The anomalous archaic Homo femur from Berg Aukas, Namibia: a biomechanical assessment

Main morphological characteristics and sexual dimorphism of hominin adult femora from the Sima de los Huesos Middle Pleistocene site (Sierra de Atapuerca, Spain)

The excellent fossil record from Sima de los Huesos (SH) includes three well-known complete adult femora and several partial specimens that have not yet been published in detail. This fossil record provides an opportunity to analyze the morphology of European pre-Neandertal adult femur and its variation with different evolution patterns. Currently, there are a minimum of five adult individuals (males or females). In this study, we compiled previously published basic anatomical and biometric characteristics of SH adult femora, emphasizing the most relevant features compared to other recent and fossil hominins. The SH femora exhibited a primitive morphological pattern common to all non-Homo sapiens femora, as well as most of the Neandertal traits. Therefore, the complete Upper Pleistocene Neandertal pattern was well-established in Middle Pleistocene ancestors long before the proper Neandertals appeared. Additionally, we highlight that the SH and Neandertal femora share some morphological traits and proportions with modern humans that hold sexual significance in our species, regardless of size. Keeping this in mind, we discussed the sex determination of the complete SH specimens and re-evaluated sex allocation in two of them.

Exploring the morphology of adult tibia and fibula from Sima de los Huesos site in sierra de Atapuerca, Burgos, Spain

The analysis of the locomotor anatomy of Late Pleistocene Homo has largely focused on changes in proximal femur and pelvic morphologies, with much attention centered on the emergence of modern humans. Although much of the focus has been on changes in the proximal femur, some research has also been conducted on tibiae and, to a lesser extent, fibulae. With this in mind, we present one of the largest samples of the same population of human tibiae and fibulae from the Middle Pleistocene to determine their main characteristic traits and establish similarities and differences, primarily with those of Neanderthals and modern humans, but also with other Middle Pleistocene specimens in the fossil record. Through this study, we established that the Middle Pleistocene population from the Sima de los Huesos (Atapuerca, Burgos, Spain) had lower leg long bones similar to those of Neanderthals, although there were some important differences, such as bone length, which this fossil individuals resembled those of modern humans and not to Neanderthals. This fact is related to the crural index and leg length, even though we do not have any true association between femora and tibiae yet, it has implications for establishing locomotor efficiency and climate adaptation.

Variation in cross-sectional indicator of femoral robusticity in Homo sapiens and Neandertals

Variations in the cross-sectional properties of long bones are used to reconstruct the activity of human groups and differences in their respective habitual behaviors. Knowledge of what factors influence bone structure in Homo sapiens and Neandertals is still insufficient thus, this study investigated which biological and environmental variables influence variations in the femoral robusticity indicator of these two species. The sample consisted of 13 adult Neandertals from the Middle Paleolithic and 1959 adult individuals of H. sapiens ranging chronologically from the Upper Paleolithic to recent times. The femoral biomechanical properties were derived from the European data set, the subject literature, and new CT scans. The material was tested using a Mantel test and statistical models. In the models, the polar moment of area (J) was the dependent variable; sex, age, chronological period, type of lifestyle, percentage of the cortical area (%CA), the ratio of second moment areas of inertia about the X and Y axes (Ix/Iy), and maximum slope of the terrain were independent covariates. The Mantel tests revealed spatial autocorrelation of the femoral index in H. sapiens but not in Neandertals. A generalized additive mixed model showed that sex, %CA, Ix/Iy, chronological period, and terrain significantly influenced variation in the robusticity indicator of H. sapiens femora. A linear mixed model revealed that none of the analyzed variables correlated with the femoral robusticity indicator of Neandertals. We did not confirm that the gradual decline in the femoral robusticity indicator of H. sapiens from the Middle Paleolithic to recent times is related to the type of lifestyle; however, it may be associated with lower levels of mechanical loading during adolescence. The lack of correlation between the analysed variables and the indicator of femoral robusticity in Neandertals may suggest that they needed a different level of mechanical stimulus to produce a morphological response in the long bone than H. sapiens.

