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Development of the African Middle Stone Age (MSA) before 300,000 years ago raises the question of how environmental change influenced the evolution of behaviors characteristic of early We used temporally well-constrained sedimentological and paleoenvironmental data to investigate environmental dynamics before and after the appearance of the early MSA in the Olorgesailie basin, Kenya. In contrast to the Acheulean archeological record in the same basin, MSA sites are associated with a markedly different faunal community, more pronounced erosion-deposition cycles, tectonic activity, and enhanced wet-dry variability. Aspects of Acheulean technology in this region imply that, as early as 615,000 years ago, greater stone material selectivity and wider resource procurement coincided with an increased pace of land-lake fluctuation, potentially anticipating the adaptability of MSA hominins.

Microbial mats, located along the margins of hot-spring pools and out¯ow channels at Lake Bogoria, Kenya, are commonly silici®ed forming friable laminated crusts. Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray. Silici®cation of the microbes involves impregnation of organic tissue by very ®ne amorphous silica particles and encrustation by small (< 2 lm) silica spheroids. Rapid silici®cation of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the ®laments, capsules and cells. Although this provides evidence of their general morphology, the biological features that are required for taxonomic identi®cations are commonly poorly preserved.The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes. Almost all the silica at the Loburu springs nucleates on microbial substrates. This af®nity of silica for functional groups on microbial surfaces contributes to the rapid silici®cation of the microbes and their preservation in modern and ancient cherts.