The human brain has undergone a threefold increase in size as well as changes in cytoarchitecture and neurochemical organization since the split with the last common ancestor with chimpanzees and bonobos. These changes have likely promoted increased cognitive flexibility but may also have come at the cost of increased vulnerability to neurodegenerative diseases like Alzheimer's disease (AD). Alzheimer's disease is the most common form of age-dependent dementia, and previous research suggests that humans may be uniquely vulnerable to AD. Primate comparative induced pluripotent stem cell models offer a unique opportunity to study the cellular and molecular mechanisms underlying this vulnerability. Plaques composed of amyloid-beta (aβ) are a hallmark pathology of AD, and while the exact role of aβ in disease pathogenesis remains incompletely understood, it is clear that aβ can have a neurotoxic effect and plays a role in the characteristic loss of neurons and synapses. Here we investigate susceptibility to aβ-induced toxicity in induced pluripotent stem cell-derived neurons from humans-both with and without AD, the two Pan species-a chimpanzee and a bonobo, and a rhesus macaque. To measure toxicity, we looked at cell death, along with other metrics of neuronal health and connectivity (neuritic beading and synaptic puncta density, respectively). We also employed transcriptomic approaches (RNA sequencing) to investigate molecular mechanisms underlying observed species differences. Our findings show that macaque neurons are less sensitive to aβ than human and ape neurons, and RNA sequencing points to molecular mechanisms that may underlie potentially protective responses in both macaque and ape neurons relative to humans. This work highlights the importance of studying AD from a comparative lens and provides insights into variation in susceptibility to AD among primates.

The Caijiagou-C (CJG-C) site is an early Middle Pleistocene site (0.59-0.75 Ma) that was discovered in the Nihewan Basin, North China. It was systematically excavated from 2020 to 2021. Hundreds of well-preserved mammalian fossils, dominated by large ungulates such as equids and bovids were unearthed. Notable cut marks and impact marks are frequently observed on these fossils. A taphonomic analysis of the site revealed that hominins rather than carnivores contributed mainly to the formation of the faunal assemblage at CJG-C. The anatomical distribution of bone surface modifications further indicated that hominins had primary access to the carcasses. Diverse butchery stages-such as skinning, disarticulation, evisceration, filleting, and marrow extraction-reflect a full-sequence carcass processing pattern, which is rarely observed in East Asia during the Early and early Middle Pleistocene. The clear anthropogenic pattern and multistage butchery sequence suggest that CJG-C can best be interpreted as a small-scale kill-butchery locality, where large ungulates were processed in situ, although we cannot rule out occasional contributions from other depositional processes. Overall, CJG-C provides one of the best-documented early Middle Pleistocene examples of systematic carcass processing in northern East Asia, holding important implications for understanding hominin subsistence strategies in this region.

Neanderthals exhibit unique morphological adaptations in their calcaneus compared to Homo sapiens, shedding light on distinct evolutionary pressures and functional requirements shaping their respective foot anatomy. This study assesses which calcaneal traits represent plesiomorphic, autapomorphic, and/or functional adaptations. Geometric morphometrics was used to analyze the external shape of 7 Neanderthal and 64 H. sapiens calcanei spanning early Upper Pleistocene hunter-gatherers to postindustrial populations. Shape, size, and allometric differences were assessed among groups, enabling detailed examination of the whole calcaneus and key subregions such as talar facets, the cuboid facet, and the tuberosity. Compared to calcanei of H. sapiens, calcanei of Neanderthals were shorter, wider, and taller-distinct adaptations likely favoring more load-bearing over the rearfoot and high mobility. Similar but less pronounced traits appear in highly mobile, unshod modern humans, while sedentary, postindustrial populations show more gracile calcanei, likely influenced by footwear and reduced activity. Neanderthals also displayed ancestral features such as a shorter sinus tarsi. Notably, Neanderthal subtalar joint shape and calcaneocuboid orientation suggest a habitually pronated foot posture. Neanderthals also exhibit a reduced lateral plantar process compared to H. sapiens, implicating differences in plantar ligament attachment and foot arch support. Differences in the cuboid facet and tuberosity of Neanderthals and H. sapiens imply alternative strategies for foot stabilization and force transmission through proximal tarsal joints, likely reflecting a different longitudinal arch expression, one adapted to Neanderthals' body mass and high-activity lifestyle.

