Radius of Paranthropus robustus from member 1, Swartkrans formation, South Africa

Recently recovered hominid postcrania from Member 1, Swartkrans Formation include the proximal and distal ends of a right radius attributed to a single individual of Paranthropus robustus. These fossils are essentially similar to Australopithecus afarensis, A. africanus, and P. boisei homologues. The head manifests an ape-like circumferentia articularis, and the distal end has prominent medial, dorsal, and lateral tubercles and a well developed brachioradialis crest, features also commonly exhibited by extant great apes. The volar set of the P. robustus radiocarpal joint, like that of Australopithecus homologues, more closely resembles the neutral condition exhibited by Homo than the greater flexion evinced by living apes. Compared with fossil and recent specimens of Homo, the configuration of the P. robustus radial head suggests enhanced stability against medial displacement during pronation and supination; the strong crest for the attachment of brachioradialis may attest to enhanced forearm flexor capability. In addition, this crest and the prominent dorsal tubercles may indicate enhanced hand extensor and, therefore, hand flexor capabilities. The differences in radial morphology between Paranthropus and Homo may relate to significant behavioral differences between these two synchronic taxa.     

Hominin first metatarsals (SKX 5017 and SK 1813) from Swartkrans: a morphometric analysis.

Two hominin metatarsals from Swartkrans, SKX 5017 and SK 1813, have been reported by Susman and Brain [1988. New first metatarsal (SKX 5017) from Swartkrans and the gait of Paranthropus robustus. Am. J. Phys. Anthropol. 79, 451-454] and Susman and de Ruiter [2004. New hominin first metatarsal (SK 1813) from Swartkrans. J. Hum. Evol. 47, 171-181]. They found these bones to have both primitive and derived traits indicating that, while being bipedal, these hominines had a unique toe-off mechanism. We have undertaken additional multivariate morphometric analyses, comparing the fossils to the first metatarsals of modern humans and extant apes. The largest proportion of discrimination lies in the different locomotor functions: apes on the one hand and the humans and fossils on the other. While the fossils have the closest affinity to humans, they have a unique biomechanical pattern suggesting a more facultative form of bipedalism. The implications of this are, while morphometric analyses do not necessarily directly capture the described primitive and derived traits, the associated functional pattern is held within the broader morphology of the bone.     

On the eruption pattern of the permanent incisors and first permanent molars in Paranthropus

It has been claimed recently that Australopithecus exhibited a pattern of permanent tooth eruption like that of extant great apes, whereas a significantly different pattern was shared by Paranthropus and Homo (Dean, 1985). More particularly, each of the four Paranthropus specimens examined in that study was held to show advanced development and eruption of the permanent incisors relative to the first molar. It is demonstrated here that the eruption sequence that was posited for at least one of these four Paranthropus specimens (SK 61) is clearly erroneous, while the developmental/eruption sequences manifested by the other three specimens would appear to be more ambiguous than was claimed. Another juvenile specimen of Paranthropus (KNM-ER 1820) that was not included in Dean's study also does not necessarily support the eruption pattern that was said to characterize that taxon.     

New hominid fossils from the Swartkrans formation (1979-1986 excavations): craniodental specimens

Seventy-two individually numbered hominid craniodental fossils from recent excavations at Swartkrans are described. All derive from in situ decalcified breccia and/or unconsolidated sediments. A total of 20 specimens, representing 13 to 16 individuals derive from Member 1 "Lower Bank," two teeth derive from sediments along the Member 1-2 Interface, 38 fossils representing 19 to 24 individuals come from Member 2, and 12 teeth representing 9 to 11 individuals derive from Member 3. All but four of the specimens are attributable at the generic level; one specimen from Member 1 "Lower Bank" and five specimens from Member 2 are attributed to Homo, while the others represent Paranthropus. The proportional representation of Homo in the Swartkrans Formation is markedly higher in Member 2 (c. 33%) than in the Member 1 "Lower Bank" (c. 8%) and Member 1 "Hanging Remnant" (c. 5%) samples.     

