RIFT-BASIN COMPARTMENTALIZATION, FLUVIAL CONNECTIVITY AND CHANGING ENVIRONMENTS: TECTONIC AND CLIMATIC FORCING OF ENVIRONMENTAL CONDITIONS DURING HOMININ EVOLUTION IN THE EAST AFRICAN RIFT SYSTEM (EARS)

In the EARS orographic forcing of rainfall, pronounced relief contrasts between shoulder areas and the axial rift sectors results in steep environmental and surface-process gradients, severed fluvial networks, and diverse vegetation types. The EARS thus comprises important physical corridors with spatially varying resource distribution, allowing for the migration of species and the interchange of geographically isolated lineages. Due to sustained Quaternary tectono-volcanic activity these basins have been further differentiated into environments where the combined effects of these processes and hydrologic changes either keep them isolated, transiently or fully connected on timescales of several 10³ to 10⁶ years. As such, they have repeatedly provided freshwater conditions and nutrients, and may have served as gateways and migration corridors for hominins and other mammals, but also aquatic organisms, fostering population expansion, gene flow and secondary contact. Lake shorelines, deltas, overflow channels and sediments of the Holocene African Humid Period have emerged as analogs for recurrent Pleistocene episodes with high lake levels and inter-basin connectivity that repeatedly linked equatorial basins with regions to the north and south, respectively. For example, fossil evidence for the Pleistocene occurrence of the Nile crocodile (Crocodylus niloticus) as far south as equatorial Lake Bogoria and its present occurrence in the now closed Lake Baringo basin indicate fluvial connectivity over several degrees of latitude during more humid episodes in the past. In addition, the presently disjunct distribution of Bat-eared fox (Otocyon megalotis), Black-backed jackal (Canis mesomelas) and of the Oryx sister taxa (O. beisa and O. gazella) in northeastern vs. southern Africa or of various forest antelopes such as Tragelaphus euryceros in the Congo basin and central Kenya, suggests that connectivity along and across the rift has been important. The main driver for this is protracted rifting, which generates the geomorphic characteristics of these corridors, but connectivity along the rift must have varied significantly due to the effects of multiple, orbitally driven climatic conditions; this ultimately determined whether the meridionally oriented rift segments acted as gateways or barriers.