The ischium and hip extensor mechanism in human evolution

Although it is commonly stated that the ischia of the late Pliocene–early Pleistocene hominid fossils are long and ape-like, new interpretations show this view to be fallacious. An important new theory proposed by Robinson concludes that the gracile form of early hominid was an efficient biped, but the robust form was a less efficient biped and was adapted for tree climbing. Interpretation of the ischium is crucial to this idea. The present study shows that (1) the gracile and robust australopithecine ischia had similar relative lengths and (2) that the hamstring mechanism was probably very similar in the two forms of South African early hominid.     

New estimates of early hominid body size

Body weights for 12 early hominid specimens are estimated based on an analysis of four variables shown to have high correlation with body size in living Old World primates. Average size estimates of around 36 kg are suggested for gracile early hominids and around 59 kg for robust early hominids. Size variation is considerably more pronounced in the robust group than in the gracile group, suggesting substantially greater sexual dimorphism in the former.     

The rise of the hominids as an adaptive shift in fallback foods: plant underground storage organs (USOs) and australopith origins

We propose that a key change in the evolution of hominids from the last common ancestor shared with chimpanzees was the substitution of plant underground storage organs (USOs) for herbaceous vegetation as fallback foods. Four kinds of evidence support this hypothesis: (1) dental and masticatory adaptations of hominids in comparison with the African apes; (2) changes in australopith dentition in the fossil record; (3) paleoecological evidence for the expansion of USO-rich habitats in the late Miocene; and (4) the co-occurrence of hominid fossils with root-eating rodents. We suggest that some of the patterning in the early hominid fossil record, such as the existence of gracile and robust australopiths, may be understood in reference to this adaptive shift in the use of fallback foods. Our hypothesis implicates fallback foods as a critical limiting factor with far-reaching evolutionary effects. This complements the more common focus on adaptations to preferred foods, such as fruit and meat, in hominid evolution.     

Experimental stress analysis of topographic diversity in early hominid gnathic morphology

Reconstructing the biomechanics of early hominid mastication is a key element in most models of hominid differentiation. Traditionally, ostelogical features marking muscle attachment surfaces have served as a reference system from which the vector geometry of the masticatory force system and resultant force distributions could be predicted. To augment traditional morphological and computational approaches, we developed a simulation system capable of replicating human and non-human primate chewing motions. The forces of occlusion are recorded as photoelastic fringes in a urethane alveolar process. Simulation experiments evaluating the functional correlates of topographic diversity in zygomatic root position and mandibular ramus height in early hominids indicated that the mandibles and dentitions of robust australopithecines are well adapted to sustain high magnitude, low gradient load distributions.     

Dental metric assessment of the omo fossils: implications for the phylogenetic position of Australopithecus africanus

The discovery of Australopithecus afarensis has led to new interpretations of hominid phylogeny, some of which reject A. africanus as an ancestor of Homo. Analysis of buccolingual tooth crown dimensions in australopithecines and Homo species by Johanson and White (Science 202:321-330, 1979) revealed that the South African gracile australopithecines are intermediate in size between Laetoli/hadar hominids and South African robust hominids. Homo, on the other hand, displays dimensions similar to those of A. afarensis and smaller than those of other australopithecines. These authors conclude, therefore, that A. africanus is derived in the direction of A. robustus and is not an ancestor of the Homo clade. However, there is a considerable time gap (ca. 800,000 years) between the Laetoli/Hadar specimens and the earliest Homo specimens; "gracile" hominids from Omo fit into this chronological gap and are from the same geographic area. Because the early specimens at Omo have been designated A. afarensis and the later specimens classified as Homo habilis, Omo offers a unique opportunity to test hypotheses concerning hominid evolution, especially regarding the phylogenetic status of A. africanus. Comparisons of mean cheek teeth breadths disclosed the significant (P less than or equal to 0.05) differences between the Omo sample and the Laetoli/Hadar fossils (P4, M2, and M3), the Homo fossils (P3, P4, M1, M2, and M1), and A. africanus (M3). Of the several possible interpretations of these data, it appears that the high degree of similarity between the Omo sample and the South African gracile australopithecine material warrants considering the two as geographical variants of A. africanus. The geographic, chronologic, and metric attributes of the Omo sample argue for its lineal affinity with A. afarensis and Homo. In conclusion, a consideration of hominid postcanine dental metrics provides no basis for removing A. africanus from the ancestry of the Homo lineage.     

How large were the australopithecines?