Femoral neck and shaft structure in Homo naledi from the Dinaledi Chamber (Rising Star System, South Africa)

The abundant femoral assemblage of Homo naledi found in the Dinaledi Chamber provides a unique opportunity to test hypotheses regarding the taxonomy, locomotion, and loading patterns of this species. Here we describe neck and shaft cross-sectional structure of all the femoral fossils recovered in the Dinaledi Chamber and compare them to a broad sample of fossil hominins, recent humans, and extant apes. Cross-sectional geometric (CSG) properties from the femoral neck (base of neck and midneck) and diaphysis (subtrochanteric region and midshaft) were obtained through CT scans for H. naledi and through CT scans or from the literature for the comparative sample. The comparison of CSG properties of H. naledi and the comparative samples shows that H. naledi femoral neck is quite derived with low superoinferior cortical thickness ratio and high relative cortical area. The neck appears superoinferiorly elongated because of two bony pilasters on its superior surface. Homo naledi femoral shaft shows a relatively thick cortex compared to the other hominins. The subtrochanteric region of the diaphysis is mediolaterally elongated resembling early hominins while the midshaft is anteroposteriorly elongated, indicating high mobility levels. In term of diaphyseal robusticity, the H. naledi femur is more gracile that other hominins and most apes. Homo naledi shows a unique combination of characteristics in its femur that undoubtedly indicate a species committed to terrestrial bipedalism but with a unique loading pattern of the femur possibly consequence of the unique postcranial anatomy of the species.

Stature, body mass, and brain size: a two-million-year odyssey

Physical size has been critical in the evolutionary success of the genus Homo over the past 2.4 million-years. An acceleration in the expansion of savannah grasslands in Africa from 1.6Ma to 1.2Ma witnessed concomitant increases in physical stature (150-170cm), weight (50-70kg), and brain size (750-900cm(3)). With the onset of 100,000year Middle Pleistocene glacial cycles ("ice ages") some 780,000years ago, large-bodied Homo groups had reached modern size and had successfully dispersed from equatorial Africa, Central, and Southeast Asia to high-latitude localities in Atlantic Europe and North East Asia. While there is support for incursions of multiple Homo lineages to West Asia and Continental Europe at this time, data does not favour a persistence of Homo erectus beyond ∼400,000years ago in Africa, west and Central Asia, and Europe. Novel Middle Pleistocene Homo forms (780,000-400,000years) may not have been substantially taller (150-170cm) than earlier Homo (1.6Ma-800,000years), yet brain size exceeded 1000cm(3) and body mass approached 80kg in some males. Later Pleistocene Homo (400,000-138,000years) were 'massive' in their height (160-190cm) and mass (70-90kg) and consistently exceed recent humans. Relative brain size exceeds earlier Homo, yet is substantially lower than in final glacial H. sapiens and Homo neanderthalensis. A final leap in absolute and relative brain size in Homo (300,000-138,000years) occurred independent of any observed increase in body mass and implies a different selective mediator to that operating on brain size increases observed in earlier Homo.

