Palaeomagnetic analysis of the Sterkfontein palaeocave deposits: implications for the age of the hominin fossils and stone tool industries

Palaeomagnetic analysis was conducted on speleothems from Members 1-5 at Sterkfontein Cave, South Africa. Palaeomagnetic analysis of siltstone and speleothem from the bulk of Member 4 indicate a reversed magnetic polarity that dates the deposits and its Australopithecus africanus fossils to between 2.58 and ∼2.16 Ma. Further confirmation of this age comes in the form of two short normal polarity events correlated to the Rèunion (∼2.16 Ma) and Huckleberry Ridge (∼2.05 Ma) events in speleothem capping the bulk of Member 4 and coeval with deposition of the final phase of Member 4, including A. africanus fossil Sts 5. At ∼2.16-2.05 Ma, Sts 5 is the youngest representative of A. africanus yet discovered. Palaeomagnetic analysis of the Silberberg Grotto deposits identifies a single short geomagnetic field event in flowstone overlying the StW 573 Australopithecus fossil, which is suggested to represent the Rèunion event at ∼2.16 Ma. This further supports the uranium lead age estimates of 2.3-2.2 Ma for the StW 573 fossil. Based on a reversed polarity for the deposits below the skeleton it cannot be older than 2.58 Ma. If StW 573 is considered to be a second species of Australopithecus then this indicates that two species of Australopithecus are present at Sterkfontein between 2.6 and 2.0 Ma. All of the Member 5 deposits date to less than 1.8 Ma based on a comparison of palaeomagnetic, faunal, and electron spin resonance age estimates. The StW 53 fossil bearing infill (M5A) is intermediate in age between Member 4 and the rest of Member 5 (B-C) at around 1.78-1.49 Ma. The rest of Member 5 (B-C) containing Oldowan and Acheulian stone tools and Homo and Paranthropus fossils was deposited gradually between 1.40 and 1.07 Ma, much younger than previously suggested.     

Sex at Sterkfontein: 'Mrs. Ples' is still an adult female

The important question of whether the Australopithecus africanus hypodigm is taxonomically heterogeneous revolves largely around the interpretation of the morphological variation exhibited by the fossils from Sterkfontein. The sex assignment of these specimens is a critical component in the evaluation of their morphological variability. The Sts 5 cranium is pivotal in this regard because it is the most complete and undistorted specimen attributed to A. africanus. Although it has traditionally been regarded as an adult female, this view has been challenged. In particular, it has been argued recently that Sts 5 is a juvenile and that this, together with alveolar bone loss that has supposedly reduced the size of the canine socket, has led to its misinterpretation as a female. Virtual reconstruction of the M³ roots (and/or alveoli) contradicts arguments that these teeth were erupting at the time of death. Regardless, canine emergence and root completion are well ahead of M³ development in juvenile australopiths from Sterkfontein. Thus, even if the M³ root of Sts 5 was incomplete, its canine root would have been fully formed. Measurements of palate depth indicate that the alveolar margins of Sts 5 have not suffered from much (if any) bone loss in the region of the C/P³; any additional bone would result in a palate of truly exceptional depth. Therefore, the dimensions of the canine alveolus of Sts 5 can be regarded as proxies for those of the canine root. The canine root of Sts 5 is among the smallest recorded for any Sterkfontein australopith, which provides strong support for Robert Broom's initial attribution of sex to this specimen. There is no evidence to contradict the assertion that ‘Mrs. Ples’ is an adult female.     

Dental development in South African australopithecines. Part II: Dental stage assessment

Dental development stages of six immature Australopithecus robustus individuals from Swarktrans (SK 61, SK 62, SK 63, SK 64, SK 438, SK 3978) and seven immature Australopithecus africanus individuals from Taung, Sterkfontein, and Makapans (Taung 1, Sts 2, Sts 8, Sts 18, Sts 24, Stw 327, MLD 2) are described. These stages were assessed using the system devised by Demirjian and colleagues and were based on a data set comprising over 350 computed tomographic (CT) scans taken at 1 and 2 mm slice intervals. It is concluded that patterns of dental development may have differed between A. robustus and A. africanus even though the chronology of development (i.e., the length of time for dental development to occur) may have proceeded relatively rapidly in both species. These data provide unique information regarding the timing and pattern of dental maturation in austral-opithecines and can be used to compare and contrast developmental patterns among early hominids, modern humans, and nonhuman primates.     