Stature of the African early hominids is estimated from most of the available fragments of fossil long bones by means of regression analysis. The average height of the South African gracile australopithecines is predicted to be 145.1 cm (4′9″) where n = 4 and of the South African robust forms, 152.7 cm (5′) where n = 3. The East African early hominids are somewhat taller (x = 163.0 cm or 5′4″, where n = 7). Variability in stature is high even within the same site which is probably a reflection of fairly strong sexual dimorphism in body size. Evidence is presented which suggests that at least in one form of early hominid the size proportions of fore- and hindlimbs are different than in modern man. There is also evidence that average stature may have increased through time. The most significant of these findings is that the two forms of early hominids in South Africa are possibly more similar in stature than is usually cited. This does not imply necessarily that the two forms did not differ significantly in robustness or weight.     

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.     

Physiology, thermoregulation and bipedalism

It has long been recognized that the bipedal posture reduces the surface area of the body exposed to the sun. In recent years, a theory has been developed by Wheeler that bipedalism evolved in the ancestor of the Hominidae in order to help relieve thermal stress on the animals in open equatorial environments. Bipedalism was said to afford a distinct adaptive advantage over quadrupedalism by permitting hominids to remain active in the open throughout the day. The heat load of the hypothetical hominid comprises the external environment as modelled by Wheeler and the animal's internal environment (i.e., the internal heat generated by its metabolic and locomotor activities, and its evaporative and respirative cooling capacities). When these factors are integrated in the calculation of the animal's thermal budget, the putative advantage of the bipedal over the quadrupedal posture is considerably reduced. The simulations conducted in this study suggest that the increased time afforded to early hominids in the open by bipedalism was relatively short and, therefore, of little or no adaptive significance. These results suggest that thermoregulatory considerations cannot be implicated as a first cause in the evolution of bipedalism in the hominid ancestor.     

A new pelvic fragment from swartkrans and the relationship between the robust and gracile australopithecines

A recently discovered hominid pelvic fragment from Swartkrans (SK 3155) is described in detail with particular reference to the relationship of the two presently recognized forms of australopithecines in South Africa. Results of this examination and metrical analysis indicate that the acetabulum and iliac blade of the early hominids are similar to Homo sapiens except for a unique pattern of traits: a relatively small sacral articular surface, a relatively small acetabulum, a relatively large iliac fossa, and wide lateral splaying of the iliac blades. The new Swartkrans fossil expresses these traits more strongly than does the gracile australopithecine (Sts 14) and is therefore somewhat less similar to Homo sapiens but it is very unlike any pongid.     

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.     

Divergent patterns of integration and reduced constraint in the human hip and the origins of bipedalism

When compared to other hominids--great apes including humans--the human pelvis reveals a fundamental reorganization of bony morphology comprised of multiple trait-level changes, many of which are associated with bipedal locomotion. Establishing how patterns of integration--correlations and covariances among traits--within the pelvis have evolved in concert with morphology is essential to explaining this evolutionary transition because integration may facilitate or constrain morphological evolution. Here, we show that the human hip bone has significantly lower levels of integration and constraint overall when compared to other hominids, that the focus of these changes is on traits hypothesized to play major functional roles in bipedalism, and we provide evidence that the human hip was reintegrated in a pattern distinct from other members of this group. Additionally, the evolutionary transition from a nonhuman great ape-like to human hip bone morphology was significantly easier to traverse using the human integration pattern in each comparison, which suggests hominin patterns may have evolved to facilitate this transition. Our results suggest natural selection for bipedalism broke down earlier hominid integration patterns, allowing relevant traits to respond to separate selection pressures to a greater extent than was previously possible, and reintegrated traits in a way that could have facilitated evolution along the vector specifying ancestral hominid and hominin morphological differences.     

Stw 441/465: a new fragmentary ilium of a small-bodied Australopithecus africanus from Sterkfontein, South Africa

In 1986 and 1987, a hominid left ilium fragment consisting of a spina iliaca anterior superior and crista iliaca was discovered during excavations at Sterkfontein, South Africa. Although the specimen is small it gives valuable hints for muscle insertions and origins at the pelvis of Australopithecus africanus. It indicates that the anatomy of the abdominal muscles and of the mm. glutei medius et minimus of A. africanus was quite different from that of the great apes and more similar to that of modern humans. This has major implications for the interpretation of the bipedalism and locomotor efficiency of the early South African hominids.     

