Early Hominin Evolution Research Articles

Spatial sampling bias influences our understanding of early hominin evolution in eastern Africa.

The eastern branch of the Eastern African Rift System (EARS) is the source of a large proportion of the early hominin fossil record, but it covers a tiny fraction (ca. 1%) of the continent. Here we investigate how this mismatch between where fossils are preserved and where hominins probably lived may influence our ability to understand early hominin evolution, using extant mammals as analogues. We show that the eastern branch of the EARS is not an environmentally representative sample of the full species range for nearly all extant rift-dwelling mammals. Likewise, when we investigate published morphometric datasets for extant cercopithecine primates, evidence from the eastern branch alone fails to capture major portions of continental-scale cercopithecine cranial morphospace. We suggest that extant rift-dwelling species should be used as analogues to place confidence intervals on hominin habitat reconstructions. Furthermore, given the north-south orientation of the eastern branch of the EARS, morphoclines that are not aligned along this major north-south axis are likely to be poorly sampled by sites in the eastern branch. There is a pressing need for research on the geography of early hominin morphoclines to estimate how morphologically representative the hominin fossil sample from the eastern branch may be.

Hominin musical sound production: palaeoecological contexts and self domestication

In this article we seek to integrate theories of music origins and dance with hominin fossil anatomy and the paleoecological contexts of hominin evolution. Based on the association between rhythm in music, dance and locomotion, we propose that early bipedal hominins may have evolved neurobiological substrates different from other great apes due to the rhythmic aspects of bipedal walking and running. Combined with the emancipation of the hands resulting from erect posture, we propose that the neurobiological changes necessary for technological innovation, cultural practices and human musical abilities may have evolved, at least in incipient form, much earlier than previously thought. The consequent ability to synchronize movement and sound production may have also proved beneficial as early bipedal hominins ventured out of late Miocene and early Pliocene woodland and forested habitats and into more open habitats with increased predation risk. We also postulate that, along with bipedalism, paedomorphic morphogenesis of the skull at the base of the hominin clade was a necessary prerequisite for the evolution of vocal modulation and singing in later varieties of hominin. To date research into the evolution of music and dance has yet to be integrated with the fossil and paleoecological evidence of early hominin evolution. This paper seeks to fill this lacuna in the extant literature on human evolution. We also suggest that autocatalytic feedback loops evolving synergistically with hominin erect posture, skull and hand morphology, neurochemical processes and the self-domestication syndrome, have been operative from early hominins some 6 Ma to the present. We document this process by reference to primatological, ethnographic, neurochemical and archaeological data.

A case of intercommunity lethal aggression by chimpanzees in an open and dry landscape, Issa Valley, western Tanzania

Intercommunity (lethal) aggression is a familiar component of the behavioural repertoire of many forest-dwelling chimpanzee (Pan troglodytes) communities. However, until now, the absence of intercommunity attacks – including killings – in communities that live in open, mosaic environments has supported hypotheses of reduced resource competition in drier habitats, and informed referential models of early hominin social dynamics in a similar habitat. In June 2020, we observed the first instance of intercommunity lethal aggression, a male-committed infanticide, by the Issa chimpanzee community, which live in a savannah-mosaic habitat in the Issa Valley, western Tanzania. The carcass was recovered by researchers after it was abandoned by the attackers. Here, we give a detailed account of the events leading up to and including the infanticide, and contextualise our observations with what has been described for other chimpanzee communities. Notably, in contrast to the majority of reported intercommunity infanticides, the infant male victim was castrated (and not cannibalised), making this the youngest reported castration. This observation of intercommunity aggression disproves its hypothesised absence in savannah-dwelling chimpanzees, which by extension, has implications for early hominin evolution. We suggest that the near absence of observations of intercommunity aggression in savannah chimpanzee communities is most likely due to the lack of long-term study communities, and in some cases geographic isolation. We hypothesise that food-rich areas within a habitat with otherwise widely distributed food sources may select for intense intercommunity aggression despite the low population density characteristic of savannah communities. Anecdotes such as this add to the comparative database available on intercommunity killings in chimpanzee society, improving our ability to draw inferences about their evolutionary significance.