Examining affinities of the Taung child by developmental simulation

As a well-preserved juvenile and the type specimen of Australopithecus africanus, the Taung child figures prominently in taxonomic, ontogenetic, and phylogenetic analyses of fossil hominins. Despite general agreement about allocation of Sterkfontein and Makapansgat fossils to this species, limited morphological comparisons have been possible between these adult specimens and the juvenile Taung. Here, we used developmental simulation to estimate the adult form of the Taung child, and directly compare its morphology to that of other fossil hominins. Specimens were represented by 50 three-dimensional landmarks superimposed by generalized Procrustes analysis. The simulation process applied developmental trajectories from extant hominine species to the Taung fossil in order to generate its adult form. Despite differences found in the developmental patterns of these modern species, simulations tested on extant juveniles-transforming them into "adults" using trajectories from other species-revealed that these differences have negligible impact on adult morphology. This indicates that morphology already present by occlusion of the first permanent molar is the primary determinant of adult form, thereby supporting use of extant trajectories to estimate the morphology of an extinct species. The simulated Taung adult was then compared to other adult fossils. As these comparisons required assumptions about the pattern and magnitude of developmental change, additional analyses were performed to evaluate these two parameters separately. Results of all analyses overwhelmingly rejected the possibility that the Taung child was a juvenile robust australopith, but were consistent with the hypothesis that the Taung and Sterkfontein fossils are conspecific. Between Sts 5 and Sts 71, the latter is more likely to resemble the adult form of the Taung child.     

The mid-face of lower Pleistocene hominins and its bearing on the attribution of SK 847 and StW 53

SK 847 and StW 53 have often been cited as evidence for early Homo in South Africa. To examine whether midfacial morphology is in agreement with these attributions, we analyze Euclidean distances calculated from 3-D coordinates on the maxillae of SK 847 and StW 53, as well as Australopithecus africanus (Sts 5, Sts 71), Paranthropus robustus (SK 46, SK 48, SK 52, SK 83), early Homo (KNM-ER 1813, KNM-ER 1805, KNM-ER 3733, KNM-WT 15000), P. boisei (KNM-ER 406, KNM-WT 17000, KNM-WT 17400), Gorilla gorilla (n = 116), Homo sapiens (n = 342), Pan paniscus (n = 21) and P. troglodytes (n = 65). Multivariate analyses separate extant hominoids suggesting we have captured taxonomic affinity. With the exception of SK 847 and SK 52, South African fossils tend to cluster together. P. robustus differs substantially from East African robust megadonts. SK 847 and StW 53 resemble the East African Homo specimens that are the most australopith-like, such as KNM-WT 15000 and KNM-ER 1813. The resemblance between StW 53 and Homo is driven partly by similarities in maxillary size. When distances are scaled, StW 53 aligns with A. africanus, while SK 847 clusters primarily with early Homo.     

Evolutionary Relationships Among Robust and Gracile Australopiths: An “Evo-devo” Perspective

Hominin fossils of gracile and robust australopith groups were found both in East and in South Africa. It is unclear, however, whether all robusts belong to a monophyletic Paranthropus clade, as the craniofacial resemblance among robust australopiths might only be a superficial correlate of similar masticatory adaptations and not evidence of shared ancestry. It has been suggested that the East African Australopithecus/Paranthropus boisei and the South African A./P. robustus might be convergent allometric variants of their gracile geographical neighbors A. afarensis and A. africanus. Here we approach the phylogenetic questions about robust and gracile australopiths from an ??evo-devo?? perspective, examining how simple alterations of development could contribute to the shape differences among hominin species. Using geometric morphometrics we compare gracile and robust australopith crania in the context of the allometric scaling patterns of Pan troglodytes, P. paniscus, and Gorilla gorilla. We examine support for two alternative evolutionary scenarios based on predictions derived from quantitative genetics models: either (1) A./P. robustus evolved in South Africa from the gracile A. africanus, or (2) A./P. robustus is a local variant of the eastern African A./P. boisei. We use developmental simulations to demonstrate that some robust characteristics (wide faces, anteriorly placed zygomatics, and facial dishing) can be predicted by allometric scaling along the ontogenetic trajectory of the gracile A. africanus. We find, however, that the facial differences between A. africanus specimens (Taung, Sts 5, Sts 71, and Stw 505) and A./P. robustus specimen SK 48 cannot be explained by allometric scaling alone. Facial shape differences between A./P. robustus SK 48 and A./P. boisei (KNM-ER 732, KNM-ER 406, OH 5) and the A./P. aethiopicus specimen KNM-WT 17000, on the other hand, can largely be explained by allometric scaling. This is consistent with a close evolutionary relationship of these robust taxa.     

