Taxonomic affinity of the early Homo cranium from Swartkrans, South Africa

Dental morphology in Homo habilis and its implications for the evolution of early Homo

The phylogenetic position of Homo habilis is central to debates over the origin and early evolution of the genus Homo. A large portion of the species hypodigm consists of dental remains, but they have only been studied at the often worn enamel surface. We investigate the morphology of the H. habilis enamel-dentine junction (EDJ), which is preserved in cases of moderate tooth wear and known to carry a strong taxonomic signal. Geometric morphometrics is used to characterise dentine crown shape and size across the entire mandibular and maxillary tooth rows, compared with a broad comparative sample (n = 712). We find that EDJ morphology in H. habilis is for the most part remarkably primitive, supporting the hypothesis that the H. habilis hypodigm has more in common with Australopithecus than later Homo. Additionally, the chronologically younger specimen OH 16 displays a suite of derived features; its inclusion in H. habilis leads to excessive levels of variation.

Making meaning from fragmentary fossils: Early Homo in the Early to early Middle Pleistocene

In celebration of the 50th anniversary of the Journal of Human Evolution, we re-evaluate the fossil record for early Homo (principally Homo erectus, Homo habilis, and Homo rudolfensis) from early diversification and dispersal in the Early Pleistocene to the ultimate demise of H. erectus in the early Middle Pleistocene. The mid-1990s marked an important historical turning point in our understanding of early Homo with the redating of key H. erectus localities, the discovery of small H. erectus in Asia, and the recovery of an even earlier presence of early Homo in Africa. As such, we compare our understanding of early Homo before and after this time and discuss how the order of fossil discovery and a focus on anchor specimens has shaped, and in many ways biased, our interpretations of early Homo species and the fossils allocated to them. Fragmentary specimens may counter conventional wisdom but are often overlooked in broad narratives. We recognize at least three different cranial and two or three pelvic morphotypes of early Homo. Just one postcranial morph aligns with any certainty to a cranial species, highlighting the importance of explicitly identifying how we link specimens together and to species; we offer two ways of visualizing these connections. Chronologically and morphologically H. erectus is a member of early Homo, not a temporally more recent species necessarily evolved from either H. habilis or H. rudolfensis. Nonetheless, an ancestral-descendant notion of their evolution influences expectations around the anatomy of missing elements, especially the foot. Weak support for long-held notions of postcranial modernity in H. erectus raises the possibility of alternative drivers of dispersal. New observations suggest that the dearth of faces in later H. erectus may mask taxonomic diversity in Asia and suggest various later mid-Pleistocene populations could derive from either Asia or Africa. Future advances will rest on the development of nuanced ways to affiliate fossils, greater transparency of implicit assumptions, and attention to detailed life history information for comparative collections; all critical pursuits for future research given the great potential they have to enrich our evolutionary reconstructions for the next fifty years and beyond.

Complete permanent mandibular dentition of early Homo from the upper Burgi Member of the Koobi Fora Formation, Ileret, Kenya

The KNM-ER 64060 dentition derives from a horizon that most likely dates to between 2.02 and 2.03 Ma. A proximate series of postcranial bones (designated KNM-ER 64061) derives from the same siltstone unit and may be associated with the dentition, but their separation on the surface of the site leaves some room for doubt. KNM-ER 64060 is one of fewer than ten hominin specimens from the Early Pleistocene of East Africa that comprises a full or nearly complete mandibular dentition. Its taxonomic attribution is potentially significant, especially if the postcranial elements are related. At least three, and probably four hominin species, including Paranthropus boisei and Homo erectus (= H. ergaster), are known at about this time in East Africa. Other penecontemporaneous fossils have been referred to a single, highly variable species, H. habilis, or two taxa, namely H. habilis and H. rudolfensis. Although the weight of evidence supports the attribution of these specimens to two species, there is notable lack of agreement over the assignation of individual fossils. We take a conservative approach and group all such specimens under the designation "early Homo sp." for comparative purposes. KNM-ER 64060 is clearly attributable to Homo rather than Paranthropus. The preponderance of the evidence suggests that the affinities of KNM-ER 64060 are with fossils assigned to the early Homo sp. category rather than with H. erectus. This is indicated by the overall sizes of the KNM-ER 64060 canine, premolar and molar crowns, the size relationships of the P₃ to P₄, the relative narrowness of its premolar crowns, the cusp proportions of the M₁ and especially those of the M₂ and M₃, and seemingly the possession of a two-rooted P₄. Some of these comparisons suggest further that among the fossils comprising the early Homo sp. sample, the KNM-ER 64060 dentition exhibits greater overall similarity to specimens such as OH 7 and OH 16 that represent Homo habilis sensu stricto.

Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa

Homo naledi is a previously-unknown species of extinct hominin discovered within the Dinaledi Chamber of the Rising Star cave system, Cradle of Humankind, South Africa. This species is characterized by body mass and stature similar to small-bodied human populations but a small endocranial volume similar to australopiths. Cranial morphology of H. naledi is unique, but most similar to early Homo species including Homo erectus, Homo habilis or Homo rudolfensis. While primitive, the dentition is generally small and simple in occlusal morphology. H. naledi has humanlike manipulatory adaptations of the hand and wrist. It also exhibits a humanlike foot and lower limb. These humanlike aspects are contrasted in the postcrania with a more primitive or australopith-like trunk, shoulder, pelvis and proximal femur. Representing at least 15 individuals with most skeletal elements repeated multiple times, this is the largest assemblage of a single species of hominins yet discovered in Africa.

