Disruptions in ecological resources as a driver for human and mammalian evolution in the South Kenya Rift: the 1 million-year-long Olorgesailie core record

The Olorgesailie Basin in the South Kenya Rift preserves the oldest known evidence for the emergence of the Middle Stone Age (MSA) in eastern Africa between 320 and 295 ka. The MSA permanently replaced Acheulean technology in the South Kenya Rift following its persistence in the Olorgesailie Basin between 1.2 Ma and 500 ka. The transition in hominin technologies between 500 and 320 ka is significant as the MSA is typical of early Homo sapiens and is furthermore distinguished by novel behaviours like social exchange networks and symbolic communication. Contemporaneous with the emergence of the MSA was a turnover in mammal species in the South Kenya Rift – one of the largest Pleistocene turnovers recorded in East Africa – from large-bodied grazers to smaller-sized species of broader ecology. This fundamental biotic and behavioural shift suggests that considerable environmental change occurred in the region during that time. To test this hypothesis, we drilled the adjacent Koora basin and recovered a 139-m-long sediment core for evidence of past environmental dynamics in the South Rift. With 22 ⁴⁰Ar-³⁹Ar ages we established the most-precisely dated core record for the past 1 Myr from East Africa. A combination of microfossil, isotope, geochemical, and sedimentological analyses was used to reconstruct freshwater availability, vegetation cover, hydroclimate and aquatic conditions. Diatom and XRF-elemental records indicate a phase of relatively stable high lake levels from 1 Ma to 470 ka dominated by freshwater conditions. The remainder of the record shows highly fluctuating lake levels with numerous periods of desiccation and shifting fresh to saline lake waters. Similarly, vegetation reconstruction based on phytoliths and carbon isotopes shows a mixed woody grassland environment, typical of savannas until 400 ka, followed by a phase in which rapid shifts between woody and grassland cover occurred, during which grassland composition shifted between C₃ and C₄ dominance. Collectively, the lake level and vegetation records point to highly variable moisture supply and associated disruptions in ecological resources after 400 ka, which impacted both hominins and mammal communities. We propose that a combination of climate variability and enhanced tectonic activity in the rift resulted in spatial and temporal variations in freshwater availability and habitats that favoured hominins and mammals with broader ecological flexibility and resilient adaptations.