Continuous dental eruption identifies Sts 5 as the developmentally oldest fossil hominin and informs the taxonomy of Australopithecus africanus

The relatively small Australopithecus africanus specimen Sts 5 has figured prominently in taxonomic debates, and the determination of this specimen as a young male or an elderly female has the potential to offer a great deal of resolution on this question. Sts 5 has been argued to be either a small, immature male or a mature female based on a variety of characters.     

Comparison of the endocranial ontogenies between chimpanzees and bonobos via temporal regression and spatiotemporal registration

This paper aims at quantifying ontogenetic differences between bonobo (Pan paniscus) and chimpanzee (Pan troglodytes) endocrania, using dental development as a timeline. We utilize a methodology based on smooth and invertible deformations combined with a metric of “currents” that defines a distance between endocranial surfaces and does not rely on correspondence between landmarks. This allows us to perform a temporal surface regression that estimates typical endocranial ontogenetic trajectories separately for bonobos and chimpanzees. We highlight non-linear patterns of endocranial ontogenetic change and significant differences between species at local anatomical levels rather than considering the endocranium as a uniform entity. A spatiotemporal registration permits the quantification of inter-species differences decomposed into a morphological deformation (accounting for size and shape differences independently of age) and a time warp (accounting for changes in the dynamics of development). Our statistical simulations suggest that patterns of endocranial volume (EV) increase may differ significantly between bonobos and chimpanzees, with an earlier phase of a relatively rapid increase (preferentially at some endocranial subdivisions) in the former and a much later phase of relatively rapid increase in the latter. As a consequence, the chimpanzee endocranium appears to reach its adult size later. Moreover, the time warp indicates that juvenile bonobos develop much slower than juvenile chimpanzees, suggesting that inter-specific ontogenetic shifts do not only concern EV increase, but also the rate of shape changes over time. Our method provides, for the first time, a quantitative estimation of inter-specific ontogenetic shifts that appear to differentiate non-linearly.     

A uniquely modern human pattern of endocranial development. Insights from a new cranial reconstruction of the Neandertal newborn from Mezmaiskaya

The globular braincase of modern humans is distinct from all fossil human species, including our closest extinct relatives, the Neandertals. Such adult shape differences must ultimately be rooted in different developmental patterns, but it is unclear at which point during ontogeny these group characteristics emerge.Here we compared internal shape changes of the braincase from birth to adulthood in Neandertals (N = 10), modern humans (N = 62), and chimpanzees (N = 62). Incomplete fossil specimens, including the two Neandertal newborns from Le Moustier 2 and Mezmaiskaya, were reconstructed using reference-based estimation methods. We used 3D geometric morphometrics to statistically compare shapes of virtual endocasts extracted from computed-tomographic scans. Throughout the analysis, we kept track of possible uncertainties due to the missing data values and small fossil sample sizes.We find that some aspects of endocranial development are shared by the three species. However, in the first year of life, modern humans depart from this presumably ancestral pattern of development. Newborn Neandertals and newborn modern humans have elongated braincases, and similar endocranial volumes. During a ‘globularization-phase’ modern human endocasts change to the globular shape that is characteristic for Homo sapiens. This phase of early development is unique to modern humans, and absent from chimpanzees and Neandertals.Our results support the notion that Neandertals and modern humans reach comparable adult brain sizes via different developmental pathways. The differences between these two human groups are most prominent directly after birth, a critical phase for cognitive development.     

