Reevaluation of the lumbosacral region of Oreopithecus bambolii

The taxonomy of Sahelanthropus tchadensis from a craniometric perspective

Sahelanthropus tchadensis has raised much debate since its initial discovery in Chad in 2001, given its controversial classification as the earliest representative of the hominin lineage. This debate extends beyond the phylogenetic position of the species, and includes several aspects of its habitual behavior, especially in what regards its locomotion. The combination of ancestral and derived traits observed in the fossils associated with the species has been used to defend different hypotheses related to its relationship to hominins. Here, the cranial morphology of Sahelanthropus tchadensis was assessed through 16 linear craniometric measurements, and compared to great apes and hominins through Principal Component Analysis based on size and shape and shape information alone. The results show that S. tchadensis share stronger morphological affinities with hominins than with apes for both the analysis that include size information and the one that evaluates shape alone. Since TM 266-01-060-1 shows a strong morphological affinity with the remaining hominins represented in the analysis, our results support the initial interpretations that S. tchadensis represents an early specimen of our lineage or a stem basal lineage more closely related to hominins than to Panini.

Lufengpithecus inner ear provides evidence of a common locomotor repertoire ancestral to human bipedalism

Various lines of evidence have been used to infer the origin of human bipedalism, but the paucity of hominoid postcranial fossils and the diversity of inferred locomotor modes have tended to confound the reconstruction of ancestral morphotypes. Examination of the bony labyrinth morphology of the inner ear of extinct and living hominoids provides independent evidence for inferring the evolution of hominoid locomotor patterns. New computed tomography data and morphometric analyses of the Late Miocene ape Lufengpithecus indicate that it and other stem great apes possess labyrinths similar to one another and show that hominoids initially evolved from a positional repertoire that included orthogrady, below-branch forelimb suspension and progression, above-branch bipedalism, climbing, clambering, and leaping (hylobatid-like) to one that comprised above-branch quadrupedalism, below-branch forelimb suspension, vertical climbing, limited leaping, terrestrial quadrupedal running and walking, possibly with knuckle walking, and short bouts of bipedalism (chimpanzee-like). The bony labyrinth morphology of Lufengpithecus indicates that it probably conforms more closely to the last common ancestors of crown hominoids and hominids in its locomotor behavior than do other Miocene hominoids. Human bipedalism evolved from this common archetypal Lufengpithecus-like locomotor repertoire. The low evolutionary rate of semicircular canal morphology suggests that Lufengpithecus experienced a relative stasis in locomotor behavior, probably due to the uplift of the Tibetan Plateau, which created a stable environment in the Miocene of southwestern China.

Systematics of Miocene apes: State of the art of a neverending controversy

Hominoids diverged from cercopithecoids during the Oligocene in Afro-Arabia, initially radiating in that continent and subsequently dispersing into Eurasia. From the Late Miocene onward, the geographic range of hominoids progressively shrank, except for hominins, which dispersed out of Africa during the Pleistocene. Although the overall picture of hominoid evolution is clear based on available fossil evidence, many uncertainties persist regarding the phylogeny and paleobiogeography of Miocene apes (nonhominin hominoids), owing to their sparse record, pervasive homoplasy, and the decimated current diversity of this group. We review Miocene ape systematics and evolution by focusing on the most parsimonious cladograms published during the last decade. First, we provide a historical account of the progress made in Miocene ape phylogeny and paleobiogeography, report an updated classification of Miocene apes, and provide a list of Miocene ape species-locality occurrences together with an analysis of their paleobiodiversity dynamics. Second, we discuss various critical issues of Miocene ape phylogeny and paleobiogeography (hylobatid and crown hominid origins, plus the relationships of Oreopithecus) in the light of the highly divergent results obtained from cladistic analyses of craniodental and postcranial characters separately. We conclude that cladistic efforts to disentangle Miocene ape phylogeny are potentially biased by a long-branch attraction problem caused by the numerous postcranial similarities shared between hylobatids and hominids-despite the increasingly held view that they are likely homoplastic to a large extent, as illustrated by Sivapithecus and Pierolapithecus-and further aggravated by abundant missing data owing to incomplete preservation. Finally, we argue that-besides the recovery of additional fossils, the retrieval of paleoproteomic data, and a better integration between cladistics and geometric morphometrics-Miocene ape phylogenetics should take advantage of total-evidence (tip-dating) Bayesian methods of phylogenetic inference combining morphologic, molecular, and chronostratigraphic data. This would hopefully help ascertain whether hylobatid divergence was more basal than currently supported.

