Cochlear shape distinguishes southern African early hominin taxa with unique auditory ecologies.

Insights into potential differences among the bony labyrinths of Plio-Pleistocene hominins may inform their evolutionary histories and sensory ecologies. We use four recently-discovered bony labyrinths from the site of Kromdraai to significantly expand the sample for Paranthropus robustus. Diffeomorphometry, which provides detailed information about cochlear shape, reveals size-independent differences in cochlear shape between P. robustus and Australopithecus africanus that exceed those among modern humans and the African apes. The cochlea of P. robustus is distinctive and relatively invariant, whereas cochlear shape in A. africanus is more variable, resembles that of early Homo, and shows a degree of morphological polymorphism comparable to that evinced by modern species. The curvature of the P. robustus cochlea is uniquely derived and is consistent with enhanced sensitivity to low-frequency sounds. Combined with evidence for selection, our findings suggest that sound perception shaped distinct ecological adaptations among southern African early hominins.

Cochlear shape reveals that the human organ of hearing is sex-typed from birth.

Sex differences in behavioral and neural characteristics can be caused by cultural influences but also by sex-based differences in neurophysiological and sensorimotor features. Since signal-response systems influence decision-making, cooperative and collaborative behaviors, the anatomical or physiological bases for any sex-based difference in sensory mechanisms are important to explore. Here, we use uniform scaling and nonparametric representations of the human cochlea, the main organ of hearing that imprints its adult-like morphology within the petrosal bone from birth. We observe a sex-differentiated torsion along the 3D cochlear curve in samples of 94 adults and 22 juvenile skeletons from cross-cultural contexts. The cochlear sexual dimorphism measured in our study allows sex assessment from the human skeleton with a mean accuracy ranging from 0.91 to 0.93 throughout life. We conclude that the human cochlea is sex-typed from an early post-natal age. This, for the first time, allows nondestructive sex determination of juveniles' skeletal remains in which the biomolecules are too degraded for study but in which the petrosal is preserved, one of the most common bone within archaeological assemblages. Our observed sex-typed cochlear shape from birth is likely associated with complex evolutionary processes in modern humans for reasons not yet fully understood.

Morphometric analyses of hominoid crania, probabilities of conspecificity and an approximation of a biological species constant.

Thackeray has previously explored the possibility of using a morphometric approach to quantify the "amount" of variation within species and to assess probabilities of conspecificity when two fossil specimens are compared, instead of "pigeon-holing" them into discrete species. In an attempt to obtain a statistical (probabilistic) definition of a species, Thackeray has recognized an approximation of a biological species constant (T=-1.61) based on the log-transformed standard error of the coefficient m (log sem) in regression analysis of cranial and other data from pairs of specimens of conspecific extant species, associated with regression equations of the form y=mx+c where m is the slope and c is the intercept, using measurements of any specimen A (x axis), and any specimen B of the same species (y axis). The log-transformed standard error of the co-efficient m (log sem) is a measure of the degree of similarity between pairs of specimens, and in this study shows central tendency around a mean value of -1.61 and standard deviation 0.10 for modern conspecific specimens. In this paper we focus attention on the need to take into account the range of difference in log sem values (Δlog sem or "delta log sem") obtained from comparisons when specimen A (x axis) is compared to B (y axis), and secondly when specimen A (y axis) is compared to B (x axis). Thackeray's approach can be refined to focus on high probabilities of conspecificity for pairs of specimens for which log sem is less than -1.61 and for which Δlog sem is less than 0.03. We appeal for the adoption of a concept here called "sigma taxonomy" (as opposed to "alpha taxonomy"), recognizing that boundaries between species are not always well defined.

Disproportionate Cochlear Length in Genus Homo Shows a High Phylogenetic Signal during Apes' Hearing Evolution.

Changes in lifestyles and body weight affected mammal life-history evolution but little is known about how they shaped species' sensory systems. Since auditory sensitivity impacts communication tasks and environmental acoustic awareness, it may have represented a deciding factor during mammal evolution, including apes. Here, we statistically measure the influence of phylogeny and allometry on the variation of five cochlear morphological features associated with hearing capacities across 22 living and 5 fossil catarrhine species. We find high phylogenetic signals for absolute and relative cochlear length only. Comparisons between fossil cochleae and reconstructed ape ancestral morphotypes show that Australopithecus absolute and relative cochlear lengths are explicable by phylogeny and concordant with the hypothetized ((Pan,Homo),Gorilla) and (Pan,Homo) most recent common ancestors. Conversely, deviations of the Paranthropus oval window area from these most recent common ancestors are not explicable by phylogeny and body weight alone, but suggest instead rapid evolutionary changes (directional selection) of its hearing organ. Premodern (Homo erectus) and modern human cochleae set apart from living non-human catarrhines and australopiths. They show cochlear relative lengths and oval window areas larger than expected for their body mass, two features corresponding to increased low-frequency sensitivity more recent than 2 million years ago. The uniqueness of the "hypertrophied" cochlea in the genus Homo (as opposed to the australopiths) and the significantly high phylogenetic signal of this organ among apes indicate its usefulness to identify homologies and monophyletic groups in the hominid fossil record.

