Astronomically forced climate change in the Kenyan Rift Valley 2.7–2.55 Ma: implications for the evolution of early hominin ecosystems

Aridity, availability of drinking water and freshwater foods, and hominin and archeological sites during the Late Pliocene-Early Pleistocene in the western region of the Turkana Basin (Kenya): A review

Although the Turkana Basin is one of the driest regions of the East African Rift, its Plio-Pleistocene sediments are rich in freshwater vertebrates and invertebrates, providing evidence that freshwater resources were available to hominins in this region during the Plio-Pleistocene (4.2-0.7 Ma). Here we provide an overview of the hydroconnectivity of the Turkana Basin. We then review the period during which freshwater river and lake systems expanded into the western region of the Turkana Basin, where hominin and archeological sites have been discovered in sediments dating back to the Late Pliocene-Pleistocene. Freshwater conditions are reconstructed from river and lake sediments and the flora and micro- and macofauna they contain. Data synthesis suggests that drinking water and freshwater foods prevailed in the western region of the Turkana Basin at 4.20-3.98 Ma, 3.70-3.10 Ma, 2.53-2.22 Ma, then between 2.10 and 1.30 Ma and intermittently from 1.27 to 0.75 Ma. Milestones in hominin evolution occurred in this context, such as the first occurrence of Australopithecus anamensis (4.20-4.10 Ma) and Kenyanthropus platyops (3.50 Ma and 3.30-3.20 Ma), the presence of Paranthropus aethiopicus (2.53-2.45 Ma), early Homo (2.33 Ma), Paranthropus boisei (2.25 Ma and 1.77-1.72 Ma) and Homo ergaster/Homo erectus (1.75 Ma, 1.47-1.42 Ma and 1.10-0.90 Ma). Developments in hominin behavior also occurred during this timeframe, including the first known stone tools (3.30 Ma), the oldest Oldowan sites (2.34 Ma and 2.25 Ma) in the Turkana Basin, the earliest known evidence for the emergence of bifacial shaping in eastern Africa (1.80 Ma), and the first known Acheulean site (1.76 Ma). Our synthesis suggests that, diachronic variation in hydroconnectivity played a role on the amount of drinking water and freshwater foods available in the western region of the Turkana Basin, despite regional aridity.

Orbital controls on eastern African hydroclimate in the Pleistocene

Understanding eastern African paleoclimate is critical for contextualizing early human evolution, adaptation, and dispersal, yet Pleistocene climate of this region and its governing mechanisms remain poorly understood due to the lack of long, orbitally-resolved, terrestrial paleoclimate records. Here we present leaf wax hydrogen isotope records of rainfall from paleolake sediment cores from key time windows that resolve long-term trends, variations, and high-latitude effects on tropical African precipitation. Eastern African rainfall was dominantly controlled by variations in low-latitude summer insolation during most of the early and middle Pleistocene, with little evidence that glacial–interglacial cycles impacted rainfall until the late Pleistocene. We observe the influence of high-latitude-driven climate processes emerging from the last interglacial (Marine Isotope Stage 5) to the present, an interval when glacial–interglacial cycles were strong and insolation forcing was weak. Our results demonstrate a variable response of eastern African rainfall to low-latitude insolation forcing and high-latitude-driven climate change, likely related to the relative strengths of these forcings through time and a threshold in monsoon sensitivity. We observe little difference in mean rainfall between the early, middle, and late Pleistocene, which suggests that orbitally-driven climate variations likely played a more significant role than gradual change in the relationship between early humans and their environment.

Eastern African environmental variation and its role in the evolution and cultural change of Homo over the last 1 million years

Characterizing eastern African environmental variability on orbital timescales is crucial to evaluating the hominin evolutionary response to past climate changes. However, there is a dearth of high-resolution, well-dated records of ecosystem dynamics from eastern Africa that cover long time intervals. In the last 1 Myr, there were significant anatomical and cultural developments in Homo, including the origin of Homo sapiens. There were also major changes in global climatic boundary conditions that may have affected eastern African environments, yet potential linkages remain poorly understood. We developed carbon isotopic records from plant waxes (δ¹³C_wax) and bulk organic matter (δ¹³C_OM) from a well-dated sediment core spanning the last ∼1 Myr extracted from the Koora Basin, located south of the Olorgesailie Basin, in the southern Kenya rift. Our record characterizes the climatic and environmental context for evolutionary events and technological advances recorded in the adjacent Olorgesailie Basin, such as the transition from Acheulean to Middle Stone Age tools by 320 ka. A significant shift toward more C₄-dominated ecosystems and arid conditions occurred near the end of the mid-Pleistocene Transition, which indicates a link between equatorial eastern African and high-latitude northern hemisphere climate. Environmental variability increases throughout the mid- to late-Pleistocene, superimposed by precession-paced packets of variability modulated by eccentricity. An interval of particularly high-amplitude climate and environmental variability occurred from ∼275 ka to ∼180 ka, synchronous with evidence for the first H. sapiens fossils in eastern Africa. These results support the 'variability selection hypothesis' that increased environmental variability selected for adaptable traits, behaviors, and technology in our hominin ancestors.

