are substantially underestimated. However, where artifacts can be dated independently by radiometric methods, hydration may be an effective paleothermometer for the last glacial era (Ambrose 1998a). Temperaturecorrected hydration dates show that the Middle Stone Age/Later Stone Age (MSA/LSA) transition is older than 46,000 BP, which supports an East African origin for the earliest Upper Paleolithic technologies of Western Eurasia (Ambrose 1998a; Bar-Yosef 1998; McBrearty and Brooks 2000). Lithic raw material site-to-source distances are widely used to infer forager mobility, trade, and exchange and interaction patterns, but inferences are rarely explicitly evaluated with respect to null models (Brantingham 2006). Obsidian source locations are usually well-defined points on the landscape and have distinctivechemicalfingerprints.Obsidianisthusideal for tracing mobility, interaction, and exchange patterns in eastern African prehistory. In southern Africa, source locations are dispersed and chemical compositions of raw materials such as silcretes are similar over great distances (Roberts 2003), and source materials can be displaced great distances by fluvial transport (Minichillo 2006). Inferences about mobility, territoriality , and regional interaction patterns drawn from frequencies of presumed “exotic” lithic raw materials in southern African sites thus remain speculations (Ambrose 2006; Minichillo 2006). Obsidian source and artifact chemical characterization research has great potential for defining early hominid home range sizes, and changes in extent of interaction spheres and boundaries of prehistoric socialnetworksduringtheevolutionofmodernhuman behavior (Ambrose 2001, 2002; Merrick et al. 1994). This research is also a useful tool for understanding the relationship between environment and social and territorial organization among Holocene hunter-gatherers . Rates of decrease in obsidian frequencies...

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