40 Ar/ 39 Ar eruption ages of Turkana Basin tuffs: millennial-scale resolution constrains palaeoclimate proxy tuning models and hominin fossil ages

The Turkana Basin in NW Kenya and SW Ethiopia hosts remarkable fossil-rich sediments that are central to our understanding of early hominin evolution, with interbedded volcanic tuffs providing critical time markers. However, the resolution of existing Early Pleistocene–Pliocene ages is limited to c. 20–60 kyr, inhibiting the evaluation of climatic and environmental drivers of evolution. We present high-precision, single-feldspar 40 Ar/ 39 Ar age and elemental data for four stratigraphically significant tuffs. These samples exhibit variably dispersed age distributions correlated with feldspar compositional trends, interpreted to indicate the partial retention of inherited 40 Ar related to crustal ‘cold storage’ and rapid melt infiltration preceding eruption. We evaluated various statistical methods and calculated astronomically calibrated Bayesian age estimates of 1879.1 ± 0.6 ka (±2.4 ka including external errors) for the Kay Behrensmeyer Site (KBS)/H2 Tuff, 1837.4 ± 0.9 ka (±2.4 ka) for the Malbe/H4 Tuff, 1357.5 ± 1.8 ka (±2.5 ka) for the Chari/L Tuff and 1315.4 ± 1.9 ka (±2.5 ka) for the Gele Tuff. Our results permit refined age constraints for important early Homo fossils, including the cranium KNM-ER1813 ( Homo habilis ) and various Homo erectus fossils. The KBS Tuff age also provides an important calibration locus for orbital tuning of palaeoclimate proxy records, revealing the complex interplays between palaeoclimate and geological drivers of sedimentation. Supplementary material : The supplementary tables and figures include previously published ages (Table S1), sample and irradiation information (Table S2), electron probe microanalytical results for tuff glasses (Table S3, Figs S2 and S3) and feldspars (Table S4), 40 Ar/ 39 Ar analytical data (Tables S5 and S6, Fig. S4), orbital tuning model parameters (Table S7), a summary of previous and revised hominin ages (Table S8) and a worked Bayesian estimation example (Fig. S1) and are available at https://doi.org/10.6084/m9.figshare.c.6602994

Aridification and orbital forcing of eastern African climate during the Plio-Pleistocene

We developed a composite outcrop proxy record of Plio-Pleistocene climate in eastern Africa using the radiometrically-dated sediments of the Omo Group that preserve evidence for early hominin evolution. New stable oxygen isotopic measurements from pedogenic carbonate nodules (δ18OPC, n = 270) are added to the existing database for a total of 759 observations spanning a ~ 3.6 Myr interval from 4.33–0.65 Ma. Linear interpolation of sample positions between tuffaceous marker horizons produced the chronostratigraphy of the δ18OPC record that was correlated to Indo-Pacific marine core sediment records and astronomical solutions of orbital climate forcing. Rainfall/evaporation changes inferred from the δ18OPC record carry orbital eccentricity signals, and the amplitude modulation of short eccentricity (~100 kyr) predicted by the astronomical solutions is evident during some intervals. δ18OPC variations recover many of the ~100 kyr cycles between ~4.3–0.7 Ma, with chronostratigraphic offsets between the Omo Group time series and astronomical solutions usually within the resolution of the radiometric uncertainties, i.e., ≤ 50 kyr; however, missing time at ~1.2 Ma contributes to nearly 150 kyr of offset. These δ18OPC data also indicate a long-term drying trend beginning at 2.11 Ma that correlates to a thicker eastern Indian Ocean mixed layer and upwelling in the eastern Pacific, implying a simultaneous response across the tropics to strengthening Walker Circulation. Published δ13CPC values from the same pedogenic carbonate nodules indicate that C4 vegetation abundances increased beginning at ~2.0 Ma, also suggesting climatic drying. Spectral analysis of the δ13CPC time series provides weaker evidence of orbital forcing as compared to the δ18OPC time series, particularly for long orbital eccentricity (405 kyr) and climatic precession (~20 kyr), suggesting other factors, such as feeding ecology of herbivores or global greenhouse gas, may have contributed to the vegetation patterns. The climatic precession component to the δ18OPC time series increases amplitude through strengthening Walker Circulation, which may signal a change in the intensity of interannual climate events. No indications were observed to suggest that the Plio-Pleistocene Northern Hemisphere glaciation resonated as either climatic cyclicity or an abrupt, stepwise shift in the regional hydroclimate. At major junctures of hominin evolution, including the origins of different genera and the earliest stone tools, environmental variability derived from the orbital forcing of the monsoon appears to have been low. However, we propose that at 2.11–1.66 Ma, directional aridity coupled with interannual climate events presented a new adaptive context where Homo erectus and Acheulian stone tools first appeared in eastern Africa.