No brain expansion in Australopithecus boisei

The endocranial volumes of robust australopithecine fossils appear to have increased in size over time. Most evidence with temporal resolution is concentrated in East African Australopithecus boisei. Including the KNM-WT 17000 cranium, this sample comprises 11 endocranial volume estimates ranging in date from 2.5 million to 1.4 million years ago. But the sample presents several difficulties to a test of trend, including substantial estimation error for some specimens and an unusually low variance. This study reevaluates the evidence, using randomization methods and a related test using an explicit model of variability. None of these tests applied to the A. boisei endocranial volume sample produces significant evidence for a trend in that species, whether or not the early KNM-WT 17000 specimen is included.     

Underestimating intraspecific variation: The problem with excluding Sts 19 from Australopithecus africanus

Two analyses conclude that Sts 19 cannot be accommodated within the Australopithecus africanus hypodigm (Kimbel and Rak [1993] In Kimbel and Martin [eds.]: Species, Species Concepts, and Primate Evolution. New York: Plenum, pp. 461–484; Sarmiento [1993] Am. J. Phys. Anthropol. [Suppl.] 16:173). Both studies exclude Sts 19 because it possesses synapomorphies with Homo. Furthermore, according to Kimbel and Rak (1993), including Sts 19 in A. africanus results in an unacceptably high degree of polymorphism. This study aims to refute the null hypothesis that Sts 19 belongs to A. africanus. Twelve basicranial characters, as defined and implemented in Kimbel and Rak's study, were scored for casts of seven A. africanus and seven Homo habilis basicranial specimens. These characters were also examined on specimens from a large (N = 87) sample of African pongids. Contrary to Kimbel and Rak's (1993) findings, the null hypothesis is not refuted. The degree of polymorphism among A. africanus with Sts 19 included is less than that seen in Pan troglodytes. In addition, Sts 19 shares only one apomorphy with Homo. However, when treated metrically, Sts 19's morphology for this character is not significantly divergent from other A. africanus specimens. Am J Phys Anthropol 105:461–480, 1998. © 1998 Wiley-Liss, Inc.     

Individual tooth macrowear pattern guides the reconstruction of Sts 52 (Australopithecus africanus) dental arches

The functional restoration of the occlusal relationship between maxillary and mandibular tooth rows is a major challenge in modern dentistry and maxillofacial surgery. Similar technical challenges are present in paleoanthropology when considering fragmented and deformed mandibular and maxillary fossils. Sts 52, an Australopithecus africanus specimen from Sterkfontein Member 4, represents a typical case where the original shape of the dental arches is no longer preserved. It includes a partial lower face (Sts 52a) and a fragmented mandible (Sts 52b), both incomplete and damaged to such an extent to thwart attempts at matching upper and lower dentitions. We show how the preserved macrowear pattern of the tooth crowns can be used to functionally reconstruct Sts 52's dental arches. High‐resolutiondental stone casts of Sts 52 maxillary and mandibular dentition were mounted and repositioned in a dental articulator. The occlusal relationship between antagonists was restored based on the analysis of the occlusal wear pattern of each preserved tooth, considering all dental contact movements represented in the occlusal compass. The reconstructed dental arches were three‐dimensional surface scanned and their occlusal kinematics tested in a simulation. The outcome of this contribution is the first functional restoration of A. africanus dental arches providing new morphometric data for specimen Sts 52. It is noteworthy that the method described in this case study might be applied to several other fossilspecimens. Am J Phys Anthropol, 2013. © 2013 Wiley Periodicals, Inc.     

The alpha taxonomy of Australopithecus at Sterkfontein: The postcranial evidence

The possibility that the fossils attributed to Australopithecus africanus represent more than a single species is of significance because of the pivotal role that A. africanus has played in discussions about hominin evolution. The A. africanus hypodigm that is currently widely recognized evinces considerable variation in a number of craniodental characters, and this has led to speculation that more than one australopith taxon may be represented among the specimens from Sterkfontein. Although crania, mandibles and teeth have dominated these taxonomic discussions, the Sterkfontein postcranial remains also have been invoked. While several workers have proposed that some of the craniodental remains from Sterkfontein can be partitioned into two groups, there is a notable lack of agreement among them as to their actual sorting. Most of the craniodental observations that have been put forward in support of arguments for taxonomic heterogeneity of the Sterkfontein australopith assemblage have been subjective and anecdotal in nature. So too, the postcranial evidence that has been cited in support of more than one australopith species at Sterkfontein has been largely subjective, and limited to a small number of elements. The results of quantitative statistical analyses of the craniodental and postcranial fossils that have been undertaken to date are not necessarily consistent with the hypothesis of taxonomic heterogeneity.     

