Fossil mammals and paleoenvironments in the Omo-Turkana Basin

Fluctuating climate and dietary innovation drove ratcheted evolution of proboscidean dental traits

Identification of the selective forces that shaped adaptive phenotypes generally relies on current habitat and function, but these may differ from the context in which adaptations arose. Moreover, the fixation of adaptive change in a fluctuating environment and the mechanisms of long-term trends are still poorly understood, as is the role of behaviour in triggering these processes. Time series of fossils can provide evidence on these questions, but examples of individual lineages with adequate fossil and proxy data over extended periods are rare. Here, we present new data on proboscidean dental evolution in East Africa over the past 26 million years, tracking temporal patterns of morphological change in relation to proxy evidence of diet, vegetation and climate (aridity). We show that behavioural experimentation in diet is correlated with environmental context, and that major adaptive change in dental traits followed the changes in diet and environment but only after acquisition of functional innovations in the masticatory system. We partition traits by selective agent, showing that the acquisition of high, multiridged molars was primarily a response to an increase in open, arid environments with high dust accumulation, whereas enamel folding was more associated with the amount of grass in the diet. We further show that long-term trends in these features proceeded in a ratchet-like mode, alternating between directional change at times of high selective pressure and stasis when the selective regime reversed. This provides an explanation for morphology adapted to more extreme conditions than current usage (Liem's Paradox). Our study illustrates how, in fossil series with adequate stratigraphic control and proxy data, environmental and behavioural factors can be mapped on to time series of morphological change, illuminating the mode of acquisition of an adaptive complex.

Pliocene hominins from East Turkana were associated with mesic environments in a semiarid basin

During the middle Pliocene (∼3.8-3.2 Ma), both Australopithecus afarensis and Kenyanthropus platyops are known from the Turkana Basin, but between 3.60 and 3.44 Ma, most hominin fossils are found on the west side of Lake Turkana. Here, we describe a new hominin locality (ET03-166/168, Area 129) from the east side of the lake, in the Lokochot Member of the Koobi Fora Formation (3.60-3.44 Ma). To reconstruct the paleoecology of the locality and its surroundings, we combine information from sedimentology, the relative abundance of associated mammalian fauna, phytoliths, and stable isotopes from plant wax biomarkers, pedogenic carbonates, and fossil tooth enamel. The combined evidence provides a detailed view of the local paleoenvironment occupied by these Pliocene hominins, where a biodiverse community of primates, including hominins, and other mammals inhabited humid, grassy woodlands in a fluvial floodplain setting. Between <3.596 and 3.44 Ma, increases in woody vegetation were, at times, associated with increases in arid-adapted grasses. This suggests that Pliocene vegetation included woody species that were resilient to periods of prolonged aridity, resembling vegetation structure in the Turkana Basin today, where arid-adapted woody plants are a significant component of the ecosystem. Pedogenic carbonates indicate more woody vegetation than other vegetation proxies, possibly due to differences in temporospatial scale and ecological biases in preservation that should be accounted for in future studies. These new hominin fossils and associated multiproxy paleoenvironmental indicators from a single locale through time suggest that early hominin species occupied a wide range of habitats, possibly including wetlands within semiarid landscapes. Local-scale paleoecological evidence from East Turkana supports regional evidence that middle Pliocene eastern Africa may have experienced large-scale, climate-driven periods of aridity. This information extends our understanding of hominin environments beyond the limits of simple wooded, grassy, or mosaic environmental descriptions.

New hominin remains and revised context from the earliest Homo erectus locality in East Turkana, Kenya

The KNM-ER 2598 occipital is among the oldest fossils attributed to Homo erectus but questions have been raised about whether it may derive from a younger horizon. Here we report on efforts to relocate the KNM-ER 2598 locality and investigate its paleontological and geological context. Although located in a different East Turkana collection area (Area 13) than initially reported, the locality is stratigraphically positioned below the KBS Tuff and the outcrops show no evidence of deflation of a younger unit, supporting an age of >1.855 Ma. Newly recovered faunal material consists primarily of C4 grazers, further confirmed by enamel isotope data. A hominin proximal 3rd metatarsal and partial ilium were discovered <50 m from the reconstructed location where KNM-ER 2598 was originally found but these cannot be associated directly with the occipital. The postcrania are consistent with fossil Homo and may represent the earliest postcrania attributable to Homo erectus.

Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia

Diet provides critical information about the ecology and environment of herbivores. Hence, understanding the dietary strategies of fossil herbivores and the associated temporal changes is one aspect of inferring paleoenvironmental conditions. Here, we present carbon isotope data from more than 1,050 fossil teeth that record the dietary patterns of nine herbivore families in the late Pliocene and early Pleistocene (3.6 to 1.05 Ma) from the Shungura Formation, a hominin-bearing site in southwestern Ethiopia. An increasing trend toward C₄ herbivory has been observed with attendant reductions in the proportions of browsers and mixed feeders through time. A high proportion of mixed feeders has been observed prior to 2.9 Ma followed by a decrease in the proportion of mixed feeders and an increase in grazers between 2.7 and 1.9 Ma, and a further increase in the proportion of grazers after 1.9 Ma. The collective herbivore fauna shows two major change points in carbon isotope values at ∼2.7 and ∼2.0 Ma. While hominin fossils from the sequence older than 2.7 Ma are attributed to Australopithecus, the shift at ∼2.7 Ma indicating the expansion of C₄ grasses on the landscape was concurrent with the first appearance of Paranthropus The link between the increased C₄ herbivory and more open landscapes suggests that Australopithecus lived in more wooded landscapes compared to later hominins such as Paranthropus and Homo, and has implications for key morphological and behavioral adaptations in our lineage.

A genotype:phenotype approach to testing taxonomic hypotheses in hominids

Paleontology has long relied on assumptions about the genetic and developmental influences on skeletal variation. The last few decades of developmental genetics have elucidated the genetic pathways involved in making teeth and patterning the dentition. Quantitative genetic analyses have refined this genotype:phenotype map even more, especially for primates. We now have the ability to define dental traits with a fair degree of fidelity to the underlying genetic architecture; for example, the molar module component (MMC) and the premolar-molar module (PMM) that have been defined through quantitative genetic analyses. We leverage an extensive dataset of extant and extinct hominoid dental variation to explore how these two genetically patterned phenotypes have evolved through time. We assess MMC and PMM to test the hypothesis that these two traits reveal a more biologically informed taxonomy at the genus and species levels than do more traditional measurements. Our results indicate that MMC values for hominids fall into two categories and that Homo is derived compared with earlier taxa. We find a more variable, species-level pattern for PMM. These results, in combination with previous research, demonstrate that MMC reflects the phenotypic output of a more evolutionarily stable, or phylogenetically congruent, genetic mechanism, and PMM is a reflection of a more evolutionarily labile mechanism. These results suggest that the human lineage since the split with chimpanzees may not represent as much genus-level variation as has been inferred from traits whose etiologies are not understood.

Mammal functional diversity and habitat heterogeneity: Implications for hominin habitat reconstruction

Hominin habitats are frequently described as 'mosaic' based on interpretations of fossil assemblages comprising taxa with divergent functional adaptations (e.g., both grazers and browsers). This interpretation rests on an assumption that mammal functional diversity is positively associated with habitat heterogeneity. We test this assumption using modern mammal data for 141 sites in Africa. Species average body mass and locomotor and dietary information was compiled for all species >500 g. The functional diversity of each species assemblage was measured using five metrics: locomotor richness, trophic richness, functional richness, functional divergence, and functional evenness. We used a high-resolution woody cover estimate for sub-Saharan Africa to compute the coefficient of variation of percentage of woody cover for each site. We used a published land cover classification to compute the number of habitat patches and the number of distinct habitat types at each site. Multiple regressions were conducted at 9 different spatial resolutions (pixel size ranging from 100 m to 30 km) to explore the relationship between functional diversity and habitat heterogeneity metrics. The overall number of species found at a site is strongly positively associated with functional richness, locomotor richness, and trophic richness at all spatial resolutions. The number of habitat types at a site and the density of habitat patches show a modest positive relationship with most functional diversity metrics at most spatial resolutions. The coefficient of variation in woody cover is a very poor predictor of functional diversity. The locomotor and trophic richness of mammal communities are positively associated with habitat heterogeneity and may be useful for reconstructing aspects of heterogeneity at hominin sites. However, the overall number of species is an important confounding variable, which in turn depends on the sample size of the overall fossil sample. Researchers should carefully consider impacts of sample size on faunal reconstructions of habitat heterogeneity.

