Hallucal tarsometatarsal joint in Australopithecus afarensis

Hallucal tarsometatarsal joints from African pongids, modern humans, and Australopithecus afarensis are compared to investigate the anatomical and mechanical changes that accompanied the transition to terrestrial bipedality. Features analyzed include the articular orientation of the medial cuneiform, curvature of the distal articular surface of the medial cuneiform, and the articular configuration of the hallucal metatarsal proximal joint surface. Morphological characteristics of the hallucal tarsometatarsal joint unequivocally segregate quadrupedal pongids and bipedal hominids.     

The calcaneus of Australopithecus afarensis and its implications for the evolution of bipedality

Calcanei from African apes, modern humans, and Australopithecus afarensis are compared to investigate the anatomical and mechanical changes that occurred in this bone as a result of the transition to terrestrial bipedality. Features analyzed include the cross-sectional area and volume of the calcaneal tuber, the geometry and orientation of the articular surfaces, and the surface topography of the calcaneal corpus. Calcaneal morphology is unequivocal in its partitioning of quadrupedal pongids and bipedal hominids.     

Talocrural joint in African hominoids: implications for Australopithecus afarensis

Talocrural joints of the African apes, modern humans, and A.L.288-1 are compared in order to investigate ankle function in the Hadar hominids. Comparisons between the hominids and African pongids clearly illustrate the anatomical and mechanical changes that occurred in this joint as a consequence of the evolutionary transition to habitual bipedality. Features which are considered include the obliquity of the distal tibial articular surface, the shape of the talar trochlea, and the location and functional implications of the talocrural axis. In every functionally significant feature examined the A.L.288-1 talocrural joint is fully bipedal. Moreover, the Hadar ankle complex also shows the functional constraints which are necessarily imposed by the adaptation to habitual bipedalism.     

Arboreality and bipedality in the Hadar hominids

Numerous studies of the locomotor skeleton of the Hadar hominids have revealed traits indicative of both arboreal climbing/suspension and terrestrial bipedalism. These earliest known hominids must have devoted part of their activities to feeding, sleeping and/or predator avoidance in trees, while also spending time on the ground where they moved bipedally. In this paper we offer new data on phalangeal length and curvature, morphology of the tarsus and metatarsophalangeal joints, and body proportions that further strengthen the argument for arboreality in the Hadar hominids. We also provide additional evidence on limb and pedal proportions and on the functional anatomy of the hip, knee and foot, indicating that the bipedality practiced at Hadar differed from that of modern humans. Consideration of the ecology at Hadar, in conjunction with modern primate models, supports the notion of arboredality in these earliest australopithecines. We speculate that selection for terrestrial bipedality may have intensified through the Plio-Pleistocene as forests and woodland patches shrunk and the need arose to move increasingly longer distances on the ground. Only with Homo erectus might body size, culture and other factors have combined to 'release' hominids from their dependence on trees.     

The locomotor anatomy of Australopithecus afarensis

The postcranial skeleton of Australopithecus afarensis from the Hadar Formation, Ethiopia, and the footprints from the Laetoli Beds of northern Tanzania, are analyzed with the goal of determining (1) the extent to which this ancient hominid practiced forms of locomotion other than terrestrial bipedality, and (2) whether or not the terrestrial bipedalism of A. afarensis was notably different from that of modern humans. It is demonstrated that A. afarensis possessed anatomic characteristics that indicate a significant adaptation for movement in the trees. Other structural features point to a mode of terrestrial bipedality that involved less extension at the hip and knee than occurs in modern humans, and only limited transfer of weight onto the medial part of the ball of the foot, but such conclusions remain more tentative than that asserting substantive arboreality. A comparison of the specimens representing smaller individuals, presumably female, to those of larger individuals, presumably male, suggests sexual differences in locomotor behavior linked to marked size dimorphism. The males were probably less arboreal and engaged more frequently in terrestrial bipedalism. In our opinion, A. afarensis from Hadar is very close to what can be called a "missing link." We speculate that earlier representatives of the A. afarensis lineage will present not a combination of arboreal and bipedal traits, but rather the anatomy of a generalized ape.     