New hominin first metatarsal (SK 1813) from Swartkrans

A recently recognized hominin hallucal metatarsal, SK 1813, from Swartkrans bears a suite of primitive and derived traits. Comparisons with extant apes, modern humans, SKX 5017, and Stw 562 reveals similar morphology in all three fossils and that these early hominins, while bipedal, possessed a unique toe-off mechanism. The implications of this are that both primitive and derived traits must be used to establish the total biomechanical pattern.     

Fossil hominin ulnae and the forelimb of Paranthropus

The discovery of Pan in the Middle Pleistocene deposits of the Kapthurin Formation of the Tugen Hills (McBrearty and Jablonski: Nature 437 (2005) 105-108) inspires new interest in the search for other chimpanzee fossils in the East African Rift Valley. Craniodental evidence of an eastward excursion of chimpanzee populations in the Plio-Pleistocene goes undetected in other hominin sites, but one enigmatic postcranial fossil, the Olduvai Hominid 36 ulna, has many chimp-like features. Analyses by Aiello et al. (Aiello et al.: Am J Phys Anthropol 109 (1999) 89-110) reveal that it is similar to extant Pan in some respects, but it also has unique traits not seen in other hominoid species. They refer it to Paranthropus boisei. In this study, we reassess the affinities of OH 36 using a different data set that includes more recently discovered hominin fossils including those attributed to Paranthropus. Despite its superficial resemblance to modern Pan, our results agree with those of Aiello et al. (Aiello et al.: Am J Phys Anthropol 109 (1999) 89-110) that OH 36 is distinctly different from modern chimpanzees. By default, it is reasonable to assign this specimen to P. boisei, but it is not at all similar to other ulnae referred to this genus. Ulnae attributed to Paranthropus from South Africa, Kenya, and Ethiopia are morphologically more heterogeneous than those within species of large-bodied Hominoidea. Although there are many apparent shared derived traits justifying a monophyletic Paranthropus clade, most if not all of these traits are related to a single functional complex (hypermastication) that may have evolved in parallel and thereby constituting a paraphyletic group of species.     

A cranial base of Australopithecus robustus from the hanging remnant of Swartkrans, South Africa

SKW 18, a partial hominin cranium recovered from the site of Swartkrans, South Africa, in 1968 is described. It is derived from ex situ breccia of the Hanging Remnant of Member 1, dated to approximately 1.5-1.8 Mya. Although partially encased in breccia, it was refit to the facial fragment SK 52 (Clarke 1977 The Cranium of the Swartkrans Hominid SK 847 and Its Relevance to Human Origins, Ph.D. dissertation, University of the Witwatersrand, Johannesburg), producing the composite cranium SKW 18/SK 52. Subsequent preparation revealed the most complete cranial base attributable to the species Australopithecus robustus. SKW 18 suffered weathering and slight postdepositional distortion, but retains considerable anatomical detail. The composite cranium most likely represents a large, subadult male, based on the incomplete fusion of the spheno-occipital synchondrosis; unerupted third molar; pronounced development of muscular insertions; and large teeth. Cranial base measures of SKW 18 expand the range of values previously recorded for A. robustus. SKW 18 provides information on anatomical features not previously visible in this taxon, and expands our knowledge of morphological variability recognizable in the cranial base. Morphological heterogeneity in the development of the prevertebral and nuchal muscular insertions is likely the result of sexual dimorphism in A. robustus, while differences in cranial base angles and the development of the occipital/marginal sinus drainage system cannot be attributed to size dimorphism.     

New hominid fossils from the Swartkrans formation (1979-1986 excavations): postcranial specimens

New postcranial fossils of Paranthropus robustus and Homo cf. erectus were recovered from Swartkrans from 1979 through 1986. These fossils are from Members 1, 2, and 3. The new fossils are described here along with their morphological affinities. Fossils that are assigned to Paranthropus indicate that the South African "robust" australopithecines engaged in tool behavior and were essentially terrestrial bipeds at around 1.8 Myr BP. The manual dexterity and bipedal locomotion of Paranthropus may have equaled that of Homo habilis in East Africa at approximately the same time.     

