Evolution of the rearfoot. A model of adaptation with evidence from the fossil record

Comparing lateral plantar process trabecular structure to other regions of the human calcaneus

Investigating skeletal adaptations to bipedalism informs our understanding of form-function relationships. The calcaneus is an important skeletal element to study because it is a weight-bearing bone with a critical locomotor role. Although other calcaneal regions have been well studied, we lack a clear understanding of the functional role of the lateral plantar process (LPP). The LPP is a bony protuberance on the inferolateral portion of the calcaneus thought to aid the tuberosity in transmission of ground reaction forces during heel-strike. Here, we analyze LPP internal trabecular structure relative to other calcaneal regions to investigate its potential functional affinities. Human calcanei (n = 20) were micro-CT scanned, and weighted spherical harmonic analysis outputs were used to position 251 volumes of interest (VOI) within each bone. Trabecular thickness (Tb.Th), spacing (Tb.Sp), degree of anisotropy (DA), and bone volume fraction (BV/TV) were calculated for each VOI. Similarities in BV/TV and DA (p = 0.2741) between the LPP and inferior tuberosity support suggestions that the LPP is a weight-bearing structure that may transmit forces in a similar direction. The LPP significantly differs from the inferior tuberosity in Tb.Th and Tb.Sp (p < 0.05). Relatively thinner, more closely spaced trabeculae in the LPP may serve to increase internal surface area to compensate for its relatively small size compared to the tuberosity. Significant differences in all parameters between LPP and joint articular surfaces indicate that trabecular morphology is differently adapted for the transmission of forces associated with body mass through joints.

Trabecular bone variation in the gorilla calcaneus

OBJECTIVES

Calcaneal external shape differs among nonhuman primates relative to locomotion. Such relationships between whole-bone calcaneal trabecular structure and locomotion, however, have yet to be studied. Here we analyze calcaneal trabecular architecture in Gorilla gorilla gorilla, Gorilla beringei beringei, and G. b. graueri to investigate general trends and fine-grained differences among gorilla taxa relative to locomotion.

MATERIALS AND METHODS

Calcanei were micro-CT scanned. A three-dimensional geometric morphometric sliding semilandmark analysis was carried out and the final landmark configurations used to position 156 volumes of interest. Trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and bone volume fraction (BV/TV) were calculated using the BoneJ plugin for ImageJ and MATLAB. Non-parametric MANOVAs were run to test for significant differences among taxa in parameter raw values and z-scores. Parameter distributions were visualized using color maps and summarized using principal components analysis.

RESULTS

There are no significant differences in raw BV/TV or Tb.Th among gorillas, however G. b. beringei significantly differs in z-scores for both parameters (p = <0.0271). All three taxa exhibit relatively lower BV/TV and Tb.Th in the posterior half of the calcaneus. This gradation is exacerbated in G. b. beringei. G. b. graueri significantly differs from other taxa in Tb.Sp z-scores (p < 0.001) indicating a different spacing distribution.

DISCUSSION

Relatively higher Tb.Th and BV/TV in the anterior calcaneus among gorillas likely reflects higher forces associated with body mass (transmitted through the subtalar joint) relative to forces transferred through the posterior calcaneus. The different Tb.Sp pattern in G. b. graueri may reflect proposed differences in foot positioning during locomotion.

