How big were early hominids?

The recent discovery of new postcranial fossils, particularly associated body parts, of several Plio-Pleistocene hominids provides a new opportunity to assess body size in human evolution.¹ Body size plays a central role in the biology of animals because of its relationship to brain size, feeding behavior, habitat preference, social behavior, and much more. Unfortunately, the prediction of body weight from fossils is inherently inaccurate because skeletal size does not reflect body size exactly and because the fossils are from species having body proportions for which there are no analogues among modern species. The approach here is to find the relationship between body size and skeletal size in ape and human specimens of known body weight at death and to apply this knowledge to the hominid fossils, using a variety of statistical methods, knowledge of the associated partial skeletons of the of early hominids, formulae derived from a modern human sample, and, finally, common sense. The following modal weights for males and females emerge: Australopithecus afarensis, 45 and 29 kg; A. africanus, 41 and 30 kg; A. robustus, 40 and 32 kg; A. boisei, 49 and 34 kg; H. habilis, 52 and 32 kg. The best known African early H. erectus were much larger with weights ranging from 55 kg on up. These estimates imply that (1) in the earliest hominid species and the “robust” australopithecines body sizes remained small relative to modern standards, but between 2.0 and 1.7 m.y.a. there was a rapid increase to essentially modern body size with the appearance of Homo erectus; (2) the earliest species had a degree of body size sexual dimorphism well above that seen in modern humans but below that seen in modern gorillas and orangs which implies (along with other evidence) a social organization characterized by kin-related, multi-male groups with females who were not kin-related; (3) relative brain sizes increased through time; (4) there were two divergent trends in relative cheek-tooth size—a steady increase through time from A. afarensis to A. africanus to the “robust” australopithecines, and a decrease beginning with H. habilis to H. erectus to H. sapiens.     

Was “Lucy” more human than her “child”? Observations on early hominid postcranial skeletons

Fully adult partial skeletons attributed to Australopithecus afarensis (AL 288-1, “Lucy”) and to Homo habilis (OH 62, “Lucy's child”), respectively, both include remains from upper and lower limbs. Relationships between various limb bone dimensions of these skeletons are compared to those of modern African apes and humans. Surprisingly, it emerges that OH 62 displays closer similarities to African apes than does AL 288-1. Yet A. afarensis, whose skeleton is dated more than 1 million years earlier, is commonly supposed to be the ancestor of Homo habilis. If OH 62, classified as Homo habilis by its discoverers, does indeed represent a stage intermediate between A. afarensis and later Homo, a revised interpretation of the course of human evolution would be necessary.     

Petite bodies of the “robust” australopithecines

The “robust” australopithecines are often depicted as having large and powerfully built bodies to match their massive masticatory apparatus, but until 1988 the sample of postcranial remains attributed with certainty to this group was very limited. Almost nothing was known about the body of the East African “robust” australopithecine because taxonomic attribution of the postcrania was so uncertain. The body of the South African “robust” australopithecine had to be reconstructed from about a dozen isolated fragments of postcrania. Now a partial skeleton is attributed with confidence to the East African “robust” group along with several isolated bones. The South African sample has more than tripled. Analyses of this vastly expanded sample reveal that a large portion of postcrania attributed to “robust” australopithecines from Swartkrans Member 1 (35%) are from extraordinarily small-bodied individuals similar in size to a modern Pygmy weighing as little as 28 kg. These small elements include parts from the forelimb, spine, and hindlimb. About 22% of these Swartkrans 1 “robust” australopithecines are about the same size as a modern human weighing about 43 kgs and about 43% are larger than this standard but less than or equal to a 54 kg modern human. Approximately the same pattern is true for the Swartkrans 2 hominids, but taxonomic attribution is less certain. All of the Member 3 specimens are similar in size to the 45 kg standard. The partial skeleton of the East African “robust” australopithecine (KNM-ER 1500) has hindlimb joints that would correspond to a modern human of 34 kgs although the actual weight may be 5 to 10 kgs greater judging from shaft robusticity and forelimb size. The largest postcranial element attributed with some certainty to the East African “robust” australopithecine group (the talus, KNM-ER 1464) is about the same overall size as a modern human of 54 kgs, although its tibial facet is slightly smaller. Although many previous studies have hinted at the possibility that “robust” australopithecines had relatively small bodies, the new fossils provide substantial evidence that these creatures ranged from quite small to only moderate in body size relative to modern humans. These were the petite-bodied vegetarian cousins of our ancestors. Sexual dimorphism in body size appears to be greater than that in modern humans, similar to that in Pan, and less than that in Gorilla or Pongo, although such comparisons are of limited value given the small samples, poorly known body proportions, time averaging, and many other problems.     

