ORIGINS OF THE GENUS HOMO AND EXPANSION OF C4-DOMINATED ENVIRONMENTS DURING THE LATEST PLIOCENE IN THE LOWER AWASH VALLEY, ETHIOPIA

Evidence for C₄-dominated savanna environments, which have often been linked to the origins of the genus Homo, is not documented in eastern Africa until ~2 Ma based on evidence from the Omo-Turkana Basin. The critical period for understanding the origins of Homo, the latest Pliocene (~3.0-2.6 Ma), however, has been poorly represented in the fossil and paleoenvironmental records of eastern Africa until now. Here we present enamel carbon and oxygen stable isotope data from the fossil mammalian fauna, including a specimen of early Homo, from the Ledi-Geraru Research Project area, lower Awash Valley, Ethiopia. These data indicate open and likely seasonally arid C₄-dominated savanna environments in the Ledi-Geraru Research Project area with > 70% of sampled individuals characterized as C₄-grazers. In contrast, earlier Pliocene Australopithecus-bearing sites in the lower Awash Valley are characterized by more abundant C₃ browsers and C₃-C₄ mixed-feeders. This shift to C₄-dominanted environments in the lower Awash Valley, however, is not accompanied by increased C₄ consumption by the single specimen of early Homo from the Ledi-Geraru Research Project Area compared to its predecessor, Australopithecus. Comparisons of enamel and paleosol carbon values between the lower Awash Valley and the Omo-Turkana Basin indicate that the trend towards more C₄-dominated environments occurred in the lower Awash Valley 500,000-800,000 years earlier than in the Omo-Turkana Basin. Evidence for significantly different patterns of environmental change between the lower Awash Valley and the Omo-Turkana Basin strongly suggests that the Plio-Pleistocene of eastern Africa was more complex than uniform shifts in environments coincident with global climate change, and that localized, basin-scale environmental drivers characterized the latest Pliocene during the evolution of the genus Homo.