Fossil primates and the evolution of some primate locomotor systems

Of Paleocene primates only Plesiadapis is complete enough to reconstruct locomotor patterns; it was an arboreal scrambler, perhaps functioning like a large squirrel. Eocene lemurs (adapids) show an array of locomotor types much like certain modern Malagasy lemurs. The European Eocene tarsiid Necrolemur and the American Hemiacodon show the beginning of saltatory specializations in possession of elongated calcaneum and astragalus. Although not a direct anthropoid ancestor Necrolemur seems one of the best models for representing the early locomotor type from which higher primates arose. The Oligocene primates of Egypt (among which are the earliest undoubted pongids) are preserved with a forest fauna. Structures of long bones suggest they were arboreal. A considerable number of Miocene ape bones are known and those of Pliopithecus and Dryopithecus indicate similar adaptations. Of African Miocene forms, Dryopithecus major was a large, gorilla-sized animal, and hence perhaps primarily terrestrial. D. africanus was somewhat more arboreally adapted and a partial brachiator. The Italian fossil Oreopithecus, a coal-swamp dweller, shows indications of bipedality in pelvic structure. Ramapithecus, which is presumably ancestral to Australopithecus, shows palatal and facial patterns much like these later hominids, and probably hence had locomotor patterns more like men than like living apes; its lack of the dental specializations of apes strongly supports this suggestion.     

The insertions of the cruropedal muscles and implications for the locomotor evolution in primates

We examined gross-anatomically the cruropedal muscles, which control the toe movements, in some species of insectivores, rodents and primates including humans, with a focus on the phylogenetic developments of these muscles including the distribution patterns of the tendons to the toes. Morphological changes corresponding to the phylogenetic advancement from primitive terrestrial mammals to arboreal primates were found in the short extensors and flexors, presumably in association with the enhancement of independent digital mobility. In contrast, the changes which correspond to the acquisition of terrestrial bipedality in humans were identified in the development of extensors and flexors which govern the first toe, as well as in establishment of the peroneus tertius that dorsi-flexes the talocrural joint.     

Morphometric affinities of the human shoulder

To analyze differences between apes and monkeys and the affinities of man, we have studied the shoulder girdle of 327 specimens of anthropoid primates. The scapula, clavicle and humerus are viewed as an integrated functional complex on the basis of 18 measurements. Several varieties of multivariate analysis show that man is clearly closer to other hominoids than to the included monkey taxa (whether terrestrial or arboreal, Old World or New World). The marked shoulder differences between apes and monkeys and similarities between apes and man correlate with the muscular anatomy, which in hominoids allows the motions involved in their locomotion and feeding behavior. As the hominid-pongid correspondence in shoulder morphology is especially detailed regarding the functionally important joint surfaces, it is consistent with a fairly recent period of common ancestry and behavior. No hypothetical evolutionary pathway or ancestral form of the human shoulder need look far beyond the model afforded by extant pongids. In contrast with previous studies on the primate shoulder, these results agree with information accumulating from other systems—comparative anatomy, primate behavior, and molecular biology — in suggesting very close relationship between man and extant African pongids.     

Knuckle-walking and the evolution of hominoid hands

Knuckle-walking is a pattern of digitigrade locomotion unique to African apes among Primates. Only chimpanzees and gorillas are specially adapted for supporting weight on the dorsal aspects of middle phalanges of flexed hand digits II–V. When forced to the ground, most orangutans assume one of a variety of flexed hand postures, but they cannot knuckle-walk. Some orangutans place their hands in palmigrade postures which are impossible to African apes. The knuckle-walking hands and plantigrade feet of African apes are both morphologically and adaptively distinct from those of Pongo, their nearest relative among extant apes. These features are associated with a common adaptive shift to terrestrial locomotion and support placing chimpanzees and gorillas in the same genus Pan. It is further suggested than Pan comprises the subgenera (a) Pan, including P. troglodytes and pygmy chimpanzees, and (b) Gorilla, including mountain and lowland populations of P. gorilla. African apes probably diverged from ancestral pongids that were specially adapted for distributing their weight in terminal branches of the forest canopy. Early adjustments to terrestrial locomotion may have involved fist-walking which later evolved into knuckle-walking. Orangutans continued to adapt to feeding and locomotion in the forest canopy and their hands and feet became highly specialized for four-digit prehension. Although chimpanzees retained arboreal feeding and nesting habits, they moved from tree to tree by terrestrial routes and became less restricted in habitat. While adapting to a diet of ground plants gorillas increased in size to the point that arboreal nesting is less frequent among them than among chimpanzees and orangutans. Early hominids probably diverged from pongids that had not developed prospective adaptations to knuckle-walking, and therefore did not evolve through a knuckle-walking stage. Initial adjustments to terrestrial quadrupedal locomotion and resting stance probably included palmigrade hand posturing. Their thumbs may have been already well developed as an adaptation for grasping during arboreal climbing. A combination of selection pressures for efficient terrestrial locomotor support and for object manipulation further advanced early hominid hands toward modern human configuration.     