Taxonomic identification of Lower Pleistocene fossil hominins based on distal humeral diaphyseal cross-sectional shape

The coexistence of multiple hominin species during the Lower Pleistocene has long presented a challenge for taxonomic attribution of isolated postcrania. Although fossil humeri are well-suited for studies of hominin postcranial variation due to their relative abundance, humeral articular morphology has thus far been of limited value for differentiating Paranthropus from Homo. On the other hand, distal humeral diaphyseal shape has been used to justify such generic distinctions at Swartkrans. The potential utility of humeral diaphyseal shape merits larger-scale quantitative analysis, particularly as it permits the inclusion of fragmentary specimens lacking articular morphology. This study analyzes shape variation of the distal humeral diaphysis among fossil hominins (c. 2-1 Ma) to test the hypothesis that specimens can be divided into distinct morphotypes. Coordinate landmarks were placed on 3D laser scans to quantify cross-sectional shape at a standardized location of the humeral diaphysis (proximal to the olecranon fossa) for a variety of fossil hominins and extant hominids. The fossil sample includes specimens attributed to species based on associated craniodental remains. Mantel tests of matrix correlation were used to assess hypotheses about morphometric relationships among the fossils by comparing empirically-derived Procrustes distance matrices to hypothetical model matrices. Diaphyseal shape variation is consistent with the hypothesis of three distinct morphotypes (Paranthropus, Homo erectus, non-erectus early Homo) in both eastern and southern Africa during the observed time period. Specimens attributed to non-erectus early Homo are unique among hominids with respect to the degree of relative anteroposterior flattening, while H. erectus humeri exhibit morphology more similar to that of modern humans. In both geographic regions, Paranthropus is characterized by a morphology that is intermediate with respect to those morphological features that differentiate the two forms of early Homo. This study demonstrates the utility of the humeral diaphysis for taxonomic identification of isolated postcranial remains and further documents a high degree of postcranial diversity in early Homo.

The role of character displacement in the molarization of hominin mandibular premolars

Closely related species are likely to experience resource competition in areas where their ranges overlap. Fossil evidence suggests that hominins in East Africa c. 2-1.5 million years ago may have lived synchronically and sympatrically, and that competition may have contributed to the different tooth sizes observed in Homo and Paranthropus. To assess the likelihood that these taxa overlapped, we applied a character displacement model to the postcanine tooth size of fossil hominins and validated this model in populations of living primates. Mandibular fourth premolar (P4 ) crown size was measured from fossil taxa and from living primate species where dietary overlap is established. Dimensions of the P4 crown were fitted to a character matrix and described as the response variables of a generalized linear model that took taxon and location as input variables. The model recovered significant divergence in samples of closely related, living primates. When applied to fossil hominins the same model detected strong indications of character displacement between early Homo and Paranthropus (P = 0.002) on the basis of their P4 crown size. Our study is an example of how ecologically informed morphologies measured in appropriate extant referents can provide a comparative context for assessing community and ecological evolution in the fossil record.

Evolution of M1 crown size and cusp proportions in the genus Homo

Previous research into tooth crown dimensions and cusp proportions has proved to be a useful way to identify taxonomic differences in Pliocene and Pleistocene fossil hominins. The present study has identified changes in both M(1) crown size and cusp proportions within the genus Homo, with M(1) overall crown size reduction apparently occurring in two main stages. The first stage (a reduction of ca. 17%) is associated with the emergence of Homo ergaster and Homo erectus sensu stricto. The second stage (a reduction of ca. 10%) occurs in Homo sapiens, but the reduced modern human M(1) tooth crown size was only attained in Upper Paleolithic times. The absolute sizes of the individual cusps are highly positively correlated with overall crown size and dental reduction produces a reduction in the absolute size of each of the cusps. Most of the individual cusps scale isometrically with crown size, but the paracone shows a negative allometric relationship, indicating that the reduction in paracone size is less than in the other M(1) cusps. Thus, the phylogenetically oldest cusp in the upper molars also seems to be the most stable cusp (at least in the M(1)). The most striking change in M(1) cusp proportions is a change in the relative size of the areas of the paracone and metacone. The combination of a small relative paracone and a large relative metacone generally characterizes specimens attributed to early Homo, and the presence of this character state in Australopithecus and Paranthropus suggests it may represent the primitive condition for the later part of the hominin clade. In contrast, nearly all later Homo taxa, with the exception of Homo antecessor, show the opposite condition (i.e. a relatively large paracone and a relatively small metacone). This change in the relationship between the relative sizes of the paracone and metacone is related to an isometric reduction of the absolute size of the metacone. This metacone reduction occurs in the context of relative stability in the paracone as crown size decreases. Among later Homo taxa, both Homo heidelbergensis and Homo neanderthalensis show a further reduction of the metacone and an enlargement of the hypocone. Fossil and contemporary H. sapiens samples show a trend toward increasing the relative size of the protocone and decreasing the relative size of the hypocone. In Europe, modern human M(1) cusp proportions are essentially reached during the Upper Paleolithic. Although some variation was documented among the fossil taxa, we suggest that the relative size of the M(1) paracone and metacone areas may be useful for differentiating the earliest members of our genus from subsequent Homo species.

Population continuity, demic diffusion and Neolithic origins in central-southern Germany: the evidence from body proportions

The transition to agro-pastoralism in central Europe has been framed within a dichotomy of "regional continuity" versus exogenous "demic diffusion". While substantial genetic support exists for a model of demographic diffusion from an ancestral source in the Near East, archaeological data furnish weak support for the "wave of advance" model. Nevertheless, archaeological evidence attests the widespread introduction of an exogenous "package" comprising ceramics, cereals, pulses and domesticated animals to central Europe at 5600calBCE. Body proportions are under strong climatic selection and evince remarkable stability within regional lineages. As such, they offer a viable and robust alternative to cranio-facial data in assessing hypothesised continuity and replacement with the transition to agro-pastoralism in central Europe. Humero-clavicular, brachial and crural indices in a large sample (n=75) of Linienbandkeramik (LBK), Late Neolithic and Early Bronze Age specimens from the middle Elbe-Saale-Werra valley (MESV) were compared with Eurasian and African terminal Pleistocene, European Mesolithic and geographically disparate recent human specimens. Mesolithic Europeans display considerable variation in humero-clavicular and brachial indices yet none approach the extreme "hyper-polar" morphology of LBK humans from the MESV. In contrast, Late Neolithic and Early Bronze Age peoples display elongated brachial and crural indices reminiscent of terminal Pleistocene and "tropically adapted" recent humans. These marked morphological changes likely reflect exogenous immigration during the terminal Fourth millennium cal BC. Population expansion and diffusion is a function of increased mobility and settlement dispersal concomitant with significant technological and subsistence changes in later Neolithic societies during the late fourth millennium cal BCE.