Morphological integration in the hominid midfoot

The calculation of morphological integration across living apes and humans may provide important insights into the potential influence of integration on evolutionary trajectories in the hominid lineage. Here, we quantify magnitudes of morphological integration among and within elements of the midfoot in great apes and humans to examine the link between locomotor differences and trait covariance. We test the hypothesis that the medial elements of the great ape foot are less morphologically integrated with one another compared to humans based on their abducted halluces, and aim to determine how adaptations for midfoot mobility/stiffness and locomotor specialization influence magnitudes of morphological integration. The study sample is composed of all cuneiforms, the navicular, the cuboid, and metatarsals 1-5 of Homo sapiens (n = 80), Pan troglodytes (n = 63), Gorilla gorilla (n = 39), and Pongo sp. (n = 41). Morphological integration was quantified using the integration coefficient of variation of interlandmark distances organized into sets of a priori-defined modules. Magnitudes of integration across these modules were then compared against sets of random traits from the whole midfoot. Results show that all nonhuman apes have less integrated medial elements, whereas humans have highly integrated medial elements, suggesting a link between hallucal abduction and reduced levels of morphological integration. However, we find considerable variation in magnitudes of morphological integration across metatarsals 2-5, the intermediate and lateral cuneiform, the cuboid, and navicular, emphasizing the influence of functional and nonfunctional factors in magnitudes of integration. Lastly, we find that humans and orangutans show the lowest overall magnitudes of integration in the midfoot, which may be related to their highly specialized functions, and suggest a link between strong diversifying selection and reduced magnitudes of morphological integration.

A new early hominin calcaneus from Kromdraai (South Africa)

The Kromdraai site in South Africa has yielded numerous early hominin fossils since 1938. As a part of recent excavations within Unit P, a largely complete early hominin calcaneus (KW 6302) was discovered. Due to its role in locomotion, the calcaneus has the potential to reveal important form/function relationships. Here, we describe KW 6302 and analyze its preserved morphology relative to human and nonhuman ape calcanei, as well as calcanei attributed to Australopithecus afarensis, Australopithecus africanus, Australopithecus sediba, Homo naledi, and the Omo calcaneus (either Paranthropus or early Homo). KW 6302 calcaneal morphology is assessed using numerous quantitative metrics including linear measures, calcaneal robusticity index, relative lateral plantar process position, Achilles tendon length reconstruction, and a three-dimensional geometric morphometric sliding semilandmark analysis. KW 6302 exhibits an overall calcaneal morphology that is intermediate between humans and nonhuman apes, although closer to modern humans. KW 6302 possesses many traits that indicate it was likely well-adapted for terrestrial bipedal locomotion, including a relatively flat posterior talar facet and a large lateral plantar process that is similarly positioned to modern humans. It also retains traits that indicate that climbing may have remained a part of its locomotor repertoire, such as a relatively gracile tuber and a large peroneal trochlea. Specimens from Kromdraai have been attributed to either Paranthropus robustus or early Homo; however, there are no definitively attributed calcanei for either genus, making it difficult to taxonomically assign this specimen. KW 6302 and the Omo calcaneus, however, fall outside the range of expected variation for an extant genus, indicating that if the Omo calcaneus was Paranthropus, then KW 6302 would likely be attributed to early Homo (or vice versa).

Covariation of proximal finger and toe phalanges in Homo sapiens: A novel approach to assess covariation of serially corresponding structures

OBJECTIVES

As hands and feet are serially repeated corresponding structures in tetrapods, the morphology of fingers and toes is expected to covary due to a shared developmental origin. The present study focuses on the covariation of the shape of proximal finger and toe phalanges of adult Homo sapiens to determine whether covariation is different in the first ray relative to the others, as its morphology is also different.

MATERIAL AND METHODS

Proximal phalanges of 76 individuals of unknown sex (Muséum national d'Histoire naturelle, Paris, and the Natural History Museum, London) were digitized using a surface scanner. Landmarks were positioned on 3D surface models of the phalanges. Generalized Procrustes analysis and two-block partial least squares (PLS) analyses were conducted. A novel landmark-based geometric morphometric approach focusing on covariation is based on a PCoA of the angles between PLS axes in morphospace. The results can be statistically evaluated.

RESULTS

The difference in PCo scores between the first and the other rays indicates that the integration between the thumb and the big toe is different from that between the lateral rays of the hand and foot.

DISCUSSION

We speculate that the results are possibly the evolutionary consequence of differential selection pressure on the big toe relative to the other toes related to the rise of bipedalism, which is proposed to have emerged very early in the hominin clade. In contrast, thumb morphology and its precision grip never ceased undergoing changes, suggesting less acute selection pressures related to the evolution of the precision grip.