The shape of the early hominin proximal femur

Postcranial skeletal variation among Plio-Pleistocene hominins has implications for taxonomy and locomotor adaptation. Although sample size constraints make interspecific comparisons difficult, postcranial differences between Australopithecus afarensis and Australopithecus africanus have been reported (McHenry and Berger: J Hum Evol 35 1998 1-22; Richmond et al.: J Hum Evol 43 [2002] 529-548; Green et al.: J Hum Evol 52 2007 187-200). Additional evidence indicates that the early members of the genus Homo show morphology like recent humans (e.g., Walker and Leakey: The Nariokotome Homo erectus skeleton. Cambridge: Harvard, 1993). Using a larger fossil sample than previous studies and novel methods, the early hominin proximal femur is newly examined to determine whether new data alter the current view of femoral evolution and inform the issue of interspecific morphological variation among australopiths. Two- and three-dimensional data are collected from large samples of recent humans, Pan, Gorilla, and Pongo and original fossil femora of Australopithecus, Paranthropus, and femora of African fossil Homo. The size-adjusted shape data are analyzed using principal components, thin plate spline analysis, and canonical variate analysis to assess shape variation. The results indicate that femora of fossil Homo are most similar to modern humans but share a low neck-shaft angle (NSA) with australopiths. Australopiths as a group have ape-like greater trochanter morphology. A. afarensis differs from P. robustus and A. africanus in attributes of the neck and NSA. However, interspecific femoral variation is low and australopiths are generally morphologically similar. Although the differences are not dramatic, when considered in combination with other postcranial evidence, the adaptive differences among australopiths in craniodental morphology may have parallels in the postcranium.

Limb length and locomotor biomechanics in the genusHomo: An experimental study

The striking variation in limb proportions within the genus Homo during the Pleistocene has important implications for understanding biomechanics in the later evolution of human bipedalism, because longer limbs and limb segments may increase bending moments about bones and joints. This research tested the hypothesis that long lower limbs and tibiae bring about increases in A-P bending forces on the lower limb during the stance phase of human walking. High-speed 3-D video data, force plates, and motion analysis software were used to analyze the walking gait of 27 modern human subjects. Limb length, as well as absolute and relative tibia length, were tested for associations with a number of kinetic and kinematic variables. Results show that individuals with longer limbs do incur greater bending moments along the lower limb during the first half of stance phase. During the second half of stance, individuals moderate bending moments through a complex of compensatory mechanisms, including keeping the knee in a more extended position. Neither absolute nor relative tibia length had any effect on the kinetic or kinematic variables tested. If these patterns apply to fossil Homo, groups with relatively long limbs (e.g. H. ergaster or early H. sapiens) may have experienced elevated bending forces along the lower limb during walking compared to those with relatively shorter limbs (e.g. the Neandertals). These increased forces could have led to greater reinforcement of joints and diaphyses. These results must be considered when formulating explanations for variation in limb morphology among Pleistocene hominins.

Temporal trends in femoral curvature and length in medieval and modern Scotland

We measured how much the radius of the anterior curvature and the length of the femoral shaft of cadaveric bones have changed from medieval to recent times. Around 20 (x, y) coordinates of a virtual coordinate system were measured at intervals of 1.5 cm along the shaft of the femur to calculate one single radius of a virtual circle in the (x, y) plane. The median radii of curvature were 119, 141, and 158 cm for medieval, early, and late 20th century femora, respectively. Early and late 20th century femora were of similar length (45 cm), but medieval femora were shorter (43.5 cm). Femora have become not only longer but also straighter since the Middle Ages. These findings account in part for the increase in height of modern generations. Size and shape changes may have significant implications for the biomechanical response of the femur to the forces to which it is subjected in everyday life, in trauma, and following surgical intervention.

Comparative morphology and paleobiology of Middle Pleistocene human remains from the Bau de l'Aubesier, Vaucluse, France

The discovery of later Middle Pleistocene human remains from the Bau de l'Aubesier, France reinforces an evolutionary model of the gradual accumulation of Neandertal-derived facial and dental features during the Middle Pleistocene of the northwestern Old World. The pronounced maxillary incisor beveling of Aubesier 4 helps to extend the antiquity of nondietary use of the anterior dentition. The interproximal "toothpick" groove on the Aubesier 10 molar increases the sample for these lesions. The pathological loss of the mandibular dentition of Aubesier 11 indicates advanced antemortem masticatory impairment, at a level previously undocumented before the Late Pleistocene. These remains support a view of later Middle Pleistocene humans able to support debilitated individuals despite the considerable use of their bodies to accomplish routine activities.