Natural history of Homo erectus

Our view of H. erectus is vastly different today than when Pithecanthropus erectus was described in 1894. Since its synonimization into Homo, views of the species and its distribution have varied from a single, widely dispersed, polytypic species ultimately ancestral to all later Homo, to a derived, regional isolate ultimately marginal to later hominin evolution. A revised chronostratigraphic framework and recent work bearing either directly or indirectly on reconstructions of life-history patterns are reviewed here and, together with a review of the cranial and postcranial anatomy of H. erectus, are used to generate a natural history of the species. Here I argue that H. erectus is a hominin, notable for its increased body size, that originates in the latest Pliocene/earliest Pleistocene of Africa and quickly disperses into Western and Eastern Asia. It is also an increasingly derived hominin with several regional morphs sustained by intermittent isolation, particularly in Southeast Asia. This view differs from several current views, most especially that which recognizes only a single hominin species in the Pleistocene, H. sapiens, and those which would atomize H. erectus into a multiplicity of taxa. Following Jolly ([2001] Yrbk Phys Anthropol 44:177-204), the regional morphs of H. erectus may be productively viewed as geographically replacing allotaxa, rather than as the focus of unresolvable species debates. Such a view allows us to focus on the adaptations and biology of local groups, including questions of biogeographic isolation and local adaptation. A number of issues remain unresolved, including the significance of diversity in size and shape in the early African and Georgian records.

Evolutionary reversals of limb proportions in early hominids? evidence from KNM-ER 3735 (Homo habilis)

Upper-to-lower limb proportions of Homo habilis are often said to be more ape-like than those of its reputed ancestor, Australopithecus afarensis. Such proportions would either imply multiple evolutionary reversals or parallel development of a relatively short upper limb in A. afarensis and later Homo. However, assessments of limb proportions are complicated by the fragmentary nature of the two known H. habilis skeletons, OH 62 and KNM-ER 3735. Initially, KNM-ER 3735 was compared to A.L. 288-1 (A. afarensis) using a single modern human and chimpanzee as reference. Here, based on a larger comparative sample, we find that the relative size of the distal humerus, radial head, and shaft of both KNM-ER 3735 and A.L. 288-1 lie within the range of variation of modern humans, whereas their sacra are small as is the case for all early hominids. In addition, their manual phalanges are similar in having a gracile base but robust midshaft. Contrary to earlier studies, the fossils are not differentiable from each other statistically with respect to all features listed above. On the other hand, they differ in robusticity of the scapular spine and relative length of the radial neck. An exact randomization test suggests only a very low probability of finding a similar degree of difference within a single species of extant hominoids. In contrast to the consensus view, we conclude that A.L. 288-1 had a short, human-like forearm, whereas KNM-ER 3735 possessed a distinctly longer forearm and more powerful shoulder girdle. This interpretation fits with earlier conclusions that suggested human-like humerofemoral proportions but chimpanzee-like brachial proportions for Homo habilis. Thus, the scenario of a unidirectional, progressive change in limb proportions within the hominid lineage is not supported by our work.

Odontometric systematic assessment of the Swartkrans SK 15 mandible

This study reports a comparison of molar crown and cusp size and shape in the Swartkrans early Homo mandible SK 15 with relevant Plio-Pleistocene taxa. Univariate and multivariate methods are employed to consider the morphological affinities of this specimen and to assess its taxonomy. The case exists for classifying SK 15 in Homo habilis with 11 features aligning it with this species. The results of multivariate studies are consistent with this hypothesis. Moreover, SK 15 lacks a number of important features that characterise the mandibular molars of Homo sapiens erectus. Considerable evidence for parallelism in the dental morphology of SK 15 and H. habilis with A. robustus is discussed. Fossil evidence for the presence of H. sapiens erectus during the Plio-Pleistocene of South Africa presently seems to be lacking. Archaeological interpretations should take greater account of this gap in the fossil record.

Bootstrap tests of significance and the case for humanlike skeletal-size dimorphism in Australopithecus afarensis

Most estimates of sexual size dimorphism in Australopithecus afarensis indicate that this early hominin was more dimorphic than modern humans. In contrast, a recent study reported that size variation in A. afarensis, as represented by postcranial remains from Hadar and Maka, Ethiopia, is statistically most similar to that of modern humans, indicating a humanlike level of sexual dimorphism. Here, we evaluate the evidence for humanlike dimorphism in A. afarensis. We argue that statistical support for this claim is not as robust as has been asserted for the following reasons: (1) the analysis from which the claim was derived does not distinguish the A. afarensis sample from either the human or chimpanzee samples; (2) for some of the comparisons made, the A. afarensis sample cannot be distinguished from the Gorilla sample using two-tailed tests; and (3) the A. afarensis postcranial sample used in the analysis may contain more male than female specimens, which precludes a straightforward interpretation of the statistical results. Thus, support for humanlike dimorphism is equivocal, and a greater level of dimorphism cannot be ruled out.

