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Understanding Climate’s Influence on Human Evolution
ity of these sapropel layers (Figure 2.5) reflect Earth’s eccentricity and precession with remarkable fidelity throughout the late Neogene (Hilgen, 1991), and this periodicity, after correlation with rapidly-deposited beds containing hominin fossils, has been used as the basis for accurately dating the fossils (McDougall et al., 2008). Precession-paced changes in ocean salinity estimated from isotopic analyses of sediment cores near the mouths of major North African rivers (e.g., Nile, Niger, and Congo) have also been used to document the timing and duration of these African humid periods (Kroon et al., 1998; Rohling, 1999; Weijers et al., 2007; Weldeab et al., 2007).
FIGURE 2.5 Left panel shows an Ocean Drilling Program core from Site 967 in the eastern Mediterranean Sea, dramatically illustrating cyclicity between sapropel layers (dark) interspersed with carbonate ooze (light). The right panel shows, on an outcrop scale, approximately 50 sapropel layers of late Miocene age (~9 Ma) from the Gibliscemi section in Sicily. SOURCE: Core photograph from Emeis, Robertson, and Richter et al. (1996). Outcrop photograph courtesy Dr. Frits Hilgen, University of Utrecht, Netherlands.
severe, persistent drought alternating with floods (Talbot and Delibrias, 1980; Street-Perrott and Perrott, 1990; Gasse, 2000).
Human population effects resulting from climate events on these timescales are well documented from the Holocene (e.g., Kropelin et al., 2008). Millennial-scale events have been little explored for older parts of the records, although a long core from Lake Tanganyika (Tierney et al., 2008) and a drill core from northern Lake Malawi (Brown et al., 2007) both provide evidence for abrupt and dramatic climatic shifts on this timescale during MIS-3.
PALEOBIOLOGICAL CONTEXT OF HUMAN EVOLUTION
An important contribution to understanding how human evolution might have been affected by climate and/or habitat change is provided by the other biota that existed both with hominins and apart from them. As climate altered vegetation habitats (determined by rainfall, evapotranspiration, soils, and other aspects of the earth system), this habitat modification applied selective pressures