The quadrupedal walking gait displayed by most primates differs from that of other mammals in several characteristics. To understand the functional implications of these characteristics, it is useful to explore known exceptions to these generalizations and to determine how widespread any such characteristic actually is. Lorises are one such exception in not exhibiting the typical primate pattern of higher peak vertical forces on their hind limbs than forelimbs. A distinctive pattern of scapulohumeral muscle recruitment is an example of a primate characteristic based only on anthropoids. This study uses electromyography (EMG) to 1) explore the activity patterns of limb retractors in the slow loris (Nycticebus coucang) to address questions related to their unusual weight support distribution and 2) determine if scapulohumeral muscle recruitment in prosimians is like that observed in anthropoids. The EMG results for the hind limb retractors in the slow loris were like those of other primates and did not match study predictions. While the forelimb retractor results did agree with predictions, their inferred contributions to locomotor mechanics were not what was expected. The recruitment patterns for the scapulohumeral muscles in the slow loris as well as four lemur species displayed a mix of similarities and differences to the EMG patterns observed in anthropoids and may relate to a shift in shoulder mechanics between prosimians and anthropoids. However, rather than explaining the higher forelimb than hind limb peak vertical forces, the limb retractor EMG results only raised additional questions about how this distribution of forces is brought about.

Bone surface modifications (BSMs) provide data for modeling predator-prey dynamics and trophic interactions in the fossil record. However, the utility of BSM data depends on our ability to correctly infer which unobservable actions or ecological agents produced these marks, and commonly used qualitative methods may be insufficient to discriminate among BSMs that exhibit substantial macromorphological overlap. Recently, quantitative BSM identification methods have been employed to overcome this taphonomic equifinality, which, despite their methodological rigor, remain underused because they require large databases of experimentally generated BSMs. In the present study, we introduce TaphoSource, an open-source database containing point cloud and measurement data for 942 experimentally generated BSMs, created by replicating five possible taphonomic and depositional actions, as well as 12 fossilized BSMs from the 1.7-million-year-old HWK EE site in Olduvai Gorge, Tanzania. To highlight the utility of this dataset, we used random forest models to distinguish among the five experimentally generated BSM categories based on measurement variability, achieving approximately 73% accuracy. These data were then used to train a random forest model to identify the actions responsible for the 12 fossilized BSMs in the TaphoSource database. We openly disseminate these data so that other researchers can conduct quantitative BSM modeling studies without the need to generate expensive and time-consuming experimental datasets. We anticipate that these data will have broader applications for reconstructing and modeling spatiotemporal trends in carnivory, providing deeper insights into the evolution of both hominin and non-hominin mammalian carnivory and behavior throughout the Plio-Pleistocene.

Mandibular ramus morphology is central to paleoanthropology, yet many quantitative workflows emphasize whole-shape summaries and do not explicitly report the intrinsic geometry of particular anatomical curves and surfaces. We present an automated pipeline that extracts key features of the notch-condyle complex from three-dimensional surface meshes, positions specimens in anatomically defined reference frames, and outputs standardized object-level measurements suitable for reproducible comparison and downstream statistics. We demonstrate the approach on a mixed sample of Pleistocene and Holocene hominin mandibles/rami. Correlation structure and principal component analysis are used to summarize covariation, and permutation-based tests evaluate whether predefined groups differ in multivariate space. Overall, the descriptors show limited redundancy, with only a small subset exhibiting moderate to strong coupling, indicating that the pipeline captures multiple, partly independent aspects of local ramus geometry. The principal component analysis indicates that variability is distributed across several dimensions rather than dominated by a single axis. In the first two components, Holocene specimens form a relatively compact cluster, whereas Pleistocene specimens are more broadly dispersed, with substantial overlap among fossil groups. Despite this overlap, permutation-based testing in reduced principal component space detects a significant effect of group membership, consistent with a conservative Holocene-Pleistocene shift combined with marked within-group dispersion in the fossil sample.

Humans are traditionally classified as diurnal primates, although field evidence from small-scale societies complicates a rigid behavioral reading of that designation. Here, I evaluate the Human Diel Flexibility hypothesis-the idea that Homo sapiens retains diurnal circadian physiology while expressing unusually flexible 24-hour activity for a large-bodied anthropoid. In study 1, I conducted actigraphy-based comparisons of diel (i.e., 24 hour) activity patterns across 10 primate species, including Hadza hunter-gatherers, gibbons (Hylobates), and seven lemur species. Functional linear modeling and Bayesian multivariate analysis revealed that Hadza individuals exhibit elevated night-time activity and a significantly lower day-to-night time ratio (DNTR) than gibbons, overlapping with some lemur genera in this dataset. In study 2, I compiled a comparative dataset of day-to-night activity ratios from 24 primate species and conducted phylogenetic generalized least squares analyses. I found that humans maintain a relatively high cathemerality index in this comparative framework by ecological and morphological variables and that endocranial volume is a significant positive predictor of diel flexibility. Evolutionary model comparisons are consistent with a two-regime Ornstein-Uhlenbeck model with distinct optima for humans and nonhuman primates, suggesting divergent selective pressures on human sleep-wake architecture. I propose the Social Sleep Theory to explain this pattern: through sociotechnological niche construction-group vigilance, fire, and shelter-humans evolved short, flexible, and high-quality sleep that enabled activity redistribution across the diel cycle. These findings challenge a rigid behavioral model of human diurnality while preserving the distinction between circadian physiology and activity expression and expand our understanding of how temporal niche plasticity has contributed to the ecological success of H. sapiens.