Dental microwear and diets of African early Homo

Conventional wisdom ties the origin and early evolution of the genus Homo to environmental changes that occurred near the end of the Pliocene. The basic idea is that changing habitats led to new diets emphasizing savanna resources, such as herd mammals or underground storage organs. Fossil teeth provide the most direct evidence available for evaluating this theory. In this paper, we present a comprehensive study of dental microwear in Plio-Pleistocene Homo from Africa. We examined all available cheek teeth from Ethiopia, Kenya, Tanzania, Malawi, and South Africa and found 18 that preserved antemortem microwear. Microwear features were measured and compared for these specimens and a baseline series of five extant primate species (Cebus apella, Gorilla gorilla, Lophocebus albigena, Pan troglodytes, and Papio ursinus) and two protohistoric human foraging groups (Aleut and Arikara) with documented differences in diet and subsistence strategies. Results confirmed that dental microwear reflects diet, such that hard-object specialists tend to have more large microwear pits, whereas tough food eaters usually have more striations and smaller microwear features. Early Homo specimens clustered with baseline groups that do not prefer fracture resistant foods. Still, Homo erectus and individuals from Swartkrans Member 1 had more small pits than Homo habilis and specimens from Sterkfontein Member 5C. These results suggest that none of the early Homo groups specialized on very hard or tough foods, but that H. erectus and Swartkrans Member 1 individuals ate, at least occasionally, more brittle or tough items than other fossil hominins studied.

Description, new reconstruction, comparative anatomy, and classification of the Sterkfontein Stw 53 cranium, with discussions about the taxonomy of other southern African early Homo remains

Specimen Stw 53 was recovered in 1976 from Member 5 of the Sterkfontein Formation. Since its incomplete initial description and comparison, the partial cranium has figured prominently in discussions about the systematics of early Homo. Despite publication of a preliminary reconstruction in 1985, Stw 53 has yet to be compared comprehensively to other Plio-Pleistocene fossils or assessed systematically. In this paper, we report on a new reconstruction of this specimen and provide a detailed description and comparison of its morphology. Our reconstruction differs in important respects from the earlier one, especially in terms of neurocranial length, breadth, and height. However, given that Stw 53 exhibits extensive damage, these dimensions are most likely prone to much error in reconstruction. In areas of well-preserved bone, Stw 53 shares many cranial features with Homo habilis, and we propose retaining it within this species. We also consider the affinities of dental remains from Sterkfontein Member 5, along with those from Swartkrans and Drimolen previously assigned to Homo. We find evidence for sympatry of H. habilis and Australopithecus robustus and possibly Plio-Pleistocene Homo sapiens sensu lato in Sterkfontein Member 5. At Swartkrans and Drimolen, we find evidence of H. habilis. We also compare the morphologies of Stw 53 and SK 847 and find compelling evidence to assign the latter specimen to H. habilis, as has been proposed.

The behavioral ecology of sympatric African apes: implications for understanding fossil hominoid ecology

The behavioral ecology of the great apes is key evidence used in the reconstruction of the behavior of extinct ape and hominid taxa. Chimpanzees and gorillas have been studied in detail in the wild, and some studies of their behavioral ecology in sympatry have also been been carried out. Although the two ape species have divergent behavior and ecology in important respects, recent studies have shown that the interspecific differences are not as stark as previously thought and subsequently urge new consideration of how they share forest resources when sympatric. These new data require re-examination of assumptions about key aspects of chimpanzee-gorilla ecological divergence, such as diet, ranging and nesting patterns, and the mating system. Diet is a key component of the species' adaptive complexes that facilitates avoidance of direct competition from the other. While the nutritional basis for chimpanzee food choice remains unclear and no doubt varies from site to site, this species is a ripe fruit specialist and ranges farther during periods of ripe fruit scarcity. Gorillas in the same habitat also feed on ripe fruit when widely available, but fall back onto fibrous plant foods during lean periods. The inclusion of animal protein in the diet of the chimpanzees and its absence in that of the gorillas also distinguish the species ecologically. It may also offer clues to aspects of ecological divergence among early members of the hominid phylogeny. The paper concludes by suggesting likely characteristics of sympatric associations of Pliocene hominids, based on field data from extant sympatric apes.

Body proportions of Homo habilis reviewed

The ratio of fore- to hindlimb size plays an important role in our understanding of human evolution. Although Homo habilis was relatively modern craniodentally, its body proportions are commonly believed to have been more apelike than in the earlier Australopithecus afarensis. The evidence for this, however, rests, on two fragmentary skeletons, OH 62 and KNM-ER 3735. The upper limb of the better-preserved OH 62 from Olduvai Gorge is long and slender, but its hindlimb is represented mainly by the proximal portion of a thin femur of uncertain length. The present analysis shows that upper-to-lower limb shaft proportions of both OH 62 and AL 288-1 (A. afarensis) fall in the modern human range of variation, although OH 62 also falls inside that of chimpanzees due to their overlap in small individuals. Despite being more fragmentary, the larger-bodied KNM-ER 3735 lies outside the chimpanzee range and close to the human mean. Because the differences between any of the three individuals are compatible with the range of variation seen in extant hominoid groups, it is not legitimate to infer more primitive upper-to-lower limb shaft proportions for either H. habilis or A. afarensis. Femur length of OH 62 can only be estimated by comparison. Its closest match in size and morphology is with the gracile OH 34 specimen, which therefore provides a better analogue for the reconstruction of OH 62 than the stocky AL 288-1 femur that is traditionally used. OH 34's slender proportions are hardly due to abrasion, but match those of a modern human of that body-size, suggesting that the relative length of OH 62's leg may have been human-like. Brachial proportions, however, remained primitive. Long legs may imply long distance terrestrial travel. Perhaps this adaptation evolved early in the genus Homo, with H. habilis providing an early representative of this important change.