The biological and chronological maturation of early hominids

Recent studies on the rate and pattern of dental development indicate that the growth and maturation of early hominids were more similar to the extant apes than to modern humans. This contrasts with the previously held opinion derived from combined dental development, pattern and attrition studies claiming that early hominids were more hominine in their development (Mann, 1975). This paper explores the origin of this difference of opinion and reviews immature hominid dentitions with the benefit of improved radiographs and new data on the pattern and rate of pongid dental development. Paranthropus and Australopithecus specimens are shown to possess an ape-like development pattern but incisor development is specialized in the former and superficially human-like in pattern. The present and recent studies on dental development rate and pattern justify the position that early hominids were more ape-like in their growth and development. Therefore, ages at death calculated from pongid dental development schedules are provided for most immature early hominids. More detailed studies of early hominid developmental biology are now possible. It is suggested that divergent heterochronic processes characterize changes in brain/body proportions during hominid evolution. Relative rates of bone remodeling processes can now be identified on early hominid skeletons. The paleodemographic analysis of early hominids is little changed by the developmental model one chooses.     

Cranial morphometry of early hominids: facial region

We report here on early hominid facial diversity, as part of a more extensive morphometric survey of cranial variability in Pliocene and early Pleistocene Hominidae. Univariate and multivariate techniques are used to summarise variation in facial proportions in South and East African hominids, and later Quaternary groups are included as comparators in order to scale the variation displayed. The results indicate that "robust" australopithecines have longer, broader faces than the "gracile" form, but that all australopithecine species show comparable degrees of facial projection. "Robust" crania are characterised by anteriorly situated, deep malar processes that slope forwards and downwards. Smaller hominid specimens, formally or informally assigned to Homo (H. habilis, KNM-ER 1813, etc.), have individual facial dimensions that usually fall within the range of Australopithecus africanus, but which in combination reveal a significantly different morphological pattern; SK 847 shows similarly hominine facial proportions, which differ significantly from those of A. robustus specimens from Swartkrans. KNM-ER 1470 possesses a facial pattern that is basically hominine, but which in some respects mimics that of "robust" australopithecines. Early specimens referred to H. erectus possess facial proportions that contrast markedly with those of other Villafranchian hominids and which suggest differing masticatory forces, possibly reflecting a shift in dietary niche. Overall the results indicate two broad patterns of facial proportions in Hominidae: one is characteristic of Pliocene/basal Pleistocene forms with opposite polarities represented by A. boisei and H. habilis; the other pattern, which typifies hominids from the later Lower Pleistocene onwards, is first found in specimens widely regarded as early representatives of H. erectus, but which differ in which certain respects from the face of later members of that species.     

From apes to humans: locomotion as a key feature for phylogeny

If bipedalism has often been considered to be of a crucial interest for understanding hominid evolution, the acceptance of locomotor features to build phylogenies is still far from being a reality in the field. Especially for hominid evolution, it still seems to be difficult to accept that traits, other than craniodental ones, can be useful for defining the major dichotomies. The recent discovery of Australopithecus anamensis suggests a challenging view of the major dichotomy between apes and humans. Whilst it is widely accepted that Ardipithecus ramidus is ancestral to Australopithecus anamensis, which in its turn is ancestral to Australopithecus afarensis and then to later hominids, the postcranial adaptations, which should be taken into account, suggest another branching pattern. Based on the fact that by 4.0 million years two different locomotor patterns can be identified in hominids, two lineages would appear to be present: the "Australopithecine" lineage (with Australopithecus afarensis or Ardipithecus ramidus if the latter is really a hominid sensu stricto) and the "Hominine" lineage (with Australopithecus anamensis = Praeanthropus africanus).     

History of American paleoanthropological research on early Hominidae, 1925–1980

Understanding of the early stages of hominid evolution prior to 1925 was based primarily on comparative morphological evidence derived from extant primates. With the publication of Australopithecus by Dart in 1925 and subsequent research in South Africa, new possibilities for empirical assessment of early hominid evolutionary history were opened. It was Gregory's work, with Hellman, reported at the first meeting of the AAPA in 1930, that convinced many workers of the hominid status of Australopithecus. The debunking of Eoanthropus as a Pliocene hominid, far from having a totally negative effect, showed that cranial expansion had occurred after bipedalism in hominid evolution, demonstrated that chemical dating had come of age, and in a broader sense, had underlined that phylogenetic hypotheses are falsifiable by recourse to the evidence. The input of biological sciences into early hominid studies, as exemplified by Washburn's “new physical anthropology,” reduced taxonomic diversity and focused attention on paleoecology and behavior. The development of the multidisciplinary approach to field research, pioneered by L. Leakey and brought to fruition by Howell, was of fundamental importance in accurately dating and understanding the context of early hominids. Archaeology, primatology, comparative and functional morphology, and morphometrics have contributed substantially in recent years to a fuller understanding of early hominid evolution. American granting agencies have heavily supported early hominid research but patterns of funding have not kept pace with the change from research based largely on individualistic enterprise to multidisciplinary research projects. Future early hominid research, if funding is available, will likely be directed toward investigating temporal and geographic gaps now known in the fossil record and in more rigorous and multidisciplinary investigations of early hominid behavior.     