Ontogeny and phylogeny of the pelvis in Gorilla, Pongo, Pan, Australopithecus and Homo

To examine the evolutionary differences between hominoid locomotor systems, a number of observations concerning the growth of the pelvis among the great apes as compared to modern and fossil hominids are reported. We are interested in the size and shape of the coxal bones at different developmental stages across species that may elucidate the relationship between ontogeny and phylogeny (i.e., heterochrony) in the hominoid pelvis. Our hypotheses are: (1) do rates of absolute growth differ?, (2) do rates of relative growth differ?, and (3) does heterochrony explain these differences? Bivariate and multivariate analyses of pelvic dimensions demonstrate both the diversity of species-specific ontogenetic patterns among hominoids, and an unequivocal separation of hominids and the great apes. Heterochrony alone fails to account for the ontogenetic differences between hominids and the great apes. Compared to recent Homo,Australopithecus can be described as 'hyper-human' from the relative size of the ischium, and short but broad ilium. Australopithecus afarensis differs from Australopithecus africanus by its relatively long pubis. In multivariate analyses of ilium shape, the most complete coxal bone attributed to Homo erectus, KNM-ER 3228, falls within the range of juvenile and adult Australopithecus, whereas Broken Hill falls within the range of modern Homo, suggesting that the modern human ilium shape arose rather recently. Among the great apes, patterns of pelvic ontogeny do not exclusively separate the African apes from Pongo.     

Climatic adaptation and hominid evolution: The thermoregulatory imperative

Anthropologists have long recognized the existence among modern humans of geographical variations in body form that parallel climatic gradients, part of more general zoological phenomena commonly referred to as Bergmann's or Allen's “Rules”. These observations have rarely been applied to earlier hominids, in part because fossil skeletons usually are so incomplete that it is difficult to reconstruct body morphology accurately. However, within the past two decades two early hominids have been discovered that preserve enough of the skeleton to allow confident assessment of their body size and shape. Comparison of these specimens—the Australopithecus afarensis A.L. 288-1 (“Lucy”) and the Homo erectus KNM-WT 15000—with others that are less complete make it evident that the evolution of Homo erectus was accompanied by not only a marked increase in body size, but also a similarly dramatic increase in the linearity of body form. That is, relative to their heights, small australopithecines had very broad bodies, whereas large early Homo had narrow bodies. This difference in body form cannot be explained on the basis of obstetric or biomechanical factors, but is consistent with thermoregulatory constraints on body shape. Specifically, to maintain the same ratio of body surface area to body mass, which is an important thermoregulatory mechanism, increases in height should be accompanied by no change in body breadth, which is exactly what is seen in comparisons of A.L. 288-1 and KNM-WT 15000. Conversely, Neandertals living in colder climates had much wider bodies, which are adaptive for heat retention. Differences in limb length proportions between fossil hominids are also consistent with thermoregulatory principles and the geographic variation observed among modern humans. Climatic adaptation during hominid evolution may have wide-ranging implications, not only with regard to interpreting body morphology, but also in relation to ecological scenarios, population movements, and the evolution of the brain.     