Energetic and endurance constraints on great ape quadrupedalism and the benefits of hominin bipedalism.

Bipedal walking was one of the first key behavioral traits that defined the evolution of early hominins. While it is not possible to identify specific selection pressures underlying bipedal evolution, we can better understand how the adoption of bipedalism may have benefited our hominin ancestors. Here, we focus on how bipedalism relaxes constraints on nonhuman primate quadrupedal limb mechanics, providing key advantages during hominin evolution. Nonhuman primate quadrupedal kinematics, especially in our closest living relatives, the great apes, are dominated by highly flexed limb joints, often associated with high energy costs, and are constrained by the need to reduce loads on mobile, but less stable forelimb joints. Bipedal walking would have allowed greater hind limb joint extension, which is associated with reduced energy costs and increased endurance. We suggest that relaxing these constraints provided bipedal hominins important benefits associated with long distance foraging and mobility.

Unsupervised learning of satellite images enhances discovery of late Miocene fossil sites in the Urema Rift, Gorongosa, Mozambique.

BackgroundPaleoanthropological research focus still devotes most resources to areas generally known to be fossil rich instead of a strategy that first maps and identifies possible fossil sites in a given region. This leads to the paradoxical task of planning paleontological campaigns without knowing the true extent and likely potential of each fossil site and, hence, how to optimize the investment of time and resources. Yet to answer key questions in hominin evolution, paleoanthropologists must engage in fieldwork that targets substantial temporal and geographical gaps in the fossil record. How can the risk of potentially unsuccessful surveys be minimized, while maximizing the potential for successful surveys?MethodsHere we present a simple and effective solution for finding fossil sites based on clustering by unsupervised learning of satellite images with the k-means algorithm and pioneer its testing in the Urema Rift, the southern termination of the East African Rift System (EARS). We focus on a relatively unknown time period critical for understanding African apes and early hominin evolution, the early part of the late Miocene, in an overlooked area of southeastern Africa, in Gorongosa National Park, Mozambique. This clustering approach highlighted priority targets for prospecting that represented only 4.49% of the total area analysed.ResultsApplying this method, four new fossil sites were discovered in the area, and results show an 85% accuracy in a binary classification. This indicates the high potential of a remote sensing tool for exploratory paleontological surveys by enhancing the discovery of productive fossiliferous deposits. The relative importance of spectral bands for clustering was also determined using the random forest algorithm, and near-infrared was the most important variable for fossil site detection, followed by other infrared variables. Bands in the visible spectrum performed the worst and are not likely indicators of fossil sites.DiscussionWe show that unsupervised learning is a useful tool for locating new fossil sites in relatively unexplored regions. Additionally, it can be used to target specific gaps in the fossil record and to increase the sample of fossil sites. In Gorongosa, the discovery of the first estuarine coastal forests of the EARS fills an important paleobiogeographic gap of Africa. These new sites will be key for testing hypotheses of primate evolution in such environmental settings.

Could woodworking have driven lithic tool selection?

