Early Origin for Human-Like Precision Grasping: A Comparative Study of Pollical Distal Phalanges in Fossil Hominins

Background

The morphology of human pollical distal phalanges (PDP) closely reflects the adaptation of human hands for refined precision grip with pad-to-pad contact. The presence of these precision grip-related traits in the PDP of fossil hominins has been related to human-like hand proportions (i.e. short hands with a long thumb) enabling the thumb and finger pads to contact. Although this has been traditionally linked to the appearance of stone tool-making, the alternative hypothesis of an earlier origin—related to the freeing of the hands thanks to the advent of terrestrial bipedalism—is also possible given the human-like intrinsic hand proportion found in australopiths.

Methodology/Principal Findings

We perform morphofunctional and morphometric (bivariate and multivariate) analyses of most available hominin pollical distal phalanges, including Orrorin, Australopithecus, Paranthropous and fossil Homo, in order to investigate their morphological affinities. Our results indicate that the thumb morphology of the early biped Orrorin is more human-like than that of australopiths, in spite of its ancient chronology (ca. 6 Ma). Moreover, Orrorin already displays typical human-like features related to precision grasping.

Conclusions

These results reinforce previous hypotheses relating the origin of refined manipulation of natural objects–not stone tool-making–with the relaxation of locomotor selection pressures on the forelimbs. This suggests that human hand length proportions are largely plesiomorphic, in the sense that they more closely resemble the relatively short-handed Miocene apes than the elongated hand pattern of extant hominoids. With the advent of terrestrial bipedalism, these hand proportions may have been co-opted by early hominins for enhanced manipulative capabilities that, in turn, would have been later co-opted for stone tool-making in the genus Homo, more encephalized than the previous australopiths. This hypothesis remains may be further tested by the finding of more complete hands of unequivocally biped early hominins.     

Oreopithecus bambolii: an unlikely case of hominid-like grip capability in a Miocene ape

Oreopithecus bambolii, an ape from the late Miocene of Italy, is said to possess a hand capable of a precision grip like that of humans. Relative hand length, proportions of the thumb, and morphological features of the thumb and wrist were adduced to support the idea that Oreopithecus had a hand that closely matched the pattern in Australopithecus. A reappraisal of earlier arguments and comparisons of Oreopithecus with humans, apes, and Old World monkeys, reveals that Oreopithecus had an essentially ape-like hand that emphasized ape-like power grasping over human-like precision grasping.     

Cortical motor asymmetry and hominid feeding strategies

Homo sapiens differs from all other primates in having a cerebrum that is markedly asymmetrical for a number of functions, including motor control of the hands. The ability to coordinate the two hands while each is engaged in a different task is not highly developed in non-human primates and may have been an important behavioral distinction between early apes and hominids. These skills are necessary for tool-making, and probably arose selectively as a feeding strategy to exploit an expanding food base. It is proposed that bimanual motor asymmetry follows bipedalism but precedes tool-making as a step in the process of hominization.     

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.     

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.     

Precision grips,hand morphology,and tools

This study asks whether there are discernable links between precision gripping, tool behaviors,

1 The term “tool behavior” has been variously used in the literature, in some cases implying exclusively tool making distinctive of humans (Susman, 1991) and in others referring variably to tool using and/or tool-making abilities, some shared with us by other animals (Susman, 1988a,b, 1994). In this paper the term is used to include both tool using and tool making behaviors of humans and non-humans; the term “tool making” is used in place of “tool behavior” whenever the discussion is focused upon distinguishing a capacity for removing flakes from stone preforms from a more general capacity to manipulate stone tools.