Patterns of resource use in early Homo and Paranthropus

Conventional wisdom concerning the extinction of Paranthropus suggests that these species developed highly derived morphologies as a consequence of specializing on a diet consisting of hard and/or low-quality food items. It goes on to suggest that these species were so specialized or stenotopic that they were unable to adapt to changing environments in the period following 1.5 Ma. The same conventional wisdom proposes that early Homo species responded very differently to the same environmental challenges. Instead of narrowing their niche it was the dietary and behavioral flexibility (eurytopy) exhibited by early Homo that enabled that lineage to persist. We investigate whether evidence taken across eleven criteria supports a null hypothesis in which Paranthropus is more stenotopic than early Homo. In six instances (most categories of direct evidence of dietary breadth, species diversity, species duration, susceptibility to dispersal, dispersal direction, and non-dietary adaptations) the evidence is inconsistent with the hypothesis. Only one line of indirect evidence for dietary breadth-occlusal morphology-is unambiguously consistent with the null hypothesis that Paranthropus' ability to process tough, fibrous food items (e.g., leaves) was reduced relative to early Homo. Other criteria (habitat preference, population density, direct and indirect evidence of dietary breadth related to incisor use) are only consistent with the hypothesis under certain conditions. If those conditions are not met, then the evidence is either inconsistent with the hypothesis, or ambiguous. On balance, Paranthropus and early Homo were both likely to have been ecological generalists. These data are inconsistent with the conventional wisdom that stenotopy was a major contributing factor in the extinction of the Paranthropus clade. Researchers will need to explore other avenues of research in order to generate testable hypotheses about the demise of Paranthropus. Ecological models that may explain the evolution of eurytopy in early hominins are discussed.

Patterns of covariation in the hominoid craniofacial skeleton: implications for paleoanthropological models

Living species are often used as analogues for fossil ones. When this is done, the implicit assumption is made that hominids and living hominoids vary in the same way. This paper addresses the validity of this assumption by comparing patterns of facial variation among humans and African apes. In particular, it addresses three major questions that underlie approaches to reconstructing hominid relationships. First, is phenotypic variation similar between closely related species? Second, if it is dissimilar, why? Third, is it feasible to use analogue species for modeling purposes? Measurements are obtained from 542 crania of adult apes and humans. Care is taken to choose homologous data, and account for differences in population size and structure. Variance/covariance and correlation matrices among the species are compared using common principal component (CPC) analysis, random skewers methods and matrix correlations. Morphological distances (D(2)) are calculated between population means, and between randomized pairs of individuals within each population, to evaluate intraspecific variation. Morphological distances are also calculated between randomized pairs of individuals using the variation patterns of analogue populations, in order to evaluate the efficacy of such substitutions. Results show that while the hominoids share a similar pattern of facial variation overall, the patterns do diverge. This difference generally corresponds to the phylogenetic relationships among these species, suggesting that patterns of variation may have diverged through time in the large bodied hominoids. Because interpretation of relationships in the fossil record is confounded by a lack of understanding of how variation changes through time, exploration of such patterns of divergence can provide important clues to understanding human evolution. Additionally, neglecting to account for this divergence when using living analogues as variation "yardsticks" can give rise to interpretations of the fossil record that are more speciose than is warranted.

Integration, phylogeny, and the hominid cranial base

Basicranial features were examined in catarrhine primates and early hominids in order to demonstrate how information about morphological integration can be incorporated into phylogenetic analysis. Hypotheses purporting to explain the functional and structural relationships of basicranial characters were tested using factor analysis. Characters found to be functionally or structurally related to each other were then further examined in order to determine whether there was evidence that they were phylogenetically independent. If phylogenetic independence could not be demonstrated, then the characters were presumed to be integrated and were grouped into a complex. That complex was then treated as if it were a single character for the purposes of cladistic analysis. Factor analysis revealed that five basicranial features may be structurally related to relative brain size in hominoids. Depending on how one defines phylogenetic independence, as few as two, or as many as all of those characters might be morphologically integrated. A cladistic analysis of early hominids based on basicranial features revealed that the use of integrated complexes had a substantial effect on the phylogenetic position of Australopithecus africanus, a species whose relationships are poorly resolved. Moreover, the use of complexes also had an effect on reanalyses of certain published cladistic data sets, implying that those studies might have been biased by patterns of basicranial integration. These results demonstrate that patterns of morphological integration need to be considered carefully in all morphology-based cladistic analyses, regardless of taxon or anatomical focus. However, an important caveat is that the functional and structural hypotheses tested here predicted much higher degrees of integration than were observed. This result warns strongly that hypotheses of integration must be tested before they can be adequately employed in phylogenetic analysis. The uncritical acceptance of an untested hypothesis of integration is likely to be as disruptive to a cladistic analysis as when integration is ignored.

Morphometric variation in the hominoid orbital aperture: a case study with implications for the use of variable characters in Miocene catarrhine systematics