Morphological modularity in the anthropoid axial skeleton

Previous research has found that hominoids have stronger modularity between limb elements than other anthropoids, suggesting that there is less constraint on morphological diversification (e.g., limb proportions) in hominoids in terms of evolutionary independence. However, degrees of modularity in the axial skeleton have not been investigated across a broad range of anthropoid taxa. Thus, it is unknown whether hominoids also have stronger modularity in the axial skeleton than other anthropoids, which has implications for the evolution of diverse torso morphologies in Miocene apes as well as the evolution of novel characteristics in the skull and vertebrae of fossil hominins. In this study, 12 anthropoid genera were sampled to examine degrees of modularity between axial skeletal elements (i.e., cranium, mandible, vertebrae, and sacrum). Covariance ratio coefficients were calculated using variance/covariance matrices of interlandmark distances for each axial skeletal element to evaluate degrees of modularity. The results showed that Alouatta, Hylobates, Gorilla, Pan, and Homo showed generally stronger modularity than other anthropoid taxa when considering all axial skeletal elements. When only considering the vertebral elements (i.e., vertebrae and sacrum), Alouatta, Hylobates, Gorilla, and Pan showed generally stronger modularity than other anthropoid taxa. Humans showed stronger modularity between the skull and vertebrae than other hominoids. Thus, the evolution of novel characteristics in the skull and vertebral column may have been less constrained in fossil hominins due to the dissociation of trait covariation between axial skeletal elements in hominoid ancestors, thus fostering more evolutionary independence between the skull and vertebral column.

New fossils of Australopithecus sediba reveal a nearly complete lower back

Adaptations of the lower back to bipedalism are frequently discussed but infrequently demonstrated in early fossil hominins. Newly discovered lumbar vertebrae contribute to a near-complete lower back of Malapa Hominin 2 (MH2), offering additional insights into posture and locomotion in Australopithecus sediba. We show that MH2 possessed a lower back consistent with lumbar lordosis and other adaptations to bipedalism, including an increase in the width of intervertebral articular facets from the upper to lower lumbar column (‘pyramidal configuration’). These results contrast with some recent work on lordosis in fossil hominins, where MH2 was argued to demonstrate no appreciable lordosis (‘hypolordosis’) similar to Neandertals. Our three-dimensional geometric morphometric (3D GM) analyses show that MH2’s nearly complete middle lumbar vertebra is human-like in overall shape but its vertebral body is somewhat intermediate in shape between modern humans and great apes. Additionally, it bears long, cranially and ventrally oriented costal (transverse) processes, implying powerful trunk musculature. We interpret this combination of features to indicate that A. sediba used its lower back in both bipedal and arboreal positional behaviors, as previously suggested based on multiple lines of evidence from other parts of the skeleton and reconstructed paleobiology of A. sediba.

One of the defining features of humans is our ability to walk comfortably on two legs. To achieve this, our skeletons have evolved certain physical characteristics. For example, the lower part of the human spine has a forward curve that supports an upright posture; whereas the lower backs of chimpanzees and other apes – which walk around on four limbs and spend much of their time in trees – lack this curvature. Studying the fossilized back bones of ancient human remains can help us to understand how we evolved these features, and whether our ancestors moved in a similar way.

Australopithecus sediba was a close-relative of modern humans that lived about two million years ago. In 2008, fossils from an adult female were discovered at a cave site in South Africa called Malapa. However, the fossils of the lower back region were incomplete, so it was unclear whether the female – referred to as Malapa Hominin 2 (MH2) – had a forward-curving spine and other adaptations needed to walk on two legs.

Here, Williams et al. report the discovery of new A. sediba fossils from Malapa. The new fossils are mainly bones from the lower back, and they fit together with the previously discovered MH2 fossils, providing a nearly complete lower spine. Analysis of the fossils suggested that MH2 would have had an upright posture and comfortably walked on two legs, and the curvature of their lower back was similar to modern females. However, other aspects of the bones’ shape suggest that as well as walking, A. sediba probably spent a significant amount of time climbing in trees.

The findings of Williams et al. provide new insights in to our evolutionary history, and ultimately, our place in the natural world around us. Our lower back is prone to injury and pain associated with posture, pregnancy and exercise (or lack thereof). Therefore, understanding how the lower back evolved may help us to learn how to prevent injuries and maintain a healthy back.