The enamel-dentine junction in the postcanine dentition of Australopithecus africanus: intra-individual metameric and antimeric variation.

We used micro-computed tomography and virtual tools to study metric and morphological features at the enamel-dentine junction and on the outer enamel surface in the postcanine dentition of an exceptionally well-preserved maxilla and mandible of an early hominin. The fossil, Sts 52 from Sterkfontein, South Africa, is attributed to Australopithecus africanus and is about 2.5 million years old. For comparative purposes in this exploratory study, we also used micro-computed tomography to analyse the dentition of a common chimpanzee (Pan troglodytes), a pygmy chimpanzee (Pan paniscus) and three extant humans. Metameric variation of the 3D enamel-dentine junction in the two chimpanzee mandibles was much smaller than in extant humans. Variation in metameric shape was high and complex. Notably, the mandibular metameric variation in extant humans can be greater within individuals, as compared with variation between individuals, with differences in shape appearing greater for M2 compared with M1. We recommend the use of a new approach in which individual metameric variation is systematically assessed before making inferences about differences between fossil hominin species. The fossil hominin examined in this study showed a metameric pattern of mandibular variation in shape that was comparable to the pattern seen in two chimpanzees. This degree of metameric variation appeared relatively small compared with the much larger patterns of variation observed within and between extant humans.

Three-dimensional molar enamel distribution and thickness in Australopithecus and Paranthropus.

Thick molar enamel is among the few diagnostic characters of hominins which are measurable in fossil specimens. Despite a long history of study and characterization of Paranthropus molars as relatively 'hyper-thick', only a few tooth fragments and controlled planes of section (designed to be proxies of whole-crown thickness) have been measured. Here, we measure molar enamel thickness in Australopithecus africanus and Paranthropus robustus using accurate microtomographic methods, recording the whole-crown distribution of enamel. Both taxa have relatively thick enamel, but are thinner than previously characterized based on two-dimensional measurements. Three-dimensional measurements show that P. robustus enamel is not hyper-thick, and A. africanus enamel is relatively thinner than that of recent humans. Interspecific differences in the whole-crown distribution of enamel thickness influence cross-sectional measurements such that enamel thickness is exaggerated in two-dimensional sections of A. africanus and P. robustus molars. As such, two-dimensional enamel thickness measurements in australopiths are not reliable proxies for the three-dimensional data they are meant to represent. The three-dimensional distribution of enamel thickness shows different patterns among species, and is more useful for the interpretation of functional adaptations than single summary measures of enamel thickness.

The hunters and the hunted revisited.

The dietary niches of extinct animals, including hominids and predators, may be constrained using stable carbon isotope ratios in fossil tooth enamel.(13)C/(12)C ratios of many of the primates abundant in the faunal assemblages of Members 1 and 2 at Swartkrans, including cercopithecoids and Australopithecus (Paranthropus) robustus, and a range of other possible prey species, have been reported previously. Resulting suggestions of a mixed, or omnivorous, diet for A. robustus raise questions about niche overlap with coeval, larger brained Homo. Here we present(13)C/(12)C data from Homo and several large predators including Panthera pardus, Dinofelis sp., Megantereon cultridens and Chasmoporthetes nitidula in Member 1, and P. pardus and P. leo in Member 2, in order to compare the two hominid species and to determine likely predators of the various primates and other macrovertebrates. Results for three Homo cf. ergaster individuals are indistinguishable from those of A. robustus, showing that proportions of C(3)- and C(4)-based foods in their diets did not differ. P. pardus, Megantereon and Crocuta are shown to be likely predators of the hominids and Papio baboons in Member 1, while the Dinofelis individual concentrated on prey which consumed C(4)grasses. The hunting hyaenid C. nitidula preyed on either mixed feeders or on a range of animals across the spectrum of C(3)and C(4)variation. The data from Members 1 and 2 confirm a shift in leopard diets towards animals that consumed C(4)grasses.

A supernumerary tooth in a 1.7 million-year-old Australopithecus robustus from Swartkrans, South Africa.

The maxillary dental arch of a partial cranium of an adult specimen of Australopithecus robustus shows the presence of a supernumerary tooth between the right first and second incisors. The fossil specimen, SK 83, is from Swartkrans Member 1 sediments, considered, on the basis of associated fauna, to be approximately 1.7 million yr old. The specimen was analyzed macroscopically, stereomacroscopically and by X-ray computed tomography. The anatomic position and the morphology of the tooth are consistent with a diagnosis of supernumerary lateral incisor rather than a mesiodens or a retained deciduous tooth. This is the first description of a supernumerary tooth found in Plio-Pleistocene hominid fossils from South African cave deposits.

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.

Enamel thickness and the topography of the enamel-dentine junction in South African Plio-Pleistocene hominids with special reference to the Carabelli trait.

