by equatorial ocean sediment cores. During this time, the equatorial ocean temperature in the Pacific, Atlantic and Indian Oceans changed from near east-west uniformity to the present-day pattern of strong east-west gradients. In modern terms, for example, the equatorial Pacific changed from being a “permanent El Niño pattern” to a dominant La Niña pattern (warm in the west, cold in the east), a change that had major effects on tropical continents worldwide (Wara et al., 2005; Fedorov et al., 2006).

These records of major climate shifts—both trends and changes in variability, and changes in climate forcing—occur within the span of important events in hominin evolution or dispersal, including the split between 3 and 2.5 Ma that produced Homo from an Australopithecus ancestor. A major new research initiative, focused on the 4- to 2-Ma time interval, would illuminate the extent to which changes in climate and/or biotic communities influenced the origin of Homo. New paleoclimate studies and faunal/vegetation analysis would also constrain the likely dispersal routes and corridors or examine the potential for long-term contact among populations of Homo across environmental boundaries (e.g., the Sahara). And, with more focused paleoecological studies, it should be possible to finally understand whether Homo first dispersed as part of an integrated faunal community, with other individual species, or on its own.

The environmental records alone are still too sparse to draw firm conclusions about geographic patterns of climate in Africa and Asia and their variability, or about climate conditions along pathways to southern Eurasia, or temporal and spatial variability of Eurasian climates. In our vision, targeted and more resolved climate simulations during the 4- to 2-Ma period, when global sea surface temperature gradients were rapidly changing and global ice volume was rapidly increasing, will play a critical interactive role with new data collection to test the likely climate system drivers underlying the new paleoenvironmental records. These will, in turn, allow us to link models of these rapidly changing earth system processes in the late Pliocene to studies of hominin history and evolution. This combination of blending current and new environmental records with new climate model experiments represents a great opportunity.

One aspect of this opportunity to provide a much improved knowledge of the environmental context in which hominin evolution occurred is that, although we understand the potential response of climate (and environment) to individual mechanisms, we have not studied combinations of these processes. For example, climate models have proven to be quite accurate in their ability to predict middle-latitude and tropical monsoonal responses to orbital forcing, the critical factors that might have triggered the onset of high-latitude glaciation, and the cause of the dramatic shift in equatorial ocean temperatures that may have had major consequences for tropical and subtropical climates. The great challenge will be to study the combination of these processes, including multiple experiments with different greenhouse gas levels to take into account uncertainties in CO2 concentrations at this time, and uncertainties about the degree of ocean transport through

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