Masato Nakatsukasa

A new Late Miocene great ape from Kenya and its implications for the origins of African great apes and humans

Extant African great apes and humans are thought to have diverged from each other in the Late Miocene. However, few hominoid fossils are known from Africa during this period. Here we describe a new genus of great ape (Nakalipithecus nakayamai gen. et sp. nov.) recently discovered from the early Late Miocene of Nakali, Kenya. The new genus resembles Ouranopithecus macedoniensis (9.6–8.7 Ma, Greece) in size and some features but retains less specialized characters, such as less inflated cusps and better-developed cingula on cheek teeth, and it was recovered from a slightly older age (9.9–9.8 Ma). Although the affinity of Ouranopithecus to the extant African apes and humans has often been inferred, the former is known only from southeastern Europe. The discovery of N. nakayamai in East Africa, therefore, provides new evidence on the origins of African great apes and humans. N. nakayamai could be close to the last common ancestor of the extant African apes and humans. In addition, the associated primate fauna from Nakali shows that hominoids and other non-cercopithecoid catarrhines retained higher diversity into the early Late Miocene in East Africa than previously recognized.

Late Miocene to Pliocene carbon isotope record of differential diet change among East African herbivores.

Stable isotope and molecular data suggest that C4 grasses first appeared globally in the Oligocene. In East Africa, stable isotope data from pedogenic carbonate and fossil tooth enamel suggest a first appearance between 15–10 Ma and subsequent expansion during the Plio-Pleistocene. The fossil enamel record has the potential to provide detailed information about the rates of dietary adaptation to this new resource among different herbivore lineages. We present carbon isotope data from 452 fossil teeth that record differential rates of diet change from C3 to mixed C3/C4 or C4 diets among East African herbivore families at seven different time periods during the Late Miocene to the Pliocene (9.9–3.2 Ma). Significant amounts of C4 grasses were present in equid diets beginning at 9.9 Ma and in rhinocerotid diets by 9.6 Ma, although there is no isotopic evidence for expansive C4 grasslands in this part of the Late Miocene. Bovids and hippopotamids followed suit with individuals that had C4-dominated (>65%) diets by 7.4 Ma. Suids adopted C4-dominated diets between 6.5 and 4.2 Ma. Gomphotheriids and elephantids had mostly C3-dominated diets through 9.3 Ma, but became dedicated C4 grazers by 6.5 Ma. Deinotheriids and giraffids maintained a predominantly C3 diet throughout the record. The sequence of differential diet change among herbivore lineages provides ecological insight into a key period of hominid evolution and valuable information for future studies that focus on morphological changes associated with diet change.

Differential Prefrontal White Matter Development in Chimpanzees and Humans

A comparison of developmental patterns of white matter (WM) within the prefrontal region between humans and nonhuman primates is key to understanding human brain evolution. WM mediates complex cognitive processes and has reciprocal connections with posterior processing regions [1, 2]. Although the developmental pattern of prefrontal WM in macaques differs markedly from that in humans [3], this has not been explored in our closest evolutionary relative, the chimpanzee. The present longitudinal study of magnetic resonance imaging scans demonstrated that the prefrontal WM volume in chimpanzees was immature and had not reached the adult value during prepuberty, as observed in humans but not in macaques. However, the rate of prefrontal WM volume increase during infancy was slower in chimpanzees than in humans. These results suggest that a less mature and more protracted elaboration of neuronal connections in the prefrontal portion of the developing brain existed in the last common ancestor of chimpanzees and humans, and that this served to enhance the impact of postnatal experiences on neuronal connectivity. Furthermore, the rapid development of the human prefrontal WM during infancy may help the development of complex social interactions, as well as the acquisition of experience-dependent knowledge and skills to shape neuronal connectivity.

Acquisition of bipedalism: the Miocene hominoid record and modern analogues for bipedal protohominids

