Biomechanics and the origins of human bipedal walking: The last 50 years.

Motion analysis, as applied to evolutionary biomechanics, has experienced its own evolution over the last 50 years. Here we review how an ever-increasing fossil record, together with continuing advancements in biomechanics techniques, have shaped our understanding of the origin of upright bipedal walking. The original, and long-established hypothesis held by Lamarck (1809), Darwin (1859) and Keith (1934), amongst others, maintained that bipedality originated in an arboreal context. However, the first field studies of gorilla and chimpanzees from the 1960's, highlighted their so-called 'knucklewalking' quadrupedalism, leading scientists to assume, semi-automatically, that knucklewalking must have been the precursor to bipedality. It would not be until the discovery of skeletons of early human relatives Australopithecus afarensis and Australopithecus prometheus, and the inclusion of methods of analysis from computer science, biomechanics, sports science and medicine, that the knucklewalking hypothesis would be most robustly challenged. Their short, but human-like lower limbs and human-like hand indicated that knucklewalking was not part of our ancestral locomotor repertoire. Rather, most current research in evolutionary biomechanics agrees it was a combination of climbing and bipedalism, both in an arboreal context, which facilitated upright, terrestrial, bipedal walking over short distances.

A biomechanical investigation of the efficiency hypothesis of hafted tool technology.

The transition from hand-held to hafted tool technology marked a significant shift in conceptualizing the construction and function of tools. Among other benefits, hafting is thought to have given users a significant biomechanical and physiological advantage in undertaking basic subsistence tasks compared with hand-held tools. It is assumed that addition of a handle improved the (bio)mechanical properties of a tool and upper limb by offering greater amounts of leverage, force and precision. This controlled laboratory study compares upper limb kinematics, electromyography and physiological performance during two subsistence tasks (chopping, scraping) using hafted and hand-held tools. Results show that hafted tool use elicits greater ranges of motion, greater muscle activity and greater net energy expenditure (EE) compared with hand-held equivalents. Importantly, however, these strategies resulted in reduced relative EE compared with the hand-held condition in both tasks. More specifically, the hafted axe prompted use of two well-known biomechanical strategies that help produce larger velocities at the distal end of the limb without requiring heavy muscular effort, thus improving the tool's functional efficiency and relative energy use. The energetic and biomechanical benefits of hafting arguably contributed to both the invention and spread of this technology.

StW 573 Australopithecus prometheus: Its Significance for an Australopith Bauplan.

The StW 573 skeleton of Australopithecus prometheus from Sterkfontein Member 2 is some 93% complete and thus by far the most complete member of that genus yet found. Firmly dated at 3.67 Ma, it is one of the earliest specimens of its genus. A crucial aspect of interpretation of locomotor behaviour from fossil remains is an understanding of the palaeoenvironment in which the individual lived and the manner in which it would have used it. While the value of this ecomorphological approach is largely accepted, it has not been widely used as a stable framework on which to build evolutionary biomechanical interpretations. Here, we collate the available evidence on StW 573's anatomy in order, as far as currently possible, to reconstruct what might have been this individual's realized and potential niche. We explore the concept of a common Australopithecus "bauplan" by comparing the morphology and ecological context of StW 573 to that of paenocontemporaneous australopiths including Australopithecus anamensis and KSD-VP-1/1 Australopithecus afarensis. Each was probably substantially arboreal and woodland-dwelling, relying substantially on arboreal resources. We use a hypothesis-driven approach, tested by: virtual experiments, in the case of extinct species; biomechanical analyses of the locomotor behaviour of living great ape species; and analogical experiments with human subjects. From these, we conclude that the habitual locomotor mode of all australopiths was upright bipedalism, whether on the ground or on branches. Some later australopiths such as Australopithecus sediba undoubtedly became more terrestrial, allowing sacrifice of arboreal stability in favour of manual dexterity. Indeed, modern humans retain arboreal climbing skills but have further sacrificed arboreal effectiveness for enhanced ability to sustain striding terrestrial bipedalism over much greater distances. We compare StW 573's locomotor adaptations to those of living great apes and protohominins, and agree with those earlier observers who suggest that the common panin-hominin last common ancestor was postcranially more like Gorilla than Pan.

Intra-subject sample size effects in plantar pressure analyses.