Evolutionary trends of the lateral foot in catarrhine primates: Contextualizing the fourth metatarsal of Australopithecus afarensis

In 2000, a complete fourth metatarsal (Mt4) of the ∼3- to 4-Million-year-old hominin Australopithecus afarensis was recovered in Hadar, Ethiopia. This metatarsal presented a mostly human-like morphology, suggesting that a rigid lateral foot may have evolved as early as ∼3.2 Ma. The lateral foot is integral in providing stability during the push off phase of gait and is key in understanding the transition to upright, striding bipedalism. Previous comparisons of this fossil were limited to Pan troglodytes, Gorilla gorilla, and modern humans. This study builds on previous studies by contextualizing the Mt4 morphology of A. afarensis (A.L. 333-160) within a diverse comparative sample of nonhuman hominoids (n = 144) and cercopithecids (n = 138) and incorporates other early hominins (n = 3) and fossil hominoids that precede the Pan-Homo split (n = 4) to better assess the polarity of changes in lateral foot morphology surrounding this divergence. We investigate seven morphological features argued to be functionally linked to human-like bipedalism. Our results show that some human-like characters used to assess midfoot and lateral foot stiffness in the hominin fossil record are present in our Miocene ape sample as well as in living cercopithecids. Furthermore, modern nonhuman hominoids can be generally distinguished from other species in most metrics. These results suggest that the possession of a rigid foot in hominins could represent a conserved trait, whereas the specialized pedal grasping mechanics of extant apes may be more derived, in which case some traits often used to infer bipedal locomotion in early hominins may, instead, reflect a lower reliance on pedal grasping. Another possibility is that early hominins reverted from modern ape Mt4 morphology into a more plesiomorphic condition when terrestrial bipedality became a dominant behavior. More fossils dating around the Pan-Homo divergence time are necessary to test these competing hypotheses.

Morphological differences in the calcaneus among extant great apes investigated by three-dimensional geometric morphometrics

Investigating the morphological differences of the calcaneus in humans and great apes is crucial for reconstructing locomotor repertories of fossil hominins. However, morphological variations in the calcaneus of the great apes (chimpanzees, gorillas, and orangutans) have not been sufficiently studied. This study aims to clarify variations in calcaneal morphology among great apes based on three-dimensional geometric morphometrics. A total of 556 landmarks and semilandmarks were placed on the calcaneal surface to calculate the principal components of shape variations among specimens. Clear interspecific differences in calcaneal morphology were extracted, corresponding to the degree of arboreality of the three species. The most arboreal orangutans possessed comparatively more slender calcaneal tuberosity and deeper pivot region of the cuboid articular surface than chimpanzees and gorillas. However, the most terrestrial gorillas exhibited longer lever arm of the triceps surae muscle, larger peroneal trochlea, more concave plantar surface, more inverted calcaneal tuberosity, more everted cuboid articular surface, and more prominent plantar process than the orangutans and chimpanzees. These interspecific differences possibly reflect the functional adaptation of the calcaneus to locomotor behavior in great apes. Such information might be useful for inferring foot functions and reconstructing the locomotion of fossil hominoids and hominids.

Scaling and relative size of the human, nonhuman ape, and baboon calcaneus

Among human and nonhuman apes, calcaneal morphology exhibits significant variation that has been related to locomotor behavior. Due to its role in weight-bearing, however, both body size and locomotion may impact calcaneal morphology. Determining how calcaneal morphologies vary as a function of body size is thus vital to understanding calcaneal functional adaptation. Here, we study calcaneus allometry and relative size in humans (n = 120) and nonhuman primates (n = 278), analyzing these relationships in light of known locomotor behaviors. Twelve linear measures and three articular facet surface areas were collected on calcaneus surface models. Body mass was estimated using femoral head superoinferior breadth. Relationships between calcaneal dimensions and estimated body mass were analyzed across the sample using phylogenetic least squares regression analyses (PGLS). Differences between humans and pooled nonhuman primates were tested using RMA ANCOVAs. Among (and within) genera residual differences from both PGLS regressions and isometry were analyzed using ANOVAs with post hoc multiple comparison tests. The relationships between all but two calcaneus dimensions and estimated body mass exhibit phylogenetic signal at the smallest taxonomic scale. This signal disappears when reanalyzed at the genus level. Calcaneal morphology varies relative to both body size and locomotor behavior. Humans have larger calcanei for estimated body mass relative to nonhuman primates as a potential adaptation for bipedalism. More terrestrial taxa exhibit longer calcaneal tubers for body mass, increasing the triceps surae lever arm. Among nonhuman great apes, more arboreal taxa have larger cuboid facet surface areas for body mass, increasing calcaneocuboid mobility.