Evolutionary Development in Australopithecus africanus

Evolutionary developmental biology is quickly transforming our understanding of how lineages evolve through the modification of ontogenetic processes. Yet, while great strides have been made in the study of neontological forms, it is much more difficult to apply the principles of evo-devo to the miserly fossil record. Because fossils are static entities, we as researchers can only infer evolution and development by drawing connections between them. The choices of how we join specimens together??juveniles to adults to study ontogeny, taxon to taxon to study evolution??can dramatically affect our results. Here, I examine paedomorphism in the fossil hominin species Australopithecus africanus. Using extant African apes as proxies for ancestral hominin morphology, I demonstrate that Sts 71 is most similar to a sub-adult African ape, suggesting that A. africanus is paedomorphic relative to the presumed ancestral form. I then plot ontogenetic size and shape in extant great apes, humans, and A. africanus in order to assess patterns of ontogenetic allometry. Results indicate that ontogenetic allometry in A. africanus, subsequent to M1 occlusion is similar to that in modern humans and bonobos; gorillas, chimpanzees, and orangutans share a different pattern of size-shape relationship. Combined with results from the analysis of paedomorphism plus knowledge about the developmental chronologies of this group, these findings suggest that paedomorphism in A. africanus arises relatively early in ontogeny.     

Where are inion and endinion? Variations of the exo- and endocranial morphology of the occipital bone during hominin evolution

The occipital bone is frequently investigated in paleoanthropological studies because it has several features that help to differentiate various fossil hominin species. Among these features is the separation between inion and endinion, which has been proposed to be an autapomorphic trait in (Asian) Homo erectus. Methodologies are developed here to quantify for the first time the location of these anatomical points, and to interpret their variation due to the complex interactions between exocranial and endocranial size and shape of the occipital and nuchal planes, as well as the occipital lobes and cerebellum. On the basis of our analysis, neither ‘the separation between inion and endinion’ nor ‘endinion below inion’ can be considered as an autapomorphic trait in H. erectus, since this feature is a condition shared by extant African great apes and fossil hominins. Moreover, our results show that the exo- and endocranial anatomy of the occipital bone differs between hominins (except Paranthropus boisei specimens and KNM-ER 1805) and great apes. For example, chimpanzees and bonobos are characterized by a very high position of inion and their occipital bone shows an antero-posterior compression. However, these features are partly correlated with their small size when compared with hominins. Asian H. erectus specimens have a thick occipital torus, but do not differ from other robust specimens, neither in this feature nor in the analysed exo- and endocranial proportions of the occipital bone. Finally, the apparent brain size reduction during the Late Pleistocene and variation between the sexes in anatomically modern humans (AMH) reflect that specimens with smaller brains have a relatively larger posterior height of the cerebellum. However, this trend is not the sole explanation for the ‘vertical shift’ of endinion above inion that appears occasionally and exclusively in AMH.     

Brief communication: new reconstruction of the Taung endocast

Earlier reconstructions of the Taung endocast, from the juvenile type specimen for Australopithecus africanus, were achieved without benefit of the advanced computer technology that is available today and before morphological differences were identified that distinguish endocasts of Paranthropus from those of A. africanus. Here, we reconstruct and measure a relatively complete virtual endocast of Taung and provide a new cranial capacity estimate of 382 cm(3) and a projected adult capacity of 406 cm(3), which are smaller than previous estimates. Linear measurements and ratios were also obtained from an endocast of Sts 5 and five Paranthropus endocasts and compared with those of Taung. A number of previously unrecognized foramina, processes, and canals are identified in the bony material that adheres to the base of the Taung endocast. The newly reconstructed virtual endocast of Taung displays a number of shape features that sort it more closely with gracile than robust australopithecines, including squared-off frontal lobes in dorsal view, and the shape of the tips of its temporal poles. The Taung endocast also shares some features with Paranthropus endocasts, while other characteristics such as small temporal lobes may be due to its juvenile status. Just how much of Taung's unique morphology is due to its juvenile status may eventually be clarified by comparing its endocast with those from other juvenile australopithecines such as the 3.3-million-year-old juvenile from Dikika, Ethiopia.     