Understanding early stone tools, particularly relationships between form and function, is fundamental to understanding the behavioral evolution of early hominins. The oldest-claimed flake tools date to ca. 3.3 million years ago, and their development may represent a key step in hominin evolution. Flake form, and its relationship to function, has long been a focus of Paleolithic studies, almost exclusively with respect to meat acquisition. However, evidence for woodworking is now known from sites dating to 1.5 Ma. Additionally, Pan troglodytes are known to manufacture wooden tools for hunting and foraging, thus creating a phylogenetic (parsimony) argument for more ancient woodworking. However, few studies examining woodworking and Paleolithic tools have been completed to date. Indeed, it remains an open question whether woodworking may have instigated specific selective demands on the form of early stone tools. Here, we conducted an experiment testing the comparative woodworking efficiency (measured by time) of small and large flakes. Two groups of participants used either a relatively small or large unretouched flake to remove a predefined area from standardized samples of wood. Those using larger flakes were significantly more efficient (i.e., required less time) during this woodworking task. Our results demonstrate that larger flakes could have been preferentially chosen by hominins for woodworking, consistent with previous data generated experimentally in other (non-woodworking) tasks. Moreover, the production of relatively large flakes, such as those at Lomekwi, could have been motivated by woodworking, rather than, or in addition to, butchery. Such issues may also have encouraged the use of Levallois production strategies in later times.

Feminist Critiques and the Modification and Persistence of Popular Androcentric Images of Early Hominin Biosocial Evolution

A feminist critical gaze in analyses of images in popular media found that a patriarchal gaze in androcentric visual reconstructions of early biosocial hominin evolution persists despite some recent modifications in response to feminist critiques. Academic patriarchal reconstructions of hominin evolution and feminist critiques and research disproving them must first be outlined in order to be able to conduct feminist critical analyses of images in popular media that are influenced by academic research. Images of hominin evolution in popular media are important to examine not only because they shape public conceptions, but because they also influence archaeological reconstructions of hominin evolution.

Palaeovegetation changes recorded in Palaeolake Olduvai OGCP Core 2A (2.09–2.12 Ma) Naibor Soit Formation Olduvai Gorge, Tanzania

For five decades Olduvai Gorge has been a key site to reconstruct and understand the relationship between environmental and landscape conditions and use of affordances by early African hominin populations. Following the first Olduvai Gorge Coring project (OGCP) during 2014, a multiproxy microbiological analysis, which includes phytoliths, pollen, diatoms, sponge spicules and chrysophyte cysts, was undertaken on samples collected from various borehole cores. The aim of the study is to better understand palaeoenvironmental and palaeovegetation conditions and changes through time and their relationship to hominin presence and evolution. This study details the first palaeobotanical and palaeoenvironmental study of Borehole 2A at Olduvai Gorge. It represents the as yet oldest known sedimentary sequence in the Olduvai Basin for a portion of the pre-Bed I Naibor Soit Formation; this is a unit that is not accessible in any natural exposures in Olduvai Gorge, and has only recently been encountered by drilling. Here we present the results from a particularly phytolith-prone portion between ~2.09 Ma and 2.12 Ma.Phytolith results indicate a savannah environment dominated by grasses, where Poaceae were a key component and where the C3 Pooideae grasses were mostly dominant, alternating with C4 grasses. Oscillations between grass subfamilies, C3 Pooideae, C4 Chloridoideae, and C4 Panicoideae to lesser degrees, indicate five substantial climatic shifts, varying between more humid and arid conditions. Associated with phytoliths, freshwater indicators such as diatoms, sponge spicules and chrysophyte cysts were also identified, suggesting the presence of wetlands in the lake catchment area. Pollen is extremely rare in the sediments but when present, comprises fungal spores and Poaceae pollen, thus supporting the wetland and grassland reconstructions, respectively. These results offer for the first time, a whole picture of the palaeovegetation and associated palaeoenvironments for this pre-Bed I period. Together with previous results from other areas and chronological periods, they improve our understanding of the evolution and adaptation of early hominins and their close relationship to the surrounding landscape.

