Richard Bernhart Owen

The Early Mid-Pleistocene Transition (EMPT) between 1,200–700 ka represents a major global climate transition from dominantly 41,000-year to 100,000-year glacial cycles. The forces and mechanisms behind this transition, and the response of African environments, is not well understood. The active volcanism and tectonics of the East African Rift System (EARS) add complexity to environmental systems and can erase important proxy records, inhibiting studies of lacustrine dynamics. As a result, there is minimal understanding of how this transition impacted the region’s lake systems, with implications for hominin migration. At paleolake Suguta in the northern Kenya Rift, however, flood basalts cap lacustrine EMPT-aged deposits and help preserve these strata and their valuable paleoenvironmental record. This research presents a high-resolution reconstruction of hydrological change from approximately 930 to 830 ka during the EMPT at the Suguta-Turkana Valley in the northern Kenya Rift. Paleolake dynamics are reconstructed from a 41 m sedimentary section using diatom morphology, sedimentology, and x-ray fluorescence analysis. Lake levels varied during the EMPT, particularly from ~885–830 ka, ranging from deep stratified lakes, shallow, well-mixed lakes, and complete desiccation. This record identifies hydroclimate variability at several thousand year-resolution within the Suguta-Turkana Valley during the EMPT, illuminating a period where generally little is known about terrestrial environmental change.

The East African Rift System (EARS) is a key location for studying Plio-Pleistocene paleoclimate and hominin inhabitance. The region experienced profound reorganization during this interval as a response to volcanism, tectonics and climate change, and arguably detailed spatiotemporally coherent climate datasets could provide evidence of causal links between geologic change and hominin evolution. However, continued tectonism, erosion, burial and volcanism obscures much of this information. Despite its rich fossil record, the Turkana basin in the northern Kenya Rift is no exception. It has been hypothesized that Lake Turkana and paleo-Lake Suguta to its south formed one 530-650 km long mega-lake before 221 ka ago, and was a major barrier for E-W dispersal of hominids and other terrestrial fauna. Here we present new information on basin development based on paleolandscape modeling and 87Sr/86Sr analysis on microfossils of newly discovered paleo-lake sequences in the Suguta Valley, permitting reconstruction of volcano-tectonic processes 900-700 ka ago. Contrary to previous assumptions, results suggest that two to three lakes separated by tectono-volcanic barriers formed instead of one mega-lake. These results have implications for previously formulated hypotheses about mega-lakes preventing W-E migration and exchange and suggest that during the early Middle Pleistocene E-W migrations were possible.

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.

Significance: We have developed an Africa-wide synthesis of paleoenvironmental variability over the Plio-Pleistocene. We show that there is strong evidence for orbital forcing of variability during this time that is superimposed on a longer trend of increasing environmental variability, supporting a combination of both low- and high-latitude drivers of variability. We combine these results with robust estimates of mammalian speciation and extinction rates and find that variability is not significantly correlated with these rates. These findings do not currently support a link between environmental variability and turnover and thus fail to corroborate predictions derived from the variability selection hypothesis.

Adaptations characteristic of early Homo sapiens prevailed during a shift to unstable environment and less predictable resources.

Modern burrowing organisms feed on microbial organic matter in matgrounds near hot springs on the margins of Lake Bogoria, a saline alkaline lake in the Kenya Rift Valley. The burrowers produce a low‐diversity trace assemblage similar to those produced by undermat miners during the Ediacaran–Cambrian transition. Despite obvious differences in body plans and phylogenetic affinities, these modern animals feed on microbes in similar ways to those inferred for primitive bilaterians. With increasing distance from hot‐spring vents, outflow channels and adjacent matgrounds, the diversity and depth of the traces increase and mixgrounds become dominant. This modern extreme environment gives clues for interpreting the heterogeneous early Cambrian seafloor, with: (1) the restriction of ‘pre‐agronomic revolution’ matground substrates; and (2) expansion of adjacent ‘post‐agronomic revolution’ mixground areas.

Significance: Previous research hypotheses have related hominin evolution to climate change. However, most theories lack basin-scale evidence for a link between environment and hominin evolution. This study documents continental, core-based evidence for a progressive increase in aridity since about 575 ka in the Magadi Basin, with a significant change from the Mid-Brunhes Event (∼430 ka). Intense aridification in the Magadi Basin corresponds with faunal extinctions and changes in toolkits in the nearby Olorgesailie Basin. Our data are consistent with climate variability as an important driver in hominin evolution, but also suggest that intensifying aridity may have had a significant influence on the origins of modern Homo sapiens and the onset of the Middle Stone Age.

The Middle Stone Age in Africa: The Olorgesailie basin in the southern Kenya rift valley contains sediments dating back to 1.2 million years ago, preserving a long archaeological record of human activity and environmental conditions. Three papers present the oldest East African evidence of the Middle Stone Age (MSA) and elucidate the system of technology and behavior associated with the origin of Homo sapiens . Potts et al. present evidence for the demise of Acheulean technology that preceded the MSA and describe variations in late Acheulean hominin behavior that anticipate MSA characteristics. The transition to the MSA was accompanied by turnover of large mammals and large-scale landscape change. Brooks et al. establish that ∼320,000 to 305,000 years ago, the populations in eastern Africa underwent a technological shift upon procurement of distantly sourced obsidian for toolmaking, indicating the early development of social exchange. Deino et al. provide the chronological underpinning for these discoveries. Science , this issue p. 86 , p. 90 , p. 95