Paleovegetation changes accompanying the evolution of a riverine system at the BK paleoanthropological site (Upper Bed II, Olduvai Gorge, Tanzania)

Paleovegetation studies are essential to ecologically frame hominin evolution. The analysis of the phytolith content of 22 paleosol samples collected along the vertical sequence of the channel containing the BK (Bell's Korongo) archaeological levels, provided evidence of the paleovegetation that accompanied the evolution of the riverine system that formed this site 1.353 million years ago, when climate was particularly arid. The analysis reveals the abundance of forest indicator phytoliths (46% to 92% of the assemblages), rare grass silica short cells and sedges phytoliths (up to 15% of assemblages), and palms (up to 10% of the phytolith assemblages). The vegetation of the BK fluvial system was therefore characterized by abundant woody plants. It did not vary significantly diachronically in the period of time under scrutiny. The alluvial paleovegetation corresponds well to the fluvial dynamic that formed BK, including the presence of sterile samples recovered in areas where the water traction changed the soils frequently. This spot of dense paleovegetation together with the fluvial watercourse (in a paleolandscape likely dominated by sparse/open vegetation) could have been attractive, seasonally, for animals and hominins.