Brian G. Richmond

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.

Pleistocene footprints show intensive use of lake margin habitats by Homo erectus groups

Reconstructing hominin paleoecology is critical for understanding our ancestors’ diets, social organizations and interactions with other animals. Most paleoecological models lack fine-scale resolution due to fossil hominin scarcity and the time-averaged accumulation of faunal assemblages. Here we present data from 481 fossil tracks from northwestern Kenya, including 97 hominin footprints attributed to Homo erectus. These tracks are found in multiple sedimentary layers spanning approximately 20 thousand years. Taphonomic experiments show that each of these trackways represents minutes to no more than a few days in the lives of the individuals moving across these paleolandscapes. The geology and associated vertebrate fauna place these tracks in a deltaic setting, near a lakeshore bordered by open grasslands. Hominin footprints are disproportionately abundant in this lake margin environment, relative to hominin skeletal fossil frequency in the same deposits. Accounting for preservation bias, this abundance of hominin footprints indicates repeated use of lakeshore habitats by Homo erectus. Clusters of very large prints moving in the same direction further suggest these hominins traversed this lakeshore in multi-male groups. Such reliance on near water environments, and possibly aquatic-linked foods, may have influenced hominin foraging behavior and migratory routes across and out of Africa.

The evolution of body size and shape in the human career

Body size is a fundamental biological property of organisms, and documenting body size variation in hominin evolution is an important goal of palaeoanthropology. Estimating body mass appears deceptively simple but is laden with theoretical and pragmatic assumptions about best predictors and the most appropriate reference samples. Modern human training samples with known masses are arguably the ‘best’ for estimating size in early bipedal hominins such as the australopiths and all members of the genus Homo, but it is not clear if they are the most appropriate priors for reconstructing the size of the earliest putative hominins such as Orrorin and Ardipithecus. The trajectory of body size evolution in the early part of the human career is reviewed here and found to be complex and nonlinear. Australopith body size varies enormously across both space and time. The pre-erectus early Homo fossil record from Africa is poor and dominated by relatively small-bodied individuals, implying that the emergence of the genus Homo is probably not linked to an increase in body size or unprecedented increases in size variation. Body size differences alone cannot explain the observed variation in hominin body shape, especially when examined in the context of small fossil hominins and pygmy modern humans. This article is part of the themed issue ‘Major transitions in human evolution’.

Human feeding biomechanics: performance, variation, and functional constraints

The evolution of the modern human (Homo sapiens) cranium is characterized by a reduction in the size of the feeding system, including reductions in the size of the facial skeleton, postcanine teeth, and the muscles involved in biting and chewing. The conventional view hypothesizes that gracilization of the human feeding system is related to a shift toward eating foods that were less mechanically challenging to consume and/or foods that were processed using tools before being ingested. This hypothesis predicts that human feeding systems should not be well-configured to produce forceful bites and that the cranium should be structurally weak. An alternate hypothesis, based on the observation that humans have mechanically efficient jaw adductors, states that the modern human face is adapted to generate and withstand high biting forces. We used finite element analysis (FEA) to test two opposing mechanical hypotheses: that compared to our closest living relative, chimpanzees (Pan troglodytes), the modern human craniofacial skeleton is (1) less well configured, or (2) better configured to generate and withstand high magnitude bite forces. We considered intraspecific variation in our examination of human feeding biomechanics by examining a sample of geographically diverse crania that differed notably in shape. We found that our biomechanical models of human crania had broadly similar mechanical behavior despite their shape variation and were, on average, less structurally stiff than the crania of chimpanzees during unilateral biting when loaded with physiologically-scaled muscle loads. Our results also show that modern humans are efficient producers of bite force, consistent with previous analyses. However, highly tensile reaction forces were generated at the working (biting) side jaw joint during unilateral molar bites in which the chewing muscles were recruited with bilateral symmetry. In life, such a configuration would have increased the risk of joint dislocation and constrained the maximum recruitment levels of the masticatory muscles on the balancing (non-biting) side of the head. Our results do not necessarily conflict with the hypothesis that anterior tooth (incisors, canines, premolars) biting could have been selectively important in humans, although the reduced size of the premolars in humans has been shown to increase the risk of tooth crown fracture. We interpret our results to suggest that human craniofacial evolution was probably not driven by selection for high magnitude unilateral biting, and that increased masticatory muscle efficiency in humans is likely to be a secondary byproduct of selection for some function unrelated to forceful biting behaviors. These results are consistent with the hypothesis that a shift to softer foods and/or the innovation of pre-oral food processing techniques relaxed selective pressures maintaining craniofacial features that favor forceful biting and chewing behaviors, leading to the characteristically small and gracile faces of modern humans.

