The appearance of Oldowan sites c. 2.5 million years ago signals one of the most important adaptive shifts in human evolution. Large mammal butchery, stone artefact manufacture and novel transport and discard behaviours led to the accumulation of the first recognized archaeological debris. Although the earliest instances of these behaviours are 2.5 million years ago, most of what we know about Oldowan palaeoecology and behaviour is derived from localities more than half a million years younger, particularly c. 1.8 million-year-old sites from Bed I Olduvai Gorge, Tanzania (Potts 1988). Sites from Kanjera South, Homa Peninsula, southwestern Kenya, yield dense concentrations of artefacts in association with the oldest (c. 2.2 million years) substantial sample of archaeological fauna known thus far from Africa. This study is the first to use a wide range of traditional and innovative techniques to investigate Oldowan hominin behaviour and site formation processes before 2 million years ago.
The sedimentary sequence at Kanjera South is approximately 12 m thick (FIGURE 1). It consists of six beds, from oldest to youngest KS-1 to KS-6 (Behrensmeyer et al. 1995; Ditchfield et al. 1999). The basal KS-1 to KS-3 sands and silts, the target layers of our excavations, were deposited by low aspect channels at the margin of a small lake or playa. They exhibit weak to moderate pedogenesis. KS-4 clays were deposited during a lake transgression while KS-5 and KS-6 sands and silts reflect a return to fluvial deposition.
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Excavations in 1996, 1997 and 2000 uncovered rich concentrations of artefacts and fossils in KS-1 to KS-3. Two large excavations (Excavations 1 and 2; 100 sq. m and 15 sq. m, respectively) and three smaller ones (Excavations 5-7, each 4 sq. m) were placed along a 70-m transect of outcrop (Plummer et al. 1999). While artefacts and fauna were recovered in each of the excavations, object density was highest in Excavations 1 and 2, suggesting that the concentrations in these sites were above average background densities. At Excavation 1, artefacts were found in association with a taxonomically diverse faunal assemblage in KS-1 and KS-2 (FIGURE 2-4). At Excavation 2, Bed KS-3 yielded a complete hippopotamus pelvis with five articulated vertebrae, a canine and two ribs in tight spatial association with five flakes, possibly representing a hippopotamus butchery site. Taxonomically diverse faunal samples with associated artefacts were found below the hippopotamus bones in KS-1 and KS-2.
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Stable carbon isotopic values from palaeosol carbonates from the archaeological layers are more strongly positive than any Miocene or Pliocene East African samples, suggesting that hominin activities at Kanjera South were carried out in an open (>75% [C.sub.4] grass) setting (Plummer et al. 1999). A palaeocommunity incorporating large tracts of secondary grassland (dry grasslands where factors such as grazing impede woody growth) is suggested by high proportions of alcelaphine antelopes and equids in the archeological fauna. In contrast, faunal and isotopic evidence suggest that the earliest archeological occurrences at Bed I Olduvai formed under much more wooded conditions (Plummer & Bishop 1994; Sikes 1994). Hominin marrow processing …