Tracking Ecological Change in Relation to the Emergence of Homo Near the Plio-Pleistocene Boundary

The emergence of species included in the genus Homo at ~2.4 Ma, as well as appearances of other hominin taxa between 3.0 and 1.8 Ma have often been attributed to global climate change (deMenocal, 1995; deMenocal and Bloemendal, 1995; Vrba, 1995), but the precise details of these climatic transitions, including causal factors, are still debated. In addition, the Plio-Pleistocene boundary (~1.8 Ma; Pasini and Colalongo, 1997) is an important benchmark in human evolution as many hominin species (e.g., Homo erec-tus/ergaster, Paranthropus robustus) have fi rst appearance data (FADs) close to this date, while other taxa (e.g., Homo rudolfensis, Paranthropus aethiopicus) appear to have last appearance data (LADs) prior to this time. There are also some species that are found on both sides of the boundary (e.g., Paranthropus boisei, Homo habilis) (Kimbel, 1995; Wood and Richmond, 2000; Cameron, 2003; Spoor et al., 2007). Perhaps the most frequently cited relationship between climate and faunal change is Vrba's turnover pulse hypothesis (Vrba, 1995; Potts, 1998a; Behrensmeyer, 2006) which refers to an intensity of turnover (speciation and extinction events) during a brief period of time as a result of environmental change. More specifi cally, synchronous change in multiple groups was said to have occurred at 2.5 Ma due to a shift from a moist, warm habitat to drier, cooler and more open conditions. More recently, this hypothesis has been countered by one positing a prolonged and gradual period of turnover between 2.5 and 1.8 Ma (Behrensmeyer et al., 1997), although this study is specifi c to the Turkana Basin. Examinations of individual site patterns in East Africa have shown that turnovers in some mammalian lineages occur during the 3.0–2.0 Ma period, but that the exact timing appears to fl uctuate depending upon site and type of analysis (Bobe and Eck, 2001; Alemseged, 2003). It has become apparent that the picture of hominin evolution is thus contex-tually diverse. Strategic analyses in different basins across both time and space are necessary to further our understanding of evolution during this time period (Behrensmeyer, 2006).