Gorilla calcaneal morphological variation and ecological divergence

OBJECTIVES

The primate foot has been extensively investigated because of its role in weight-bearing; however, the calcaneus has been relatively understudied. Here we examine entire gorilla calcaneal external shape to understand its relationship with locomotor behavior.

MATERIALS AND METHODS

Calcanei of Gorilla gorilla gorilla (n = 43), Gorilla beringei graueri (n = 20), and Gorilla beringei beringei (n = 15) were surface or micro-CT scanned. External shape was analyzed through a three-dimensional geometric morphometric sliding semilandmark analysis. Semilandmarks were slid relative to an updated Procrustes average in order to minimize the bending energy of the thin plate spline interpolation function. Shape variation was summarized using principal components analysis of shape coordinates. Procrustes distances between taxa averages were calculated and resampling statistics run to test pairwise differences. Linear measures were collected and regressed against estimated body mass.

RESULTS

All three taxa exhibit statistically different morphologies (p < .001 for pairwise comparisons). G. g. gorilla demonstrates an anteroposteriorly elongated calcaneus with a deeper cuboid pivot region and mediolaterally flatter posterior talar facet. G. b. beringei possesses the flattest cuboid and most medially-angled posterior talar facets. G. b. graueri demonstrates intermediate articular facet morphology, a medially-angled tuberosity, and an elongated peroneal trochlea.

DISCUSSION

Articular facet differences separate gorillas along a locomotor gradient. G. g. gorilla is adapted for arboreality with greater joint mobility, while G. b. beringei is adapted for more stereotypical loads associated with terrestriality. G. b. graueri's unique posterolateral morphology may be due to a secondary transition to greater arboreality from a more terrestrial ancestor.

Morphometric analysis of the hominin talus: Evolutionary and functional implications

The adoption of bipedalism is a key benchmark in human evolution that has impacted talar morphology. Here, we investigate talar morphological variability in extinct and extant hominins using a 3D geometric morphometric approach. The evolutionary timing and appearance of modern human-like features and their contributions to bipedal locomotion were evaluated on the talus as a whole, each articular facet separately, and multiple combinations of facets. Distinctive suites of features are consistently present in all fossil hominins, despite the presence of substantial interspecific variation, suggesting a potential connection of these suites to bipedal gait. A modern human-like condition evolved in navicular and lateral malleolar facets early in the hominin lineage compared with other facets, which demonstrate more complex morphological variation within Homininae. Interestingly, navicular facet morphology of Australopithecus afarensis is derived in the direction of Homo, whereas more recent hominin species such as Australopithecus africanus and Australopithecus sediba retain more primitive states in this facet. Combining the navicular facet with the trochlea and the posterior calcaneal facet as a functional suite, however, distinguishes Australopithecus from Homo in that the medial longitudinal arch had not fully developed in the former. Our results suggest that a more everted foot and stiffer medial midtarsal region are adaptations that coincide with the emergence of bipedalism, whereas a high medial longitudinal arch emerges later in time, within Homo. This study provides novel insights into the emergence of talar morphological traits linked to bipedalism and its transition from a facultative to an obligate condition.