The evolution of human bipedalism has traditionally been reconstructed from the pelvis and lower limb, whereas the contribution of the vertebral motion segment has received far less attention. This study introduces the vertebral apophyseal ring (AR) as a novel proxy for reconstructing spinal biomechanics in fossil hominins. Several vertebrae with complete ARs from Australopithecus afarensis A.L. 288-1 were compared with vertebrae from 240 adult modern humans, a Neanderthal (Kebara), an Upper Paleolithic modern human (Ohalo II), 20 chimpanzees (Pan troglodytes), and 24 gorillas (Gorilla gorilla). Vertebral body and AR dimensions were measured from vertebrae T4-L5 (L4 in apes), and corresponding ratios were calculated. Statistical differences between modern humans and African apes were assessed using Welch's analysis of variance followed by Games-Howell post hoc tests. Relative AR size in A.L. 288-1 consistently falls within the range of chimpanzees and gorillas rather than modern humans, indicating that A. afarensis spinal motion segments differed functionally from those of modern humans. Because AR reduction is associated with enlargement of the nucleus pulposus, our findings suggest that spinal adaptations necessary for efficient load absorption, torsional flexibility, and endurance locomotion evolved later in human evolution. These results indicate that the evolution of humanlike bipedalism was a gradual and mosaic process. Although Australopithecus possessed the fundamental capacity for upright walking, it retained primitive spinal features indicative of a less optimized locomotor system. The emergence of a fully modern human gait, therefore, involved progressive reorganization of the vertebral motion segment rather than a single evolutionary transition.

We report a new Middle Pleistocene paleoanthropological locality located in southern Iberian Peninsula, Ruidera, and describe a newly discovered partial parietal bone (RVH-1). The fossil was chronologically constrained through a combination of uranium-series and optically stimulated luminescence dating methods. In particular, the human fossil-bearing breccia may be tentatively dated to between 224 ± 14 ka and 160 ± 10 ka, although an older age cannot be entirely excluded, given the inherent uncertainty in the gamma dose rate evaluation. When considering these new results together with those previously obtained on Caprinae fossil teeth from that same layer, we can reasonably propose a late (second half) Middle Pleistocene age for the Ruidera fossil locality. Morphological analysis shows that RVH-1 exhibits relatively high cranial vault thickness and parietal angle compared with Iberian Middle Pleistocene samples such as the preneanderthals from Sima de los Huesos from Atapuerca (Spain). However, both variables fall within the broader range of variation observed across Homo heidelbergensis (both s.l. and s.s.). In this context, RVH-1 appears to be closer to specimens often described as more plesiomorphic in Europe (e.g., Ceprano) and to some African Middle Pleistocene individuals (e.g., Kabwe). These results highlight the considerable variability in cranial morphology during the Middle Pleistocene and caution against strong taxonomic or phylogenetic interpretations based on isolated traits. RVH-1 is therefore best interpreted within a framework of morphological variability in Europe. RVH-1 provides a critical addition to the European human fossil record, which remains very fragmented for the Middle Pleistocene.

The evolutionary transformation of the hominin hand is of central interest due to its critical role in mediating interaction with both the physical and cultural environments. Beyond its functional significance, however, hand morphology can also reflect broader anatomical trends at the organism level. This study analyzes 31 metacarpal remains from the Neandertal site of El Sidrón (Asturias, Spain), using a combination of discrete morphological traits, linear measurements, and three-dimensional geometric morphometrics based on a 32-landmark template. The dataset is examined within a broad comparative and evolutionary framework. Our results confirm that Neandertal metacarpals differ markedly from those of modern Homo sapiens. They display increased robustness, more strongly developed muscle attachment sites, and enlarged heads, resulting in the characteristic 'lollipop' morphology. Significant differences are also observed in the carpometacarpal joint complex, including a flattened proximal articular facet on the first metacarpal, a reduced styloid process on the third metacarpal, and the absence of torsion in the fifth metacarpal, which also affects its articulation with the fourth metacarpal. Although these traits have often been interpreted as localized functional adaptations related to grip strength, joint loading, and manipulative performance, our results support a complementary anatomical interpretation. Comparable patterns of skeletal hypertrophy and joint reorganization (e.g., articular surface flattening and surface rotation) have been described in other Neandertal anatomical regions (including the clavicle, elbow, sacrolumbar spine, wrist, and femur), suggesting that metacarpal morphology may form part of a broader systemic pattern of skeletal organization. We therefore interpret the Neandertal metacarpals as compatible with an organism-level perspective, while emphasizing that behavioral and functional inferences require additional lines of evidence.