巨猿牙齿釉质厚度及对食性适应与系统演化的意义

步氏巨猿(Gigantopithecus blacki)是更新世时期生活于我国华南地区的一种超大型猿类, 它的体态特征和演化分类倍受关注。牙齿釉质厚度在探讨灵长类食性、环境适应以及系统演化方面具有重要意义。本文利用显微CT技术构建18颗巨猿臼齿虚拟模型, 测量其釉质厚度。将巨猿釉质厚度与现代人、现生类人猿、古人类、中新世古猿及其他现生灵长类进行比较, 从牙齿釉质厚度探讨巨猿的食性适应和系统演化问题。结果发现巨猿的实测釉质厚度是目前所有已知现生和化石灵长类中最厚的, 只有傍人、南非早期人属及奥兰诺古猿三种化石灵长类与之接近; 如果考虑不同物种牙齿与身体大小的关联因素, 相对釉质厚度指数显示巨猿属于"厚"釉质类型, 但非"超厚"类型, 低于奥兰诺古猿、傍人、南非早期人属; 巨猿与某些中新世古猿 (如原康修尔猿尼安萨种、非洲古猿)、南方古猿、东非早期人属、亚洲直立人以及现代人、现生卷尾猴的相对釉质厚度指数相近。巨猿的厚釉质特征与其食性和环境适应密切相关, 使得牙齿具有非常强的抗磨损功能, 能够适应长时间的咀嚼和研磨食物。从釉质厚度的系统演化角度推测, 厚釉质应该是人类祖先的特征性状, 巨猿有可能是早期人类支系演化过程中的一个特化旁支, 同时也不排除巨猿是从某种具有厚釉质的中新世古猿旁支平行演化而来的可能性。     

The posterior border of the sphenoid greater wing and its phylogenetic usefulness in human evolution

The elucidation of patterns of cranial skeletal maturation and growth in fossil hominids is possible not only through dental studies but also by mapping different aspects of ossification in both extant African apes and humans. However, knowledge of normal skeletal development in large samples of extant great apes is flimsy. To remedy this situation, this paper offers an extensive survey and thorough discussion of the ossification of the posterior border of the sphenoid greater wing. Indeed, this area provides much information about basicranial skeletal maturation. We investigate three variants: the absence of the foramen spinosum and the position of both the foramen spinosum and the foramen ovale in relation to the sphenosquamosal suture. Providing original data about humans and 1,425 extant great ape skulls and using a sample of 64 fossil hominids, this study aimed to test whether different ossification patterns occurred during the course of human evolution. The incidence of three derived morphologies located on the posterior border of the sphenoid greater wing increases during human evolution at different geological periods. The evolutionary polarity of these three derived morphologies is assessed by outgroup comparison and ontogenetic methods. During human evolution, there is a clear trend for the foramen spinosum to be present and wholly located on the posterior area of the sphenoid greater wing. Moreover, in all the great ape species and in Australopithecus afarensis, the sphenosquamosal suture may split the foramen ovale. Inversely, the foramen ovale always lies wholly within the sphenoid greater wing in Australopithecus africanus, robust australopithecines, early Homo, H. erectus (and/or H. ergaster), and Homo sapiens. From ontogenetic studies in humans, we conclude that, during human evolution, the ossification of the posterior area of the sphenoid greater wing progressively surrounded the middle meningeal artery (passing through the foramen spinosum) and the small meningeal artery (passing through the foramen ovale). Am J Phys Anthropol 107:387–399, 1998. © 1998 Wiley-Liss, Inc.     

Early hominid body weight and encephalization

The body weight of the Plio-Pleistocene hominids of Africa is estimated by predicting equations derived from the Terry Collection of human skeletons with known body weights. About 50% of the variance in body weight can be accounted for by vertebral and femoral size. Predicted early hominid weights range from 27.6 kg (61 lb) to 54.3 kg (119 lb). The average weight for Australopithecus is 43.2 kg (95 lb) and for Homo sp. indet. from East Rudolf, Kenya, is 52.8 kg (116 lb). These estimates are consistent even if pongid proportions are assumed. Indices of encephalization show that the brain to body weight ratio in Australopithecus is above the great ape averages but well below Homo sapiens. The Homo sp. indet. represented by the KNM-ER 1470, O.H. 7 and O.H. 13 crania have encephalization indices above Australopithecus despite the greater body weight of the former.     