and hand morphology in modern hominoids, which may guide functional interpretation of early hominid hand morphology. Findings from a three-pronged investigation answer this question in the affirmative, as follows. (1) Experimental manufacture of early prehistoric tools provides evidence of connections between distinctive human precision grips and effective tool making. (A connection is not found between the “fine” thumb/index finger pad precision grip and early tool making.) (2) Manipulative behavior studies of chimpanzees, hamadryas baboons, and humans show that human precision grips are distinguished by the greater force with which objects may be secured by the thumb and fingers of one hand (precision pinching) and the ability to adjust the orientation of gripped objects through movements at joints distal to the wrist (precision handling). (3) Morphological studies reveal eight features distinctive of modern humans which facilitate use of these grips. Among these features are substantially larger moment arms for intrinsic muscles that stabilize the proximal thumb joints. Examination of evidence for these reveals that three of the eight features occur in Australopithecus afarensis, but limited thumb mobility would have compromised tool making. Also, Olduvai hand morphology strongly suggests a capacity for stone tool making. However, functional and behavioral implications of Sterkfontein and Swartkrans hand morphology are less clear. At present, no single skeletal feature can be safely relied upon as an indicator of distinctively human capabilities for precision gripping or tool making in fossil hominids. Am J Phys Anthropol 102:91–110, 1997. © 1997 Wiley-Liss, Inc.     

Intrinsic hand proportions of euarchontans and other mammals: implications for the locomotor behavior of plesiadapiforms

Arboreal primates have distinctive intrinsic hand proportions compared with many other mammals. Within Euarchonta, platyrrhines and strepsirrhines have longer manual proximal phalanges relative to metacarpal length than colugos and terrestrial tree shrews. This trait is part of a complex of features allowing primates to grasp small-diameter arboreal substrates. In addition to many living and Eocene primates, relative elongation of proximal manual phalanges is also present in most plesiadapiforms. In order to evaluate the functional and evolutionary implications of manual similarities between crown primates and plesiadapiforms, we measured the lengths of the metacarpal, proximal phalanx, and intermediate phalanx of manual ray III for 132 extant mammal species (n=702 individuals). These data were compared with measurements of hands in six plesiadapiform species using ternary diagrams and phalangeal indices. Our analyses reveal that many arboreal mammals (including some tree shrews, rodents, marsupials, and carnivorans) have manual ray III proportions similar to those of various arboreal primates. By contrast, terrestrial tree shrews have hand proportions most similar to those of other terrestrial mammals, and colugos are highly derived in having relatively long intermediate phalanges. Phalangeal indices of arboreal species are significantly greater than those of the terrestrial species in our sample, reflecting the utility of having relatively long digits in an arboreal context. Although mammals known to be capable of prehensile grips demonstrate long digits relative to palm length, this feature is not uniquely associated with manual prehension and should be interpreted with caution in fossil taxa. Among plesiadapiforms, Carpolestes, Nannodectes, Ignacius, and Dryomomys have manual ray III proportions that are unlike those of most terrestrial species and most similar to those of various arboreal species of primates, tree shrews, and rodents. Within Euarchonta, Ignacius and Carpolestes have intrinsic hand proportions most comparable to those of living arboreal primates, while Nannodectes is very similar to the arboreal tree shrew Tupaia minor. These results provide additional evidence that plesiadapiforms were arboreal and support the hypothesis that Euarchonta originated in an arboreal milieu.     

Forelimb segment length proportions in extant hominoids and Australopithecus afarensis

Forelimb proportions have been used to infer locomotor adaptation in Australopithecus afarensis. However, little is known about proportions among individual forelimb segments in extant or fossil hominoids. The partial A. afarensis skeleton A.L. 438-1 and the more complete skeleton A.L. 288-1 provide the opportunity to assess relative length of the arm, forearm, wrist, and palm. We compare scaling relationships between pairs of forelimb bones of extant hominoids and A. afarensis, and length of individual forelimb elements to a body size surrogate. Hylobatids, and to a lesser extent orangutans, have the longest forelimb bones relative to size, although the carpus varies little among taxa, perhaps due to functional constraints of the wrist. Pan species are unique in having long metacarpals relative to ulnar length, demonstrating that they probably differ from the common chimp-human ancestor, and also that developmental mechanisms can be altered to results in differential growth of individual forelimb segments. A. afarensis has no forelimb bones that are significantly longer than those of humans for its size. It falls within the range of variation seen in modern humans for all comparisons relative to size, but appears to differ from the typical human brachial index due to a slightly shorter humerus and/or slightly longer ulna. It has short metacarpals like humans only among hominoids. Thus, while Pan may have elongated its metacarpus relative to ulnar length, A. afarensis may have reduced the length of its metacarpals and possibly its humerus relative to body size from the primitive condition.     

Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use

In this paper, we explore how the results of a primate-wide higher-level phylogenetic analysis of muscle characters can improve our understanding of the evolution and homologies of the forearm and hand muscles of modern humans. Contrary to what is often suggested in the literature, none of the forearm and hand muscle structures usually present in modern humans are autapomorphic. All are found in one or more extant non-human primate taxa. What is unique is the particular combination of muscles. However, more muscles go to the thumb in modern humans than in almost all other primates, reinforcing the hypothesis that focal thumb movements probably played an important role in human evolution. What makes the modern human thumb myology special within the primate clade is not so much its intrinsic musculature but two extrinsic muscles, extensor pollicis brevis and flexor pollicis longus, that are otherwise only found in hylobatids. It is likely that these two forearm muscles play different functional roles in hylobatids and modern humans. In the former, the thumb is separated from elongated digits by a deep cleft and there is no pulp-to-pulp opposition, whereas modern humans exhibit powerful thumb flexion and greater manipulative abilities, such as those involved in the manufacture and use of tools. The functional and evolutionary significance of a third peculiar structure, the intrinsic hand structure that is often called the ‘interosseous volaris primus of Henle’ (and which we suggest is referred to as the musculus adductor pollicis accessorius) is still obscure. The presence of distinct contrahentes digitorum and intermetacarpales in adult chimpanzees is likely the result of prolonged or delayed development of the hand musculature of these apes. In relation to these structures, extant chimpanzees are more neotenic than modern humans.     

Hand pressure distribution during Oldowan stone tool production

Modern humans possess a highly derived thumb that is robust and long relative to the other digits, with enhanced pollical musculature compared with extant apes. Researchers have hypothesized that this anatomy was initially selected for in early Homo in part to withstand high forces acting on the thumb during hard hammer percussion when producing stone tools. However, data are lacking on loads experienced during stone tool production and the distribution of these loads across the hand.Here we report the first quantitative data on manual normal forces (N) and pressures (kPa) acting on the hand during Oldowan stone tool production, captured at 200 Hz. Data were collected from six experienced subjects replicating Oldowan bifacial choppers. Our data do not support hypotheses asserting that the thumb experiences relatively high loads when making Oldowan stone tools. Peak normal force, pressure, impulse, and the pressure/time integral are significantly lower on the thumb than on digits 2 and/or digit 3 in every subject. Our findings call into question hypotheses linking modern human thumb robusticity specifically to load resistance during stone tool production.     

EMG study of hand muscle recruitment during hard hammer percussion manufacture of Oldowan tools

The activity of 17 hand muscles was monitored by electromyography (EMG) in three subjects during hard hammer percussion manufacture of Oldowan tools. Two of the subjects were archaeologists experienced in the replication of prehistoric stone tools. Simultaneous videotapes recorded grips associated with the muscle activities. The purpose of the study was to identify the muscles most likely to have been strongly and repeatedly recruited by early hominids during stone tool-making. This information is fundamental to the identification of skeletal features that may reliably predict tool-making capabilities in early hominids. The muscles most frequently recruited at high force levels for strong precision pinch grips required to control the hammerstone and core are the intrinsic muscles of the fifth finger and the thumb/index finger regions. A productive search for skeletal evidence of habitual Oldowan tool-making behavior will therefore be in the regions of the hand stressed by these intrinsic muscles and in the joint configurations affecting the relative lengths of their moment arms. Am J Phys Anthropol 105:315–332, 1998. © 1998 Wiley-Liss, Inc.     

Bipedalism as a brachiating adaptation

The original hominoid brachiators probably used typical terrestrial quadrupedalism as their alternate mode of ground locomotion. The development of knuckle walking made possible a shortening of the flexor muscles to improve the hand grip for more efficient arm-swinging. Bipedalism equally served to free the hands from palmar application to the ground, and likewise permits manual flexor shortening. Recently discovered australopithecine limb bones may be interpreted as indicating a primarily arboreal adaptation, emphasizing brachiation, with bipedalism being no more than an alternate mode. Thus the origin of bipedal adaptations might be viewed not as a means of leaving the trees, but rather as a step in perfecting brachiation.     