Dental microwear texture analysis of Pliocene Suidae from Hadar and Kanapoi in the context of early hominin dietary breadth expansion

Stable carbon isotope studies suggest that early hominins may have diversified their diet as early as 3.76 Ma. Early Pliocene hominins, including Australopithecus anamensis, had diets that were dominated by C₃ resources while Late Pliocene hominins, including Australopithecus afarensis-a putative descendant of A. anamensis-had diets that included both C₃ and C₄ resources. It has been hypothesized that the expansion of C₄ grasslands in eastern Africa during the Pliocene could have prompted hominins to incorporate C₄ resources in their diets. However, dental microwear analyses suggest that diet diversification did not involve changes in the mechanical properties of the foods consumed. To provide contextual and comparative information on this issue, the diet of suids from the A. anamensis site of Kanapoi and the A. afarensis site of Hadar is investigated. Using dental microwear texture analyses, it is shown that despite significant dietary overlap, there is evidence for dietary niche partitioning among suids. Based on comparisons with the diet of extant African suids, it is inferred that Nyanzachoerus pattersoni (n = 21) was a mixed feeder, Nyanzachoerus jaegeri (n = 4) and Notochoerus euilus (n = 61) were habitual grazers, and Kolpochoerus afarensis (n = 34) had a broad diet that included hard brittle foods and underground resources. The dental microwear of Ny. pattersoni and Ny. jaegeri/No. euilus do not differ significantly between Kanapoi and Hadar. Most differences are driven by K. afarensis, a suid absent at Kanapoi but present at Hadar. Food availability probably differed between Hadar and Kanapoi, and it is likely that A. afarensis did not exploit some of the foods (e.g., underground resources) consumed by suids. It is hypothesized that despite the incorporation of C₄ resources in the diet, a significant dietary change towards flexible diets in the hominin lineage had yet to come.

Diversity analysis of Plio-Pleistocene large mammal communities in the Omo-Turkana Basin, eastern Africa

Knowing how the diversity of large mammal communities changes across space and time provides an important ecological framework for studying hominin evolution. However, diversity studies that apply methods currently used by neoecologists are rare in paleoanthropology and are also challenging due to diversity's unusual statistical properties. Here, we apply up-to-date analytical methods for understanding how species- and genus-level large mammalian diversity in the Omo-Turkana Basin changed through time and across space at multiple spatiotemporal scales (within each formation:10^2-3 km² and 10^4-5 years; and within the basin as a whole: 10³ km² and 10⁵ years). We found that, on average, Koobi Fora's large mammal community was more diverse than Nachukui's, which in turn was more diverse than Shungura's. Diversity was stable through time within each of these formations (alpha diversity), as was diversity in the basin as a whole (gamma diversity). Compositional dissimilarity between these three formations (beta diversity) was relatively low through time, with a 0.6 average proportion of shared species, suggesting dispersal acted to homogenize the region. Though alpha and gamma diversity were fairly stable through time, we do observe several notable peaks: during the KBS Member in Koobi Fora (30% increase), the Lokalalei Member in Nachukui (120% increase), and at 1.7 Ma in the entire basin (100% increase). We conclude by (1) demonstrating that habitat heterogeneity was an important factor influencing alpha diversity within each of the three formations, and (2) hypothesizing that diversity stability may have been driven by equilibrial dynamics in which overall diversity was constrained by resource availability, implying biotic interactions were an important factor in structuring the communities that included hominins as members. Our findings demonstrate the need to quantify how large mammal diversity changes across time and space in order to further our understanding of hominin ecology and evolution.

An ecometric analysis of the fossil mammal record of the Turkana Basin

Although ecometric methods have been used to analyse fossil mammal faunas and environments of Eurasia and North America, such methods have not yet been applied to the rich fossil mammal record of eastern Africa. Here we report results from analysis of a combined dataset spanning east and west Turkana from Kenya between 7 and 1 million years ago (Ma). We provide temporally and spatially resolved estimates of temperature and precipitation and discuss their relationship to patterns of faunal change, and propose a new hypothesis to explain the lack of a temperature trend. We suggest that the regionally arid Turkana Basin may between 4 and 2 Ma have acted as a ‘species factory’, generating ecological adaptations in advance of the global trend. We show a persistent difference between the eastern and western sides of the Turkana Basin and suggest that the wetlands of the shallow eastern side could have provided additional humidity to the terrestrial ecosystems. Pending further research, a transient episode of faunal change centred at the time of the KBS Member (1.87–1.53 Ma), may be equally plausibly attributed to climate change or to a top-down ecological cascade initiated by the entry of technologically sophisticated humans.

This article is part of the themed issue ‘Major transitions in human evolution’.