Comparative 3D quantitative analyses of trapeziometacarpal joint surface curvatures among living catarrhines and fossil hominins

Comparisons of joint surface curvature at the base of the thumb have long been made to discern differences among living and fossil primates in functional capabilities of the hand. However, the complex shape of this joint makes it difficult to quantify differences among taxa. The purpose of this study is to determine whether significant differences in curvature exist among selected catarrhine genera and to compare these genera with hominin¹ fossils in trapeziometacarpal curvature. Two 3D approaches are used to quantify curvatures of the trapezial and metacarpal joint surfaces: (1) stereophotogrammetry with nonuniform rational B‐spline (NURBS) calculation of joint curvature to compare modern humans with captive chimpanzees and (2) laser scanning with a quadric‐based calculation of curvature to compare modern humans and wild‐caught Pan, Gorilla, Pongo, and Papio. Both approaches show that Homo has significantly lower curvature of the joint surfaces than does Pan. The second approach shows that Gorilla has significantly more curvature than modern humans, while Pongo overlaps with humans and African apes. The surfaces in Papio are more cylindrical and flatter than in Homo. Australopithecus afarensis resembles African apes more than modern humans in curvatures, whereas the Homo habilis trapezial metacarpal surface is flatter than in all genera except Papio. Neandertals fall at one end of the modern human range of variation, with smaller dorsovolar curvature. Modern human topography appears to be derived relative to great apes and Australopithecus and contributes to the distinctive human morphology that facilitates forceful precision and power gripping, fundamental to human manipulative activities. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc. ¹ The term “hominin” refers to members of the tribe Hominini, which includes modern humans and fossil species that are related more closely to modern humans than to extant species of chimpanzees, Wood and Lonergan (2008). Hominins are in the family Hominidae with great apes.     

Endocranial volumes of early African hominids, and the role of the brain in human mosaic evolution

Volumetric data are presented for 16 of the early hominids from both South and East Africa. Although the sample sizes are small, the statistical data support the conclusion that at least three taxa are represented; Australopithecus africanus, A. robustus, and Homo habilis. These data, plus certain morphological attributes, indicate that the brains of early hominids were reorganized to a human pattern, regardless of their small endocranial capacities. Some speculative suggestions are made regarding the possible relationship between brain and body weights, as well as Stephan's (1972) “progression indices”. If the speculations are correct, they provide additional support for the idea that brain reorganization occurred early in human evolution, and that concepts which regard the brain as having a more terminal role in human mosaic evolution are incorrect, as all of the fossil encephalization or “progression indices” are in the range of modern Homo sapiens.     

Metric variation and species recognition in the fossil record

One of the first and most important tasks of the paleontologist is classifying specimens into species. Species recognition commonly involves sorting specimens on the basis of qualitative and quantitative similarities and differences. Often, however, variation in simple metric characters like tooth size or jaw length plays an important role in debates about whether a sample comprises a single species or more than one morphologically similar species. For example, Simpson, Roe, and Lewontin 1 suggested that a fossil sample showing a coefficient of variation greater than 10.0 was likely to comprise more than one species. Well‐known controversies over species recognition in which metric sample variation has been important have simmered for years, focusing on hominids, hominoids, and other extinct primates. Some of these have been resolved; others have not. For example, Pilbeam and Zwell 2 convincingly demonstrated multiple species among South African hominids by showing that metric tooth size variation was too great to be reasonably interpreted as sexual dimorphism. But metric variation continues to play a role in debates about whether Australopithecus afarensis 3 , 4 and Homo habilis 5 - 9 each comprise a single species or two or more separate species. Similarly, there has been steady debate about the number of species present in African Proconsul. Some favor an interpretation of a single extremely dimorphic species, 10 - 12 while others favor an interpretation of two or more species. 13     

Evolution of mandibular arcades: a statistical study

The alveolar arcades of a large number of fossil mandibles including Australopithecus and hominids fromHomo habilis andHomo erectus up to modern man have been characterized by fourteen cartesian points each representing a tooth. From these points, dimensions and angles have been calculated. These values are correlated to the geological age of the fossils. A linear dependance of dimensions and angles on the logarithm of age has been found. These results are discussed in the framework of a continuous gradual development within genus Homo and contrasted to prehominid data. Using these mean arcades and selected angles thereon the European and the AfroasiaticHomo erectus are compared and contrasted to the Neandertalians.     

