Extant Hominoid Species Research Articles

Bony birth‐canal dimensions and obstetric constraints in hominoids

AbstractObjectivesComparative studies of the birth process in humans and other primates have focused on the pelvic inlet. This is a region of birth‐canal constraint in humans, but not in other primates. Thus, the true obstetric differences between humans and other apes remain unknown. This research seeks to document nonhuman ape birth‐canal morphologies and their relationships to critical dimensions of neonates, in order to determine what aspects of human birth are unique.Materials and MethodsComputer‐generated images of the scanned pelves of six extant hominoid species are used to compare entire birth canals as three‐dimensional entities, documenting and analyzing the functionally relevant metrics of the maternal pelvis and the obstetric constraints for each species.ResultsThe inlet is a poor basis of comparison for evaluating tightness of fit in nonhuman primates, whose birth canal is most constricted at the lower end of the sacrum. Previous studies have overestimated the capaciousness of the nonhuman hominoid birth canal (which appears to be just as tight a fit for the fetal head in hylobatids as it is in Homo sapiens) and also misrepresented it as a simple straight tube. In fact, its cross‐sectional shape changes markedly from inlet to outlet in most hominoids.ConclusionNonhuman hominoids exhibit obstetric constraints unlike those seen in modern humans. Human adaptations to maternal‐fetal craniopelvic disproportion are unique owing to the constraints imposed by bipedality, and probably predate the origins of the genus Homo.

Morphometric analysis of hominin teeth attributed to Australopithecus, Paranthropus and Homo


 
 
 Teeth are the most common element in the fossil record and play a critical role in taxonomic assessments. Variability in extant hominoid species is commonly used as a basis to gauge expected ranges of variability in fossil hominin species. In this study, variability in lower first molars is visualised in morphospace for four extant hominoid species and seven fossil hominin species. A size-versus-shape-based principle component analysis plot was used to recognise spatial patterns applicable to sexual dimorphism in extant species for comparison with fossil hominin species. In three African great ape species, variability occurs predominantly according to size (rather than shape), with the gorilla sample further separating into a male and a female group according to size. A different pattern is apparent for the modern human sample, in which shape variability is more evident. There is overlap between male and female modern humans and some evidence of grouping by linguistic/tribal populations. When fossil hominin species are analysed using equivalent axes of variance, the specimens group around species holotypes in quite similar patterns to those of the extant African great apes, but six individual fossil molars fall well outside of polygons circumscribing holotype clusters; at least three of these specimens are of interest for discussion in the context of sexual dimorphism, species variability and current species classifications. An implication of this study is that, especially in the case of modern humans, great caution needs to be exercised in using extant species as analogues for assessing variability considered to be a result of sexual dimorphism in fossil hominin species.
 
 
 
 
 Significance: 
 
 
 
 Caution should be exercised in using modern analogue species as proxies for fossil hominin species variability.
 Exceptionally wide ranges of molar variability between certain fossil hominin specimens currently allocated to the same species might indicate possible misclassification.
 Molar morphology in gorillas tends to reflect primarily size, rather than shape, variability between the sexes, which is a consideration in the context of assessing possible sexual dimorphism in fossil hominin species. 
 
 
 

Three‐dimensional shape variation of talar surface morphology in hominoid primates