BACKGROUND: Recent work using large datasets (>500 records per subject) has demonstrated seemingly high levels of step-to-step variation in peak plantar pressure within human individuals during walking. One intuitive consequence of this variation is that smaller sample sizes (e.g., 10 steps per subject) may be quantitatively and qualitatively inaccurate and fail to capture the variance in plantar pressure of individuals seen in larger data sets. However, this remains quantitatively unexplored reflecting a lack of detailed investigation of intra-subject sample size effects in plantar pressure analysis. METHODS: Here we explore the sensitivity of various plantar pressure metrics to intra-subject sample size (number of steps per subject) using a random subsampling analysis. We randomly and incrementally subsample large data sets (>500 steps per subject) to compare variability in three metric types at sample sizes of 5-400 records: (1) overall whole-record mean and maximum pressure; (2) single-pixel values from five locations across the foot; and (3) the sum of pixel-level variability (measured by mean square error, MSE) from the whole plantar surface. RESULTS: Our results indicate that the central tendency of whole-record mean and maximum pressure within and across subjects show only minor sensitivity to sample size >200 steps. However, <200 steps, and particularly <50 steps, the range of overall mean and maximum pressure values yielded by our subsampling analysis increased considerably resulting in potential qualitative error in analyses of pressure changes with speed within-subjects and in comparisons of relative pressure magnitudes across subjects at a given speed. Our analysis revealed considerable variability in the absolute and relative response of the single pixel centroids of five regions to random subsampling. As the number of steps analysed decreased, the absolute value ranges were highest in the areas of highest pressure (medial forefoot and hallux), while the largest relative changes were seen in areas of lower pressure (the midfoot). Our pixel-level measure of variability by MSE across the whole-foot was highly sensitive to our manipulation of sample size, such that the range in MSE was exponentially larger in smaller subsamples. Random subsampling showed that the range in pixel-level MSE only came within 5% of the overall sample size in subsamples of >400 steps. The range in pixel-level MSE at low subsamples (<50) was 25-75% higher than that of the full datasets of >500 pressure records per subject. Overall, therefore, we demonstrate a high probability that the very small sample sizes (n < 20 records), which are routinely used in human and animal studies, capture a relatively low proportion of variance evident in larger plantar pressure data set, and thus may not accurately reflect the true population mean.

The pectoral girdle of StW 573 ('Little Foot') and its implications for shoulder evolution in the Hominina.

The ca. 3.67 Ma adult skeleton known as 'Little Foot' (StW 573), recovered from Sterkfontein Member 2 breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula; an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa; and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows: smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e., A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431. Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and inferior angles. As the StW 573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of apes-Gorilla in particular-rather than modern humans, we suggest that the StW 573 morphological pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned above the head, more than human-like manipulatory capabilities. When compared with less complete pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1 (Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573 pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown hominin last common ancestor.

Keep your head down: Maintaining gait stability in challenging conditions.

BACKGROUND: Peripheral vision often deteriorates with age, disrupting our ability to maintain normal locomotion. Laboratory based studies have shown that lower visual field loss, in particular, is associated with changes in gaze and gait behaviour whilst walking and this, in turn, increases the risk of falling in the elderly. Separately, gaze and gait behaviours change and fall risk increases when walking over complex surfaces. It seems probable, but has not yet been established, that these challenges to stability interact. RESEARCH QUESTION: How does loss of the lower visual field affect gaze and gait behaviour whilst walking on a variety of complex surfaces outside of the laboratory? Specifically, is there a synergistic interaction between the effects on behaviour of blocking the lower visual field and increased surface complexity? METHODS: We compared how full vision versus simulated lower visual field loss affected a diverse range of behavioural measures (head pitch angle, eye angle, muscle coactivation, gait speed and walking smoothness as measured by harmonic ratios) in young participants. Participants walked over a range of surfaces of different complexity, including pavements, grass, steps and pebbles. RESULTS: In both full vision and blocked lower visual field conditions, surface complexity influenced gaze and gait behaviour. For example, more complex surfaces were shown to be associated with lowered head pitch angles, increased leg muscle coactivation, reduced gait speed and decreased walking smoothness. Relative to full vision, blocking the lower visual field caused a lowering of head pitch, especially for more complex surfaces. However, crucially, muscle coactivation, gait speed and walking smoothness did not show a significant change between full vision and blocked lower visual field conditions. Finally, head pitch angle, muscle coactivation, gait speed and walking smoothness were all correlated highly with each other. SIGNIFICANCE: Our study showed that blocking the lower visual field did not significantly change muscle coactivation, gait speed or walking smoothness. This suggests that young people cope well when walking with a blocked lower visual field, making minimal behavioural changes. Surface complexity had a greater effect on gaze and gait behaviour than blocking the lower visual field. Finally, head pitch angle was the only measure that showed a significant synergistic interaction between surface complexity and blocking the lower visual field. Together our results indicate that, first, a range of changes occur across the body when people walk over more complex surfaces and, second, that a relatively simple behavioural change (to gaze) suffices to maintain normal gait when the lower visual field is blocked, even in more challenging environments. Future research should assess whether young people cope as effectively when several impairments are simulated, representative of the comorbidities found with age.