The influence of mobility strategy on the modern human talus

OBJECTIVES

The primate talus is known to have a shape that varies according to differences in locomotion and substrate use. While the modern human talus is morphologically specialized for bipedal walking, relatively little is known on how its morphology varies in relation to cultural and environmental differences across time. Here we compare tali of modern human populations with different subsistence economies and lifestyles to explore how cultural practices and environmental factors influence external talar shape.

MATERIALS AND METHODS

The sample consists of digital models of 142 tali from 11 archaeological and post-industrial modern human groups. Talar morphology was investigated through 3D (semi)landmark based geometric morphometric methods.

RESULTS

Our results show distinct differences between highly mobile hunter-gatherers and more sedentary groups belonging to a mixed post-agricultural/industrial background. Hunter-gatherers exhibit a more "flexible" talar shape, everted posture, and a more robust and medially oriented talar neck/head, which we interpret as reflecting long-distance walking strictly performed barefoot, or wearing minimalistic footwear, along uneven ground. The talus of the post-industrial population exhibits a "stable" profile, neutral posture, and a less robust and orthogonally oriented talar neck/head, which we interpret as a consequence of sedentary lifestyle and use of stiff footwear.

DISCUSSION

We suggest that talar morphological variation is related to the adoption of constraining footwear in post-industrial society, which reduces ankle range of motion. This contrasts with hunter-gatherers, where talar shape shows a more flexible profile, likely resulting from a lack of footwear while traversing uneven terrain. We conclude that modern human tali vary with differences in locomotor and cultural behavior.

The foot of Homo naledi

Modern humans are characterized by a highly specialized foot that reflects our obligate bipedalism. Our understanding of hominin foot evolution is, although, hindered by a paucity of well-associated remains. Here we describe the foot of Homo naledi from Dinaledi Chamber, South Africa, using 107 pedal elements, including one nearly-complete adult foot. The H. naledi foot is predominantly modern human-like in morphology and inferred function, with an adducted hallux, an elongated tarsus, and derived ankle and calcaneocuboid joints. In combination, these features indicate a foot well adapted for striding bipedalism. However, the H. naledi foot differs from modern humans in having more curved proximal pedal phalanges, and features suggestive of a reduced medial longitudinal arch. Within the context of primitive features found elsewhere in the skeleton, these findings suggest a unique locomotor repertoire for H. naledi, thus providing further evidence of locomotor diversity within both the hominin clade and the genus Homo.

Kinematics of primate midfoot flexibility

This study describes a unique assessment of primate intrinsic foot joint kinematics based upon bone pin rigid cluster tracking. It challenges the assumption that human evolution resulted in a reduction of midfoot flexibility, which has been identified in other primates as the "midtarsal break." Rigid cluster pins were inserted into the foot bones of human, chimpanzee, baboon, and macaque cadavers. The positions of these bone pins were monitored during a plantarflexion-dorsiflexion movement cycle. Analysis resolved flexion-extension movement patterns and the associated orientation of rotational axes for the talonavicular, calcaneocuboid, and lateral cubometatarsal joints. Results show that midfoot flexibility occurs primarily at the talonavicular and cubometatarsal joints. The rotational magnitudes are roughly similar between humans and chimps. There is also a similarity among evaluated primates in the observed rotations of the lateral cubometatarsal joint, but there was much greater rotation observed for the talonavicular joint, which may serve to differentiate monkeys from the hominines. It appears that the capability for a midtarsal break is present within the human foot. A consideration of the joint axes shows that the medial and lateral joints have opposing orientations, which has been associated with a rigid locking mechanism in the human foot. However, the potential for this same mechanism also appears in the chimpanzee foot. These findings demonstrate a functional similarity within the midfoot of the hominines. Therefore, the kinematic capabilities and restrictions for the skeletal linkages of the human foot may not be as unique as has been previously suggested.

A cross-disciplinary approach to understanding flatfoot

As form follows function, pedal anatomy is embedded in a history of evolution. This literature review seeks to further the understanding of physiologic and pathologic flatfoot through cross-disciplinary research of expired and extant members of the Homininae subfamily. Archaeological, anthropological, paleontological, and ontogenetic evidence presents multiple biomechanical similarities and anatomical parallels between flatfooted hominins and humans. Recognizing an evolutionary pattern in flatfoot pathologic disorders enhances anticipation and effective treatment.