The hominoid foot is of particular interest to biological anthropologists, as changes in its anatomy through time reflect the adoption of terrestrial locomotion, particularly in species of Australopithecus and Homo. Understanding the osteological morphology associated with changes in whole foot function and the development of the plantar medial longitudinal foot arch are key to understanding the transition through habitual bipedalism in australopithecines to obligate bipedalism and long-distance running in Homo. The talus is ideal for studying relationships between morphology and function in this context, as it is a major contributor to the adduction-abduction, plantar-dorsal flexion and inversion-eversion of the foot, and transmits all forces encountered from the foot to the leg. The talar surface is predominantly covered by articular facets, which have different quantifiable morphological characters, including surface area, surface curvature and orientation. The talus also presents challenges to the investigator, as its globular shape is very difficult to quantify accurately and reproducibly. Here we apply a three-dimensional approach using type 3 landmarks (slid semilandmarks) that are geometrically homologous to determine overall talar shape variations in a range of living and fossil hominoid taxa. Additionally, we use novel approaches to quantify the relative orientations and curvatures of talar articular facets by determining the principal vectors of facet orientation and fitting spheres to articular facets. The resulting metrics are analysed using phylogenetic regressions and principal components analyses. Our results suggest that articular surface curvatures reflect locomotor specialisations with, in particular, orangutans having more highly curved facets in all but the calcaneal facet. Similarly, our approach to quantifying articular facet orientation appears to be effective in discriminating between extant hominoid species, and may therefore provide a sound basis for the study of fossil taxa and evolution of bipedalism in Australopithecus and Homo.

A probabilistic approach to the craniometric variability of the genus Homo and inferences on the taxonomic affinities of the first human population dispersing out of Africa

There is probably no paleoanthropological issue with deeper disagreements than the taxonomic status of the Early–Middle Pleistocene members of the genus Homo One reason could be the difficulty of estimating the relationship between morphological and taxonomic diversity. In an attempt to contribute new evidence to this debate, bootstrapping techniques are used for analyzing the cranial variability of Homo. The results indicate that: (i) the size of the neurocranium relative to the viscerocranium discriminates better among extant hominoid species than skull size; (ii) no cluster of fossil specimens of Homo exceeds the morphological variability of Gorilla gorilla, with the only exception of the one that comprises all members of Homo except modern humans; and (iii) some clusters are taxonomically more consistent than others, as long as they show a range of morphological variability similar to that found in both Pan troglodytes and Homo sapiens. According to these results, three taxa are tentatively suggested for Homo: (i) anatomically modern humans; (ii) an “erectine” morphotype plus Neanderthals; and (iii) a “habiline” cluster. Finally, the results indicate a greater taxonomic affinity for the human population involved in the first dispersal “Out of Africa” with the “habiline” group, which agrees with the early age reported for Dmanisi (1.77 Ma), a Georgian site placed at the gates of Europe.

Quantifying Mental Foramen Position in Extant Hominoids and Australopithecus: Implications for its Use in Studies of Human Evolution

The location of the mental foramen on the mandibular corpus has figured prominently in debates concerning the taxonomy of fossil hominins and Gorilla gorilla. In this study we quantify the antero/posterior (A/P) position of the mental foramen across great apes, modern humans and Australopithecus. Contrary to most qualitative assessments, we find significant differences between some extant hominoid species in mental foramen A/P position supporting its potential usefulness as a character for taxonomic and phylogenetic analyses of fossil hominoids. Gorilla gorilla, particularly the eastern subspecies, with a comparatively longer dental arcade and fossil and extant hominins with reduced canines and incisors tend to exhibit more anteriorly positioned mental foramina. Conversely, Pan troglodytes exhibits more posteriorly positioned mental foramina. Variation in this character among Gorilla gorilla subspecies supports recent taxonomic assessments that separate eastern and western populations. In all taxa other than Pan troglodytes the A/P position of the mental foramen is positively allometric with respect to dental arcade length. Thus, within each of these species, specimens with longer dental arcades tend to have more posteriorly positioned mental foramina. Those species with greater sexual dimorphism in canine size and dental arcade length (i.e., Gorilla gorilla and Pongo pygmaeus) exhibit more extreme differences between smaller and larger individuals. Moreover, among great apes those individuals with greater anterior convergence of the dental arcade tend to exhibit more posteriorly positioned mental foramina. Dental arcade length, canine crown area and anterior convergence are all significantly associated with mental foramen A/P position, suggesting that these traits may influence taxonomic variation in the A/P position of the mental foramen.

