Until about 1.8 Ma ago, when early Homo erectus and perhaps other archaic hominins strode into Eurasia, our forerunners lived and evolved on only one continent – Africa. The physical and environmental conditions underlying the steps from a common ancestor with modern chimpanzees through a growing number of upright species are not well charted by the Pliocene and early Pleistocene terrestrial evidence. All that is know of this formative period is that global climate cooled in an oscillating fashion that culminated in the onset of Northern Hemisphere glaciations in the late Pliocene (~3 Ma) and a shift to drier conditions in East Africa around 2.8 Ma suggested by pollen records off the east coast. Marine sediments of the Indian Ocean, Red Sea and Gulf of Aden still offer the most convenient means of charting environmental change in detail for this crucial episode in human history. As well as oxygen-isotope and pollen-type variations, modern core analysis offers a growing number of wind-blown proxies for onshore vegetation. These include organic geochemistry plus carbon and hydrogen isotopes from trace amounts of leaf waxes. During the May to September East African Monsoon, high speed winds in the upper atmosphere drag dusty continental air from the East African Rift System over the Gulf of Aden, making sea-floor sediment an important target for tracking variations in the proxies (Liddy, H.M. et al. 2016. Cooling and drying in northeast Africa across the Pliocene. Earth and Planetary Science Letters, v. 449, p. 430-438. doi:10.1016/j.epsl.2016.05.005). Hannah Liddy and colleagues from the Universities of Southern California and Arizona, USA, applied these techniques to a Gulf of Aden core from offshore Somaliland to open a window on this crucial period.
Early Pliocene East Africa (5.3 to 4.3 Ma), the time of Ardepithecus ramidus, was characterized by evidence for a climate wet enough to sustain grasses and riverine woodlands. Yet around 4.3 Ma conditions had shifted to ecosystems more dominated by shrubby plants able to thrive in more arid conditions. At about that time the earliest australopithecines appear in the fossil record, with A. anamensis. Yet the later Pliocene was not devoid of grasses or herbivores. There is ample carbon-isotope evidence from the teeth of hominins that shows that after 3.4 Ma the diet of A. afarensis and A. africanus included increasing amounts whose carbon derived from grasses, when. This apparent paradox can be explained by a major turn to eating meat from herbivores as vegetable foods declined with increasing aridity. This is all very interesting, especially the detailed record of δ13C in plant waxes, but there is little to indicate that steps in hominin speciation or extinction had much direct connection with fluctuations in climatic conditions. Environmental change may have formed a background to other influences that may have been wholly down to early hominin’s social and technological behaviour.