Examination of magnitudes of integration in the catarrhine vertebral column

The evolution of novel vertebral morphologies observed in humans and other extant hominoids may be related to changes in the magnitudes and/or patterns of covariation among traits. To examine this, we tested magnitudes of integration in the vertebral column of cercopithecoids and hominoids, including humans. Three-dimensional surface scans of 14 vertebral elements from 30 Cercopithecus, 32 Chlorocebus, 39 Macaca, 45 Hylobates, 31 Pan, and 86 Homo specimens were used. A resampling method was used to generate distributions of integration coefficient of variation scores for vertebral elements individually using interlandmark distances. Interspecific comparisons of mean integration coefficient of variation were conducted using Mann-Whitney U tests with Bonferroni adjustment. The results showed that hominoids generally had lower mean integration coefficient of variation than cercopithecoids. In addition, humans showed lower mean integration coefficient of variation than other hominoids in their last thoracic and lumbar vertebrae. Cercopithecoids and Hylobates showed relatively lower mean integration coefficient of variation in cervical vertebrae than in thoracolumbar vertebrae. Pan and Homo showed relatively lower mean integration coefficient of variation in the last thoracic and lumbar vertebrae in the thoracolumbar region, except for the L1 of Pan. The results suggest fewer integration-mediated constraints on the evolution of vertebral morphology in hominoids when compared with cercopithecoids. The weaker magnitudes of integration in lumbar vertebrae in humans when compared with chimpanzees likewise suggest fewer constraints on the evolution of novel lumbar vertebrae morphology in humans.

Insights into the lower torso in late Miocene hominoid Oreopithecus bambolii

Oreopithecus bambolii (8.3-6.7 million years old) is the latest known hominoid from Europe, dating to approximately the divergence time of the Pan-hominin lineages. Despite being the most complete nonhominin hominoid in the fossil record, the O. bambolii skeleton IGF 11778 has been, for decades, at the center of intense debate regarding the species' locomotor behavior, phylogenetic position, insular paleoenvironment, and utility as a model for early hominin anatomy. Here we investigate features of the IGF 11778 pelvis and lumbar region based on torso preparations and supplemented by other O. bambolii material. We correct several crucial interpretations relating to the IGF 11778 anterior inferior iliac spine and lumbar vertebrae structure and identifications. We find that features of the early hominin Ardipithecus ramidus torso that are argued to have permitted both lordosis and pelvic stabilization during upright walking are not present in O. bambolii However, O. bambolii also lacks the complete reorganization for torso stiffness seen in extant great apes (i.e., living members of the Hominidae), and is more similar to large hylobatids in certain aspects of torso form. We discuss the major implications of the O. bambolii lower torso anatomy and how O. bambolii informs scenarios of hominoid evolution.

Potential adaptations for bipedalism in the thoracic and lumbar vertebrae of Homo sapiens: A 3D comparative analysis

A number of putative adaptations for bipedalism have been identified in the hominin spine. However, it is possible that some have been overlooked because only a few studies have used 3D and these studies have focused on cervical vertebrae. With this in mind, we used geometric morphometric techniques to compare the 3D shapes of three thoracic and two lumbar vertebrae of Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. The study had two goals. One was to confirm the existence of traits previously reported to distinguish the thoracic and lumbar vertebrae of H. sapiens from those of the great apes. The other was to, if possible, identify hitherto undescribed traits that differentiate H. sapiens thoracic and lumbar vertebrae from those of the great apes. Both goals were accomplished. Our analyses not only substantiated a number of traits that have previously been discussed in the literature but also identified four traits that have not been described before: (1) dorsoventrally shorter pedicles in the upper thoracic vertebrae; (2) dorsoventrally longer laminae in all five of the vertebrae examined; (3) longer transverse processes in the upper thoracic vertebrae; and (4) craniocaudally 'pinched' spinous process tips in all of the vertebrae examined. A review of the biomechanical literature suggests that most of the traits highlighted in our analyses can be plausibly linked to bipedalism, including three of the four new ones. As such, the present study not only sheds further light on the differences between the spines of H. sapiens and great apes but also enhances our understanding of how the shift to bipedalism affected the hominin vertebral column.

