Covariation between the shape and mineralized tissues of the rib cross section in Homo sapiens, Pan troglodytes and Sts 14

OBJECTIVES

Studying rib torsion is crucial for understanding the evolution of the hominid ribcage. Interestingly, there are variables of the rib cross section that could be associated with rib torsion and, consequently, with the morphology of the thorax. The aim of this research is to conduct a comparative study of the shape and mineralized tissues of the rib cross section in different hominids to test for significant differences and, if possible, associate them to different thoracic morphotypes.

MATERIALS AND METHODS

The sample consists of the rib cross sections at the midshaft taken from 10 Homo sapiens and 10 Pan troglodytes adult individuals, as well as from A. africanus Sts 14. The shape of these rib cross sections was quantified using geometric morphometrics, while the mineralized tissues were evaluated using the compartmentalization index. Subsequently, covariation between both parameters was tested by a Spearman's ρ test, a permutation test and a linear regression.

RESULTS

Generally, P. troglodytes individuals exhibit rib cross sections that are rounder and more mineralized compared to those of H. sapiens. However, the covariation between both parameters was only observed in typical ribs (levels 3-10). Although covariation was not found in the rib cross sections of Sts 14, their parameters are closer to P. troglodytes.

DISCUSSION

On the one hand, the differences observed in the rib cross sections between H. sapiens and P. troglodytes might be related to different degrees of rib torsion and, consequently, to different thoracic 3D configurations. These findings can be functionally explained by considering their distinct modes of breathing and locomotion. On the other hand, although the rib cross sections belonging to Sts 14 are more similar to those of P. troglodytes, previous publications determined that their overall morphology is closer to modern humans. This discrepancy could reflect a diversity of post-cranial adaptations in Australopithecus.

Evolutionary assembly and disassembly of the mammalian sternum

Evolutionary transitions are frequently associated with novel anatomical structures,¹ but the origins of the structures themselves are often poorly known. We use developmental, genetic, and paleontological data to demonstrate that the therian sternum was assembled from pre-existing elements. Imaging of the perinatal mouse reveals two paired sternal elements, both composed primarily of cells with lateral plate mesoderm origin. Location, articulations, and development identify them as homologs of the interclavicle and the sternal bands of synapsid outgroups. The interclavicle, not previously recognized in therians,² articulates with the clavicle and differs from the sternal bands in both embryonic HOX expression and pattern of skeletal maturation. The sternal bands articulate with the ribs in two styles, most clearly differentiated by their association with sternebrae. Evolutionary trait mapping indicates that the interclavicle and sternal bands were independent elements throughout most of synapsid history. The differentiation of rib articulation styles and the subdivision of the sternal bands into sternebrae were key innovations likely associated with transitions in locomotor and respiratory mechanics.^3,4 Fusion of the interclavicle and the anterior sternal bands to form a presternum anterior to the first sternebra was a historically recent innovation unique to therians. Subsequent disassembly of the radically reduced sternum of mysticete cetaceans was element specific, reflecting the constraints that conserved developmental programs exert on composite structures.

Variation in human 3D trunk shape and its functional implications in hominin evolution

This study investigates the contribution of external trunk morphology and posture to running performance in an evolutionary framework. It has been proposed that the evolution from primitive to derived features of torso shape involved changes from a mediolaterally wider into a narrower, and antero-posteriorly deeper into a shallower, more lightly built external trunk configuration, possibly in relation to habitat-related changes in locomotor and running behaviour. In this context we produced experimental data to address the hypothesis that medio-laterally narrow and antero-posteriorly shallow torso morphologies favour endurance running capacities. We used 3D geometric morphometrics to relate external 3D trunk shape of trained, young male volunteers (N = 27) to variation in running velocities during different workloads determined at 45–50%, 70% and 85% of heart rate reserve (HRR) and maximum velocity. Below 85% HRR no relationship existed between torso shape and running velocity. However, at 85% HRR and, more clearly, at maximum velocity, we found highly statistically significant relations between external torso shape and running performance. Among all trained subjects those with a relatively narrow, flat torso, a small thoracic kyphosis and a more pronounced lumbar lordosis achieved significantly higher running velocities. These results support the hypothesis that external trunk morphology relates to running performance. Low thoracic kyphosis with a flatter ribcage may affect positively respiratory biomechanics, while increased lordosis affects trunk posture and may be beneficial for lower limb biomechanics related to leg return. Assuming that running workload at 45–50% HRR occurs within aerobic metabolism, our results may imply that external torso shape is unrelated to the evolution of endurance running performance.