Dental variability of the Liberian chimpanzee,Pan troglodytes verus

Dental variability was studied in a collection of Liberian chimpanzee (Pan troglodytes verus) crania from one geographic area of Central Liberia. Morphological and metric data were compared to another population of the same subspecies studied by Schuman and Brace (1955) as well as to other pongid taxa. It appears that of all living pongids, chimpanzees are the most derived in their lower molar patterns, particularly, P. t. verus. It is clear, however that the mandibular molar patterns of contemporary chimpanzees are more similar to other pongids that to humans which is contra to the suggestions of Schuman and Brace. Hypocone reduction from M1 to M3 is the common pattern in all hominoids. Complete absence of the M3 hypocone is rare in pongids but it is present in the Frankfurt collection. Of living pongids, the gorilla expresses the least amount of hypocone reduction from M1 to M3. A cusp of Carabelli is recorded bilaterally in one P.t. verus. There is less odontometric variation in P.t. verus than in other pongids as indicated by the CV’s which may suggest the greater dental variability present when different geographic groups are included in the sample.     

Mediolateral reaction forces and forelimb anatomy in quadrupedal primates: implications for interpreting locomotor behavior in fossil primates

The forelimb joints of terrestrial primate quadrupeds appear better able to resist mediolateral (ML) shear forces than those of arboreal quadrupedal monkeys. These differences in forelimb morphology have been used extensively to infer locomotor behavior in extinct primate quadrupeds. However, the nature of ML substrate reaction forces (SRF) during arboreal and terrestrial quadrupedalism in primates is not known. This study documents ML-SRF magnitude and orientation and forelimb joint angles in six quadrupedal anthropoid species walking across a force platform attached to terrestrial (wooden runway) and arboreal supports (raised horizontal poles). On the ground all subjects applied a lateral force in more than 50% of the steps collected. On horizontal poles, in contrast, all subjects applied a medially directed force to the substrate in more than 75% of the steps collected. In addition, all subjects on arboreal supports combined a lower magnitude peak ML-SRF with a change in the timing of the ML-SRF peak force. As a result, during quadrupedalism on the poles the overall SRF resultant was relatively lower than it was on the runway. Most subjects in this study adduct their humerus while on the poles. The kinetic and kinematic variables combine to minimize the tendency to collapse or translate forelimbs joints in an ML plane in primarily arboreal quadrupedal primates compared to primarily terrestrial quadrupedal ones. These data allow for a more complete understanding of the anatomy of the forelimb in terrestrial vs. arboreal quadrupedal primates. A better understanding of the mechanical basis of morphological differences allows greater confidence in inferences concerning the locomotion of extinct primate quadrupeds.     

Climbing,brachiation, and terrestrial quadrupedalism: Historical precursors of hominid bipedalism

The vertical-climbing account of the evolution of locomotor behavior and morphology in hominid ancestry is reexamined in light of recent behavioral, anatomical, and paleontological findings and a more firmly established phylogeny for the living apes. The behavioral record shows that African apes, when arboreal, are good vertical climbers, and that locomotion during traveling best separates the living apes into brachiators (gibbons), scrambling/climbing/brachiators (orangutans), and terrestrial quadrupeds (gorillas and chimpanzees). The paleontological record documents frequent climbing as an ancestral catarrhine ability, while a reassessment of the morphology of the torso and forelimb in living apes and Atelini suggests that their shared unique morphological pattern is best explained by brachiation and forelimb suspensory positional behavior. Further, evidence from the hand and foot points to a terrestrial quadrupedal phase in hominoid evolution prior to the adoption of bipedalism. The evolution of positional behavior from early hominoids to hominids appears to have begun with an arboreal quadrupedal-climbing phase and proceeded though an orthograde, brachiating, forelimb-suspensory phase, which was in turn followed by arboreal and terrestrial quadrupedal phases prior to the advent of hominid bipedality. The thesis that protohominids climbed down from the trees to become terrestrial bipeds needs to be reexamined in light of a potentially long history of terrestriality in the ancestral protohominid. © 1996 Wiley-Liss, Inc.     