Normal genetic variation of the human foot: part 1: the paradox of normal anatomical alignment in an evolutionary epigenetic context

Molecular genetics is changing our understanding of the developmental translation of genotype to phenotype between and within different phylogenetic groups. Together with a growing understanding of our own evolutionary relationships to common ancestors, the epigenetic processes involved enforce a reexamination of what is regarded as a normal foot structure. A revised populationist approach is proposed and supported by paleoanthropologic evidence that reflects a picture of emerging suitability for bipedalism that is driven by natural genetic divergence.

Metatarsal torsion in monkeys, apes, humans and australopiths

This paper presents an analysis of metatarsal torsion in apes, cercopithecoids and humans, compares australopiths with these species, and discusses their inferred foot morphology and function relative to prehensility, arboreality and the presence or absence of a longitudinal arch. Our results show that locomotor modes are reflected in metatarsal torsion values. Apes, which climb vertically with their foot inverted, have hallucal metatarsal heads that are turned toward the other toes and lateral toes that are inverted. Cercopithecoids, which tend to orient their feet in an axis more parallel to the line of motion, present signs of prehensility by having inverted 2nd metatarsals that oppose the hallux, while their two lateral-most metatarsals are strongly everted. Humans, with their rigid feet and longitudinal arches, have all toes that present their plantar surface toward the ground, resulting in hallucal and 2nd metatarsals that are relatively untwisted and the others that are strongly everted. Humans are different from all taxa only for the 2nd and 3rd metatarsal. It is hypothesized that the untwisted 2nd metatarsal reflects the lack of digit opposability of the medial foot and the strongly everted 3rd metatarsal reflects the longitudinal arch. Australopithecus afarensis was characterized by an everted lateral foot, the prerequisite for the development, but not necessarily an indicator, of a longitudinal arch. In Australopithecus africanus, torsion of fragmentary and complete 1st, 2nd, 3rd and 5th metatarsals suggest that the species did not have a foot with monkey- or ape-like prehensile capabilities and did not have a human-like longitudinal arch. In the Swartkrans remains, torsion is consistent with an unprehensile foot. The morphology of the fossils indicates that there was strong selection to orient the plantar surface of the toes facing the ground at the expense of a grasping foot and inversion ability.

The foot and ankle of Australopithecus sediba

A well-preserved and articulated partial foot and ankle of Australopithecus sediba, including an associated complete adult distal tibia, talus, and calcaneus, have been discovered at the Malapa site, South Africa, and reported in direct association with the female paratype Malapa Hominin 2. These fossils reveal a mosaic of primitive and derived features that are distinct from those seen in other hominins. The ankle (talocrural) joint is mostly humanlike in form and inferred function, and there is some evidence for a humanlike arch and Achilles tendon. However, Au. sediba is apelike in possessing a more gracile calcaneal body and a more robust medial malleolus than expected. These observations suggest, if present models of foot function are correct, that Au. sediba may have practiced a unique form of bipedalism and some degree of arboreality. Given the combination of features in the Au. sediba foot, as well as comparisons between Au. sediba and older hominins, homoplasy is implied in the acquisition of bipedal adaptations in the hominin foot.

Revisiting the "midtarsal break"