Variable characters are ubiquitous in hominoid systematics and present a number of unique problems for phylogenetic analyses that include extinct taxa. As yet, however, few studies have quantified ranges of variation in complex morphometric characters within extant taxa and then used those data to assess the consistency with which discrete character states can be applied to poorly represented fossil species. In this study, ranges of intrageneric morphometric variation in the shape of the hominoid orbital aperture are estimated using exact randomization of average pairwise taxonomic distances (ATDs) derived from size-adjusted centroid, height-width, and elliptic Fourier (EF) variables. Using both centroid and height-width variables, 19 of the 21 possible ATDs between individuals representing seven extinct catarrhine taxa (Aegyptopithecus, Afropithecus, Ankarapithecus, Ouranopithecus, Paranthropus, Sivapithecus and Turkanapithecus) can be observed within a single extant hominoid subspecies, although generally with low probabilities. A resampling study is employed as a means for gauging the effect that this intrataxonomic variation may have on the consistency with which discrete orbital shape character states can be delimited given the small sample sizes available for most Miocene catarrhine taxa preserving this feature (i.e., n=1). For each type of morphometric variable, 100 cluster (UPGMA) analyses of pairwise ATDs are performed in which a single individual is randomly selected from each hominoid genus and analyzed alongside known extinct taxa; consensus trees are computed in order to obtain the frequencies with which different shape clusters appeared in each of the three analyses. The two major clusters appearing most frequently in all three consensus trees are found in only 57% (centroid variables), 49% (height-width variables), and 36% (EF variables) of these trees. If ranges of variation within represented extinct taxa could also be estimated, these frequencies would certainly be far lower. Hominoids clearly exhibit considerable intrageneric, intraspecific, and even intrasubspecific variation in orbit shape, and substantial morphometric overlap exists between taxa; consequently, discrete character states delimiting these patterns of continuous variation are likely to be highly unreliable in phylogenetic analyses of living and extinct species, particularly as the number of terminal taxa increases. Morphological phylogenetic studies of extant catarrhines that assess the effect of different methods (e.g., use of objective a priori weighting or frequency coding of variable characters, inclusion vs. exclusion of variable characters, use of specific vs. supraspecific terminal taxa) on phylogenetic accuracy may help to improve the techniques that systematists employ to make phylogenetic inferences about extinct taxa.

Human evolution: taxonomy and paleobiology

This review begins by setting out the context and the scope of human evolution. Several classes of evidence, morphological, molecular, and genetic, support a particularly close relationship between modern humans and the species within the genus Pan, the chimpanzee. Thus human evolution is the study of the lineage, or clade, comprising species more closely related to modern humans than to chimpanzees. Its stem species is the so-called 'common hominin ancestor', and its only extant member is Homo sapiens. This clade contains all the species more closely-related to modern humans than to any other living primate. Until recently, these species were all subsumed into a family, Hominidae, but this group is now more usually recognised as a tribe, the Hominini. The rest of the review sets out the formal nomenclature, history of discovery, and information about the characteristic morphology, and its behavioural implications, of the species presently included in the human clade. The taxa are considered within their assigned genera, beginning with the most primitive and finishing with Homo. Within genera, species are presented in order of geological age. The entries conclude with a list of the more important items of fossil evidence, and a summary of relevant taxonomic issues.

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.P(3)mesiodistal length, M(3)size, 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.

Morphological and taxonomic affinities of the Olduvai ulna (OH 36)

The OH 36 ulna derives from Upper Bed II in the Olduvai Gorge, and is dated to circa 1.1-1.2 Myr. Multivariate analyses incorporating data from samples of modern humans, common and pygmy chimpanzees, gorillas, orangutans, and two other early hominin ulnae, Omo L40-19 and KNM-BK 66, suggest that OH 36 belonged to an individual with powerful forearms consistent with a locomotor repertoire that included arboreal locomotion. However, there is no compelling evidence that it made regular use of its forelimbs as supports when travelling on the ground. When compared with levels of intra- and intertaxon size and shape variation in the comparative sample (humans, chimpanzees, gorillas), the differences between OH 36, KNM-BK 66, and Omo L40-19 are compatible with OH 36 differing from the other two fossil hominin ulnae to the extent that modern humans differ from modern great apes. KNM-BK 66 and Omo L40-19 differ from each other in overall size and shape only to the degree that would be expected within any of the individual modern comparative samples. Based on these analyses, there is no evidence to support the hypothesis that OH 36 and Omo L40-19 belong to the same species of fossil hominin, or to two species that shared a similar forelimb locomotor repertoire. We suggest that OH 36 has the greater claim to be assigned to Paranthropus boisei, and we recommend that for the time being the latter be referred to the tribe Hominini gen. et sp. indet. The surprising result of these analyses is the overall size and shape similarity between Omo L40-19 and KNM-BK 66, two fossils that are separated in time by more than 1.5 million years, and which have traditionally been assumed to represent hominin species with quite different locomotor patterns.

Kinematic data on primate head and neck posture: implications for the evolution of basicranial flexion and an evaluation of registration planes used in paleoanthropology

Kinematic data on primate head and neck posture were collected by filming 29 primate species during locomotion. These were used to test whether head and neck posture are significant influences on basicranial flexion and whether the Frankfurt plane can legitimately be employed in paleoanthropological studies. Three kinematic measurements were recorded as angles relative to the gravity vector, the inclination of the orbital plane, the inclination of the neck, and the inclination of the Frankfurt plane. A fourth kinematic measurement was calculated as the angle between the neck and the orbital plane (the head-neck angle [HNA]). The functional relationships of basicranial flexion were examined by calculating the correlations and partial correlations between HNA and craniometric measurements representing basicranial flexion, orbital kyphosis, and relative brain size (Ross and Ravosa [1993] Am. J. Phys. Anthropol. 91:305-324). Significant partial correlations were observed between relative brain size and basicranial flexion and between HNA and orbital kyphosis. This indicates that brain size, rather than head and neck posture, is the primary influence on flexion, while the degree of orbital kyphosis may act to reorient the visual field in response to variation in head and neck posture. Regarding registration planes, the Frankfurt plane was found to be horizontal in humans but inclined in all nonhuman primates. In contrast, nearly all primates (including humans) oriented their orbits such that they faced anteriorly and slightly inferiorly. These results suggest that for certain functional craniometric studies, the orbital plane may be a more suitable registration plane than Frankfurt "Horizontal."