The evolution of human fatigue resistance

Humans differ from African great apes in numerous respects, but the chief initial difference setting hominins on their unique evolutionary trajectory was habitual bipedalism. The two most widely supported selective forces for this adaptation are increased efficiency of locomotion and improved ability to feed in upright contexts. By 4 million years ago, hominins had evolved the ability to walk long distances but extreme selection for endurance capabilities likely occurred later in the genus Homo to help them forage, power scavenge and persistence hunt in hot, arid conditions. In this review we explore the hypothesis that to be effective long-distance walkers and especially runners, there would also have been a strong selective benefit among Homo to resist fatigue. Our hypothesis is that since fatigue is an important factor that limits the ability to perform endurance-based activities, fatigue resistance was likely an important target for selection during human evolution for improved endurance capabilities. We review the trade-offs between strength, power, and stamina in apes and Homo and discuss three biological systems that we hypothesize humans evolved adaptations for fatigue resistance: neurological, metabolic and thermoregulatory. We conclude that the evolution of endurance at the cost of strength and power likely also involved the evolution of mechanisms to resist fatigue.

Comparing high versus low-altitude populations to test human adaptations for increased ventilation during sustained aerobic activity

Despite aerobic activity requiring up to tenfold increases in air intake, human populations in high-altitude hypoxic environments can sustain high levels of endurance physical activity. While these populations generally have relatively larger chest and lung volumes, how thoracic motions actively increase ventilation is unknown. Here we show that rib movements, in conjunction with chest shape, contribute to ventilation by assessing how adulthood acclimatization, developmental adaptation, and population-level adaptation to high-altitude affect sustained aerobic activity. We measured tidal volume, heart rate, and rib-motion during walking and running in lowland individuals from Boston (~ 35 m) and in Quechua populations born and living at sea-level (~ 150 m) and at high altitude (> 4000 m) in Peru. We found that Quechua participants, regardless of birth or testing altitudes, increase thoracic volume 2.0-2.2 times more than lowland participants (p < 0.05). Further, Quechua individuals from hypoxic environments have deeper chests resulting in 1.3 times greater increases in thoracic ventilation compared to age-matched, sea-level Quechua (p < 0.05). Thus, increased thoracic ventilation derives from a combination of acclimatization, developmental adaptation, and population-level adaptation to aerobic demand in different oxygen environments, demonstrating that ventilatory demand due to environment and activity has helped shape the form and function of the human thorax.

Cultural variation in running techniques among non-industrial societies

Research among non-industrial societies suggests that body kinematics adopted during running vary between groups according to the cultural importance of running. Among groups in which running is common and an important part of cultural identity, runners tend to adopt what exercise scientists and coaches consider to be good technique for avoiding injury and maximising performance. In contrast, among groups in which running is not particularly culturally important, people tend to adopt suboptimal technique. This paper begins by describing key elements of good running technique, including landing with a forefoot or midfoot strike pattern and leg oriented roughly vertically. Next, we review evidence from non-industrial societies that cultural attitudes about running associate with variation in running techniques. Then, we present new data from Tsimane forager–horticulturalists in Bolivia. Our findings suggest that running is neither a common activity among the Tsimane nor is it considered an important part of cultural identity. We also demonstrate that when Tsimane do run, they tend to use suboptimal technique, specifically landing with a rearfoot strike pattern and leg protracted ahead of the knee (called overstriding). Finally, we discuss processes by which culture might influence variation in running techniques among non-industrial societies, including self-optimisation and social learning.