Savanna tree evolutionary ages inform the reconstruction of the paleoenvironment of our hominin ancestors

Ideas on hominin evolution have long invoked the emergence from forests into open habitats as generating selection for traits such as bipedalism and dietary shifts. Though controversial, the savanna hypothesis continues to motivate research into the palaeo-environments of Africa. Reconstruction of these ancient environments has depended heavily on carbon isotopic analysis of fossil bones and palaeosols. The sparsity of the fossil record, however, imposes a limit to the strength of inference that can be drawn from such data. Time-calibrated phylogenies offer an additional tool for dating the spread of savanna habitat. Here, using the evolutionary ages of African savanna trees, we suggest an initial tropical or subtropical expansion of savanna between 10 and 15 Ma, which then extended to higher latitudes, reaching southern Africa ca. 3 Ma. Our phylogenetic estimates of the origin and latitudinal spread of savannas broadly correspond with isotopic age estimates and encompass the entire hominin fossil record. Our results are consistent with the savanna hypothesis of early hominin evolution and reignite the debate on the drivers of savanna expansion. Our analysis demonstrates the utility of phylogenetic proxies for dating major ecological transitions in geological time, especially in regions where fossils are rare or absent or occur in discontinuous sediments.

Early hominins evolved within non-analog ecosystems

Present-day African ecosystems serve as referential models for conceptualizing the environmental context of early hominin evolution, but the degree to which modern ecosystems are representative of those in the past is unclear. A growing body of evidence from eastern Africa's rich and well-dated late Cenozoic fossil record documents communities of large-bodied mammalian herbivores with ecological structures differing dramatically from those of the present day, implying that modern communities may not be suitable analogs for the ancient ecosystems of hominin evolution. To determine when and why the ecological structure of eastern Africa's herbivore faunas came to resemble those of the present, here we analyze functional trait changes in a comprehensive dataset of 305 modern and fossil herbivore communities spanning the last ∼7 Myr. We show that nearly all communities prior to ∼700 ka were functionally non-analog, largely due to a greater richness of non-ruminants and megaherbivores (species >1,000 kg). The emergence of functionally modern communities precedes that of taxonomically modern communities by 100,000s of years, and can be attributed to the combined influence of Plio-Pleistocene C4 grassland expansion and pulses of aridity after ∼1 Ma. Given the disproportionate ecological impacts of large-bodied herbivores on factors such as vegetation structure, hydrology, and fire regimes, it follows that the vast majority of early hominin evolution transpired in the context of ecosystems that functioned unlike any today. Identifying how past ecosystems differed compositionally and functionally from those today is key to conceptualizing ancient African environments and testing ecological hypotheses of hominin evolution.

Relevance of the eastern African coastal forest for early hominin biogeography

The influence of climate change on hominin evolution is much debated. Two issues hamper our understanding of this process: the limited hominin fossil record, and incomplete knowledge about hominin spatial occupation of Africa. Here, we analyze the presently known hominin fossil distribution pattern and explore the potential geographic distribution of hominins between ∼4.5 and ∼2.5 Ma. We focus on assessing the relevance of the Coastal Forest of Eastern Africa (CFEA) along the Indian Ocean as a core area for early hominin evolution. Based on biogeographic-phylogeographic data we propose the coastal refuge hypothesis: the CFEA provided a refugium for early hominins in periods of variable climate and strong seasonality during eccentricity maxima. From this refuge, evolved species could disperse inland (e.g. to rift basins) via vegetated humid corridors, whenever onset of stable climate periods with low seasonality during eccentricity minima allowed expansion out of the coastal enclave. We develop a conceptual model in time and space, comparing predictions with climatic and hominin fossil records. The results imply that:1) between ∼4.5 and 3 Ma, ongoing (mostly anagenetic) hominin evolution occurred in the CFEA, punctuated by inland dispersal events at ∼4.4, 4.2, 3.8, 3.5, and 3.2 Ma;2) before ∼3 Ma, the Afar Basin was a (sub)core area often connected to and relatively similar to the CFEA, while other inland areas were more or less marginal for early hominin habitation;3) after ∼3 Ma, Northern Hemisphere Glaciation exerted strong influence by causing latitudinal contraction of the CFEA, leading to habitat fragmentation, isolation of hominin populations and possible cladogenetic evolution.A major challenge for the coastal refuge model is the fact that at present, no (hominin) fossils are known from the CFEA. We consider how this can be explained, and possibly overcome with targeted search efforts. Furthermore we discuss how the model can be tested, e.g. with molecular phylogeography approaches, and used to predict new hominin fossil locations. With this study, we hope to contribute a fresh perspective to the climate-evolution debate, emphasizing the role of climatic stability, length of dry season and vegetation cover to facilitate connectivity between hominin core and marginal habitats.

