Although climate change is considered to have been a large-scale driver of African human evolution, landscape-scale shifts in ecological resources that may have shaped novel hominin adaptations are rarely investigated. We use well-dated, high-resolution, drill-core datasets to understand ecological dynamics associated with a major adaptive transition in the archeological record ~24 km from the coring site. Outcrops preserve evidence of the replacement of Acheulean by Middle Stone Age (MSA) technological, cognitive, and social innovations between 500 and 300 thousand years (ka) ago, contemporaneous with large-scale taxonomic and adaptive turnover in mammal herbivores. Beginning ~400 ka ago, tectonic, hydrological, and ecological changes combined to disrupt a relatively stable resource base, prompting fluctuations of increasing magnitude in freshwater availability, grassland communities, and woody plant cover. Interaction of these factors offers a resource-oriented hypothesis for the evolutionary success of MSA adaptations, which likely contributed to the ecological flexibility typical of Homo sapiens foragers.

Asymmetric rifting, which is characterized by a dominant single border fault, is known to have played a major role in the evolution of many past rift systems (e.g., Schlische et al., 2003;Withjack et al., 2013). It can also be observed in many presently active extensional tectonic settings (e.g., Gawthorpe & Leeder, 2008;Ebinger & Scholz, 2011), where it governs basin geometry, topography evolution, erosion, and sedimentation patterns. One such setting is the largest continental rift system in existence today: the East African Rift System (EARS). It exhibits a wide array of developmental stages from youthful extension with incipient faulting to final continental breakup (Corti, 2009;Ebinger & Scholz, 2011;Ring, 2014) and is thus an ideal location for studying the stages of early rifting that involve asymmetric normal fault activity followed by hanging-wall segmentation. In this study, we focus on the southern and central sectors of the Kenya Rift, which are in an early phase of active continental rifting (Ebinger et al., 2017) characterized by the transition from waning border fault activity to enhanced intra-basinal faulting and subsidence (Muirhead et al., 2016).The first-order tectonic characteristics of continental rifts are known to be influenced by a large range of structural, petrological, and thermal parameters, which have been investigated in previous numerical modeling studies (e.g.,

The structural evolution of calderas in rifts helps to characterize the spatiotemporal relationships between magmatism, long wavelength crustal deformation and the formation of tectonic deformation zones along the rift axis. We document the structural characteristics of the c. 36 ka old Menengai Caldera located within a young zone of extension in the central Kenya Rift. Field mapping and high-resolution digital surface models show that NNE-striking Holocene normal faults perpendicular to the regional ESE–WNW extension direction dominate the interior sectors of the rift. Inside the caldera, these structures are overprinted by post-collapse doming and faulting of the magmatic centre, resulting in obliquely slipping normal faults bounding a resurgence horst. Radiocarbon dating of faulted units as young as 5 ka cal BP and the palaeo-shorelines of a lake formed during the African Humid Period in the Nakuru Basin indicate that volcanism and fault activity inside and in the vicinity of Menengai must have been sustained during the Holocene. Our analysis confirms that the caldera is located at the centre of an extending rift segment and suggests that other magmatic centres and young zones of faulting along the volcano-tectonic axis of the Kenya Rift may constitute nucleation points of faulting that ultimately foster future continental break-up. Supplementary material: 40 Ar/ 39 Ar radiometric dating data table is available at: https://doi.org/10.6084/m9.figshare.c.4647818

East Africa is a global biodiversity hotspot and exhibits distinct longitudinal diversity gradients from west to east in freshwater fishes and forest mammals. The assembly of this exceptional biodiversity and the drivers behind diversity gradients remain poorly understood, with diversification often studied at local scales and less attention paid to biotic exchange between Afrotropical regions. Here, we reconstruct a river system that existed for several millennia along the now semiarid Kenya Rift Valley during the humid early Holocene and show how this river system influenced postglacial dispersal of fishes and mammals due to its dual role as a dispersal corridor and barrier. Using geomorphological, geochronological, isotopic, and fossil analyses and a synthesis of radiocarbon dates, we find that the overflow of Kenyan rift lakes between 12 and 8 ka before present formed a bidirectional river system consisting of a “Northern River” connected to the Nile Basin and a “Southern River,” a closed basin. The drainage divide between these rivers represented the only viable terrestrial dispersal corridor across the rift. The degree and duration of past hydrological connectivity between adjacent river basins determined spatial diversity gradients for East African fishes. Our reconstruction explains the isolated distribution of Nilotic fish species in modern Kenyan rift lakes, Guineo-Congolian mammal species in forests east of the Kenya Rift, and recent incipient vertebrate speciation and local endemism in this region. Climate-driven rearrangements of drainage networks unrelated to tectonic activity contributed significantly to the assembly of species diversity and modern faunas in the East African biodiversity hotspot.

Magmatic continental rifts often constitute nascent plate boundaries, yet long‐term extension rates and transient rate changes associated with these early stages of continental breakup remain difficult to determine. Here, we derive a time‐averaged minimum extension rate for the inner graben of the Northern Kenya Rift (NKR) of the East African Rift System for the last 0.5 m.y. We use the TanDEM‐X science digital elevation model to evaluate fault‐scarp geometries and determine fault throws across the volcano‐tectonic axis of the inner graben of the NKR. Along rift‐perpendicular profiles, amounts of cumulative extension are determined, and by integrating four new 40Ar/39Ar radiometric dates for the Silali volcano into the existing geochronology of the faulted volcanic units, time‐averaged extension rates are calculated. This study reveals that in the inner graben of the NKR, the long‐term extension rate based on mid‐Pleistocene to recent brittle deformation has minimum values of 1.0–1.6 mm yr−1, locally with values up to 2.0 mm yr−1. A comparison with the decadal, geodetically determined extension rate reveals that at least 65% of the extension must be accommodated within a narrow, 20‐km‐wide zone of the inner rift. In light of virtually inactive border faults of the NKR, we show that extension is focused in the region of the active volcano‐tectonic axis in the inner graben, thus highlighting the maturing of continental rifting in the NKR.