Rene ´ Bobe & Anna K. Behrensmeyer Evolution of Terrestrial Ecosystems Program, Department of Paleobiology, MRC 121, Smithsonian Institution, Washington, DC 20560-0121. E-mail: bobe.rene@nmnh.si.edu & behrensmeyer.kay@nmnh.si.edu Ralph E. Chapman Applied Morphometrics Laboratory, MRC 136, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560. E-mail: chapman.ralph@nmnh.si.edu Received 7 June 2001 Revision received 26 November 2001 Accepted 5 December 2001 and published electronically 14 February 2002 Keywords: climate change, faunal change, Pliocene mammals, environmental variability, lower Omo valley, Shungura Formation, hominid evolution, Homo. Faunal change, environmental variability and late Pliocene hominin evolution Global change during the late Pliocene was manifested in declining temperatures, increased amplitude of climate cycles, and shifts in the periodicity of orbital climate forcing. Linking these changes to the evolution of African continental faunas and to hominin evolution requires well-documented fossil evidence that can be examined through substantial periods of time. The Omo sequence of southern Ethiopia provides such a database, and we use it to analyze change in the abundances of mammal taxa at different levels of temporal and taxonomic resolution between 4 and 2 Ma. This study provides new evidence for shifts through time in the ecological dominance of suids, cercopithecids, and bovids, and for a trend from more forested to more open woodland habitats. Superimposed on these long-term trends are two episodes of faunal change, one involving a marked shift in the abundances of different taxa at about 2·8 0·1 Ma, and the second the transition at 2·5 Ma from a 200-ka interval of faunal stability to marked variability over intervals of about 100 ka. The first appearance of Homo, the earliest artefacts, and the extinction of non-robust Australopithecus in the Omo sequence coincide in time with the beginning of this period of high variability. We conclude that climate change caused significant shifts in vegetation in the Omo paleo-ecosystem and is a plausible explanation for the gradual eco- logical change from forest to open woodland between 3·4 and 2·0 Ma, the faunal shift at 2·8 0·1 Ma, and the change in the tempo of faunal variability of 2·5 Ma. Climate forcing in the late Pliocene is more clearly indicated by population shifts within the Omo mammal community than by marked turnover at the species level. Journal of Human Evolution (2002) 42, 475–497 doi:10.1006/jhev.2001.0535 Available online at http://www.idealibrary.com on Introduction Some of the most important events in early human evolution, including major taxo- nomic diversification and the advent of stone tool manufacture, occurred in the late Pliocene of Africa. Various hypotheses have linked these events, as well as a turnover pulse of speciation in mammals, to global- scale climatic and environmental shifts (Vrba, 1985, 1988, 1995, 2000; Stanley, 1992; deMenocal, 1995). However, many key aspects of African faunal evolution and its relationship to climatic change remain unknown or poorly understood. The climatic record derives primarily from marine sediments and provides a relatively continuous signal of ocean temperatures, continental aridity in Saharan Africa, and high-latitude ice accumulation (Shackleton et al., 1984; deMenocal & Bloemendal, 1995; Kennett, 1995; Shackleton, 1995; Denton, 1999; Zachos et al., 2001). The terrestrial record provides a more discon- tinuous and taphonomically altered signal that has been difficult to interpret with respect to the marine climate record. The central issue we address in this study is the role of climate change in late Pliocene African faunal evolution. For the purposes of this paper, faunal evolution refers to changes in attributes of whole faunas or 0047–2484/02/040475+23$35.00/0