This study explores the internal morphology of early hominid teeth using high-resolution computed tomography. Data on Carabelli feature size, enamel thickness, and the topography of the enamel-dentine junction are considered together in order to examine the relationship among these variables in the maxillary molars of gracile and robust australopithecines from South Africa. In particular, one aim is to investigate the degree to which Carabelli feature size influences enamel thickness in the plane of the mesial cusps. The results demonstrate that maxillary molars attributed to Australopithecus africanus from Sterkfontein, Taung and Makapansgat possess larger Carabelli features and thinner enamel along the lingual wall of the protocone than do specimens attributed to Paranthropus robustus from Swartkrans and Kromdraai. Distinct differences in the position of the Carabelli feature at the level of both the enamel-dentine junction and tooth crown surface between early hominid species may help explain the observed disparity in enamel thickness at that region of the tooth crown as well as offer clues to the functional role of Carabelli's cusp. As the size and position of the Carabelli feature affects the linear thickness of enamel at this one particular region of the tooth crown, future comparative studies focusing on taxa that possess moderate to strong development of the Carabelli complex should use the linear thickness of enamel taken close to the protoconal dentine horn or at the maximum projection of the Carabelli's cusp.

Faunal assemblage seriation of southern African Pliocene and Pleistocene fossil deposits.

Fossil assemblages from the Pliocene and Pleistocene of southern Africa were seriated in order to give a better idea of their relative chronology. Time-sensitive mammals were selected for calculation of the Faunal Resemblance Index among 17 site units. On the basis of a logistical seriation and subsequent site analysis, the following sequence of sites was deemed most probable: Makapansgat Member 3, Makapansgat Member 4, Taung Dart deposits, Sterkfontein Member 4 and Taung Hrdlicka deposits, Sterkfontein Member 5 (in part) and Kromdraai B, Kromdraai A and Swartkrans Member 1, Swartkrans Member 2, Swartkrans Member 3, Plovers Lake, Cornelia, Elandsfontein Main Site, Cave of Hearths Acheulian levels, Florisbad and Equus Cave and Klasies River Mouth.

Computed tomography and intercuspal angulation of maxillary molars of Plio-Pleistocene hominids from Sterkfontein, Swartkrans and Kromdraai (South Africa): an exploratory study.

The non-destructive technique of Computed Tomography (CT) is used in an exploratory study to investigate variability in angles measured at the crown surface and the dentino-enamel junction of maxillary molars of Plio-Pleistocene hominids from Swartkrans, Sterkfontein and Kromdraai. Discriminant analysis of six angles used together successfully separates 'gracile' and 'robust' australopithecines, although individual angles do not. This study indicates that differences between teeth of Australopithecus africanus and A. robustus are not fully accounted for by gross differences in enamel thickness.

Histological reconstruction of dental development and age at death of a juvenile Paranthropus robustus specimen, SK 63, from Swartkrans, South Africa.

There has been disagreement about whether the earliest hominids grew in a similar manner to great apes or modern humans. This has important biological implications, since it may have been inappropriate to apply modern human developmental standards to early hominids. The aim of the present study was to combine data from replicas of tooth surfaces, computed tomographic (CT) scans, and radiographs with data from a histological section of the canine crown, in order to provide a complete description of tooth crown and tooth root development in a single early hominid specimen (SK 63). Although partially destructive in nature, we have been able to determine the most reliable data yet for aspects of dental development in an important juvenile early hominid specimen. Appositional enamel formation time in the permanent right canine was estimated at between 305 and 418 days, imbricational enamel formation time at 819 days, and total crown formation time at between 3.18 and 3.48 years. The most likely age at death was estimated at around 4 years with a range of ages calculated between 3.18 and 4.23 years based on differences in timing of initial mineralization of the canine and differences in appositional enamel formation times. Crown formation times of the lower central and lateral incisors were estimated between 2.35-2.68 years and 2.57-2.91 years, respectively. Crown formation time of the first permanent molar was estimated at 2.4 years. Wear facets on the first permanent molars indicate that gingival emergence had occurred sometime prior to death, between 3 and 4 years of age. Estimates of root extension rates in the first permanent molars and in the permanent incisors are fast, and either within or above ranges of rates estimated for modern great apes. While we recognize that data for one individual may not be representative of data for a whole population of early hominids, the data for age at death, for age of M1 emergence, and for root extension rates presented here accord with those known for modern great apes and fall beyond the known ranges for modern humans.

Computed tomography and enamel thickness of maxillary molars of Plio-Pleistocene hominids from Sterkfontein, Swartkrans, and Kromdraai (South Africa): An exploratory study.

This paper is one in a series which explores the possibility of using the non-destructive CT technique to identify patterns in tooth enamel distribution and structure of hominid molars from Plio-Pleistocene sites in South Africa, notably Swartkrans, Sterkfontein, and Kromdraai. Whereas previous investigators have emphasised gross differences in absolute and relative or average enamel thickness between hominid taxa, the present study highlights differences in enamel thickness over functionally significant regions of the crown. Differences in the distribution of enamel in A. robustus, A. africanus, and Homo sp. are identified through the use of bivariate and multivariate analyses, and are interpreted in terms of dietary regimes.

Dated rock engravings from wonderwerk cave, South Africa.

Radiocarbon dates associated with engraved stones from sealed archeological deposits at Wonderwerk Cave in the northern Cape Province indicate that rock engraving in South Africa is at least 10,000 years old.