Species boundaries are central to evolutionary biology, yet the reliability of skeletal morphology, particularly isolated postcranial elements, for diagnosing species remains unresolved. Long bones are commonly recovered in the fossil record and often used as operational taxonomic units but are shaped by functional and developmental constraints that may decouple morphological differentiation from species diagnosability. Here, morphological variation in the humerus and femur is examined across extant great apes to assess their utility for species delimitation, with implications for extinct hominins. At the genus level, differences between groups are expressed through significant differences in morphology and commonly in disparity. Among species-level taxa, mean shape differences were consistently detected, although without corresponding differences in disparity, indicating morphological differentiation within a constrained envelope. The results further indicate that the femur may be more informative than the humerus for detecting taxonomic and functional signals among hominids and that data availability can influence results obscuring morphological differences, as demonstrated by simulations on fragmentary bones and analyses of landmark taxonomic signals. Overall, these findings show that although postcranial long bones can track morphological changes and retain valuable taxonomic signals, they remain weak proxies for species boundaries when studied in isolation, with direct implications for taxonomic identification in the hominin fossil record.

Hominin lumbar vertebrae are characterized by adaptations for bipedal locomotion, which are mostly known from adults. However, vertebrae of Homo erectus are exclusively known from immature individuals. As such, understanding the ontogeny of lumbar vertebrae is warranted. Here, we use three-dimensional geometric morphometrics to quantify the morphology of three H. erectus upper lumbar vertebrae, D2672 from Dmanisi, Georgia; KNM-WT 15000 from Nariokotome, Kenya; and SK 853 from Swartkrans, South Africa, as well as those of two Australopithecus sediba U.W.88-92 and U.W.88-232 from Malapa, South Africa, in a comparative developmental context to assess the likely vertebral positions of D2672 and SK 853. The result suggests that the vertebral position of D2672 cannot be distinguished between the first and second lumbar vertebrae, while SK 853 most likely represents a first lumbar vertebra. We also quantified the ontogenetic variation in the first and second lumbar vertebrae of modern humans, from late infancy to full adulthood, and compared these with H. erectus. In modern humans, the lumbar vertebrae increase in size and undergo a series of shape changes across ontogeny. In particular, the vertebral canal relatively decreases in dorsoventral length but increases in mediolateral length with age, thus juveniles possess relatively narrower vertebral canals than adults. All H. erectus individuals fall within the range of modern humans at a comparable developmental stage in the morphospace, despite their smaller size, suggesting that the vertebral traits associated with humanlike bipedal locomotion were established relatively early in ontogeny and in the evolutionary process.

A richly fossiliferous sandstone outcrop in the Wasatch Formation, Great Divide Basin, southern Wyoming, has yielded exceptionally well-preserved fossils of three omomyid primates. Here we report new occurrences of two rarely documented omomyids, Arapahovius gazini and Anemorhysis savagei, describe a new species of Tetonius, and perform a phylogenetic analysis of Tetonius and its allies; the two previously described omomyids are new additions to the fossil record of the Great Divide Basin and represent only the second and third occurrences of these species, respectively. Tetonius, Arapahovius, and Anemorhysis are not known to co-occur elsewhere, particularly in the well-documented Bighorn Basin of northwestern Wyoming, where the Tetonius + Pseudotetonius clade is locally extirpated prior to the first appearances of Arapahovius and Anemorhysis. Morphological support for the new species of Tetonius is corroborated by biostratigraphic data suggesting it is younger than Pseudotetonius ambiguus from the Bighorn Basin. However, the new species of Tetonius retains key dental symplesiomorphies that signal a relatively early divergence from the Tetonius + Pseudotetonius clade as it is recorded in the Bighorn Basin. Results from our maximum parsimony analysis indicate that Tetonius varleyorum sp. nov. from the Great Divide Basin is more closely related to P. ambiguus than either of the species of Tetonius known from the Bighorn Basin, underscoring the paraphyletic status of Tetonius with respect to Pseudotetonius proposed by earlier workers. The evolutionary history of the Tetonius + Pseudotetonius clade was more complex than previously recognized, and this added complexity can only be recognized through broader geographic sampling.