Body size and proportions in early hominids.

The discovery of several associated body parts of early hominids whose taxonomic identity is known inspires this study of body size and proportions in early hominids. The approach consists of finding the relationship between various measures of skeletal size and body mass in modern ape and human specimens of known body weight. This effort leads to 78 equations which predict body weight from 95 fossil specimens ranging in geological age between 4 and 1.4 mya. Predicted weights range from 10 kg to over 160 kg, but the partial associated skeletons provide the essential clues as to which predictions are most reliable. Measures of hindlimb joint size are the best and probably those equations based on the human samples are better than those based on all Hominoidea. Using hindlimb joint size of specimens of relatively certain taxonomy and assuming these measures were more like those of modern humans than of apes, the male and female averages are as follows: Australopithecus afarensis, 45 and 29 kg; A. africanus, 41 and 30 kg; A. robustus, 40 and 32 kg; A. boisei, 49 and 34 kg; H. habilis, 52 and 32 kg. These values appear to be consistent with the range of size variation seen in the entire postcranial samples that can be assigned to species. If hominoid (i.e., ape and human combined) proportions are assumed, the males would be 10 to 23 kg larger and the females 4 to 10 kg larger.     

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 ‘archiac’ to ‘modern’ Homo may have taken place in Africa.     

The crescent of foramina in Australopithecus afarensis and other early hominids

The crescent of foramina of the cerebral surface of the sphenoid bone (superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum) differs morphologically in the African great apes and modern humans. New discoveries of Australopithecus afarensis at Hadar, Ethiopia, draw attention to the similarity of the crescent, particularly the “foramen” shape of the superior orbital fissure and its close proximity to the foramen rotundum, in this species, the African apes, and many other primates. Australopithecus africanus also shows this primitive pattern, whereas “robust” australopiths and humans share a configuration in which a true, laterally extended superior orbital fissure intervenes between the greater and lesser wings of the sphenoid and a broad bridge of bone separates the fissure from the foramen rotundum. This shared morphology may be added to the list of putative “robust” australopith-Homo synapomorphies. © 1996 Wiley-Liss, Inc.     

Origin of the Genus <Emphasis Type="Italic">Homo</Emphasis>

The origin of the genus Homo in Africa signals the beginning of the shift from increasingly bipedal apes to primitive, large-brained, stone tool-making, meat-eaters that traveled far and wide. This early part of the human genus is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus. H. habilis is known for retaining primitive features that link it to australopiths and for being the first stone tool makers. Little is known about H. rudolfensis except that it had a relatively large brain and large teeth compared to H. habilis and that it overlapped in time and space with other early Homo. Our understanding of the paleobiology and evolution of the larger-brained H. erectus is enhanced due to its rich fossil record. H. erectus was the first obligate, fully committed biped, and with a body adapted for modern striding locomotion, it was also the first in the human lineage to disperse outside of Africa. The early members of the genus Homo are the first to tip the scale from the more apish side of our evolutionary history toward the more human one.     

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.     