The Boltysh impact structure: An early Danian impact event during recovery from the K-Pg mass extinction

Both the Chicxulub and Boltysh impact events are associated with the K-Pg boundary. While Chicxulub is firmly linked to the end-Cretaceous mass extinction, the temporal relationship of the ~24-km-diameter Boltysh impact to these events is uncertain, although it is thought to have occurred 2 to 5 ka before the mass extinction. Here, we conduct the first direct geochronological comparison of Boltysh to the K-Pg boundary. Our ⁴⁰Ar/³⁹Ar age of 65.39 ± 0.14/0.16 Ma shows that the impact occurred ~0.65 Ma after the mass extinction. At that time, the climate was recovering from the effects of the Chicxulub impact and Deccan trap flood volcanism. This age shows that Boltysh has a close temporal association with the Lower C29n hyperthermal recorded by global sediment archives and in the Boltysh crater lake sediments. The temporal coincidence raises the possibility that even a small impact event could disrupt recovery of the Earth system from catastrophic events.

Multiple radiations of spiny mice (Rodentia: Acomys) in dry open habitats of Afro-Arabia: evidence from a multi-locus phylogeny

Background

Spiny mice of the genus Acomys are distributed mainly in dry open habitats in Africa and the Middle East, and they are widely used as model taxa for various biological disciplines (e.g. ecology, physiology and evolutionary biology). Despite their importance, large distribution and abundance in local communities, the phylogeny and the species limits in the genus are poorly resolved, and this is especially true for sub-Saharan taxa. The main aims of this study are (1) to reconstruct phylogenetic relationships of Acomys based on the largest available multilocus dataset (700 genotyped individuals from 282 localities), (2) to identify the main biogeographical divides in the distribution of Acomys diversity in dry open habitats in Afro-Arabia, (3) to reconstruct the historical biogeography of the genus, and finally (4) to estimate the species richness of the genus by application of the phylogenetic species concept.

Results

The multilocus phylogeny based on four genetic markers shows presence of five major groups of Acomys called here subspinosus, spinosissimus, russatus, wilsoni and cahirinus groups. Three of these major groups (spinosissimus, wilsoni and cahirinus) are further sub-structured to phylogenetic lineages with predominantly parapatric distributions. Combination of alternative species delimitation methods suggests the existence of 26 molecular operational taxonomic units (MOTUs), potentially corresponding to separate species. The highest genetic diversity was found in Eastern Africa. The origin of the genus Acomys is dated to late Miocene (ca. 8.7 Ma), when the first split occurred between spiny mice of eastern (Somali-Masai) and south-eastern (Zambezian) savannas. Further diversification, mostly in Plio-Pleistocene, and the current distribution of Acomys were influenced by the interplay of global climatic factors (e.g., Messinian salinity crisis, intensification of Northern Hemisphere glaciation) with local geomorphology (mountain chains, aridity belts, water bodies). Combination of divergence dating, species distribution modelling and historical biogeography analysis suggests repeated “out-of-East-Africa” dispersal events into western Africa, the Mediterranean region and Arabia.

Conclusions

The genus Acomys is very suitable model for historical phylogeographic and biogeographic reconstructions of dry non-forested environments in Afro-Arabia. We provide the most thorough phylogenetic reconstruction of the genus and identify major factors that influenced its evolutionary history since the late Miocene. We also highlight the urgent need of integrative taxonomic revision of east African taxa.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1380-9) contains supplementary material, which is available to authorized users.

Aridity and hominin environments

Aridification is often considered a major driver of long-term ecological change and hominin evolution in eastern Africa during the Plio-Pleistocene; however, this hypothesis remains inadequately tested owing to difficulties in reconstructing terrestrial paleoclimate. We present a revised aridity index for quantifying water deficit (WD) in terrestrial environments using tooth enamel δ¹⁸O values, and use this approach to address paleoaridity over the past 4.4 million years in eastern Africa. We find no long-term trend in WD, consistent with other terrestrial climate indicators in the Omo-Turkana Basin, and no relationship between paleoaridity and herbivore paleodiet structure among fossil collections meeting the criteria for WD estimation. Thus, we suggest that changes in the abundance of C₄ grass and grazing herbivores in eastern Africa during the Pliocene and Pleistocene may have been decoupled from aridity. As in modern African ecosystems, other factors, such as rainfall seasonality or ecological interactions among plants and mammals, may be important for understanding the evolution of C₄ grass- and grazer-dominated biomes.