The well‐known fossil hominoid Proconsul from the Early Miocene of Kenya was a non‐specialized arboreal quadruped with strong pollicial/hallucial assisted grasping capability. It lacked most of the suspensory specializations acquired in living hominoids. Nacholapithecus, however, from the Middle Miocene of Kenya, although in part sharing with Proconsul the common primitive anatomical body design, was more specialized for orthograde climbing, ‘hoisting’ and bridging, with the glenoid fossae of the scapula probably being cranially orientated, the forelimbs proportionally large, and very long toes. Its tail loss suggests relatively slow movement, although tail loss may already have occurred in Proconsul. Nacholapithecus‐like positional behaviour might thus have been a basis for development of more suspensory specialized positional behaviour in later hominoids. Unfortunately, after 13 Ma, there is a gap in the hominoid postcranial record in Africa until 6 Ma. Due to this gap, a scenario for later locomotor evolution prior to the divergence of Homo and Pan cannot be determined with certainty. The time gap also causes difficulties when we seek to determine polarities of morphological traits in very early hominids. Interpretation of the form–function relationships of postcranial features in incipient hominids will be difficult because it is predicted that they had incorporated bipedalism only moderately into their total positional repertoires. However, Japanese macaques, which are trained in traditional bipedal performance, may provide useful hints about bipedal adaptation in the protohominids. Kinematic analyses revealed that these macaques walked bipedally with a longer stride and lower stride frequency than used by ordinary macaques, owing to a more extended posture of the hindlimb joints. The body centre of gravity rises during the single‐support phase of stance. Energetic studies of locomotion in these bipedal macaques revealed that energetic expenditure was 20–30% higher in bipedalism than in quadrupedalism, regardless of walking velocity.

Human Origins and Environmental Backgrounds.

Hidemi Ishida: 40 Years of Footprints in Japanese Primatology and Paleoanthropology.- Hidemi Ishida: 40 Years of Footprints in Japanese Primatology and Paleoanthropology.- Fossil Hominoids and Paleoenvironments.- Seven Decades of East African Miocene Anthropoid Studies.- Evolution of the Vertebral Column in Miocene Hominoids and Plio-Pleistocene Hominids.- Terrestriality in a Middle Miocene Context: Victoriapithecus from Maboko, Kenya.- Late Cenozoic Mammalian Biostratigraphy And Faunal Change.- The Ages and Geological Backgrounds of Miocene Hominoids Nacholapithecus, Samburupithecus, and Orrorin from Kenya.- Functional Morphology.- Patterns of Vertical Climbing in Primates.- Functional Morphology of the Midcarpal Joint in Knuckle-Walkers and Terrestrial Quadrupeds.- Morphological Adaptation of Rat Femora to Different Mechanical Environments.- A Hallmark of Humankind: The Gluteus Maximus Muscle.- Primates Trained for Bipedal Locomotion as a Model for Studying the Evolution of Bipedal Locomotion.- Locomotor Energetics in Nonhuman Primates.- Computer Simulation of Bipedal Locomotion.- Theoretical Approaches.- Paleoenvironments, Paleoecology, Adaptations, and the Origins of Bipedalism in Hominidae.- Arboreal Origin of Bipedalism.- Neontological Perspectives on East African Middle and Late Miocene Anthropoidea.- The Prehominid Locomotion Reflected: Energetics, Muscles, and Generalized Bipeds.- Evolution of the Social Structure of Hominoids.- Are Human Beings Apes, or are Apes People too?.- Current Thoughts on Terrestrialization in African Apes and the Origin of Human Bipedalism.

Three-dimensional musculoskeletal kinematics during bipedal locomotion in the Japanese macaque, reconstructed based on an anatomical model-matching method.

Studying the bipedal locomotion of non-human primates is important for clarifying the evolution of habitual bipedalism in the human lineage. However, quantitative descriptions of three-dimensional kinematics of bipedal locomotion in non-human primates are very scarce, due to difficulties associated with measurements. In this study, we performed a kinematic analysis of bipedal locomotion on two highly trained (performing) Japanese macaques walking on a treadmill at different speeds and estimated three-dimensional angular motions of hindlimb and trunk segments, based on a model-based registration method. Our results demonstrated a considerable degree of axial rotation occurring at the trunk and hip joints during bipedal locomotion, suggesting that bipedal locomotion in Japanese macaques is essentially three-dimensional. In addition, ranges of angular motions at the hip and ankle joints were larger and the knee joint was more flexed in the mid-stance phase with increasing speed, indicating that gait kinematics are modulated depending on speed. Furthermore, macaques were confirmed to have actually acquired, at least to some extent, the energy conservation mechanism of walking due to pendular exchange of potential and kinetic energy, but effective utilization of this mutual exchange of energy was found to occur only at comparatively low velocity. Spring-like running mechanics were probably more exploited at higher speed because the duty factor was above 0.5. Fundamental differences in bipedal strategy seem to exist between human and non-human primate bipedal locomotion.

Development of an anatomically based whole-body musculoskeletal model of the Japanese macaque (Macaca fuscata).