Shared Brain Lateralization Patterns in Language and Acheulean Stone Tool Production: A Functional Transcranial Doppler Ultrasound Study

Background

The popular theory that complex tool-making and language co-evolved in the human lineage rests on the hypothesis that both skills share underlying brain processes and systems. However, language and stone tool-making have so far only been studied separately using a range of neuroimaging techniques and diverse paradigms.

Methodology/Principal Findings

We present the first-ever study of brain activation that directly compares active Acheulean tool-making and language. Using functional transcranial Doppler ultrasonography (fTCD), we measured brain blood flow lateralization patterns (hemodynamics) in subjects who performed two tasks designed to isolate the planning component of Acheulean stone tool-making and cued word generation as a language task. We show highly correlated hemodynamics in the initial 10 seconds of task execution.

Conclusions/Significance

Stone tool-making and cued word generation cause common cerebral blood flow lateralization signatures in our participants. This is consistent with a shared neural substrate for prehistoric stone tool-making and language, and is compatible with language evolution theories that posit a co-evolution of language and manual praxis. In turn, our results support the hypothesis that aspects of language might have emerged as early as 1.75 million years ago, with the start of Acheulean technology.     

Does body posture influence hand preference in an ancestral primate model?

Background  

The origin of human handedness and its evolution in primates is presently under debate. Current hypotheses suggest that body posture (postural origin hypothesis and bipedalism hypothesis) have an important impact on the evolution of handedness in primates. To gain insight into the origin of manual lateralization in primates, we studied gray mouse lemurs, suggested to represent the most ancestral primate condition. First, we investigated hand preference in a simple food grasping task to explore the importance of hand usage in a natural foraging situation. Second, we explored the influence of body posture by applying a forced food grasping task with varying postural demands (sit, biped, cling, triped).     

Metacarpal proportions in Australopithecus africanus

Recent work has shown that, despite being craniodentally more derived, Australopithecus africanus had more apelike limb-size proportions than A. afarensis. Here, we test whether the A. africanus hand, as judged by metacarpal shaft and articular proportions, was similarly apelike. More specifically, did A. africanus have a short and narrow first metacarpal (MC1) relative to the other metacarpals? Proportions of both MC breadth and length were considered: the geometric mean (GM) of articular and midshaft measurements of MC1 breadth was compared to those of MC2-4, and MC1 length was compared to MC3 length individually and also to the GM of MC2 and 3 lengths. To compare the extant hominoid sample with an incomplete A. africanus fossil record (11 attributed metacarpals), a resampling procedure imposed sampling constraints on the comparative groups that produced composite intrahand ratios. Resampled ratios in the extant sample are not significantly different from actual ratios based on associated elements, demonstrating the methodological appropriateness of this technique. Australopithecus africanus metacarpals do not differ significantly from the great apes in the comparison of breadth ratios but are significantly greater than chimpanzees and orangutans in both measures of relative length. Conversely, A. africanus has a significantly smaller breadth ratio than modern humans, but does not significantly differ from this group in either measure of relative length. We conclude that the first metacarpals of A. africanus are more apelike in relative breadth while also being more humanlike in relative length, a finding consistent with previous work on A. afarensis hand proportions. This configuration would have likely promoted a high degree of manipulative dexterity, but the relatively slender, apelike first metacarpal suggests that A. africanus did not place the same mechanical demands on the thumb as more recent, stone-tool-producing hominins.     

Three-dimensional kinematics of capuchin monkey bipedalism

Capuchin monkeys are known to use bipedalism when transporting food items and tools. The bipedal gait of two capuchin monkeys in the laboratory was studied with three-dimensional kinematics. Capuchins progress bipedally with a bent-hip, bent-knee gait. The knee collapses into flexion during stance and the hip drops in height. The knee is also highly flexed during swing to allow the foot which is plantarflexed to clear the ground. The forefoot makes first contact at touchdown. Stride frequency is high, and stride length and limb excursion low. Hind limb retraction is limited, presumably to reduce the pitch moment of the forward-leaning trunk. Unlike human bipedalism, the bipedal gait of capuchins is not a vaulting gait, and energy recovery from pendulum-like exchanges is unlikely. It extends into speeds at which humans and other animals run, but without a human-like gait transition. In this respect it resembles avian bipedal gaits. It remains to be tested whether energy is recovered through cyclic elastic storage and release as in bipedal birds at higher speeds. Capuchin bipedalism has many features in common with the facultative bipedalism of other primates which is further evidence for restrictions on a fully upright striding gait in primates that transition to bipedalism. It differs from the facultative bipedalism of other primates in the lack of an extended double-support phase and short aerial phases at higher speeds that make it a run by kinematic definition. This demonstrates that facultative bipedalism of quadrupedal primates need not necessarily be a walking gait.     