Dental microwear and Pliocene paleocommunity ecology of bovids, primates, rodents, and suids at Kanapoi

Reconstructions of habitat at sites like Kanapoi are key to understanding the environmental circumstances in which hominins evolved during the early Pliocene. While Australopithecus anamensis shows evidence of terrestrial bipedality traditionally associated with a more open setting, its enamel has low δ¹³C values consistent with consumption of C₃ foods, which predominate in wooded areas of tropical Africa. Habitat proxies, ranging from paleosols and their carbonates to associated herbivore fauna and their carbon isotope ratios, suggest a heterogeneous setting with both grass and woody plant components, though the proportions of each have been difficult to pin down. Here we bring dental microwear texture analysis of herbivorous fauna to bear on the issue. We present texture data for fossil bovids, primates, rodents, and suids (n = 107 individuals in total) from the hominin bearing deposits at Kanapoi, and interpret these in the light of closely related extant mammals with known differences in diet. The Kanapoi bovid results, for example, are similar to those for extant variable grazers or graze-browse intermediate taxa. The Kanapoi suid data vary by taxon, with one similar to the pattern of extant grazers and the other more closely resembling mixed feeders. The Kanapoi primates and rodents are more difficult to associate with a specific environment, though it seems that grass was likely a component in the diets of both. All taxa evince microwear texture patterns consistent with a mosaic of discrete microhabitats or a heterogeneous setting including both tree and grass components.

Mechanical evidence that Australopithecus sediba was limited in its ability to eat hard foods

Australopithecus sediba has been hypothesized to be a close relative of the genus Homo. Here we show that MH1, the type specimen of A. sediba, was not optimized to produce high molar bite force and appears to have been limited in its ability to consume foods that were mechanically challenging to eat. Dental microwear data have previously been interpreted as indicating that A. sediba consumed hard foods, so our findings illustrate that mechanical data are essential if one aims to reconstruct a relatively complete picture of feeding adaptations in extinct hominins. An implication of our study is that the key to understanding the origin of Homo lies in understanding how environmental changes disrupted gracile australopith niches. Resulting selection pressures led to changes in diet and dietary adaption that set the stage for the emergence of our genus.

Dietary adaptations of extinct early humans are often inferred from dental microwear data. Here, the authors employ mechanical analyses to show that Australopithecus sediba had limited ability to consume hard foods.

Paleodietary reconstruction using stable isotopes and abundance analysis of bovids from the Shungura Formation of South Omo, Ethiopia

Preservation of the stable carbon isotopic composition of fossil tooth enamel enables us to estimate the relative proportion of C3 versus C4 vegetation in an animal's diet, which, combined with analysis of faunal abundance, may provide complementary methods of paleoenvironmental reconstruction. To this end, we analyzed stable carbon isotopic composition (δ(13)C values) of tooth enamel from four bovid tribes (Tragelaphini, Aepycerotini, Reduncini, and Alcelaphini) derived from six members of the Shungura Formation (Members B, C, D, F, G, and L; ages from ca. 2.90-1.05 Ma (millions of years ago) in the Lower Omo Valley of southwestern Ethiopia. The bovids show a wide range of δ(13)C values within taxa and stratigraphic members, as well as temporal changes in the feeding strategies of taxa analyzed throughout the middle to late Pliocene and early Pleistocene. Such variation suggests that the use of actualistic approaches for paleoenvironmental reconstruction may not always be warranted. Alcelaphini was the only taxon analyzed that retained a consistent dietary preference throughout the sequence, with entirely C4-dominated diets. Reduncini had a mixed C3/C4 to C4-dominated diet prior to 2.4 Ma, after which this taxon shifted to a largely C4-dominated diet. Aepycerotini generally showed a mixed C3/C4 diet, with a period of increased C4 diet from 2.5 to 2.3 Ma. Tragelaphini showed a range of mixed C3/C4 diets, with a median value that was briefly nearer the C4 end member from 2.9 to 2.4 Ma but was otherwise towards the C3 end member. These isotopic results, combined with relative abundance data for these bovids, imply that the environment of the Lower Omo Valley consisted of a mosaic of closed woodlands, with riverine forests and open grasslands. However, our data also signify that the overall environment gradually became more open, and that C4 grasses became more dominant. Finally, these results help document the range and extent of environments and potential diets that were available to the four hominin species encountered in the Shungura sequence.

Human locomotion and heat loss: an evolutionary perspective

Humans are unique in many respects including being furless, striding bipeds that excel at walking and running long distances in hot conditions. This review summarizes what we do and do not know about the evolution of these characteristics, and how they are related. Although many details remain poorly known, the first hominins (species more closely related to humans than to chimpanzees) apparently diverged from the chimpanzee lineage because of selection for bipedal walking, probably because it improved their ability to forage efficiently. However, because bipedal hominins are necessarily slow runners, early hominins in open habitats likely benefited from improved abilities to dump heat in order to forage safely during times of peak heat when predators were unable to hunt them. Endurance running capabilities evolved later, probably as adaptations for scavenging and then hunting. If so, then there would have been strong selection for heat-loss mechanisms, especially sweating, to persistence hunt, in which hunters combine endurance running and tracking to drive their prey into hyperthermia. As modern humans dispersed into a wide range of habitats over the last few hundred thousand years, recent selection has helped populations cope better with a broader range of locomotor and thermoregulatory challenges, but all humans remain essentially adapted for long distance locomotion rather than speed, and to dump rather than retain heat.