Are bonobos (Pan paniscus) really more bipedal than chimpanzees (Pan troglodytes)?

Of the living apes, the chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) are often presented as possible models for the evolution of hominid bipedalism. Bipedality in matched pairs of captive bonobos and chimpanzees was analyzed to test hypotheses for the evolution of bipedalism, derived from a direct referential model. There was no overall species difference in rates of bipedal positional behavior, either postural or locomotory. The hominoid species differed in the function or use of bipedality, with bonobos showing more bipedality for carrying and vigilance, and chimpanzees showing more bipedality for display.     

The scansorius muscle in pongids

The hip musculature of each of the pongids was dissected in order to determine the presence, location, and relationships of the scansorius muscle. The literature on scansorius is reviewed and certain inconsistencies with regard to this muscle are noted.The author's dissections revealed the presence of scansorius only in the orang-utan. In the other 3 pongids, only a single undivided gluteus minimus muscle was found to occur in the place that both gluteus minimus proper and scansorius occupied in the orang. On the basis of these dissections and accounts of the muscle from the literature, it is suggested that scansorius always occurs as an individual muscle in the orang, but that it is usually partly or totally fused with gluteus minimus proper in the other 3 pongids. A discussion of the possible reasons for these differences is included.     

The Maka femur and its bearing on the antiquity of human walking: applying contemporary concepts of morphogenesis to the human fossil record

MAK-VP-1/1, a proximal femur recovered from the Maka Sands (ca. 3.4 mya) of the Middle Awash, Ethiopia, and attributed to Australopithecus afarensis, is described in detail. It represents the oldest skeletal evidence of locomotion in this species, and is analyzed from a morphogenetic perspective. X-ray, CT, and metric data are evaluated, using a variety of methods including discriminant function. The specimen indicates that the hip joint of A. afarensis was remarkably like that of modern humans, and that the dramatic muscle allocation shifts which distinguish living humans and African apes were already present in a highly derived form in this species. Its anatomy provides no indication of any form of locomotion save habitual terrestrial bipedality, which very probably differed only trivially from that of modern humans.     

Trauma, degenerative disease, and other pathologies among the Gombe chimpanzees

The well-known and extremely well-documented chimpanzees from Gombe National Park were analyzed for presence of skeletal pathologies. Of the 15 animals available for study, 11 were old and complete enough to permit systematic analysis. Of these, 10 showed some evidence of skeletal pathological involvement. The most common type of lesion seen resulted from trauma. Those chimps with the most fractures (Old Female, 3; Flo, 4; Hugo, 8) are consistently the oldest individuals in the sample. In addition to accidental falls, the most common cause of trauma was from interpersonal violence, resulting in bite wounds (see in two individuals) and fractures (see in three individuals). Conversely to trauma, degenerative disease was exceedingly rare in this population, found in no large intervertebral joints (N = 344) and only two major synovial joints (N = 186). In fact, the complete lack of osteophytosis, even in older individuals, stands in stark contrast to the situation seen in modern humans, perhaps in our species reflecting a biomechanical cost of bipedality.     

Bipedality in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus): testing hypotheses on the evolution of bipedalism

A host of ecological, anatomical, and physiological selective pressures are hypothesized to have played a role in the evolution of hominid bipedalism. A referential model, based on the chimpanzee (Pan troglodytes) and bonobo (Pan paniscus), was used to test through experimental manipulation four hypotheses on the evolution of hominid bipedalism. The introduction of food piles (Carry hypothesis) increased locomotor bipedality in both species. Neither the introduction of branches (Display hypothesis) nor the construction of visual barriers (Vigilance hypothesis) altered bipedality in either species. Introduction of raised foraging structures (Forage hypothesis) increased postural bipedality in chimpanzees. These experimental manipulations provided support for carrying of portable objects and foraging on elevated food-items as plausible mechanisms that shaped bipedalism in hominids.     

