Limb proportions and positional behavior

Major changes in body shape occurred during human evolution, but questions remain about body shape in australopiths. The present study investigates the specifics of the presumed relationships between limb indices and positional behavior underlying prior work that compared proportions among extant hominids in order to make inferences about extinct hominins. We find that although both intermembral index or ratio of diaphyseal and articular proportions distinguish humans from great apes, neigher correlates well with variation in the degree of arboreality in the locomotor repertoire of extant hominids. Brachial index and a ratio of diaphyseal and articular dimensions from the fore- and hindlimb, however, do correlate with degree of arboreality, and scale slightly positively allometrically within species in all extant taxa. These two observations are taken into consideration in a more nuanced interpretation of a reanalysis of articular-diaphyseal limb proportions in an expanded sample of the Sterkfontein postcrania. This study confirms previous findings that Australopithecus africanus had larger forelimb dimensions in relation to hindlimb dimensions than modern humans and A. afarensis, similar to the patterns seen in extant apes, particularly western gorillas. However, data presented here suggest that interpreting a particular taxon as “human-like” or “ape-like” may be overly simplistic. Instead, while both A. africanus and A. afarensis were almost certainly committed bipeds that incorporated some arboreality into their locomotor repertoire, A. africanus apparently used a set of locomotor behaviors that was more distinct from that of A. afarensis than Pan troglodytes and Gorilla gorilla are from each other.

Long bone cross-sectional geometry

Long bone diaphyseal cross-sectional geometries reflect the mechanical properties of the bones, and can be used to aid in inferences of locomotor behavior in extinct hominins. This chapter considers all available long bone diaphyseal and femoral neck cross-sections of specimens from Sterkfontein Member 4, and presents comparisons of these section properties and other cross-sectional dimensions with those of other early hominins as well as modern samples. The cross-sectional geometry of the Sterkfontein Member 4 long bone specimens suggests some similarities to, but also interesting differences in, mechanical loading of these elements relative to modern humans. The less asymmetric cortical bone distribution in the Sterkfontein femoral necks is consistent with other evidence above indicating an altered gait pattern involving lateral displacement of the body center of mass over the stance limb. The relatively very strong upper limb of StW 431 implies that arboreal behavior formed a significant component of its locomotor repertoire. Bipedal gait may have been less efficient and arboreal climbing more prevalent in the Sterkfontein hominins.

Metacarpals and manual phalanges

This chapter presents description and analysis of the metacarpals and manual phalanges from Sterkfontein. Although the morphology is generally similar across the sample where there are duplicates of the same element, there are differences in size that are quite remarkable within the context of all South African hominins. Some very large specimens suggest the presence of individuals at Sterkfontein with much larger hands, and presumably larger body size, at Sterkfontein than those of A. sediba MH2, H. naledi and the Swartkrans hominins. Australopithecus africanus had human-like proportions, but this may be plesiomorphic within the hominoid clade. The potentially less mobile trapezium-Mc1 joint, absence of a fully developed palmar pulp on the distal thumb, more limited pronation of the index finger, and potentially more wedge-shaped trapezoid inferred from the preserved external morphology, is consistent with lower manipulative loading of the thumb than is typical of later Homo. As for other forelimb elements, moderately curved manual phalanges suggests a greater reliance on forelimb-dominated locomotor behaviors and perhaps selection for more frequent use of an arboreal environment in A. africanus than is found in A. afarensis. Thus, within this broader context, the Sterkfontein fossil hominin remains are not unusual. The Sterkfontein hand fossils suggest an overall manipulative and locomotor loading regime that was more similar to that of other South African australopiths and distinct from that of later Homo, but more refined functional interpretations require additional fossil evidence, particularly from associated hand skeletons

Distal humerus

Anatomical descriptions are presented for six distal humeri from Sterkfontein. These fossil specimens, and one humerus from Makapansgat, are compared to those of extant hominids and a variety of fossil hominins using geometric morphometric data from a transverse section through the distal diaphysis. Principal components analysis is used to summarize morphometric affinities among specimens. Procrustes distances are used to measure shape dissimilarity between pairs, and total sample variation is quantified as the sum of squared distances (SSD) of pairwise Procrustes. Bootstrap resampling of SSD is used to evaluate the possibility of taxonomic heterogeneity in the Sterkfontein assemblage. Humeral variation at Sterkfontein/Makapansgat reflects three distinct groups, one of which resembles Homo erectus (Stw 150, 182), one of which resembles Australopithecus sediba (Stw 339, MLD 14), and one of which resembles non-sediba australopiths (Stw 38, Stw 124, Stw 431c). Based on variation within extant species, we cannot reject the null hypothesis that the Sterkfontein specimens are conspecific. On the other hand, Sterkfontein SSD is significantly high with reference to a mixed-species sample of australopiths from across Africa. On balance, we suggest that only those specimens in the last group (non-sediba autralopiths) should be attributed to A. africanus.