Expanded character sampling underscores phylogenetic stability of Ardipithecus ramidus as a basal hominin

Phylogenetic relationships among hominins provide a necessary framework for assessing their evolution. Reconstructing these relationships hinges on the strength of the character data analyzed. The phylogenetic position of Ardipithecus ramidus is critical to understanding early hominin evolution, and while many accept that it is most likely the sister taxon to all later hominins, others have argued that Ar. ramidus was ancestral to Pan. Although the study by Strait and Grine (2004) suggested the former, available evidence permitted only 26% of characters in their matrix to be assessed for Ar. ramidus. Fossils described subsequently by Suwa, White and colleagues in 2009 have enabled the number of characters that can be coded for this species to be expanded to 78% of the matrix. Here, we incorporate these new character data to evaluate their impact on the phylogenetic relationships of Ar. ramidus. Moreover, we have further revised the Strait and Grine (2004) matrix as necessitated by additions to the hypodigms of other fossil taxa. This updated matrix was analyzed using both parsimony and Bayesian techniques in a sequence of four iterative steps to independently evaluate the impact of matrix and expanded character revisions on tree topology. Despite the new data and matrix revisions, tree topology has remained remarkably stable. The addition of new craniodental material has served to markedly strengthen the support for the placement of Ar. ramidus as being derived relative to Sahelanthropus, and as the sister taxon of all later hominins. These findings support the phylogenetic hypothesis originally proposed by White and colleagues in 1994. This updated matrix provides a basis for the assessment of additional extinct species.

The bony labyrinth of StW 573 (“Little Foot”): Implications for early hominin evolution and paleobiology

Because of its exceptional degree of preservation and its geological age of ∼3.67 Ma, StW 573 makes an invaluable contribution to our understanding of early hominin evolution and paleobiology. The morphology of the bony labyrinth has the potential to provide information about extinct primate taxonomic diversity, phylogenetic relationships and locomotor behaviour. In this context, we virtually reconstruct and comparatively assess the bony labyrinth morphology in StW 573. As comparative material, we investigate 17 southern African hominin specimens from Sterkfontein, Swartkrans and Makapansgat (plus published data from two specimens from Kromdraai B), attributed to Australopithecus, early Homo or Paranthropus, as well as 10 extant human and 10 extant chimpanzee specimens. We apply a landmark-based geometric morphometric method for quantitatively assessing labyrinthine morphology. Morphology of the inner ear in StW 573 most closely resembles that of another Australopithecus individual from Sterkfontein, StW 578, recovered from the Jacovec Cavern. Within the limits of our sample, we observe a certain degree of morphological variation in the Australopithecus assemblage of Sterkfontein Member 4. Cochlear morphology in StW 573 is similar to that of other Australopithecus as well as to Paranthropus specimens included in this study, but it is substantially different from early Homo. Interestingly, the configuration of semicircular canals in Paranthropus specimens from Swartkrans differs from other fossil hominins, including StW 573. Given the role of the cochlea in the sensory-driven interactions with the surrounding environment, our results offer new perspectives for interpreting early hominin behaviour and ecology. Finally, our study provides additional evidence for discussing the phylogenetic polarity of labyrinthine traits in southern African hominins.