Physical and perceptual measures of walking surface complexity strongly predict gait and gaze behaviour.

BACKGROUND: Walking surfaces vary in complexity and are known to affect stability and fall risk whilst walking. However, existing studies define surfaces through descriptions only. OBJECTIVE: This study used a multimethod approach to measure surface complexity in order to try to characterise surfaces with respect to locomotor stability. METHODS: We assessed how physical measurements of walking surface complexity compared to participant's perceptual ratings of the effect of complexity on stability. Physical measurements included local slope measures from the surfaces themselves and shape complexity measured using generated surface models. Perceptual measurements assessed participants' perceived stability and surface roughness using Likert scales. We then determined whether these measurements were indicative of changes to stability as assessed by behavioural changes including eye angle, head pitch angle, muscle coactivation, walking speed and walking smoothness. RESULTS: Physical and perceptual measures were highly correlated, with more complex surfaces being perceived as more challenging to stability. Furthermore, complex surfaces, as defined from both these measurements, were associated with lowered head pitch, increased muscle coactivation and reduced walking smoothness. SIGNIFICANCE: Our findings show that walking surfaces defined as complex, based on physical measurements, are perceived as more challenging to our stability. Furthermore, certain behavioural measures relate better to these perceptual and physical measures than others. Crucially, for the first time this study defined walking surfaces objectively rather than just based on subjective descriptions. This approach could enable future researchers to compare results across walking surface studies. Moreover, perceptual measurements, which can be collected easily and efficiently, could be used as a proxy for estimating behavioural responses to different surfaces. This could be particularly valuable when determining risk of instability when walking for individuals with compromised stability.

Look out: an exploratory study assessing how gaze (eye angle and head angle) and gait speed are influenced by surface complexity.

BACKGROUND: Most research investigating the connection between walking and visual behaviour has assessed only eye movements (not head orientation) in respect to locomotion over smooth surfaces in a laboratory. This is unlikely to reflect gaze changes found over the complex surfaces experienced in the real world, especially given that eye and head movements have rarely been assessed simultaneously. RESEARCH QUESTION: How does gaze (eye and head) angle and gait speed change when walking over surfaces of different complexity? METHODS: In this exploratory study, we used a mobile eye tracker to monitor eye movements and inertia measurement unit sensors (IMUs) to measure head angle whilst subjects (n = 11) walked over surfaces with different complexities both indoors and outdoors. Gait speed was recorded from ankle IMUs. RESULTS: Overall, mean gaze angle was lowest over the most complex surface and this surface also elicited the slowest mean gait speed. The head contributed increasingly to the lowering of gaze with increased surface complexity. Less complex surfaces showed no significant difference between gaze and gait behaviour. SIGNIFICANCE: This study supports previous research showing that increased surface complexity is an important factor in determining gaze and gait behaviour. Moreover, it provides the novel finding that head movements provide important contributions to gaze location. Our future research aims are to further assess the role of the head in determining gaze location during locomotion across a greater range of complex surfaces to determine the key surface characteristics that influence gaze during gait.

The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism.

Functional morphology of the atlas reflects multiple aspects of an organism's biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 ('Little Foot') skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.

A Three-Dimensional Musculoskeletal Model of the Western Lowland Gorilla Foot: Examining Muscle Torques and Function.

Due to difficulty of obtaining accurate quantitative data on foot muscles, relatively little has been done to study foot muscle function in non-human apes. Gorilla feet are known to be similar in bony proportions and mechanics to those of humans, hence are key to understanding human foot evolution and its ecological context. We present the first 3D musculoskeletal computer model of a western lowland gorilla foot, giving muscle torques about the tarsometatarsal, metatarsophalangeal and interphalangeal joints of digits 2-5. Peak flexor torque around the fifth metatarsophalangeal joint occurs at a highly flexed position, suggesting an ability to maintain flexed postures around lateral metatarsophalangeal joints, useful for grasping vertical supports. For distal interphalangeal joints, flexor torques peaked the more medial the digit at relatively flexed postures. We report, for the first time, interossei acting upon proximal and distal interphalangeal joints. All these facilitate maintenance of flexed positions around distal interphalangeal joints, likely used for grasping of small supports/objects. Humans lack these features, suggesting that semi-arboreal early hominins made less use of the peripheral canopy than gorillines. Information here could be used in gorilla enclosure design to encourage wild-type locomotor repertoires in captivity.