Variations and asymmetries in regional brain surface in the genus Homo

Paleoneurology is an important field of research within human evolution studies. Variations in size and shape of an endocast help to differentiate among fossil hominin species whereas endocranial asymmetries are related to behavior and cognitive function. Here we analyse variations of the surface of the frontal, parieto-temporal and occipital lobes among different species of Homo, including 39 fossil hominins, ten fossil anatomically modern Homo sapiens and 100 endocasts of extant modern humans. We also test for the possible asymmetries of these features in a large sample of modern humans and observe individual particularities in the fossil specimens.This study contributes important new information about the brain evolution in the genus Homo. Our results show that the general pattern of surface asymmetry for the different regional brain surfaces in fossil species of Homo does not seem to be different from the pattern described in a large sample of anatomically modern H. sapiens, i.e., the right hemisphere has a larger surface than the left, as do the right frontal, the right parieto-temporal and the left occipital lobes compared with the contra-lateral side. It also appears that Asian Homo erectus specimens are discriminated from all other samples of Homo, including African and Georgian specimens that are also sometimes included in that taxon. The Asian fossils show a significantly smaller relative size of the parietal and temporal lobes. Neandertals and anatomically modern H. sapiens, who share the largest endocranial volume of all hominins, show differences when considering the relative contribution of the frontal, parieto-temporal and occipital lobes. These results illustrate an original variation in the pattern of brain organization in hominins independent of variations in total size. The globularization of the brain and the enlargement of the parietal lobes could be considered derived features observed uniquely in anatomically modern H. sapiens.     

Size and shape variation in the proximal femur of Australopithecus africanus

Aside from use as estimates of body mass dimorphism and fore to hind limb joint size comparisons, postcranial elements have not often contributed to assessments of variation in Australopithecus africanus. Meanwhile, cranial, facial, and dental size variation is interpreted to be high or moderately high. Further, the cranial base and face express patterns of structural (shape) variation, which are interpreted by some as evidence for the presence of multiple species. Here, the proximal femur is used to consider postcranial size and shape variation in A. africanus. Original fossils from Makapansgat and Sterkfontein, and samples from Homo, Pan, Gorilla, and Pongo were measured. Size variation was assessed by comparing the A. africanus coefficient of variation to bootstrapped distributions of coefficient of variation samples for each taxon. Shape variation was assessed from isometrically adjusted shape variables. First, the A. africanus standard deviation of log transformed shape variables was compared to bootstrapped distributions of logged standard deviations in each taxon. Second, shape variable based Euclidean distances between fossil pairs were compared to pairwise Euclidean distance distributions in each reference taxon. The degree of size variation in the A. africanus proximal femur is consistent with that of a single species, and is most comparable to Homo and Pan, lower than A. afarensis, and lower than some estimates of cranial and dental variation. Some, but not all, shape variables show more variation in A. africanus than in extant taxa. The degree of shape difference between some fossils exceeds the majority of pairwise differences in the reference taxa. Proximal femoral shape, but not size, variation is consistent with high estimates of A. africanus cranial variation.     

Endocranial casts of early archosaurian reptiles

The first endocranial casts from non-crown-group archosaurian reptiles are described. Synthetic casts were made of the preserved parts of the endocranial cavities ofErythrosuchus africanus,Vjushkovia triplicostata andXilousuchus sapingensis. These casts are incomplete, but they reveal a substantial amount of information on the form of the early archosaur brain and inner ear. Cranial and pontine flexures are equal in bothErythrosuchus (45°) andV. triplicostata (35–40°). The endocasts ofXilousuchus andV. triplicostata are characterised by the possession of a pseudolagena and a quantifiable down-slope from the myelencephalon to the metencephalon — features that have yet to be described for other reptilian endocranial casts.