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At the northwest end of the Lake Turkana Basin (northern Kenya Rift), intensive fieldwork conducted on the Plio-Pleistocene fluvio-lacustrine Nachukui Formation by the National Museums of Kenya and the West Turkana Archaeological Project (WTAP), led to the discovery of more than 50 archaeological sites aged between 2.4 and 0.7 Ma. Among them is the Lokalalei archaeological site complex, which includes the two oldest archaeological sites (2.34 Ma) found in the Kenyan segment of the East African Rift System. The environmental background of the two sites was described as a succession of ephemeral streams with floodplain palaeosols in which the archaeological sites are situated, bordering the western bank of a large axial meandering river flowing southward. The Lokalalei 1 (LA1) and Lokalalei 2C (LA2C) archaeological sites are of extreme importance in terms of knowledge of hominins’ knapping activities. The stratigraphic position of the LA1 and LA2C sites as well as implications on the technical differences between the two sites have been successively discussed by Roche et al. (1999), Brown and Gathogo (2002), and Delagnes and Roche (2005). In terms of stratigraphic position, Lokalalei 2C was estimated to be slightly higher in the section (i.e. younger) than Lokalalei I. An alternative stratigraphic correlation was proposed by Brown and Gathogo (2002), who suggested that LA2C site should have been approximately 100,000 years younger than LA1.

New considerations on the stratigraphy and environmental context of the Lokalalei sites have been developed following controversy on the stratigraphic position and time interval between the LA1 and LA2C sites. High-resolution lithostratigraphic work based on bed-to-bed field correlations, fades sedimentology and tephra geochemistry confirms that the LA2C site is slightly higher in the section than the LA1 site by about 11.20 m. This represents a time interval of similar to 74,000 years based on an assumed sedimentation rate of 152 mm/ka. Sedimentary facies analysis indicates that the hominin occupation sites were situated in the vicinity of a lagoon/embayment environment close to the shoreline of an open lake. This lake was part of the broad lake development described across East Africa during the 2.7-2.4 Ma period. Palaeontological data associated with the sites suggest at the LA1 site a lake margin habitat, and at the LA2 sites an habitat with poor and sparse vegetation along channels of an alluvial fan system landward of the lake margin. They also confirm the aridity trend mentioned at the global scale for this period, which is demonstrated in the upper Lokalalei sediment sequence by progradation of an alluvial fan environment over the lake shoreline. Associated conglomeratic deposits could have been the source from which the knappers collected their raw materials. In addition, rapid variations from humid to arid episodes in a unique environment such as the East African Rift may have had a major influence in controlling hominin evolution. (C) 2010 Elsevier Ltd. All rights reserved.