华南化石猩猩前部牙齿釉面横纹与牙冠形成时间研究

釉面横纹的分布与数目可以反映牙齿生长发育的时间和速率变化, 在化石研究中能为复原个体生活史提供重要依据。本研究运用扫描电子显微镜观察华南化石猩猩门齿、犬齿釉面横纹分布与数目, 并估算门齿和犬齿牙冠形成时间, 结果如下: 牙冠从牙尖至牙颈方向釉面横纹分布密度有疏密变化, 牙尖釉面横纹密度小于10条/mm, 中间至牙颈釉面横纹密度较尖部增大, 大约10-15条/mm; 犬齿釉面横纹数目多于门齿, 雄性犬齿釉面横纹数目多于雌性; 根据釉面横纹计数及其生长周期的组织切片观察结果, 估算门齿牙冠形成时间大约为2.97-6.66年, 犬齿雄性长于雌性, 分别为6.25-11.31年和4.28-7.29年。与一些古猿、早期人类、现代人以及现生大猿比较, 华南化石猩猩釉面横纹整体密度稍大于南方古猿和傍人, 小于黑猩猩、大猩猩、现代人和禄丰古猿; 除侧门齿外, 华南化石猩猩釉面横纹数目明显多于南方古猿、傍人和现代人, 与大猩猩接近; 华南猩猩前部牙齿牙冠形成时间与现生大猿、禄丰古猿差别不大, 与现生猩猩最相近, 长于南方古猿和傍人。     

A new morphometric analysis of the hominid pelvic bone

This study is based upon a new morphometric technique providing both size and shape variables. It has been applied to 189 pelvic bones of extant humans and African apes as well as to 13 hominid pelvic bones of various taxonomic status. The main aim of this work is to include such fossil bones in the same study in order to set a synthetic comparison of their shape in the light of the yardstick given by the African ape/human pelvic bone comparison. To do so, ratio diagrams are chosen because they are simple and very expressive tools with which to present such comparisons. Shape differences are very well illustrated and quantified by this technique. The ilium appears to be the most different of the three parts of the pelvic bone. Compared to these differences, discrepancies between fossil hominid and extant human bones are of a totally different scale. This shows the architectural unity related to the acquisition of bipedalism by hominids. It is nonetheless possible to detect two levels of difference. The first separates Australopithecus from Homo and could be seen as reflecting locomotor differences between both genera. The second splits both Homo erectus and Neanderthal from modern human pelvic bones. It appears from the hominid fossil record of pelvic bones that two periods of stasis exist and are separated by a period of very rapid evolution corresponding to the emergence of the genus Homo. We are of the opinion that the same could be true for the split between African ape and hominid lineages at the end of the Miocene.     

Limb-size proportions in Australopithecus afarensis and Australopithecus africanus

Previous analyses have suggested that Australopithecus africanus possessed more apelike limb proportions than Australopithecus afarensis. However, due to the errors involved in estimating limb length and body size, support for this conclusion has been limited. In this study, we use a new Monte Carlo method to (1) test the hypothesis that A. africanus had greater upper:lower limb-size proportions than A. afarensis and (2) assess the statistical significance of interspecific differences among these taxa, extant apes, and humans. Our Monte Carlo method imposes sampling constraints that reduce extant ape and human postcranial measurements to sample sizes comparable to the fossil samples. Next, composite ratios of fore- and hindlimb geometric means are calculated for resampled measurements from the fossils and comparative taxa. Mean composite ratios are statistically indistinguishable (alpha=0.05) from the actual ratios of extant individuals, indicating that this method conserves each sample's central tendency. When applied to the fossil samples, upper:lower limb-size proportions in A. afarensis are similar to those of humans (p=0.878) and are significantly different from all great ape proportions (p< or =0.034), while Australopithecus africanus is more similar to the apes (p> or =0.180) and significantly different from humans and A. afarensis (p< or =0.031). These results strongly support the hypothesis that A. africanus possessed more apelike limb-size proportions than A. afarensis, suggesting that A. africanus either evolved from a more postcranially primitive ancestor than A. afarensis or that the more apelike limb-size proportions of A. africanus were secondarily derived from an A. afarensis-like ancestor. Among the extant taxa, limb-size proportions correspond with observed levels of forelimb- and hindlimb-dominated positional behaviors. In conjunction with detailed anatomical features linked to arboreality, these results suggest that arboreal posture and locomotion may have been more important components of the A. africanus behavioral repertoire relative to that of A. afarensis.