The hominid ilium is shaped by a synapomorphic growth mechanism that is unique within primates

The human ilium is significantly shorter and broader than those of all other primates. In addition, it exhibits an anterior inferior iliac spine (AIIS) that emerges via a secondary center of ossification, which is unique to hominids (i.e., all taxa related to the human clade following their phyletic separation from the African apes). Here, we track the ontogeny of human and other primate ossa coxae. The human pattern is unique, from anlage to adulthood, and fusion of its AIIS is the capstone event in a repositioning of the anterior gluteals that maximizes control of pelvic drop during upright walking. It is therefore a hominid synapomorphy that can be used to assess the presence and age of bipedal locomotion in extinct taxa.

Response of the Axial Skeleton to Bipedal Loading Behaviors in an Experimental Animal Model

Many derived aspects of modern human axial skeletal morphology reflect our reliance on obligate bipedal locomotion. Insight into the adaptive significance of features, particularly in the spine, has been gained through experimental studies that induce bipedal standing or walking in quadrupedal mammals. Using an experimental animal model (Rattus norvegicus), the present study builds on earlier work by incorporating additional metrics of the cranium, employing quantitative methods established in the paleoanthropological literature, and exploring how variation in mechanical loading regimes impacts axial anatomy. Rats were assigned to one of five experimental groups, including "fully loaded bipedal walking," "partially loaded bipedal walking," "standing bipedally," "quadrupedal walking," and "no exercise control," and engaged in the behavior over 12-weeks. From μCT data obtained at the beginning and end of the experiment, we measured foramen magnum position and orientation, lumbar vertebral body wedging, cranial surface area of the lumbar and first sacral vertebral bodies, and sacral mediolateral width. Results demonstrate that bipedal rodents generally have more anteriorly positioned foramina magna, more dorsally wedged lumbar vertebrae, greater articular surface areas of lumbar and first sacral vertebral bodies, and sacra that exhibit greater mediolateral widths, compared to quadrupedal rodents. We further document variation among bipedal loading behavior groups (e.g., bipedal standing vs. walking). Our experimental animal model reveals how loading behaviors and adaptations may be specifically linked, and implicates a potential role for developmental plasticity in the evolutionary acquisition of bipedal adaptations in the hominin lineage. Anat Rec, 2018. © 2018 American Association for Anatomy.

Trabecular Bone Structural Variation in the Proximal Sacrum Among Primates

The sacrum occupies a functionally important anatomical position as part of the pelvic girdle and vertebral column. Sacral orientation and external morphology in modern humans are distinct from those in other primates and compatible with the demands of habitual bipedal locomotion. Among nonhuman primates, however, how sacral anatomy relates to positional behaviors is less clear. As an alternative to evaluation of the sacrum's external morphology, this study assesses if the sacrum's internal morphology (i.e., trabecular bone) differs among extant primates. The primary hypothesis tested is that trabecular bone parameters with established functional relevance will differ in the first sacral vertebra (S1) among extant primates that vary in positional behaviors. Results for analyses of individual variables demonstrate that bone volume fraction, degree of anisotropy, trabecular number, and size-corrected trabecular thickness differ among primates grouped by positional behaviors to some extent, but not always in ways consistent with functional expectations. When examined as a suite, these trabecular parameters distinguish obligate bipeds from other positional behavior groups; and, the latter three trabecular bone variables further distinguish knuckle-walking terrestrial quadrupeds from manual suspensor-brachiators, vertical clingers and leapers, and arboreal quadrupeds, as well as between arboreal and terrestrial quadrupeds. As in other regions of the skeleton in modern humans, trabecular bone in S1 exhibits distinctively low bone volume fraction. Results from this study of extant primate S1 trabecular bone structural variation provide a functional context for interpretations concerning the positional behaviors of extinct primates based on internal sacral morphology. Anat Rec, 302:1354-1371, 2019. © 2018 Wiley Periodicals, Inc.

The role of allometry and posture in the evolution of the hominin subaxial cervical spine

Cervical vertebrae not only protect the spinal cord but also are the insertion and origin points for muscles related to the movement of the head, upper limb, and trunk, among others, and are thus important elements in primate evolution. While previous work has been undertaken on the first two cervical vertebrae, there is a dearth of studies on the subaxial cervical spine in hominines. In this paper, we provide detailed morphological information on two important aspects of the subaxial cervical vertebrae (C3 - C7): mid-sagittal morphology and superior facet orientation. We studied large samples of African apes including modern humans and the most complete fossil hominin subaxial cervical vertebrae using both traditional and geometric morphometrics. There are significant differences between extant hominoids related to the relative length and orientation of the spinous process as well as to the orientation of the articular facets, which are related to size, locomotion, and neck posture. In fact, fossil hominins do not completely conform to any of the extant groups. Our assessment of mid-sagittal morphology and superior articular facet orientation shows that australopiths have more Homo-like upper subaxial cervical vertebrae coupled with more "primitive" lower cervical vertebrae. Based on these results, we hypothesize that those changes, maybe related to postural changes derived from bipedalism, did not affect the entire subaxial cervical spine at once. From a methodological point of view, the combination of traditional and geometric morphometric data provides a more integrative perspective of morphological change and evolution, which is certainly useful in human evolutionary studies.