Are the differences between Stw 431 ( Australopithecus africanus) and A.L. 288-1 ( A. afarensis) significant?

Recent studies of early hominin body proportions paint a complex evolutionary picture, with multiple instances of reversal in body shape. These interpretations rest heavily upon the inferred limb joint proportions of Australopithecus africanus. For example, the partial skeleton Stw 431 has been suggested to show apelike joint proportions compared to the A. afarensis specimen A.L. 288-1. This suggests an evolutionary reversal in the more recent A. africanus. However, no study has examined the probability of sampling the differences between Stw 431 and A.L. 288-1 from a single extant hominoid species. The present study compares elbow/hip and elbow/lumbosacral joint size ratios between Stw 431 and A.L. 288-1 using exact randomization, based on chimpanzee and human models of variation. Results indicate that differences in elbow/hip proportions between Stw 431 and A.L. 288-1 can be sampled from a single species. In contrast, differences in elbow/lumbosacral proportions between Stw 431 and A.L. 288-1 show a significantly low probability of being sampled from a single species. Thus, Stw 431 and A.L. 288-1 are not significantly different from each with regard to limb joint proportions, but Stw 431 has a significantly smaller lumbosacral joint. This pattern does not conform to previous interpretations of limb proportions in A. africanus. Low statistical power in the present study may account for the discrepancy. Further research is needed to illuminate the functional implications of variation in relative lumbosacral joint size in early hominins.

Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis

The Pliocene hominin samples from Hadar and Laetoli are thought to represent one species,Australopithecus afarensis , that exhibits stasis throughout its temporal range and has high levels of skeletal sexual dimorphism. In this paper, we test the hypothesis of stasis in dental and mandibular dimensions using nonparametric rank correlation methods to detect temporal trends and randomization tests to evaluate their statistical significance. We then use two methods (CV resampling; Fligner–Killeen test) to compare overall levels of variation in the fossil sample to those of extant hominoid species. Together, these analyses allow us to gauge the effects of changes through time on variation in mandibles and teeth of A. afarensis.P3mesiodistal length, M3size, and canine shape change through time but do not appear unusually variable in the sample as a whole. These temporal trends possibly reflect differences between the Laetoli and Hadar site-samples. For mandibles, a pronounced trend towards greater corpus size occurs late in the temporal sequence and contributes to high levels of variation compared to African apes. These results show that significant directional changes do occur in the A. afarensis mandibles and teeth, and in these elements, at least, the species is not static. Temporal variation is clearly an important component of overall variation in the A. afarensis lineage, even though other factors, such as sexual dimorphism, may also play a part.

The brain and its main anatomical subdivisions in living hominoids using magnetic resonance imaging.

Primary comparative data on the hominoid brain are scarce and major neuroanatomical differences between humans and apes have not yet been described satisfactorily, even at the gross level. Basic questions that involve the evolution of the human brain cannot be addressed adequately unless the brains of all extant hominoid species are analyzed. Contrary to the scarcity of original data, there is a rich literature on the topic of human brain evolution and several debates exist on the size of particular sectors of the brain, e.g., the frontal lobe. In this study we applied a non-invasive imaging technique (magnetic resonance) on living human, great ape and lesser ape subjects in order to investigate the overall size of the hominoid brain. The images were reconstructed in three dimensions and volumetric estimates were obtained for the brain and its main anatomical sectors, including the frontal and temporal lobes, the insula, the parieto-occipital sector and the cerebellum.A remarkable homogeneity is present in the relative size of many of the large sectors of the hominoid brain, but interspecific and intraspecific variation exists in certain parts of the brain. The human cerebellum is smaller than expected for an ape brain of human size. It is suggested that the cerebellum increased less than the cerebrum after the split of the human lineage from the African ancestral hominoid stock. In contrast, humans have a slightly larger temporal lobe and insula than expected, but differences are not statistically significant. Humans do not have a larger frontal lobe than expected for an ape brain of human size and gibbons have a relatively smaller frontal lobe than the rest of the hominoids. Given the fact that the frontal lobe in humans and great apes has similar relative size, it is parsimonious to suggest that the relative size of the whole of the frontal lobe has not changed significantly during hominid evolution in the Plio-Pleistocene.