A synthesis of the theories and concepts of early human evolution

Current evidence suggests that many of the major events in hominin evolution occurred in East Africa. Hence, over the past two decades, there has been intensive work undertaken to understand African palaeoclimate and tectonics in order to put together a coherent picture of how the environment of Africa has varied over the past 10 Myr. A new consensus is emerging that suggests the unusual geology and climate of East Africa created a complex, environmentally very variable setting. This new understanding of East African climate has led to the pulsed climate variability hypothesis that suggests the long-term drying trend in East Africa was punctuated by episodes of short alternating periods of extreme humidity and aridity which may have driven hominin speciation, encephalization and dispersals out of Africa. This hypothesis is unique as it provides a conceptual framework within which other evolutionary theories can be examined: first, at macro-scale comparing phylogenetic gradualism and punctuated equilibrium; second, at a more focused level of human evolution comparing allopatric speciation, aridity hypothesis, turnover pulse hypothesis, variability selection hypothesis, Red Queen hypothesis and sympatric speciation based on sexual selection. It is proposed that each one of these mechanisms may have been acting on hominins during these short periods of climate variability, which then produce a range of different traits that led to the emergence of new species. In the case of Homo erectus (sensu lato), it is not just brain size that changes but life history (shortened inter-birth intervals, delayed development), body size and dimorphism, shoulder morphology to allow thrown projectiles, adaptation to long-distance running, ecological flexibility and social behaviour. The future of evolutionary research should be to create evidence-based meta-narratives, which encompass multiple mechanisms that select for different traits leading ultimately to speciation.

Early human speciation, brain expansion and dispersal influenced by African climate pulses

Early human evolution is characterised by pulsed speciation and dispersal events that cannot be explained fully by global or continental paleoclimate records. We propose that the collated record of ephemeral East African Rift System (EARS) lakes could be a proxy for the regional paleoclimate conditions experienced by early hominins. Here we show that the presence of these lakes is associated with low levels of dust deposition in both West African and Mediterranean records, but is not associated with long-term global cooling and aridification of East Africa. Hominin expansion and diversification seem to be associated with climate pulses characterized by the precession-forced appearance and disappearance of deep EARS lakes. The most profound period for hominin evolution occurs at about 1.9 Ma; with the highest recorded diversity of hominin species, the appearance of Homo (sensu stricto) and major dispersal events out of East Africa into Eurasia. During this period, ephemeral deep-freshwater lakes appeared along the whole length of the EARS, fundamentally changing the local environment. The relationship between the local environment and hominin brain expansion is less clear. The major step-wise expansion in brain size around 1.9 Ma when Homo appeared was coeval with the occurrence of ephemeral deep lakes. Subsequent incremental increases in brain size are associated with dry periods with few if any lakes. Plio-Pleistocene East African climate pulses as evinced by the paleo-lake records seem, therefore, fundamental to hominin speciation, encephalisation and migration.

Partial skeleton of Theropithecus brumpti (Primates, Cercopithecidae) from the Chemeron Formation of the Tugen Hills, Kenya

Here we describe a complete skull and partial skeleton of a large cercopithecoid monkey (KNM-TH 46700) discovered in the Chemeron Formation of the Tugen Hills at BPRP Site #152 (2.63 Ma). Associated with the skeleton was a mandible of an infant cercopithecoid (KNM-TH 48364), also described here. KNM-TH 46700 represents an aged adult female of Theropithecus brumpti, a successful Pliocene papionin taxon better known from the Omo Shungura Formation in Ethiopia and sites east and west of Lake Turkana, Kenya. While the morphology of male T. brumpti is well-documented, including a partial skeleton with both cranial and postcranial material, the female T. brumpti morphotype is not well-known. This skeleton represents some of the first associated evidence of cranial and postcranial female T. brumpti remains. In addition to the complete skull, postcranial material includes elements of the axial skeleton and lower limb. While aspects of the skeleton conform to those of specimens previously assigned to T. brumpti, other features on the femur and tibia appear to differ from those previously described for this species. It is unclear whether these differences represent general variation within the T. brumpti population, variation between the sexes in T. brumpti, or the incorrect assignment of previous isolated hindlimb specimens. In total, the observable morphological features of the hindlimb suggest that KNM-TH 46700 was a terrestrial quadruped similar to modern savannah baboons (Papio). From the available evidence, it is difficult to assess whether or not KNM-TH 46700 frequently engaged in the specialized squatting and shuffling behavior observed in extant geladas (Theropithecus gelada).

