Middle to Late Miocene

East Africa is home to the first documented members of the human family. There is much debate about the causal factors in human origins. Many hypotheses hinge on environmental change as the primary driving force behind the evolution of the physical attributes that characterize the family, and that are thought to have first been present around 6-7 million years ago. A long held view of this change entailed progressive spread of savanna environments at the expense of lowland forest. Plant fossils from the Tugen Hills in the Kenya rift valley have demonstrated that a variety of habitats were available to hominids and their immediate precursors, and they are not consistent with progressive opening of habitats during the time period just prior to the origin of Hominidae.

Climate estimates for the Tugen Hills fossil localities based on fossil leaves indicate that annual rainfall was approximately 1000-1200 mm/yr at Kabarsero (12.6 Ma), 500-700 mm/yr at Waril (~10 Ma), and 750-1000 mm/yr at Kapturo (6.8 Ma). These data are the first quantitative estimates of paleoclimate for the time period during which the human family evolved and support previous paleoenvironmental estimates, based on qualitative interpretation of plant fossils, for varied environments between 12 and 6 million years ago.