Can enamel microstructure be used to establish the presence of different species of Plio-Pleistocene hominids from Omo, Ethiopia?

Microstructural characteristics of enamel are minimally influenced by extrinsic / non-genomic factors and are thus potentially important in any attempt to establish the taxonomic attribution of Plio-Pleistocene hominids. The aim of this work is to assess the potential use of enamel microstructural characteristics in attributing teeth from the Omo to different Plio-Pleistocene hominid species. First, the results from this study are compared to data for extant hominoids in order to observe whether Omo teeth show similar or larger variation. This might suggest that more than one species is represented in the Omo sample. Alternatively, a similar or smaller variation would suggest that no more than one species could be reliably recognized in the sample using enamel microstructural characteristics. Secondly, since all previous studies suggest that more than one species is present in the large sample of teeth from the Omo, enamel microstructure characteristics with the largest variability are used to group teeth into morphs. These morphs are compared with previous taxonomic attributions of Omo teeth in order to determine if the morphs grouped teeth in a similar way to previous studies. The results of this study demonstrate that enamel microanatomy characteristics in Plio-Pleistocene hominid teeth from the Omo do not present a larger variation than that observed in extant hominoid species. This suggests that no more than one species can be recognized by using these characteristics. These results also indicate a large overlap in enamel microstructure characteristics between Plio-Pleistocene hominid species from Omo and place serious doubt on the theoretical potential for using enamel microstructural characteristics as a taxonomic tool. The morphs defined in this study do not correspond with any previous taxonomic attributions based on macrostructural analysis of the same teeth and cannot be referred to different species. However, macrostructural characteristics have never been assessed in the same way in extant hominoids and it remains to be seen to what extent these characters reflect interspecific, intraspecific or even intra-individual variation. For now therefore, the taxonomy of Plio-Pleistocene hominids from the Omo still remains unresolved.

A morphometric analysis of extant and early miocene fossil hominoid maxillo-dental specimens

It is demonstrated in this paper that before we can hope to formulate phylogenetic relationships between and amongst fossil hominoid material it is first necessary to sex the material accurately. In order to determine whether the morphological and morphometrical variability seen in fossil specimens is due to sexual or inter species dimorphism, it is necessary to calibrate fossil specimens against extant hominoid species' morphologies. Only after fossil specimens have been sexed is it possible to differentiate between morphologies that are related to sex and those that are species specific. This will help reduce fossil misallocation. A morphometric analysis of extant and fossilProconsul hominoid material is presented. Each fossil specimen has been sexed according to symplesiomorphic sex morphologies as defined in this paper. After the fossil specimens have been sexed they are analyzed using multivariate statistics. The identification of differing sex patterns within the specimens examined here suggests that a new species ofProconsul may have to be considered.

Bite force, diet, and cranial morphology of fossil hominids

Functional parameters that approximate the maximal adducting force of the jaw were measured on cranial samples of extant hominoids and well preserved fossil hominid skulls. Estimates of bite force based on these measurements were calculated for the hominid fossils and for extant hominoid species. Relative to known or estimated body weight, only the robust australopithecines proved capable of generating bite forces that are exceptionally high compared to those characterizing living species. Forces developed by gracile australopithecines and early Homo resemble those of chimpanzees. When bite force is plotted against the occlusal area of the molar teeth, neither robust nor gracile forms deviate substantially from the regression line based on living hominoid species. We suggest that the high bite force of the robust australopithecines was a necessary consequence of the enlargement of molar crown surface. Bite force reduction is a late trend in hominid evolution that parallels a reduction in tooth size. Some of the characteristics of early hominid skull shape can be interpreted as adaptations to augment bite force with increasing tooth size.