A Geometric Morphometric Examination of Hominoid Third Metacarpal Shape and Its Implications for Inferring the Precursor to Terrestrial Bipedalism.

The third metacarpal has been a focus of study when examining questions surrounding early hominin locomotion since this bone is adapted to the diverse range of positional behaviors performed by extant hominoids. The shape of this bone is potentially under strong selective pressure related to the biomechanical demands of terrestrial knuckle-walking, arboreal clambering, and brachiation performed by extant hominoids since the hand directly interacts with the substrate during the performance of these movements. The objective of the present study was to explore shape variation of the third metacarpal and examine how different parts of the bone discriminated between hominoid genera that perform these different locomotor behaviors. In addition to examining general interspecies variation, shape analysis was applied to testing the knuckle-walking hypothesis for human evolution. Fourteen 3D landmark coordinates were collected on hominoid third metacarpals, and principal component analysis and Procrustes distances were used to examine metacarpal shape. Comparable measurements were collected on fossilized third metacarpals of Australopithecus afarensis as an early hominin test case for examining the knuckle-walking hypothesis. Analyses that included landmarks collected on both ends of the bone distinguished humans from great apes and presented a strong functional signal related to suspensory locomotion among nonhuman hominoids, whereas the distal articular surface provided the strongest knuckle-walking signal. The shapes of Australopithecus afarensis metacarpals examined in the current study did not provide evidence for a trajectory of shape change in early hominin evolution that started from a metacarpal adapted for terrestrial knuckle-walking. Anat Rec, 302:983-998, 2019. © 2018 Wiley Periodicals, Inc.

The MIS 13 interglacial at Ceprano, Italy, in the context of Middle Pleistocene vegetation changes in southern Europe

Climatic and environmental changes of the Middle Pleistocene in Europe provide the context for an important phase in the evolution and dispersal of early hominins. Pollen records from terrestrial and marine sediment sequences reveal patterns not usually visible in palaeoenvironmental reconstructions from archaeological sites alone and show that hominin evolution took place against a background of marked environmental change as forests expanded and contracted in concert with global and regional climatic shifts. It is rare to find archaeological material in stratigraphically long and continuous palaeoenvironmental sequences, yet this is what is needed to enable particular phases of hominin evolution to be securely associated with climatic and environmental changes. At the well-known Middle Pleistocene archaeological site of Ceprano in Italy we have been able to produce a pollen record from the sedimentary unit directly below the stratum of the hominin remains. The new pollen data from Ceprano are presented here in full for the first time and provide a detailed picture of interglacial vegetation development during Marine Isotope Stage (MIS) 13. This evidence contributes significantly to our knowledge of the spatial heterogeneity of ecosystem responses to climatic change during this period and helps to further constrain the age of the hominin remains. New sites combining both archaeology and long, continuous high quality palaeoecological records would help clarify the relationship between hominin presence and palaeoclimatic and palaeoenvironmental conditions. In the meantime, better communication between the research communities can also go a long way to improving our understanding of the links between the two.

A two-million-year-long hydroclimatic context for hominin evolution in southeastern Africa.

The past two million years of eastern African climate variability is currently poorly constrained, despite interest in understanding its assumed role in early human evolution1-4. Rare palaeoclimate records from northeastern Africa suggest progressively drier conditions2,5 or a stable hydroclimate6. By contrast, records from Lake Malawi in tropical southeastern Africa reveal a trend of a progressively wetter climate over the past 1.3 million years7,8. The climatic forcings that controlled these past hydrological changes are also a matter of debate. Some studies suggest a dominant local insolation forcing on hydrological changes9-11, whereas others infer a potential influence of sea surface temperature changes in the Indian Ocean8,12,13. Here we show that the hydroclimate in southeastern Africa (20-25°S) is controlled by interplay between low-latitude insolation forcing (precession and eccentricity) and changes in ice volume at high latitudes. Our results are based on a multiple-proxy reconstruction of hydrological changes in the Limpopo River catchment, combined with a reconstruction of sea surface temperature in the southwestern Indian Ocean for the past 2.14 million years. We find a long-term aridification in the Limpopo catchment between around 1 and 0.6 million years ago, opposite to the hydroclimatic evolution suggested by records from Lake Malawi. Our results, together with evidence of wetting at Lake Malawi, imply that the rainbelt contracted toward the Equator in response to increased ice volume at high latitudes. By reducing the extent of woodland or wetlands in terrestrial ecosystems, the observed changes in the hydroclimate of southeastern Africa-both in terms of its long-term state and marked precessional variability-could have had a role in the evolution of early hominins, particularly in the extinction of Paranthropus robustus.