制作工具在人类演化中的地位与作用

制作工具曾经被视作人类独有的行为能力,"人类"曾经据此而定义。但目前学术界将直立行走作为人类区别于其他灵长类最重要的体质与行为特征。少量其他动物种类,尤其是非人高等灵长类,也能使用工具乃至简单制作工具。如何认识制作工具在人类演化中的作用?人类制作工具的能力与其他动物有何区别?考古学是否有能力分辨人类的工具和其他灵长类的产品?本文通过对现代巴西猴群敲砸石头的行为及其产品、4300年前黑猩猩的"石制品"和早期人类石制品的比较研究,指出人类的工具与其他动物制作和使用的工具存在根本的区别;工具制作和使用对确定人类的演化方向,增强人类的适应生存能力,塑造人类的大脑与心智及行为方式,提升语言和交流能力,形成现代人类的身心和社会,至关重要,不可或缺。考古工作者一方面需要谨慎分辨、研究人类工具制作初期的产品,不使其与自然的产物和其他动作的作品相混淆,另一方面应该认识到人类工具制作在计划性、目的性、预见性、规范性和精美度上具有唯一性,有内在的智能控制、思维逻辑和规律可循。学科发展的积累和现代科技的支撑使考古学者具有多方面的利器,能够把人类工具制作的历史挖掘、复原出来,能够破译特定的石器技术和功能,进而将人类演化的历史画卷描绘得更加精细,更加完整。     

Side steps: the erratic pattern of hominin postcranial change through time

The hominin fossil record reveals brain-size expansion, canine reduction, premolar metaconid development, and numerous other craniodental features that become more human-like through time. In general, the postcranial skeleton also gets more human-like through time, but in some respects it does not. This is particularly apparent in the overall morphology of one of the most frequently preserved elements, the distal humerus. Some of the earliest hominins display quite human-like morphologies, whereas later specimens are quite unusual among extant species of Hominoidea: when described metrically and subjected to multivariate discriminant analyses in the context of large samples of extant hominoid humeri, the shapes of the earliest hominin fossils are more human-like than many of the later specimens. The Mahalanobis distances between many of the 1.5-2Ma hominin humeri and Homo sapiens are remarkably large. Many of the less well-represented postcranial specimens do not follow a linear path through time of increasing hominization either. This is particularly noticeable in the fore-to-hind limb joint-size proportions, ulnar morphology, and pelvic architecture. The hominin postcranial fossil record reveals many side-steps: there appears to be no simple march toward our human bodies, but a pattern better explained as adaptations to proximate conditions and constrained by ontogeny and history.     

Primate origins: evolutionary change in digital ray patterning and segmentation

This study presents evidence that the first primates share with extant lemurs, tarsiers, and anthropoids hand proportions unlike those of their close relatives, the tree shrews (Scandentia), colugos (Dermoptera), and plesiadapiforms. Specifically, early primates as well as modern strepsirhines and haplorhines have relatively short metacarpals and long proximal phalanges giving them a grasping, prehensile hand. Limb development was studied in the primate Microcebus murinus and a comparative sample of rodents, artiodactyls, and marsupials to investigate the role of embryonic patterning in the morphogenesis and evolution of primate hand proportions. Comparative analysis shows that the derived finger proportions of primates are generated during the early phases of digital ray patterning and segmentation, when the interzone cells marking the presumptive metacarpo- and interphalangeal joints first appear. Interspecific variation in relative digit and metapodial proportions therefore has high developmental penetrance; that is, adult differences are observed at early ontogenetic stages. The paleontological, comparative, and developmental data are therefore consistent with the hypothesis that the early Cenozoic origin of primates involved an evolutionary change in digital ray pattern formation ultimately yielding a grasping, prehensile hand.     

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.