We constructed a three-dimensional whole-body musculoskeletal model of the Japanese macaque (Macaca fuscata) based on computed tomography and dissection of a cadaver. The skeleton was modeled as a chain of 20 bone segments connected by joints. Joint centers and rotational axes were estimated by joint morphology based on joint surface approximation using a quadric function. The path of each muscle was defined by a line segment connecting origin to insertion through an intermediary point if necessary. Mass and fascicle length of each were systematically recorded to calculate physiological cross-sectional area to estimate the capacity of each muscle to generate force. Using this anatomically accurate model, muscle moment arms and force vectors generated by individual limb muscles at the foot and hand were calculated to computationally predict muscle functions. Furthermore, three-dimensional whole-body musculoskeletal kinematics of the Japanese macaque was reconstructed from ordinary video sequences based on this model and a model-based matching technique. The results showed that the proposed model can successfully reconstruct and visualize anatomically reasonable, natural musculoskeletal motion of the Japanese macaque during quadrupedal/bipedal locomotion, demonstrating the validity and efficacy of the constructed musculoskeletal model. The present biologically relevant model may serve as a useful tool for comprehensive understanding of the design principles of the musculoskeletal system and the control mechanisms for locomotion in the Japanese macaque and other primates.

Muscle dimensions in the chimpanzee hand

We dissected the forearms and hands of a female chimpanzee and systematically recorded mass, fiber length, and physiological cross-sectional area (PCSA) of all muscles including those of intrinsic muscles that have not been reported previously. The consistency of our measurements was confirmed by comparison with the published data on chimpanzees. Comparisons of the hand musculature of the measured chimpanzee with corresponding published human data indicated that the chimpanzee has relatively larger forearm flexors but smaller thenar eminence muscles, as observed in previous studies. The interosseous muscles were also confirmed to be relatively larger in the chimpanzee. However, a new finding was that relative PCSA, which reflects a muscle’s capacity to generate force, might have increased slightly in humans as a result of relatively shorter muscle fiber length. This suggests that the human intrinsic muscle architecture is relatively more adapted to dexterous manipulative functions. Shortening of the metacarpals and the intervening interosseous muscles might accordingly be a prerequisite for the evolution of human precision-grip capabilities.

Trabecular bone anisotropy and orientation in an Early Pleistocene hominin talus from East Turkana, Kenya

Among the structural properties of trabecular bone, the degree of anisotropy is most often found to separate taxa with different habitual locomotor modes. This study examined the degree of anisotropy, the elongation, and primary orientation of trabecular bone in the KNM-ER 1464 Early Pleistocene hominin talus as compared with extant hominoid taxa. Modern human tali were found to have a pattern of relatively anisotropic and elongated trabeculae on the lateral aspect, which was not found in Pan, Gorilla, Pongo, or KNM-ER 1464. Trabecular anisotropy in the fossil talus most closely resembled that of the African apes except for a region of high anisotropy in the posteromedial talus. The primary orientation of trabeculae in the anteromedial region of KNM-ER 1464 was strikingly different from that of the great apes and very similar to that of modern humans in being directed parallel to the talar neck. These results suggest that, relative to that of modern humans, the anteromedial region of the KNM-ER 1464 talus may have transmitted body weight to the midfoot in a similar manner while the lateral aspect may have been subjected to more variable loading conditions.

Developmental patterns of chimpanzee cerebral tissues provide important clues for understanding the remarkable enlargement of the human brain.

Developmental prolongation is thought to contribute to the remarkable brain enlargement observed in modern humans (Homo sapiens). However, the developmental trajectories of cerebral tissues have not been explored in chimpanzees (Pan troglodytes), even though they are our closest living relatives. To address this lack of information, the development of cerebral tissues was tracked in growing chimpanzees during infancy and the juvenile stage, using three-dimensional magnetic resonance imaging and compared with that of humans and rhesus macaques (Macaca mulatta). Overall, cerebral development in chimpanzees demonstrated less maturity and a more protracted course during prepuberty, as observed in humans but not in macaques. However, the rapid increase in cerebral total volume and proportional dynamic change in the cerebral tissue in humans during early infancy, when white matter volume increases dramatically, did not occur in chimpanzees. A dynamic reorganization of cerebral tissues of the brain during early infancy, driven mainly by enhancement of neuronal connectivity, is likely to have emerged in the human lineage after the split between humans and chimpanzees and to have promoted the increase in brain volume in humans. Our findings may lead to powerful insights into the ontogenetic mechanism underlying human brain enlargement.