Pleistocene animal communities of a 1.5 million-year-old lake margin grassland and their relationship to Homo erectus paleoecology

The ecological and selective forces that sparked the emergence of Homos adaptive strategy remain poorly understood. New fossil and archaeological finds call into question previous interpretations of the grade shift that drove our ancestors evolutionary split from the australopiths. Furthermore, issues of taphonomy and scale have limited reconstructions of the hominin habitats and faunal communities that define the environmental context of these behavioral changes. The multiple ∼1.5xa0Ma track surfaces from the Okote Member of the Koobi Fora Formation at East Turkana provide unique windows for examining hominin interactions with the paleoenvironment and associated faunas at high spatiotemporal resolution. These surfaces preserve the tracks of many animals, including cf. Homo erectus. Here, we examine the structure of the animal community that inhabited this landscape, considering effects of preservation bias by comparing the composition of the track assemblage to a skeletal assemblage from the same time and place. We find that the track and skeletal assemblages are similar in their representation of the vertebrate paleocommunity, with comparable levels of taxonomic richness and diversity. Evenness (equitability of the number of individuals per taxon) differs between the two assemblages due to the very different circumstances of body fossil versus track preservation. Both samples represent diverse groups of taxa including numerous water-dependent species, consistent with geological interpretations of the track site environments. Comparisons of these assemblages also show a pattern of non-random hominin association with a marginal lacustrine habitat relative to other vertebrates in the track assemblage. This evidence is consistent with behavior that included access to aquatic foods and possibly hunting by H.xa0erectus in lake margins/edaphic grasslands. Such behaviors may signal the emergence of the adaptative strategies that define our genus.

African Genesis: Hominin proximal femur morphology from the Tugen Hills to Flores

The proximal femur has played a prominent role in our understanding of the origin and evolution of human gait because of its functional importance and relatively good representation in the fossil record. This study examines the morphology of femora from the fossil record, including those attributed to Orrorin tugenensis (BAR 1002’00) and Homo fl oresiensis (LB1/9). Considerable debate surrounds both of these taxa, focusing primarily on the evidence that the former is a hominin and shows convincing adaptations for bipedalism , and over whether or not the latter is a pathological diminutive modern human or a distinct species (and what the anatomy suggests regarding the evolutionary history of H. fl oresiensis ). This study addresses the questions of whether Orrorin femoral morphology more closely resembles femora of humans and fossil hominins than apes, and whether it is more similar to the femora of Homo among the hominins. Our study also tests the hypothesis that the femoral morphology of LB1/9 is consistent with that of a small-bodied modern human, or more closely resembles fossil hominins. To test these questions, we compare the proximal femoral morphology of BAR 1002’00 and LB1/9 to a large sample of adult humans, chimpanzees, bonobos, gorillas, orang-utans and most available early hominin taxa. Importantly, the human sample includes individuals from largeand small-bodied populations that overlap with the small sizes of BAR 1002’00 and LB1/9. The results show that the external morphology of the Orrorin femur more closely resembles the femora of Australopithecus and Paranthropus than those of great apes, or fossil and modern Homo . Its morphology is not consistent with it being characterised as more like human than australopith femora, or a phylogenetic hypothesis that Orrorin is ancestral to Homo to the exclusion of the australopithecines. However, its morphology is consistent with its 6 13 Hominin proximal femur morphology from the Tugen Hills to Flores

Facial biomechanics in Australopithecus africanus: implications for feeding ecology

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.

Masticatory biomechanics in Australopithecus africanus examined using finite element analysis: A preliminary study based on Sts 5

Understanding the relationship between bone strain and bone growth is critical for interpreting variations in skeletal robusticity. Recently we presented a model for interactions between estrogen, strain, and periosteal bone growth, in which high estrogen (E2) increases, and low (E2) decreases, osteogenic responses to strain. We compared cortical growth in expercised and sedentary sheep (Ovis aries) with higher vs. lower estrogen levels, and showed that exercised animals with high E2 added substantially more bone than those with lower E2. However, without normal controls, it was unclear whether exercise-induced cortical growth was stimulated by high E2, suppressed by lower E2, or both. Here we present a broader test of interactions between E2 levels (normal, low, high) and loading (exercised and sedentary). Low E2 animals were vaccinated against GnRH to suppress estrogen, while high E2 animals received estrogen implants. After 45 days, periosteal bone growth was measured at hindlimb midshafts. The results support the hypothesis that estrogen upregulates strain-induced cortical bone growth: exercised, high-E2 animals grew 6-27% more bone than exercised animals with lower E2 levels, or sedentary animals (p<.05). The effects of the anti-GnRH vaccine on bone growth are more complex. Assays showed that vaccinated animals had normal, not decreased, E2 levels, but grew 34-39% less bone in response to exercise than normal controls (p<.05). This suggests the vaccine affected strain-induced bone growth without changing circulating E2, an unexpected finding. These results demonstrate that variation in E2 levels may produce differential growth response to similar mechanical loading through complex mechanisms.