Sporadic sampling, not climatic forcing, drives observed early hominin diversity

The role of climate change in the origin and diversification of early hominins is hotly debated. Most accounts of early hominin evolution link observed fluctuations in species diversity to directional shifts in climate or periods of intense climatic instability. None of these hypotheses, however, have tested whether observed diversity patterns are distorted by variation in the quality of the hominin fossil record. Here, we present a detailed examination of early hominin diversity dynamics, including both taxic and phylogenetically corrected diversity estimates. Unlike past studies, we compare these estimates to sampling metrics for rock availability (hominin-, primate-, and mammal-bearing formations) and collection effort, to assess the geological and anthropogenic controls on the sampling of the early hominin fossil record. Taxic diversity, primate-bearing formations, and collection effort show strong positive correlations, demonstrating that observed patterns of early hominin taxic diversity can be explained by temporal heterogeneity in fossil sampling rather than genuine evolutionary processes. Peak taxic diversity at 1.9 million years ago (Ma) is a sampling artifact, reflecting merely maximal rock availability and collection effort. In contrast, phylogenetic diversity estimates imply peak diversity at 2.4 Ma and show little relation to sampling metrics. We find that apparent relationships between early hominin diversity and indicators of climatic instability are, in fact, driven largely by variation in suitable rock exposure and collection effort. Our results suggest that significant improvements in the quality of the fossil record are required before the role of climate in hominin evolution can be reliably determined.

A computer simulation to investigate the association between gene-based gifting and pair-bonding in early hominins

This article describes simulation research based on the Hamiltonian theory of gene-based altruism. It investigates the origin of semipermanent breeding bonds during hominin evolution. The research framework is based on a biologically detailed, ecologically situated, multi-agent microsimulation of emergent sociality. The research question tested is whether semipermanent breeding bonds (an emergent homoplastic social construct) might emerge among primate-like agents as the consequence of a mutation capable of supporting involuntary prosocial behavior. The research protocol compared several, single independent-variable longitudinal studies wherein hundreds of generations of autonomous, initially promiscuous, biologically detailed, hominin-like artificial life software agents were born, allowed to forage, reproduce, and die during experimental intervals lasting several simulated millennia. The temporal setting of the experiment was roughly contemporaneous with, or slightly after the time of, the Pan-Homo split. The simulation investigated what would happen if, within a population, a single gene for prosocial behavior (the independent variable in the experiment) was either switched on or switched-off. The null hypothesis predicted that, if the gene was switched off, then semipermanent breeding bonds (the dependent variable) would nonetheless emerge within the population. The results of the simulation rejected this null hypothesis, by showing that semipermanent breeding bonds would reliably emerge among the experimental populations but not among the control groups. Moreover, it was found that, across all experimental settings having constrained population numbers, the portion of each population having no prosocial trait would die out early, whereas the portion with the prosocial trait would survive. Large control populations had no discernible loss. The results of this research imply that, during the early stages of hominin evolution, there might have been a set of initially gene-based, altruistic excess forage-sharing social traits that contributed to the onset of morphological and additional complex social changes characteristic of this group. This work also demonstrates that modern computational technologies can extend our ability to test ‘what if’ hypotheses appropriate to the study of early hominin evolution.