Abstract: Early hominids have long been considered to have evolved in response to a directional shift from forested to open habitats (e.g., woodland, savanna grassland, glaciated terrain). Long-term paleoenvironmental records during the span of hominid evolution, however, contradict the idea of a simple directional trend followed by open-habitat stability. Rather, evidence from deep ocean cores, paleovegetation, and paleolakes all suggest a high degree and erratic pace of environmental fluctuation. The degree of fluctuation was higher during the period of hominid evolution than any earlier time during the Cenozoic. Thus adaptation to disparity, or to the conflicting demands posed by natural selection, may provide a better explanation of hominid evolution than adaptation to a single directional trend or stable environment. An example from the Pleistocene of southern Kenya illustrates how species survival and change may have been affected by environmental variability.

Key words: Hominid evolution; Paleoecology; Savanna hypothesis; Environmental variability; East Africa; Olorgesailie; Mammalian evolution