Seasonal-to-interannual hydroclimatic variability in Afromontane environments during the late Early Pleistocene hominin occupations on the Southeastern Ethiopian Highlands.

Climate variability is a key factor influencing the distribution and adaptive response of mammalian species, including hominins. Here, we investigate seasonal and interannual hydroclimatic and environmental variability associated with the late Early Pleistocene hominin occupations on the Southeastern Ethiopian Highlands. We present stable carbon and oxygen isotope analyses of sequentially sampled tooth enamel from herbivores recovered from the archaeological contexts of Melka Wakena and Gadeb (2300-2400 m a.s.l.) and dated between 1.6 and ~ 1.0 Ma. Carbon isotope data indicate predominantly C4 and mixed C3-C4 diets, consistent with mosaic vegetation as part of the Dry evergreen Afromontane Forest and grassland complex (DAF). Intra-tooth oxygen isotope profiles suggest seasonal variability and interannual fluctuations in water balance, reflecting changes in precipitation, evaporation, and local hydrology. Although the temporal resolution of individual tooth records precludes identification of specific climatic drivers, the isotopic data document recurring hydroclimatic variability within high-elevation environments characterized by broadly comparable ecological conditions. Herbivore diets show limited long-term change, suggesting sustained resource availability across seasons and over hundreds of thousands of years. These results indicate that the Ethiopian Highlands repeatedly supported mammalian communities and recurrent Acheulean hominin occupations during the late Early Pleistocene, providing sustained access to water and vegetation resources within an Afromontane environmental framework.