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

Paleoanthropologists have long been intrigued by the observed patterns of human evolution, including species diversity, and often invoked climatic change as the principal driver of evolutionary change. Here, we investigate whether the early hominin fossil record is of suitable quality to test these climate-forcing hypotheses. Specifically, we compare early hominin diversity to sampling metrics that quantify changes in fossil preservation and sampling intensity between 7 and 1 million years ago. We find that observed diversity patterns are governed by sporadic sampling and do not yield a genuine evolutionary signal. Many more fossil discoveries are required before existing hypotheses linking climate and evolution can be meaningfully tested.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

The evolution of mammalian brain size

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size

Phylogeny, ecology, and morphological evolution in the atelid cranium

Reconstructing evolutionary relationships of living and extinct primate groups requires reliable phylogenetic inference based on morphology, as DNA is rarely preserved in fossil specimens. Atelids (family Atelidae) are a monophyletic clade and one of the three major adaptive radiations of south and central American primates (platyrrhines), including the genera Alouatta, Ateles, Brachyteles, and Lagothrix, and are diverse in morphology, body and brain size, locomotion, diet, social systems, and behavioral ecology. Molecular phylogenetic relationships of the extant atelid genera are well resolved, yet morphological analyses often support alternative phylogenetic relationships to molecular data. We collected geometric morphometric data from the crania of atelid taxa for phylogenetic analysis of the cranium, cranial base, and face and tested the hypotheses that cranial data maintain a phylogenetic signal, cranial base morphology most closely reflects the atelid molecular phylogeny, and facial and overall cranial morphology are shaped by diet and have experienced greater homoplasy. All analyses supported genus monophyly, and facial morphology maintained a strong phylogenetic signal inferring the atelid molecular phylogeny and a sister relationship between Brachyteles and Lagothrix, whereas results from the cranial base and whole cranium supported Ateles–Lagothrix and/or Alouatta–Brachyteles clades reflecting homoplasy and ecological and dietary similarities. A phylogenetic signal in the atelid face is important for future studies integrating fossil taxa and supports evidence that congruence between molecular and morphological phylogenetics in primates is module and clade specific

Data from: 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, in order 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 artefact, 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

Functional morphology of the cave bear (Ursus spelaeus) mandible: a 3D geometric morphometric analysis

The diet of the fossil cave bears (Ursus spelaeus group) has been debated extensively. Thought traditionally to be herbivorous, more recent studies have proposed more meat in the cave bear diet. To test this, the mandibular morphology of cave bears was analysed using 3D geometric morphometrics and compared to that of extant Ursidae. Landmarks for 3D digitisation of the mandible were chosen to reflect functional morphology relating to the temporalis and masseter muscles. Extant and extinct Pleistocene Ursidae were digitised with a MicroScribe G2. Generalised Procrustes superimposition was performed, and data were allometrically and phylogenetically corrected. Principal component analysis (PCA), two-block partial least squares analysis (2B-PLS), regression analysis and discriminant function analysis were performed. PCA and 2B-PLS differentiate between known dietary niches in extant Ursidae. The lineage of the cave bear runs parallel to that of the panda (Ailuropoda melanoleuca) in morphospace, implying the development of morphological adaptations for eating foliage. A regression of shape onto foliage content in the diet and a discriminant function analysis also indicate that the cave bear diet consisted primarily of foliage

Functional morphology of the cave bear (Ursus spelaeus) cranium: a three-dimensional geometric morphometric analysis

The diet of the fossil bear Ursus spelaeus has been debated extensively. U. spelaeus is thought to have been herbivorous, but the exact composition of its diet remains unclear. To test this, the cranial morphology of U. spelaeus was analysed using 3D geometric morphometrics and compared to that of extant Ursidae. An approach including the Ursus species with a varied diet (brown, and American and Asiatic black bears) allows for a more precise focus on food items that are directly relevant to the question of the diet of U. spelaeus. Landmarks for 3D digitisation of the cranium were chosen to reflect functional morphology relating to the attachment of the musculus temporalis and m. masseter and general shape of the cranium. Extant Ursidae and the extinct U. spelaeus were digitised with a Microscribe G2. Generalised Procrustes superimposition was performed on the coordinates and allometry corrected for using pooled regression analysis. Principal Component Analysis (PCA) was conducted and interpreted with respect to diet. PCA differentiates between genera in Ursidae and known dietary composition in extant Ursus on principal components 1 and 2. The position of U. spelaeus in morphospace suggests more foliage intake, relative to the extant brown bear Ursus arctos. These results suggest that the diet of U. spelaeus likely consisted mostly of foliage. Two cave bear specimens from Moravský Kras and Merkensteinhöhle are separate from the others on PC2; this may be due to dietary differentiation, but the exact interpretation remains elusive, because precise geographical and temporal data are missing for the specimen from Moravský Kras

Sexual dimorphism and laterality in the evolution of the primate prefrontal cortex

Social selective pressures are commonly considered as the main driving force of primate brain evolution. Primate social behaviour is, however, known to be sexually dimorphic, and no previous study has made a direct comparison between male and female brain structures across species. We quantify sex-specific evolutionary trends in the prefrontal cortex of anthropoid primates (including humans) to investigate how sexual selection has shaped brain evolution in primates. The prefrontal cortex is of particular importance to the investigation of sexual dimorphism in primate brain evolution because of its association to those cognitive capacities central to primate (and human) evolution: sociality and higher-order cognitive processing. Our results demonstrate sex-by-hemisphere differences in the evolution of the prefrontal cortex in humans and non-human anthropoid primates congruent with the principal selective pressures considered to underlie anthropoid behavioural evolution. Our findings further show how sexual selection can shape brain adaptation in primates and provide an evolutionary framework for interpreting sex and sex-by-hemisphere differences in cortical organization in humans and non-human primates