The long limb bones of the StW 573 Australopithecus skeleton from Sterkfontein Member 2: Descriptions and proportions.

Due to its completeness, the A.L. 288-1 ('Lucy') skeleton has long served as the archetypal bipedal Australopithecus. However, there remains considerable debate about its limb proportions. There are three competing, but not necessarily mutually exclusive, explanations for the high humerofemoral index of A.L. 288-1: (1) a retention of proportions from an Ardipithecus-like chimp/human last common ancestor (CLCA); (2) indication of some degree of climbing ability; (3) allometry. Recent discoveries of other partial skeletons of Australopithecus, such as those of Australopithecus sediba (MH1 and MH2) and Australopithecus afarensis (KSD-VP-1/1 and DIK-1/1), have provided new opportunities to test hypotheses of early hominin body size and limb proportions. Yet, no early hominin is as complete (>90%), as is the ∼3.67 Ma 'Little Foot' (StW 573) skeleton from Sterkfontein Member 2. Here, we provide the first descriptions of its upper and lower long limb bones, as well as a comparative context of its limb proportions. We found that StW 573 possesses absolutely longer limb lengths than A.L. 288-1, but both skeletons show similar limb proportions. This finding seems to argue against a purely allometric explanation for A.L. 288-1 limb proportions. In fact, our multivariate allometric analysis suggests that limb lengths of Australopithecus, as represented by StW 573 and A.L. 288-1, exhibit a significantly different (p < 0.001) allometric pattern than that which typifies modern humans and African apes. Like some previous analyses, our results also suggest that hominin limb evolution occurred in two stages with: first, a modest increase in lower limb length and a concurrent shortening of the antebrachium between Ardipithecus and Australopithecus, followed by a considerable lengthening of the lower limb along with a decrease of both upper limb elements occurring between Australopithecus and Homo sapiens.

A multiscale stratigraphic investigation of the context of StW 573 'Little Foot' and Member 2, Sterkfontein Caves, South Africa.

The Sterkfontein Caves is currently the world's richest Australopithecus-bearing site. Included in Sterkfontein's hominin assemblage is StW 573 ('Little Foot'), a near-complete Australopithecus skeleton discovered in Member 2 in the Silberberg Grotto. Because of its importance to the fossil hominin record, the geological age of StW 573 has been the subject of significant debate. Three main hypotheses have been proposed regarding the formation and age of Member 2 and by association StW 573. The first proposes that Member 2 (as originally defined in the type section in the Silberberg Grotto) started to accumulate at around 2.58 Ma and that the unit is contained within the Silberberg Grotto. The second proposes that Member 2 started forming before 3.67 ± 0.16 Ma and that the deposit extends into the Milner Hall and close to the base of the cave system. The third proposes a 'two-stage burial scenario', in which some sediments and StW 573 represent a secondary and mixed-age accumulation reworked from a higher cave. The stratigraphic and sedimentological implications of these hypotheses are tested here through the application of a multiscale investigation of Member 2, with reference to the taphonomy of the StW 573 skeleton. The complete infilling sequence of Member 2 is described across all exposures of the deposit in the Silberberg Grotto and into the Milner Hall. Sediments are generally stratified and conformably deposited in a sequence of silty sands eroded from well-developed lateritic soils on the landscape surface. Voids, clasts and bioclasts are organized consistently across and through Member 2 conforming with the underlying deposit geometry, indicating gradual deposit accretion with no distinct collapse facies evident and only localized intra-unit postdepositional modification. The stratigraphy and sedimentology of Member 2 support a simple single-stage accumulation process of Member 2 and a primary association between the sediments of Member 2 and the StW 573 'Little Foot' skeleton.

The bony labyrinth of StW 573 ("Little Foot"): Implications for early hominin evolution and paleobiology.