First steps of bipedality in hominids: evidence from the atelid and proconsulid pelvis

Upright walking absent a bent-hip-bent-knee gait requires lumbar lordosis, a ubiquitous feature in all hominids for which it can be observed. Its first appearance is therefore a central problem in human evolution. Atelids, which use the tail during suspension, exhibit demonstrable lordosis and can achieve full extension of their hind limbs during terrestrial upright stance. Although obviously homoplastic with hominids, the pelvic mechanisms facilitating lordosis appear largely similar in both taxa with respect to abbreviation of upper iliac height coupled with broad sacral alae. Both provide spatial separation of the most caudal lumbar(s) from the iliac blades. A broad sacrum is therefore a likely facet of earliest hominid bipedality. All tailed monkeys have broad alae. By contrast all extant apes have very narrow sacra, which promote “trapping” of their most caudal lumbars to achieve lower trunk rigidity during suspension. The alae in the tailless proconsul Ekembo nyanzae appear to have been quite broad, a character state that may have been primitive in Miocene hominoids not yet adapted to suspension and, by extension, exaptive for earliest bipedality in the hominid/panid last common ancestor. This hypothesis receives strong support from other anatomical systems preserved in Ardipithecus ramidus.

Genetic and developmental basis for parallel evolution and its significance for hominoid evolution

Greater understanding of ape comparative anatomy and evolutionary history has brought a general appreciation that the hominoid radiation is characterized by substantial homoplasy.(1-4) However, little consensus has been reached regarding which features result from repeated evolution. This has important implications for reconstructing ancestral states throughout hominoid evolution, including the nature of the Pan-Homo last common ancestor (LCA). Advances from evolutionary developmental biology (evo-devo) have expanded the diversity of model organisms available for uncovering the morphogenetic mechanisms underlying instances of repeated phenotypic change. Of particular relevance to hominoids are data from adaptive radiations of birds, fish, and even flies demonstrating that parallel phenotypic changes often use similar genetic and developmental mechanisms. The frequent reuse of a limited set of genes and pathways underlying phenotypic homoplasy suggests that the conserved nature of the genetic and developmental architecture of animals can influence evolutionary outcomes. Such biases are particularly likely to be shared by closely related taxa that reside in similar ecological niches and face common selective pressures. Consideration of these developmental and ecological factors provides a strong theoretical justification for the substantial homoplasy observed in the evolution of complex characters and the remarkable parallel similarities that can occur in closely related taxa. Thus, as in other branches of the hominoid radiation, repeated phenotypic evolution within African apes is also a distinct possibility. If so, the availability of complete genomes for each of the hominoid genera makes them another model to explore the genetic basis of repeated evolution.

A partial hominoid innominate from the Miocene of Pakistan: description and preliminary analyses

We describe a partial innominate, YGSP 41216, from a 12.3 Ma locality in the Siwalik Group of the Potwar Plateau in Pakistan, assigned to the Middle Miocene ape species Sivapithecus indicus. We investigate the implications of its morphology for reconstructing positional behavior of this ape. Postcranial anatomy of extant catarrhines falls into two distinct groups, particularly for torso shape. To an extent this reflects different although variable and overlapping positional repertoires: pronograde quadrupedalism for cercopithecoids and orthogrady for hominoids. The YGSP innominate (hipbone) is from a primate with a narrow torso, resembling most extant monkeys and differing from the broader torsos of extant apes. Other postcranial material of S. indicus and its younger and similar congener Sivapithecus sivalensis also supports reconstruction of a hominoid with a positional repertoire more similar to the pronograde quadrupedal patterns of most monkeys than to the orthograde patterns of apes. However, Sivapithecus postcranial morphology differs in many details from any extant species. We reconstruct a slow-moving, deliberate, arboreal animal, primarily traveling above supports but also frequently engaging in antipronograde behaviors. There are no obvious synapomorphic postcranial features shared exclusively with any extant crown hominid, including Pongo.