The feeding biomechanics and dietary ecology of Paranthropus boisei

The African Plio-Pleistocene hominins known as australopiths evolved derived craniodental features frequently interpreted as adaptations for feeding on either hard, or compliant/tough foods. Among australopiths, Paranthropus boisei is the most robust form, exhibiting traits traditionally hypothesized to produce high bite forces efficiently and strengthen the face against feeding stresses. However, recent mechanical analyses imply that P. boisei may not have been an efficient producer of bite force and that robust morphology in primates is not necessarily strong. Here we use an engineering method, finite element analysis, to show that the facial skeleton of P. boisei is structurally strong, exhibits a strain pattern different from that in chimpanzees (Pan troglodytes) and Australopithecus africanus, and efficiently produces high bite force. It has been suggested that P. boisei consumed a diet of compliant/tough foods like grass blades and sedge pith. However, the blunt occlusal topography of this and other species suggests that australopiths are adapted to consume hard foods, perhaps including grass and sedge seeds. A consideration of evolutionary trends in morphology relating to feeding mechanics suggests that food processing behaviors in gracile australopiths evidently were disrupted by environmental change, perhaps contributing to the eventual evolution of Homo and Paranthropus.

Chimpanzee isotopic ecology: a closed canopy C3 template for hominin dietary reconstruction

The most significant hominin adaptations, including features used to distinguish and/or classify taxa, are critically tied to the dietary environment. Stable isotopic analyses of tooth enamel from hominin fossils have provided intriguing evidence for significant C4/CAM (crassulacean acid metabolism) resource consumption in a number of Plio-Pleistocene hominin taxa. Relating isotopic tooth signatures to specific dietary items or proportions of C3 versus C4/CAM plants, however, remains difficult as there is an ongoing need to document and quantify isotopic variability in modern ecosystems. This study investigates the ecological variables responsible for carbon isotopic discrimination and variability within the C3-dominated dietary niche of a closed canopy East African hominoid, Pan troglodytes, from Ngogo, Kibale National Park, Uganda. δ(13)C values among C3 resources utilized by Ngogo chimpanzees were highly variable, ranging over 13‰. Infrequent foraging on papyrus (the only C4 plant consumed by chimpanzees at the site) further extended this isotopic range. Variation was ultimately most attributable to mode of photosynthesis (C3 versus C4), food type, and elevation, which together accounted for approximately 78% of the total sample variation. Among C3 food types, bulk carbon values ranged from -24.2‰ to -31.1‰ with intra-plant variability up to 12.1‰. Pith and sapling leaves were statistically more (13)C depleted than pulp, seeds, flowers, cambium, roots, leaf buds, and leaves from mature trees. The effect of elevation on carbon variation was highly significant and equivalent to an approximately 1‰ increase in δ(13)C for every 150 m of elevation gain, likely reflecting habitat variability associated with topography. These results indicate significant δ(13)C variation attributable to food type and elevation among C3 resources and provide important data for hominin dietary interpretations based on carbon isotopic analyses.

Seasonal mortality patterns in primates: implications for the interpretation of dental microwear

The microscopic traces of use wear on teeth have been extensively studied to provide information that will assist in elucidating the dietary habits of extinct hominin species. It has been amply documented that dental microwear provides information pertaining to diet for living animals, where there is a strong and consistent association between dental microwear patterns and different types of foods that are chewed. The details of occlusal surface wear patterns are capable of distinguishing among diets when the constituent food items differ in their fracture properties. For example, the microwear traces left on the teeth of mammals that crush hard, brittle foods such as nuts are generally dominated by pits, whereas traces left on the teeth of mammals that shear tough items such as leaves tend to be characterized by scratches. These microwear features result from and thus record actual chewing events. As such, microwear patterns are expected to be variably ephemeral, as individual features are worn away and replaced or overprinted by others as the tooth wears down in subsequent bouts of mastication. Indeed, it has been demonstrated, both in the laboratory and the wild, that short-term dietary variation can result in the turnover of microwear. Because occlusal microwear potentially reflects an individual's diet for a short time (days, weeks, or months, depending on the nature of the foods being masticated), tooth surfaces sampled at different times will display differences that relate to temporal (for example, seasonal) differences in diet.