New estimates of tooth mark and percussion mark frequencies at the FLK Zinj site: the carnivore-hominid-carnivore hypothesis falsified

Traditional interpretations of hominid carcass acquisition strategies revolve around the debate over whether early hominids hunted or scavenged. A popular version of the scavenging scenario is the carnivore-hominid-carnivore hypothesis, which argues that hominids acquired animal resources primarily through passive opportunistic scavenging from felid-defleshed carcasses. Its main empirical support comes from the analysis of tooth mark frequency and distribution at the FLK Zinj site reported by Blumenschine (Blumenschine, 1995, J. Hum. Evol. 29, 21-51), in which it was shown that long bone mid-shafts exhibited a high frequency of tooth marks, only explainable if felids had preceded hominids in carcass defleshing. The present work shows that previous estimates of tooth marks on the FLK Zinj assemblage were artificially high, since natural biochemical marks were mistaken for tooth marks. Revised estimates are similar to those obtained in experiments in which hyenas intervene after humans in bone modification. Furthermore, analyses of percussion marks, notches, and breakage patterns provide data which are best interpreted as the results of hominid activity (hammerstone percussion and marrow extraction), based on experimentally-derived referential frameworks. These multiple lines of evidence support previous analyses of cut marks and their anatomical distribution; all indicate that hominids had early access to fleshed carcasses that were transported, processed, and accumulated at the FLK Zinj site.     

Bipedal behavior of olive baboons (Papio anubis) and its relevance to an understanding of the evolution of human bipedalism

The bipedal behavior of a troop of olive baboons (Papio anubis) is described. Bipedalism is relatively rare but nevertheless occurs in a wide variety of situations, although bipedalism during feeding occurs much more frequently than in other situations. The incidence of bipedalism varies between different age-sex classes and between individuals within age-sex classes. This pattern of bipedalism occurred within an overall adaptive response, particularly in feeding behavior, which was similar to that of the gelada baboon (Theropithecus gelada). The data on bipedalism is used together with an existing model of early hominid differentiation based on T. gelada to indicate the types of bipedal behavior which might have occurred in early hominid small object feeders and to suggest how a bipedal pattern of this type might have served as a basis for the action of selection for a more committedly bipedal pattern at later stages of hominid evolution.     

The relative cost of bent-hip bent-knee walking is reduced in water

The debate about how early hominids walked may be characterised as two competing hypotheses: They moved with a fully upright (FU) gait, like modern humans, or with a bent-hip, bent-knee (BK) gait, like apes. Both have assumed that this bipedalism was almost exclusively on land, in trees or a combination of the two. Recent findings favoured the FU hypothesis by showing that the BK gait is 50–60% more energetically costly than a FU human gait on land. We confirm these findings but show that in water this cost differential is markedly reduced, especially in deeper water, at slower speeds and with greater knee flexion. These data suggest that the controversy about australopithecine locomotion may be eased if it is assumed that wading was a component of their locomotor repertoire and supports the idea that shallow water might have been an environment favourable to the evolution of early forms of “non-optimal” hominid bipedalism.     

Early hominid feeding mechanisms.

Three predominant influences mark the evolution of human head form: big brain, erect bipedalism, modified oral apparatus. Confusing interplay between different adaptive requirements of each feature has made explanation of skull structure extremely difficult in the past. It now seems possible to isolate each influence in early fossil forms. A model of mammalian modes of feeding adaptation is proposed in the form of a “Natural Experiment” for tighter analysis of fossil forms. Two forms of australopithecines are recognized, “gracile” and “robust.” Both had closely similar brains, both had erect bipedalism, but each had different masticatory construction. Separation of the first two similar influences isolates the adaptive differences in oral mechanics. The gracile form had a projecting oral apparatus, distinct canine and zygomatic buttresses, moderate jaw-lever development, jaw joint not unlike most higher primates, large unusual anterior teeth, moderately sized posterior teeth. The robust form had a retruded, greatly deepened oral apparatus, “dished-in” face with fused canine and zygomatic buttresses, powerful jaw-lever development, distinctively different joint construction, remarkably small anterior teeth, enormous posterior teeth. Striking evidence for extraordinary jaw movements emerges from these features in the robust form. This is strongly supported by remarkably close parallels in Ursidae: grizzly bear and giant panda.