The eruption pattern of the permanent incisors and first permanent molars in Australopithecus (Paranthropus) robustus

This study aims to reassess the claim that the eruption sequence of the permanent incisor and first permanent molar teeth of Australopithecus (Paranthropus) robustus is identical with that in modern Homo sapiens. Eight fossil hominid mandibles of equivalent dental developmental age were chosen for comparative study. Emphasis has been placed upon the comparative timing of events within the growth period rather than eruption sequence alone. The results of this study indicate that Homo sapiens and Australopithecus (Paranthropus) robustus share the same pattern of permanent molar and incisor eruption and that this is significantly different from the pattern of eruption shared by the great apes, Australopithecus africanus and Australopithecus afarensis.     

Species attribution of the Swartkrans member 1 first metacarpals: SK84 and SKX 5020

Susman (Am. J. Phys. Anthropol. 75:277-278, 79:451-474; Science 240:781-784; In FE Grine (ed): Evolutionary History of the "Robust" Australopithecines. New York: Aldine de Gruyter, pp. 149-172) has attributed the morphologically similar SK 84 and SKX 5020 hominid first metacarpals to Homo erectus and Australopithecus robustus, respectively, and has inferred that both species exhibited derived pollical morphologies, indicating refined precision grips. Consideration of the structure of his taphonomic arguments indicates that there are no adequate nonmorphological reasons to attribute these specimens securely to one or the other of the craniodentally represented species at Swartkrans. His morphological arguments fail to note any significant differences between the two specimens. Only the contrast in size between the small SK 84 and large SKX 5020 bones might warrant a species distinction; yet comparison of their length ratio to distributions of modern human first metacarpal length ratios indicates that it is not possible to reject conclusively the null hypothesis that they are conspecific. Therefore, early hominid adaptive scenarios based on a derived Homo-like manual functional morphology in A. robustus remain without a secure paleontological basis.     

Hominid carpal,metacarpal, and phalangeal bones recovered from the Hadar formation: 1974–1977 collections

The fossil hominid hand bone collection from the Pliocene Hadar Formation, Ethiopia, is described anatomically. These hand bones, all from A.L. (Afar Locality) 333 and 333w, constitute the largest sample of hominid manus remains thus far recovered from the Plio-Pleistocene of Africa.     

Enamel-dentine junction (EDJ) morphology distinguishes the lower molars of Australopithecus africanus and Paranthropus robustus

Tooth crown morphology plays a central role in hominin systematics, but the removal of the original outer enamel surface by dental attrition often eliminates from consideration the type of detailed crown morphology that has been shown to discriminate among hominin taxa. This reduces the size of samples available for study. The enamel-dentine junction (EDJ) is the developmental precursor and primary contributor to the morphology of the unworn outer enamel surface, and its morphology is only affected after considerable attrition. In this paper, we explore whether the form of the EDJ can be used to distinguish between the mandibular molars of two southern African fossil hominins: Paranthropus (or Australopithecus) robustus and Australopithecus africanus. After micro-computed tomographic scanning the molar sample, we made high-resolution images of the EDJ and used geometric morphometrics to compare EDJ shape differences between species, in addition to documenting metameric variation along the molar row within each species. Landmarks were collected along the marginal ridge that runs between adjacent dentine horns and around the circumference of the cervix. Our results suggest that the morphology of the EDJ can distinguish lower molars of these southern African hominins, and it can discriminate first, second, and third molars within each taxon. These results confirm previous findings that the EDJ preserves taxonomically valuable shape information in worn teeth. Mean differences in EDJ shape, in particular dentine horn height, crown height, and cervix shape, are more marked between adjacent molars within each taxon than for the same molar between the two taxa.     

Cross-sectional morphology of the SK 82 and 97 proximal femora

Computed tomography scans of the proximal femoral shaft of the South African “robust” australopithecine, A. robustus, reveal a total morphological pattern that is similar to the specimen attributed to A. boisei in East Africa but unlike that of Homo erectus or modern human femora. Like femora attributed to H. erectus, SK 82 and 97 have very thick cortices, although they do not have the extreme increase in mediolateral buttressing that is so characteristic of H. erectus. And unlike H. erectus or modern humans, their femoral heads are very small relative to shaft strength. These features are consistent with both increased overall mechanical loading of the postcranial skeleton and a possibly slightly altered pattern of bipedal gait relative to that of H. erectus and modern humans. Am J Phys Anthropol 109:509–521, 1999. © 1999 Wiley-Liss, Inc.     