The partial skeletons

Sterkfontein is notable not only for its wealth of isolated bones but also for the fortuitous preservation of two partial skeletons, both attributed to Australopithecus africanus, Sts 14 and StW 431 from Member 4. This short chapter reviews the history of recovery and preservation of these skeletons. Sts 14 was the first partial skeleton of any australopith ever found, including 19 associated bones recovered in a single block of breccia in 1947. The skeleton was too fragile to be removed by mechanical preparation techniques, so was only removed from the block using acid preparation in the late 1960s. Additional rib fragments were only recently identified and accessioned with this skeleton. StW 431 was recovered in 1987, with four additional elements identified and published in 2003. Although this skeleton did not derive from a single block of breccia, their proximity within the sediments and similar preservation indicate that these bones are associated. Given the small size of Sts 14 and compared to the much larger size of StW 431, they are likely female and male individuals respectively. Subsequent chapters describe individual skeletal elements by anatomical region.

Summary and synthesis

This chapter summarizes the findings presented in the preceding chapters. To our knowledge, this volume represents the first source presenting all these fossils together in one collection. The chapters in this book provide photographs, measurements, and basic descriptions of each postcranial fossil. They present comparative observations and analyses and statistical analysis of taxonomically and functionally relevant aspects of morphology, and in some cases they provide reconstructions of the bones. Each of the chapters in this volume addresses key questions surrounding the Sterkfontein hominins. The Sterkfontein specimens share with other hominins features of the vertebral column, thorax, hip, knee, ankle, and foot indicating fully upright, human-like posture. Among many anatomical regions, though, the A. africanus fossils appear distinct from those attributed to A. afarensis, yet the variation between these samples often does not exceed that observed within single species of extant hominoids. Still, there may be evidence of more climbing behavior but also improved manual manipulatory behaviors, and perhaps a slightly different gait than typical of other hominins. Each chapter in this volume comes to a similar conclusion, that although variation in size or morphology may exceed that found in other australopith species, or even among mixed samples of other fossil hominins, the hypothesis that only one species is represented by the Sterkfontein Member 4 hominins cannot be falsified. The australopith species that “started it all” back in 1925 still has more to teach us, and we all look forward to learning what the future holds for Australopithecus africanus.

Pelvis

The hominin pelvis is a remarkable structure that plays a central role in many critical biological processes, most notably bipedal locomotion and parturition. Here we describe each pelvic element from Sterkfontein and reconstruct the pelvis of the partial skeletons of Sts 14 and StW 431. Based on preserved contours, if human sexing methods can be accurately applied to australopiths, it is likely that Sts 14 represents a female individual, with the larger size, narrower greater sciatic notch of Sts 65 and StW 431 and other morphologies suggesting that these individuals were male. Some degenerative defects of the StW 431 acetabulum also suggest this may have been an older adult. This chapter also presents complete pelvic reconstructions of all of these fragmentary hipbone fossils.

Scapula, clavicle, and proximal humerus

This chapter describes parts of four scapulae, two clavicles, and three proximal humeri, and provides information on the functional anatomy of the Sterkfontein hominin pectoral girdle and shoulder joint. The clavicle resembles that of other hominins, which indicates a more horizontal orientation than in apes. Comparative analysis of these scapulae, though, suggests that the glenohumeral joint of Australopithecus africanus is more cranially oriented than typical for humans, perhaps even more so than seen in East African early hominins. This morphology may also indicate features of dorsal scapular shape, which could impact potential areas of attachment as well as line of action of dorsal rotator cuff muscles. The glenohumeral joint morphology and orientation implies the possibility of more arboreality in the A. africanus locomotor repertoire.

A new multidisciplinary age of 2.61–2.07 Ma for the Sterkfontein Member 4 australopiths

Sterkfontein Caves is the single richest early hominin site in the world, with deposits yielding two potential species of Australopithecus, Paranthropus robustus, and early Homo, as well as an extensive faunal collection and stone tools. Recent advances in uranium-lead (U-Pb) dating of speleothems and palaeomagnetic analysis at Sterkfontein provide the first consistent chronological framework for Member 4 (MB4) and so the interned australopith fossils. Current data suggest that the MB4 deposit and so australopith remains accumulated over at least 400,000 years (2.4–2.0 Ma) if not 500,000–800,000 years. This long period of deposition should be taken into account when studying the MB4 australopith remains and looking at variability in both anatomy and other data such as isotopic evidence for diet.

The geological setting, cave formation, and stratigraphy of the fossil-bearing deposits at Sterkfontein Caves

Understanding the formation of the Sterkfontein Caves is critical for interpreting the geologic age and patterns of accumulation of the fossils found there. The complex stratigraphy of the caves has been investigated for decades, often yielding conflicting interpretation. Current research at Sterkfontein aims to apply new mutlidisciplinary stratigraphic analyses to new stratigraphically sensitive excavations, thereby providing high-resolution contextual support to recovered assemblages and attempts to increase our understanding of excavated deposit morphology through identification and study of sedimentary and stratigraphic features preserved in the remnants of previous excavations. The Sterkfontein deposits separated into six members, Member 1 to Member 6, of which Member 2, 4, and 5 have yielded hominin remains. Partial collapses and localized erosion of deposits are common, and the spaces created are often filled with sediments from distinct sources, complicating interpretation. This is especially the case with Members 4 and 5. Characterizing taxonomic, morphological, or cultural variability from excavated deposits where no stratigraphy was recognized is difficult but is the subject of ongoing work.