Sexual dimorphism in the face of Australopithecus africanus

Recently discovered crania of Australopithecus africanus from Sterkfontein Member 4 and Makapansgat enlarge the size range of the species and encourage a reappraisal of both the degree and pattern of sexual dimorphism. Resampling methodology (bootstrapping) is used here to establish that A. africanus has a greater craniofacial size range than chimpanzees or modern humans, a range which is best attributed to a moderately high degree of sexual dimorphism. Compared to other fossil hominins, this variation is similar to that of Homo habilis (sensu lato) but less than that of A. boisei. The finding of moderately high dimorphism is corroborated by a CV-based estimate and ratios between those specimens considered to be male and those considered to be female. Inferences about the pattern of craniofacial dimorphism in the A. africanus face currently rely on the relationship of morphology and size. Larger specimens, particularly Stw 505, show prominent superciliary eminences and glabellar regions, but in features related in part to canine size, such as the curvature of the infraorbital surface, large and small specimens of A. africanus are similar. In this respect, the pattern resembles that of modern humans more so than chimpanzees or lowland gorillas. A. africanus may also show novel patterns of sexual dimorphism when compared to extant hominines, such as in the form of the anterior pillar. However, males of the species do not exhibit characteristics of more derived hominins, such as A. robustus.

Intrapopulational body size variation and cranial capacity variation in Middle Pleistocene humans: the Sima de los Huesos sample (Sierra de Atapuerca, Spain)

A sexual dimorphism more marked than in living humans has been claimed for European Middle Pleistocene humans, Neandertals and prehistoric modern humans. In this paper, body size and cranial capacity variation are studied in the Sima de los Huesos Middle Pleistocene sample. This is the largest sample of non-modern humans found to date from one single site, and with all skeletal elements represented. Since the techniques available to estimate the degree of sexual dimorphism in small palaeontological samples are all unsatisfactory, we have used the bootstraping method to asses the magnitude of the variation in the Sima de los Huesos sample compared to modern human intrapopulational variation. We analyze size variation without attempting to sex the specimens a priori. Anatomical regions investigated are scapular glenoid fossa; acetabulum; humeral proximal and distal epiphyses; ulnar proximal epiphysis; radial neck; proximal femur; humeral, femoral, ulnar and tibial shaft; lumbosacral joint; patella; calcaneum; and talar trochlea. In the Sima de los Huesos sample only the humeral midshaft perimeter shows an unusual high variation (only when it is expressed by the maximum ratio, not by the coefficient of variation). In spite of that the cranial capacity range at Sima de los Huesos almost spans the rest of the European and African Middle Pleistocene range. The maximum ratio is in the central part of the distribution of modern human samples. Thus, the hypothesis of a greater sexual dimorphism in Middle Pleistocene populations than in modern populations is not supported by either cranial or postcranial evidence from Sima de los Huesos.

Size variation in Middle Pleistocene humans

It has been suggested that European Middle Pleistocene humans, Neandertals, and prehistoric modern humans had a greater sexual dimorphism than modern humans. Analysis of body size variation and cranial capacity variation in the large sample from the Sima de los Huesos site in Spain showed instead that the sexual dimorphism is comparable in Middle Pleistocene and modern populations.

The Sima de los Huesos crania (Sierra de Atapuerca, Spain). A comparative study

The Sima de los Huesos (Sierra de Atapuerca) cranial remains found up to and including the 1995 field season are described and compared with other fossils in order to assess their evolutionary relationships. The phenetic affinities of the Sima de los Huesos crania and a large sample of Homo fossils are investigated through principal component analyses. Metrical comparisons of the Sima de los Huesos and other European and African Middle Pleistocene fossils with Neandertals are performed using Z-scores relative to the Neandertal sample statistics. The most relevant cranial traits are metrically and morphologically analyzed and cladistically evaluated. The Sima de los Huesos crania exhibit a number of primitive traits lost in Upper Pleistocene Neandertals (especially in the braincase, but also in the facial skeleton), as well as other traits that are transitional to the Neandertal morphology (particularly in the occipital bone), and features close to what is found in Neandertals (as the supraorbital morphology and midfacial prognathism). Different combinations of primitive and derived traits (shared with Neandertals) are also displayed by the other European Middle Pleistocene fossils. In conclusion, the Sima de los Huesos sample is evolutionarily related to Neandertals as well as to the other European Middle Pleistocene fossils. In our opinion, all the European Middle Pleistocene fossils belong to the Neandertal lineage, and none can be included in an Afroeuropean common ancestor of Neandertals and modern humans.

Systematic assessment of a maxilla of Homo from Hadar, Ethiopia

The Hadar site in Ethiopia is a prolific source of hominid fossils attributed to the species Australopithecus afarensis, which spans the period 3.4-3.0 million years (myr) in the Sidi Hakoma, Denen Dora and lower Kada Hadar Members of the Hadar Formation. Since 1992 a major focus of field work conducted at Hadar has centered on sediments younger than 3.0 myr, comprising the bulk of the Kada Hadar Member. Witnessing the rise of the "robust" Australopithecus clade(s), the origin of Homo, and the first record of lithic artifacts, the period between 3.0 and 2.0 myr is strategically vital for paleoanthropology. However, in eastern Africa it is a particularly poorly sampled temporal interval. This paper provides a detailed comparative description of a hominid maxilla with partial dentition found at Hadar in 1994. The specimen, A.L. 666-1, derives from a lithic artifact-bearing horizon high in the Kada Hadar Member, 0.8 m below the BKT-3 tephra, dated by the 40Ar/39Ar method to 2.33 +/- 0.07 myr. Our preliminary investigation of the hominid specimen showed unambiguous affinities with early representatives of the Homo clade (Kimbel et al. [1996] J. Hum. Evol. 31:549-561). Further studies on maxillary and dental morphology lead us to attribute A.L. 666-1 to Homo aff. H. habilis. The new Hadar jaw is the first paleontological evidence for the projection of the H. habilis maxillofacial morphotype well back into the Pliocene. It may represent a male of this species, whose maxillary hypodigm consists chiefly of females. A subsidiary finding of our study is that of the three earliest recorded species of Homo (H. habilis, H. rudolfensis, H. erectus), it is H. habilis that exhibits facial morphology closest to that expected in their last common ancestor.