A complete second metatarsal (StW 89) from Sterkfontein Member 4, South Africa

The functional anatomy of the hominin foot has played a crucial role in studies of locomotor evolution in human ancestors and extinct relatives. However, foot fossils are rare, often isolated, and fragmentary. Here, we describe a complete hominin second metatarsal (StW 89) from the 2.0-2.6 million year old deposits of Member 4, Sterkfontein Cave, South Africa. Like many other fossil foot bones, it displays a mosaic of derived human-like features and primitive ape-like features. StW 89 possesses a domed metatarsal head with a prominent sulcus, indicating dorsiflexion at the metatarsophalangeal joint during bipedal walking. However, while the range of motion at the metatarsophalangeal joint is human-like in dorsiflexion, it is ape-like in plantarflexion. Furthermore, StW 89 possesses internal torsion of the head, an anatomy decidedly unlike that found in humans today. Unlike other hominin second metatarsals, StW 89 has a dorsoplantarly gracile base, perhaps suggesting more midfoot laxity. In these latter two anatomies, the StW 89 second metatarsal is quite similar to the recently described second metatarsal of the partial foot from Burtele, Ethiopia. We interpret this combination of anatomies as evidence for a low medial longitudinal arch in a foot engaged in both bipedal locomotion, but also some degree of pedal, and perhaps even hallucal, grasping. Additional fossil evidence will be required to determine if differences between this bone and other second metatarsals from Sterkfontein reflect normal variation in an evolving lineage, or taxonomic diversity.     

Alleged synapomorphy of the M1/I1 eruption pattern in robust australopithecines and Homo: evidence from high-resolution computed tomography

Ever since Broom and Robinson (1951) published their claim that the eruption pattern of permanent incisors in robust australopithecines was most similar to that of modern man and different from that of gracile australopithecines and apes, the accuracy of this observation has been the subject of periodic debate (e.g., Wallace: Ph.D. thesis, 1972; Dean: Am. J. Phys. Anthropol. 67:251-257, 1985; Grine: Am. J. Phys. Anthropol. 72:353-359, 1987). Part of the problem is that the developing incisors in one of the specimens most crucial to this argument (SK61) are difficult to visualize clearly by conventional radiographic techniques because of the heavy mineralization in the fossil. This study reanalyzes SK 61 by high-resolution computed tomography in order to contribute to the final resolution of its incisor development. Grine's (op. cit.) assessment of the incisors as the deciduous ones, not the permanent ones, is fully confirmed. This fact, in conjunction with the observation that permanent incisor root formation had only just commenced in this specimen, further weakens the argument of M1/I1 eruption pattern synapomorphy between Homo and robust australopithecines.     

Gluteus maximus muscle function and the origin of hominid bipedality

Bipedality not only frees the hands for tool use but also enhances tool use by allowing use of the trunk for leverage in applying force and thus imparting greater final velocity to tools. Since the weight and acceleration of the trunk and forelimbs on the hindlimbs must be counteracted by muscles such as m. gluteus maximus that control pelvic and trunk movements, it is suggested that the large size of the cranial portion of the human gluteus maximus muscle and its unique attachment to the dorsal ilium (which is apparent in the Makapan australopithecine ilium) may have contributed to the effectiveness with which trunk movement was exploited in early hominid foraging activities. To test this hypothesis, the cranial portions of both right and left muscles were investigated in six human subjects with electromyography during throwing, clubbing, digging, and lifting. The muscles were found to be significantly recruited when the trunk is used in throwing and clubbing, initiating rotation of the pelvis and braking it as trunk rotation ceases and the forelimb accelerates. They stabilize the pelvis during digging and exhibit marked and prolonged activity when the trunk is maintained in partial flexion during lifting of heavy objects.