PALEOENVIRONMENTAL CONTEXTS OF HOMININ EVOLUTION: RECONCILING AFRICAN WET-DRY CYCLES AND SECULAR VEGETATION TRENDS OVER THE NEOGENE

Since the latest Miocene divergence between ancestral chimp and human lineages, subtropical Africa experienced large changes in regional hydroclimate and vegetation. We compile new and published plant wax biomarker stable isotopic (dD and d13C) data from terrestrial and marine sediment archives to explore linkages between African hydroclimate and vegetation change over the late Neogene. Where records are sufficiently resolved from NW and East Africa, there is clear evidence for precessionally-paced wet-dry cycles extending at least from the latest Miocene. Although there remain large sampling gaps, there is little evidence for changes in monsoonal precipitation (dD amplitude) responses to orbital forcing, after correcting for ice volume and plant functional type. As suggested by the continuous Mediterranean sapropel record, the paleoclimate history of NW and East Africa has been dominated by eccentricity modulation of precessional monsoon wet-dry cycles, with modest high-latitude influences. In contrast, plant wax d13C records document a clear, synchronous onset of C4 grassland expansion for NW and East Africa at 10 Ma with modern d13C values only attained by the latest Pliocene. This decoupling between hydroclimate and vegetation change is also evident at the orbital scale. Collectively, these results suggest that the expansion of African C4 grasslands - a critical factor influencing Neogene African mammalian evolution - had a separate primary causal mechanism.