A reappraisal of early hominid phylogeny

We report here on the results of a new cladistic analysis of early hominid relationships. Ingroup taxa included Australopithecus afarensis, Australopithecus africanus, Australopithecus aethiopicus, Australopithecus robustus, Australopithecus boisei, Homo habilis, Homo rudolfensis, Homo ergaster and Homo sapiens. Outgroup taxa included Pan troglodytes and Gorilla gorilla. Sixty craniodental characters were selected for analysis. These were drawn from the trait lists of other studies and our own observations. Eight parsimony analyses were performed that differed with respect to the number of characters examined and the manner in which the characters were treated. Seven employed ordered characters, and included analyses in which (1) taxa that were variable with respect to a character were coded as having an intermediate state, (2) characters with variable states in any taxon were excluded; (3) a variable taxon was coded as having the state exhibited by the majority of its hypodigm, (4) variable taxa were coded as missing data for that character, (5) some characters were considered irreversible, (6) masticatory characters were excluded, and (7) characters whose states were unknown in some taxa were excluded. In the final analysis, (8) all characters were unordered. All analyses were performed using PAUP 3.0s. Despite the fact that the eight analyses differed with respect to methodology, they produced several consistent results. All agreed that the "robust" australopithecines form a clade, A. afarensis is the sister taxon of all other hominids, and the genus Australopithecus, as conventionally defined, is paraphyletic. All eight also supported trees in which A. africanus is the sister taxon of a joint Homo+ "robust" clade, although in one analysis an equally parsimonious topology found A. africanus to be the sister of the "robust" species. In most analyses, the relationships of A. africanus and H. habilis were unstable, in the sense that their positions vary in trees that are marginally less parsimonious than the favored one. Trees in which "robust" australopithecines are paraphyletic were found to be extremely unparsimonious.

Human evolution

The common ancestor of modern humans and the great apes is estimated to have lived between 5 and 8 Myrs ago, but the earliest evidence in the human, or hominid, fossil record is Ardipithecus ramidus, from a 4.5 Myr Ethiopian site. This genus was succeeded by Australopithecus, within which four species are presently recognised. All combine a relatively primitive postcranial skeleton, a dentition with expanded chewing teeth and a small brain. The most primitive species in our own genus, Homo habilis and Homo rudolfensis, are little advanced over the australopithecines and with hindsight their inclusion in Homo may not be appropriate. The first species to share a substantial number of features with later Homo is Homo ergaster, or 'early African Homo erectus', which appears in the fossil record around 2.0 Myr. Outside Africa, fossil hominids appear as Homo erectus-like hominids, in mainland Asia and in Indonesia close to 2 Myr ago; the earliest good evidence of 'archaic Homo' in Europe is dated at between 600-700 Kyr before the present. Anatomically modern human, or Homo sapiens, fossils are seen first in the fossil record in Africa around 150 Kyr ago. Taken together with molecular evidence on the extent of DNA variation, this suggests that the transition from 'archaic' to 'modern' Homo may have taken place in Africa.

Morphometric variation in Plio-Pleistocene hominid distal humeri

The magnitude and meaning of morphological variation among Plio-Pleistocene hominid distal humeri have been recurrent points of disagreement among paleoanthropologists. Some researchers have found noteworthy differences among fossil humeri that they believe merit taxonomic separation, while others question the possibility of accurately sorting these fossils into different species and/or functional groups. Size and shape differences among fossil distal humeri are evaluated here to determine whether the magnitude and patterns of these differences can be observed within large-bodied, living hominoids. Specimens analyzed in this study have been assigned to various taxa (Australopithecus afarensis, A. africanus, A. anamensis, Paranthropus, and early Homo) and include AL 288-1m, AL 288-1s, AL 137-48a, AL 322-1, Gomboré IB 7594, TM 1517, KNM-ER 739, KNM-ER 1504, KMN-KP 271 (Kanapoi), and Stw 431. Five extant hominoid populations are sampled to provide a standard by which to consider differences found between the fossils, including two modern human groups (Native American and African American), one group of Pan troglodytes, and two subspecies of Gorilla gorilla (G.g. beringei, G.g. gorilla). All possible pairwise d values (average Euclidena distances) are calculated within each of the reference populations using an exact randomization procedure. This is done using both raw linear measurements as well as scale-free shape data created as ratios of each measurement to the geometric mean. Differences between each pair of fossil humeri are evaluated by comparing their d values to the distribution of d values found within each of the reference populations. Principal coordinate analysis and an unweighted pair group method with arithmetic averages (UPGMA) cluster analysis are utilized to further assess similarities and differences among the fossils. Finally, canonical variates analysis and discriminant analysis are employed using all hominoid samples in order to control for correlations among variables and to identify those variables that discriminate among groups; possible affinities of individual fossils with specific extant species are also examined. The largest size differences, those between the small Hadar specimens and the two largest fossils (KNM-ER 739, IB 7594), can be accommodated easily within the ranges of variation of the two Gorilla samples, but are extreme relative to the other reference samples. The d values between most of the fossils based on shape data, with the notable exception of those associated with KNM-ER 739 and KNM-ER 1504, can be sampled safely within all five reference samples. Subsequent analyses further support the inference that KNM-ER 739 and KNM-ER 1504 are different from the other hominid humeri and possess a unique total morphometric pattern. In overall shape, the distal humeri of the other fossils (non-Koobi Fora) are most similar to living chimpanzees. The distal humerus of Paranthropus from Kromdraai (TM 1517e) is most similar to one of the Hadar specimens of A. afarensis (AL 137-48a), whereas the first specimen of A. africanus from Sterkfontein (Stw 431) is not closely linked to any of the other australopithecines. The A. anamensis humerus from Kanapoi exhibits no special affinities to A. afarensis or to modern humans.

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.