the Panama and Indonesian seaways. It will also be important to investigate how likely scenarios of moisture transport would have impacted the vegetation and water resources on which early hominins depended. This combined use of models and environmental records should then provide, for the first time, the opportunity to compare our best estimates of spatial and temporal patterns of environmental change with the fossil record of hominins.

Time Window 3: Longer and Larger Glacial/Interglacial Cycles (2-0.5 Ma)

A major shift in the tempo of climate and its extremes began about 1.2 Ma, and was in full swing by 0.9 Ma. The dominant period of glacial/interglacial cycles switched dramatically to the 100,000-year eccentricity cycle, although the 23,000 and 41,000-year components remained present. Although details of the geographic extremes of these glacial boundaries are uncertain, we know from the most recent extremes (22 ka) that ice sheets probably extended from the European Arctic southward as far as western and central Europe, with belts of tundra reaching into middle latitudes and with dramatic drops in global sea level of well over 100 meters. In the tropics, the 23,000-year precession cycle influencing tropical wet/dry cycles remained strong, but with perhaps even more pronounced episodes of aridity in some parts of the tropics. During much of this period, starting at about 0.8 Ma, ice cores provide a key additional environmental variable—we know CO2 and methane concentrations of the atmosphere through extraction and analysis of fossil air samples from the ice cores. Lowered CO2 helped amplify the cold swings and higher levels of ρCO2 occurred during the warmer interglacials. Both ice sheet changes and changes in CO2 levels could also have influenced tropical and subtropical precipitation.

What we do not understand, and would be the focus of the new research initiative proposed here, is how these climatic changes and resulting vegetation and water resource changes were transmitted to the specific regions where hominins lived. Another dispersal of Homo from Africa to Eurasia happened around the time of the climatic transition to large and long glacial/interglacial cycles (1.2-0.8 Ma) and these major climate swings must have modified the environments inhabited by Homo in Africa and Eurasia. Moreover, the changing amplitude and duration of the orbitally forced changes in climate, other quasi-periodic changes of shorter duration, or relatively gradual changes in forcing, may have had different effects on ecosystems than the abrupt changes that are also a characteristic of the climate record. With improved density, accuracy, and dating of environmental records during this period, there is a major opportunity to use climate models to ask more detailed questions and to obtain more detailed information about both the climate and vegetation comprising hominin habitats. In particular, the availability of accurate estimates of atmospheric CO2 will permit simulation of both the direct effects of greenhouse gases on climate (and vegetation) and the possible physiological effects on vegetation of changing levels of CO2. These models can only

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