The midtarsal break was first described in this journal nearly 75 years ago to explain the ability of non-human primates to lift their heel independently of the rest of the foot. Since the initial description of the midtarsal break, the calcaneocuboid joint has been assumed to be the anatomical source of this motion. Recently, however, it has been suggested that the midtarsal break may occur at the cuboid-metatarsal joint, rather than at the calcaneocuboid joint. Data compiled from X-rays, dissections, manual manipulation of living primate feet, video of captive catarrhines, and osteological specimens concur that the midtarsal break is a complex motion caused by dorsiflexion at both joints with the cuboid-metatarsal joint contributing roughly 2/3 of total midfoot dorsiflexion, and the calcaneocuboid joint only about 1/3 of total midfoot dorsiflexion. The convexity of the proximal articular surface of the fourth and fifth metatarsals and corresponding concave cuboid facets provide skeletal correlates for the presence of midfoot dorsiflexion at the cuboid-metatarsal joint. Study of hominin metatarsals from Australopithecus afarensis, A. africanus, Homo erectus, and the metatarsals and a cuboid from the OH 8 foot show little capacity for dorsiflexion at the cuboid-metatarsal joint. These results suggest that hominins may have already evolved a stable midfoot region well adapted for the push-off phase of bipedalism by at least 3.2 million years ago. These data illuminate the evolution of the longitudinal arch and show further evidence of constraints on the arboreal capacity in early hominins.

Little Foot and big thoughts—a re-evaluation of the Stw573 foot from Sterkfontein, South Africa

The part of the fossil assemblage Stw573 consisting of some medial foot bones was initially reported by Clarke & Tobias (Science 269 (2002) 521). They found it to have both ape- and human-like qualities, being human-like proximally and ape-like distally. We have undertaken a re-examination of this pedal assemblage using a multivariate analysis; while we also found ape- and human-like qualities, they are in direct conflict with the original findings of Clarke and Tobias. We report an essentially ape-like morphology proximally and a human-like morphology distally; the talus and navicular were found to be ape-like and the medial cuneiform human-like. We also undertook a morphometric analysis of the medial cuneiform from the fossil assemblage OH8, as this was not included in the original OH8 study of Kidd et al (J Hum Evol 31 (1996) 269); this cuneiform was found to have a human-like morphology. Thus, the medial column findings from the two assemblages are very similar. This finding, coupled with the re-evaluation of the stratigraphy at Sterkfontein (Am J Phys Anthrop 119 (2002) 192), suggests that the two may have been contemporaneous. We also note that three broad patterns of modification have been identified, equating to proximal-distal lateral-medial (cranio-caudal) and dorsal plantar (posterior-anterior). It has not escaped our notice that these patterns are each controlled by specific genes or growth factors; we thus see a morphometric expression of our developmental past.

Fossils, feet and the evolution of human bipedal locomotion

We review the evolution of human bipedal locomotion with a particular emphasis on the evolution of the foot. We begin in the early twentieth century and focus particularly on hypotheses of an ape-like ancestor for humans and human bipedal locomotion put forward by a succession of Gregory, Keith, Morton and Schultz. We give consideration to Morton's (1935) synthesis of foot evolution, in which he argues that the foot of the common ancestor of modern humans and the African apes would be intermediate between the foot of Pan and Hylobates whereas the foot of a hypothetical early hominin would be intermediate between that of a gorilla and a modern human. From this base rooted in comparative anatomy of living primates we trace changing ideas about the evolution of human bipedalism as increasing amounts of postcranial fossil material were discovered. Attention is given to the work of John Napier and John Robinson who were pioneers in the interpretation of Plio-Pleistocene hominin skeletons in the 1960s. This is the period when the wealth of evidence from the southern African australopithecine sites was beginning to be appreciated and Olduvai Gorge was revealing its first evidence for Homo habilis. In more recent years, the discovery of the Laetoli footprint trail, the AL 288-1 (A. afarensis) skeleton, the wealth of postcranial material from Koobi Fora, the Nariokotome Homo ergaster skeleton, Little Foot (Stw 573) from Sterkfontein in South Africa, and more recently tantalizing material assigned to the new and very early taxa Orrorin tugenensis, Ardipithecus ramidus and Sahelanthropus tchadensis has fuelled debate and speculation. The varying interpretations based on this material, together with changing theoretical insights and analytical approaches, is discussed and assessed in the context of new three-dimensional morphometric analyses of australopithecine and Homo foot bones, suggesting that there may have been greater diversity in human bipedalism in the earlier phases of our evolutionary history than previously suspected.