Because of its exceptional degree of preservation and its geological age of ∼3.67 Ma, StW 573 makes an invaluable contribution to our understanding of early hominin evolution and paleobiology. The morphology of the bony labyrinth has the potential to provide information about extinct primate taxonomic diversity, phylogenetic relationships and locomotor behaviour. In this context, we virtually reconstruct and comparatively assess the bony labyrinth morphology in StW 573. As comparative material, we investigate 17 southern African hominin specimens from Sterkfontein, Swartkrans and Makapansgat (plus published data from two specimens from Kromdraai B), attributed to Australopithecus, early Homo or Paranthropus, as well as 10 extant human and 10 extant chimpanzee specimens. We apply a landmark-based geometric morphometric method for quantitatively assessing labyrinthine morphology. Morphology of the inner ear in StW 573 most closely resembles that of another Australopithecus individual from Sterkfontein, StW 578, recovered from the Jacovec Cavern. Within the limits of our sample, we observe a certain degree of morphological variation in the Australopithecus assemblage of Sterkfontein Member 4. Cochlear morphology in StW 573 is similar to that of other Australopithecus as well as to Paranthropus specimens included in this study, but it is substantially different from early Homo. Interestingly, the configuration of semicircular canals in Paranthropus specimens from Swartkrans differs from other fossil hominins, including StW 573. Given the role of the cochlea in the sensory-driven interactions with the surrounding environment, our results offer new perspectives for interpreting early hominin behaviour and ecology. Finally, our study provides additional evidence for discussing the phylogenetic polarity of labyrinthine traits in southern African hominins.

The endocast of StW 573 ("Little Foot") and hominin brain evolution.

One of the most crucial debates in human paleoneurology concerns the timing and mode of the emergence of the derived cerebral features in the hominin fossil record. Given its exceptional degree of preservation and geological age (i.e., 3.67 Ma), StW 573 ('Little Foot') has the potential to shed new light on hominin brain evolution. Here we present the first detailed comparative description of the external neuroanatomy of StW 573. The endocast was virtually reconstructed and compared to ten southern African hominin specimens from Makapansgat, Malapa, Sterkfontein and Swartkrans attributed to Australopithecus and Paranthropus. We apply an automatic method for the detection of sulcal and vascular imprints. The endocranial surface of StW 573 is crushed and plastically deformed in a number of locations. The uncorrected and therefore minimum cranial capacity estimate is 408 cm3 and plots at the lower end of Australopithecus variation. The endocast of StW 573 approximates the rostrocaudally elongated and dorsoventrally flattened endocranial shape seen in Australopithecus and displays a distinct left occipital petalia. StW 573 and the comparative early hominin specimens share a similar sulcal pattern in the inferior region of the frontal lobes that also resembles the pattern observed in extant chimpanzees. The presumed lunate sulcus in StW 573 is located above the sigmoid sinus, as in extant chimpanzees, while it is more caudally positioned in SK 1585 and StW 505. The middle branch of the middle meningeal vessels derives from the anterior branch, as in MH 1, MLD 37/38, StW 578. Overall, the cortical anatomy of StW 573 displays a less derived condition compared to the late Pliocene/early Pleistocene southern African hominins (e.g., StW 505, SK 1585).

A 3D musculoskeletal model of the western lowland gorilla hind limb: moment arms and torque of the hip, knee and ankle.

Three-dimensional musculoskeletal models have become increasingly common for investigating muscle moment arms in studies of vertebrate locomotion. In this study we present the first musculoskeletal model of a western lowland gorilla hind limb. Moment arms of individual muscles around the hip, knee and ankle were compared with previously published data derived from the experimental tendon travel method. Considerable differences were found which we attribute to the different methodologies in this specific case. In this instance, we argue that our 3D model provides more accurate and reliable moment arm data than previously published data on the gorilla because our model incorporates more detailed consideration of the 3D geometry of muscles and the geometric constraints that exist on their lines-of-action about limb joints. Our new data have led us to revaluate the previous conclusion that muscle moment arms in the gorilla hind limb are optimised for locomotion with crouched or flexed limb postures. Furthermore, we found that bipedalism and terrestrial quadrupedalism coincided more regularly with higher moment arms and torque around the hip, knee and ankle than did vertical climbing. This indicates that the ability of a gorilla to walk bipedally is not restricted by musculoskeletal adaptations for quadrupedalism and vertical climbing, at least in terms of moment arms and torque about hind limb joints.

Practice makes perfect: Performance optimisation in 'arboreal' parkour athletes illuminates the evolutionary ecology of great ape anatomy.