The environmental context of human evolutionary history in Eurasia and Africa

This review has three main aims: (1) to make specific predictions about the habitat of the hypothetical last common ancestor of the chimpanzee/bonobo-human clade; (2) to outline the major trends in environments between 8-6 Ma and the late Pleistocene; and (3) to pinpoint when, and in some cases where, human ancestors evolved to cope with the wide range of habitats they presently tolerate. Several lines of evidence indicate that arboreal environments, particularly woodlands, were important habitats for late Miocene hominids and hominins, and therefore possibly for the last common ancestor of the chimpanzee/bonobo-human clade. However, as there is no clear candidate for this last common ancestor, and because the sampling of fossils and past environments is inevitably patchy, this prediction remains a working hypothesis at best. Nonetheless, as a primate, it is expected that the last common ancestor was ecologically dependent on trees in some form. Understanding past environments is important, as palaeoenvironmental reconstructions provide the context for human morphological and behavioural evolution. Indeed, the impact of climate on the evolutionary history of our species has long been debated. Since the mid-Miocene, the Earth has been experiencing a general cooling trend accompanied by aridification, which intensified during the later Pliocene and Pleistocene. Numerous climatic fluctuations, as well as local, regional and continental geography that influenced weather patterns and vegetation, created hominin environments that were dynamic in space and time. Behavioural flexibility and cultural complexity were crucial aspects of hominin expansion into diverse environments during the Pleistocene, but the ability to exploit varied and varying habitats was established much earlier in human evolutionary history. The development of increasingly complex tool technology facilitated re-expansion into tropical forests. These environments are difficult for obligate bipeds to negotiate, but their exploitation was accomplished by archaic and/or anatomically modern humans independently in Africa and south-east Asia. Complex social behaviour and material culture also allowed modern humans to reach some of the most hostile regions of the globe, above the Arctic Circle, by the late Pleistocene. This, with colonization of the Americas and Australasia, established Homo sapiens as a truly cosmopolitan species.

High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution

The late Cenozoic climate of East Africa is punctuated by episodes of short, alternating periods of extreme wetness and aridity, superimposed on a regime of subdued moisture availability exhibiting a long-term drying trend. These periods of extreme climate variability appear to correlate with maxima in the 400-thousand-year (kyr) component of the Earth's eccentricity cycle. Prior to 2.7 Ma the wet phases appear every 400 kyrs, whereas after 2.7 Ma, the wet phases appear every 800 kyrs, with periods of precessional-forced extreme climate variability at 2.7-2.5 Ma, 1.9-1.7 Ma, and 1.1-0.9 Ma before present. The last three major lake phases occur at the times of major global climatic transitions, such as the onset of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1.0-0.7 Ma). High-latitude forcing is required to compress the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in East Africa.

Tectonics, orbital forcing, global climate change, and human evolution in Africa: introduction to the African paleoclimate special volume

The late Cenozoic climate of Africa is a critical component for understanding human evolution. African climate is controlled by major tectonic changes, global climate transitions, and local variations in orbital forcing. We introduce the special African Paleoclimate Issue of the Journal of Human Evolution by providing a background for and synthesis of the latest work relating to the environmental context for human evolution. Records presented in this special issue suggest that the regional tectonics, appearance of C(4) plants in East Africa, and late Cenozoic global cooling combined to produce a long-term drying trend in East Africa. Of particular importance is the uplift associated with the East African Rift Valley formation, which altered wind flow patterns from a more zonal to more meridinal direction. Results in this volume suggest a marked difference in the climate history of southern and eastern Africa, though both are clearly influenced by the major global climate thresholds crossed in the last 3 million years. Papers in this volume present lake, speleothem, and marine paleoclimate records showing that the East African long-term drying trend is punctuated by episodes of short, alternating periods of extreme wetness and aridity. These periods of extreme climate variability are characterized by the precession-forced appearance and disappearance of large, deep lakes in the East African Rift Valley and paralleled by low and high wind-driven dust loads reaching the adjacent ocean basins. Dating of these records show that over the last 3 million years such periods only occur at the times of major global climatic transitions, such as the intensification of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1-0.7 Ma). Authors in this volume suggest this onset occurs as high latitude forcing in both Hemispheres compresses the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in Africa. In particular, hominin species seem to differentially originate and go extinct during periods of extreme climate variability. Results presented in this volume may represent the basis of a new theory of early human evolution in Africa.