Size- and locomotion-related aspects of hominid and anthropoid pelves: An osteometrical multivariate analysis

Two multivariate methods — the logarithmic principal component analysis (LPCA), and the logarithmic factorial analysis (LFA) — have been used tocompare the hip bone proportions of hominoids biometrically. The results have shown that size effects among apes and hominids interact to a centain extent with locomotor specializations, which are related to the attainment of more or less terrestrial behaviors. The pelvic morphology of great apes (Pongo, Pan, Gorilla) has retained numerous morphological traits — such as a gracile and elongated hip bone —, which were inherited from common adaptations to arboreal locomotion. In spite of these common traits, the African pongids (Pan, Gorilla) present two very different pelvic morphologies corresponding to two adaptative modes of terrestrial quadrupedalism. The hip bone of humans is proportionnally short and robust, most particularly at the level of its axial part. These characteristics, as well as the whole pelvic proportions, clearly indicate that gravitational forces exert a strong pressure on the pelvic walls during bipedalism. Among hominids, the transition from an australopithecine-like pelvic pattern to a human-like one corresponds to an increase of loading constraints on the hip jiont. This seems to indicate an evident change in locomotor behavior. Progression apparently became exclusively terrestrial with the genusHomo.     

Functional morphology of the forelimb of tupaiids (Mammalia, Scandentia) and its phylogenetic implications

In this study, the forelimb of 12 species of tupaiids was analyzed functionally and compared to that of other archontan mammals. Several differences that relate to differential substrate use were found in the forelimb morphology of tupaiids. These differences included shape of the scapula, length and orientation of the coracoid process, size of the lesser tuberosity, shape of the capitulum, length of the olecranon process, and shape of the radial head and central fossa. The forelimb of the arboreal Ptilocercus lowii, the only ptilocercine, is better adapted for arboreal locomotion, while that of tupaiines is better adapted for terrestrial (or scansorial) locomotion. While the forelimb of the arboreal Ptilocercus appears to be habitually flexed and exhibits more mobility in its joints, a necessity for movement on uneven, discontinuous arboreal supports, all tupaiines are characterized by more extended forelimbs and less mobility in their joints. These restricted joints limit movements more to the parasagittal plane, which increases the efficiency of locomotion on a more even and continuous surface like the ground. Even the most arboreal tupaiines remain similar to their terrestrial relatives in their forelimb morphology, which probably reflects the terrestrial ancestry of Tupaiinae (but not Tupaiidae). The forelimb of Urogale everetti is unique among tupaiines in that it exhibits adaptations for scratch-digging. Several features of the tupaiid forelimb reflect the arboreal ancestry of Tupaiidae and it is proposed that the ancestral tupaiid was arboreal like Ptilocercus. Also, compared to the forelimb character states of tupaiines, those of Ptilocercus are more similar to those of other archontans and it is proposed that the attributes of the forelimb of Ptilocercus are primitive for the Tupaiidae. Hence, Ptilocercus should be considered in any phylogenetic analysis that includes Scandentia.     

Humeral outlines in some hominoid primates and in Plio-Pleistocene hominids

A method of drawing outlines of the distal end of the humerus is presented and carried out on some pongids (Pan troglodytes, Pan paniscus, Pongo pygmaeus), on modern man, and on some casts of Plio-Pleistocene hominids. It appears that these outlines are good indicators of the overall morphology and permit the distinguishing of the different hominoids. For example, the morphology of the pillars surrounding the fossa olecrani is useful for this purpose. In modern man, the lateral pillar is quadrangular, contrasting with the triangular medial one. In pongids, both of them are triangular; however, it is possible to note differences between Pongo and Pan. In the South Asian ape, there is a stronger anteroposterior flattening of the pillars as well as the diaphysis. The similarity of the shape of the pillars might be considered as a result of an adaptation to suspension. The differences might be due to different weights of the animals. Plio-Pleistocene hominids are variable with regard to the morphology of this region. For example, Gombore IB 7594 is similar to Homo. KNM ER 739 exhibits features intermediate between hominids and pongids. Finally, AL 288.1M is closer to pongids. These results confirm a previous anatomical work.