An animal's size is central to its ecology, yet remarkably little is known about the selective pressures that drive this trait. A particularly compelling example is how ancestral apes evolved large body mass in such a physically and energetically challenging environment as the forest canopy, where weight-bearing branches and lianas are flexible, irregular and discontinuous, and the majority of preferred foods are situated on the most flexible branches at the periphery of tree crowns. To date the issue has been intractable due to a lack of relevant fossil material, the limited capacity of the fossil record to reconstruct an animal's behavioural ecology and the inability to measure energy consumption in freely moving apes. We studied the oxygen consumption of parkour athletes while they traversed an arboreal-like course as an elite model ape, to test the ecomorphological and behavioural mechanisms by which a large-bodied ape could optimize its energetic performance during tree-based locomotion. Our results show that familiarity with the arboreal-like course allowed the athletes to substantially reduce their energy expenditure. Furthermore, athletes with larger arm spans and shorter legs were particularly adept at finding energetic savings. Our results flesh out the scanty fossil record to offer evidence that long, strong arms, broad chests and a strong axial system, combined with the frequent use of uniform branch-to-branch arboreal pathways, were critical to off-setting the mechanical and energetic demands of large mass in ancestral apes.

The nature of functional variability in plantar pressure during a range of controlled walking speeds.

During walking, variability in step parameters allows the body to adapt to changes in substrate or unexpected perturbations that may occur as the feet interface with the environment. Despite a rich literature describing biomechanical variability in step parameters, there are as yet no studies that consider variability at the body-environment interface. Here, we used pedobarographic statistical parametric mapping (pSPM) and two standard measures of variability, mean square error (m.s.e.) and the coefficient of variation (CV), to assess the magnitude and spatial variability in plantar pressure across a range of controlled walking speeds. Results by reduced major axis, and pSPM regression, revealed no consistent linear relationship between m.s.e. and speed or m.s.e. and Froude number. A positive linear relationship, however, was found between CV and walking speed and CV and Froude number. The spatial distribution of variability was highly disparate when assessed by m.s.e. and CV: relatively high variability was consistently confined to the medial and lateral forefoot when measured by m.s.e., while the forefoot and heel show high variability when measured by CV. In absolute terms, variability by CV was universally low (less than 2.5%). From these results, we determined that variability as assessed by m.s.e. is independent of speed, but dependent on speed when assessed by CV.

The hominins: a very conservative tribe? Last common ancestors, plasticity and ecomorphology in Hominidae. Or, What's in a name?

In the early 20th century the dominant paradigm for the ecological context of the origins of human bipedalism was arboreal suspension. In the 1960s, however, with recognition of the close genetic relationship of humans, chimpanzees and bonobos, and with the first field studies of mountain gorillas and common chimpanzees, it was assumed that locomotion similar to that of common chimpanzees and mountain gorillas, which appeared to be dominated by terrestrial knuckle-walking, must have given rise to human bipedality. This paradigm has been popular, if not universally dominant, until very recently. However, evidence that neither the knuckle-walking or vertical climbing of these apes is mechanically similar to human bipedalism, as well as the hand-assisted bipedality and orthograde clambering of orang-utans, has cast doubt on this paradigm. It now appears that the dominance of terrestrial knuckle-walking in mountain gorillas is an artefact seen only in the extremes of their range, and that both mountain and lowland gorillas have a generalized orthogrady similar to that seen in orang-utans. These data, together with evidence for continued arboreal competence in humans, mesh well with an increasing weight of fossil evidence suggesting that a mix of orang-utan and gorilla-like arboreal locomotion and upright terrestrial bipedalism characterized most australopiths. The late split date of the panins, corresponding to dates for separation of Homo and Australopithecus, leads to the speculation that competition with chimpanzees, as appears to exist today with gorillas, may have driven ecological changes in hominins and perhaps cladogenesis. However, selection for ecological plasticity and morphological conservatism is a core characteristic of Hominidae as a whole, including Hominini.

Locomotor Ecology of Propithecus verreauxi in Kirindy Mitea National Park.

The locomotor behaviour of 2 groups of Propithecus verreauxi (Verreaux's sifaka) was studied over an 8-month period in Kirindy Mitea National Park (KMNP), Madagascar. This paper assesses the major characteristics of their locomotion, focusing on the extent that seasonal variation in climate and habitat, and local variation in habitat, is reflected in changes in locomotor behaviour. P. verreauxi is a committed leaper with a strong preference for vertical and angled supports. We found clear between-group differences in support orientation and diameter suggesting local variation in habitat. During the dry season, P. verreauxi utilizes smaller-diameter supports than in the rainy season. While this difference cannot yet be ascribed to any single cause, we discuss the factors which may contribute to this result.