New evidence from the Tsimane underscores that running techniques vary between societies according to the cultural importance of running

Neuromechanical linkage between the head and forearm during running

OBJECTIVES

The main objective was to test the hypothesis of a neuromechanical link in humans between the head and forearm during running mediated by the biceps brachii and superior trapezius muscles. We hypothesized that this linkage helps stabilize the head and combats rapid forward pitching during running which may interfere with gaze stability.

MATERIALS AND METHODS

Thirteen human participants walked and ran on a treadmill while motion capture recorded body segment kinematics and electromyographic sensors recorded muscle activation. To test perturbations to the linkage system we compared participants running normally as well as with added mass to the face and the hand.

RESULTS

The results confirm the presence of a neuromechanical linkage between the head and forearm mediated by the biceps and superior trapezius during running but not during walking. In running, the biceps and superior trapezius activations were temporally linked during the stride cycle, and adding mass to either the head or hand increased activation in both muscles, consistent with our hypothesis. During walking the forces acting on the body segments and muscle activation levels were much smaller than during running, indicating no need for a linkage to keep the head and gaze stable.

DISCUSSION

The results suggest that the evolution of long distance running in early Homo may have favored selection for reduced rotational inertia of both the head and forearm through synergistic muscle activation, contributing to the transition from australopith head and forelimb morphology to the more human-like form of Homo erectus. Selective pressures from the evolution of bipedal walking were likely much smaller, but may explain in part the intermediate form of the australopith scapula between that of extant apes and humans.

An overview of de novo bone generation in animal models

Some of the earliest success in de novo tissue generation was in bone tissue, and advances, facilitated by the use of endogenous and exogenous progenitor cells, continue unabated. The concept of one health promotes shared discoveries among medical disciplines to overcome health challenges that afflict numerous species. Carefully selected animal models are vital to development and translation of targeted therapies that improve the health and well-being of humans and animals alike. While inherent differences among species limit direct translation of scientific knowledge between them, rapid progress in ex vivo and in vivo de novo tissue generation is propelling revolutionary innovation to reality among all musculoskeletal specialties. This review contains a comparison of bone deposition among species and descriptions of animal models of bone restoration designed to replicate a multitude of bone injuries and pathology, including impaired osteogenic capacity.

Rib cage anatomy in Homo erectus suggests a recent evolutionary origin of modern human body shape

The tall and narrow body shape of anatomically modern humans (Homo sapiens) evolved via changes in the thorax, pelvis and limbs. It is debated, however, whether these modifications first evolved together in African Homo erectus, or whether H. erectus had a more primitive body shape that was distinct from both the more ape-like Australopithecus species and H. sapiens. Here we present the first quantitative three-dimensional reconstruction of the thorax of the juvenile H. erectus skeleton, KNM-WT 15000, from Nariokotome, Kenya, along with its estimated adult rib cage, for comparison with H. sapiens and the Kebara 2 Neanderthal. Our three-dimensional reconstruction demonstrates a short, mediolaterally wide and anteroposteriorly deep thorax in KNM-WT 15000 that differs considerably from the much shallower thorax of H. sapiens, pointing to a recent evolutionary origin of fully modern human body shape. The large respiratory capacity of KNM-WT 15000 is compatible with the relatively stocky, more primitive, body shape of H. erectus.

Late subadult ontogeny and adult aging of the human thorax reveals divergent growth trajectories between sexes

Sexual dimorphism is an important feature of adult thorax morphology, but when and how sex-related differences in the ribcage arise during ontogeny is poorly known. Previous research proposed that sex-related size differences in the nasal region arise during puberty. Therefore, we explore whether ribcage sexual dimorphism also arises at that time and whether this sexual dimorphism is maintained until old age. We measured 526 (semi)landmarks on 80 CT-based human ribcage reconstructions, on individuals ranging from 7 to 65 year-old. The 3D coordinates were submitted to the Procrustes superimposition and analyzed. Our results show that the trajectories of thorax size and shape between sexes diverge at around 12 years of age, and continue slightly diverging until old age. The differential ontogenetic trends cause adult male ribcages to become deeper, shorter, and wider than female. Our results are consistent with the evidence from the cranial respiratory system, with the development of sexual dimorphism probably related to changes in body composition during puberty combined with changes in the reproductive system.