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Evolution of bipedalism. The body form from which bipedalism evolved remains unknown. Scholars have proposed models based on (counterclockwise from top) brachiating hylobatids, pronograde monkeys, knuckle-walking African apes or quadrumanous orangutans. These models have deep roots in the anthropological literature and continue to be debated today. Figure based on Richmond et al. (2001), redrawn using PhyloPics Creative Commons Attribution-ShareAlike 3.0 Unported licence (https://creativecommons.org/licenses/by/3.0/), courtesy of Gareth Monger, T. Michael Keesey and Nobu Tamura.
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Bipedal locomotion is a hallmark of being human. Yet, the body form from which bipedalism evolved remains unclear. Specifically, the positional behavior (i.e., orthograde vs. pronograde) and the length of the lumbar spine (i.e., long and mobile vs. short and stiff) of the last common ancestor (LCA) of the African great apes and humans require furth...
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Previous research has suggested that magnitudes of integration may be distinct in the postcranium of hominoids when compared to other primate species. To test this hypothesis, we estimated and compared magnitudes of integration of eight postcranial bones from three-dimensional surface scans for 57 Hylobates lar, 58 Gorilla gorilla, 60 Pan troglodyt...
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... Their anatomy allows bipedal walking in the so-called 'bent-hip, bent-knee' posture, which acknowledges that they do not use extended limb postures as observed in humans (Alexander, 2004;Hirasaki et al., 2004;Ogihara et al., 2010;Demes and O'Neill, 2013;Pontzer et al., 2014;Demes et al., 2015;Thompson et al., 2015;O'Neill et al., 2018;Blickhan et al., 2021;Thompson et al., 2021). As recently shown in a comparative study in captivity, extant catarrhines (i.e., bonobos, chimpanzees, gorillas, orangutans, hylobatids, siamangs, baboons, and mandrills in this study) are using bipedal walking for very short bouts during their daily activities (Rosen et al., 2022; see also Rose, 1976;Hunt, 1994;Stanford, 2006;Thorpe et al., 2007;Druelle et al., 2016), but the evolutionary transition toward habitual bipedalism obviously required a stronger involvement into this mode. As a result, such a widespread behavior observed in many extant species let us suggest that it was also the case in Miocene hominoids. ...
Bipedal locomotion was a major functional change during hominin evolution, yet, our understanding of this gradual and complex process remains strongly debated. Based on fossil discoveries, it is possible to address functional hypotheses related to bipedal anatomy, however, motor control remains intangible with this approach. Using comparative models which occasionally walk bipedally has proved to be elevant to shed light on the evolutionary transition toward habitual bipedalism. Here, we explored the organization of the neuromuscular control using surface electromyography (sEMG) for six extrinsic muscles in two baboon individuals when they walk quadrupedally and bipedally on the ground. We compared their muscular coordination to five human subjects walking bipedally. We extracted muscle synergies from the sEMG envelopes using the nonnegative matrix factorization algorithm which allows decomposing the sEMG data in the linear combination of two non-negative matrixes (muscle weight vectors and activation coefficients). We calculated different parameters to estimate the complexity of the sEMG signals, the duration of the activation of the synergies, and the generalizability of the muscle synergy model across species and walking conditions. We found that the motor control strategy is less complex in baboons when they walk bipedally, with an increased muscular activity and muscle coactivation. When comparing the baboon bipedal and quadrupedal pattern of walking to human bipedalism, we observed that the baboon bipedal pattern of walking is closer to human bipedalism for both baboons, although substantial differences remain. Overall, our findings show that the muscle activity of a nonadapted biped effectively fulfills the basic mechanical requirements (propulsion and balance) for walking bipedally, but substantial refinements are possible to optimize the efficiency of bipedal locomotion. In the evolutionary context of an expanding reliance on bipedal behaviors, even minor morphological alterations, reducing muscle coactivation, could have faced strong selection pressure, ultimately driving bipedal evolution in hominins.
... Interestingly, all extant Catarrhini species, while largely relying on the quadrupedal locomotor system, are also capable of occasional bipedal walking and use this locomotor mode spontaneously in their daily activities (e.g. Druelle and Berillon, 2014;Rosen et al., 2022). The proportion of their bipedal walking has been widely quantified in natural (Carvalho et al., 2012;Hunt, 1994;Rose, 1976;Stanford, 2006;Wrangham, 1980) and experimental contexts (Druelle et al., 2016;McGrew, 2001, 2002), and also the kinematics of bipedalism in non-human primates has been studied extensively (e.g. ...
We investigated how baboons transition from quadrupedal to bipedal walking without any significant interruption in their forward movement (i.e. transition ‘on the fly’). Building on basic mechanical principles (momentum only changes when external forces/moments act on the body), insights into possible strategies for such a dynamical mode transition are provided and applied first to the recorded planar kinematics of an example walking sequence (including several continuous quadrupedal, transition and subsequent bipedal steps). Body dynamics are calculated from the kinematics. The strategy used in this worked example boils down to: crouch the hind parts and sprint them underneath the rising body centre of mass. Forward accelerations are not in play. Key characteristics of this transition strategy were extracted: progression speed, hip height, step duration (frequency), foot positioning at touchdown with respect to the hip and the body centre of mass (BCoM), and congruity between the moments of the ground reaction force about the BCoM and the rate of change of the total angular moment. Statistical analyses across the full sample (15 transitions of 10 individuals) confirm this strategy is always used and is shared across individuals. Finally, the costs (in J kg−1 m−1) linked to on the fly transitions were estimated. The costs are approximately double those of both the preceding quadrupedal and subsequent bipedal walking. Given the short duration of the transition as such (<1 s), it is argued that the energetic costs to change walking posture on the fly are negligible when considered in the context of the locomotor repertoire.
... Soft tissue plasticity developed over a lifetime of practice enables extreme ankle flexion well beyond what would cause injury or muscle tearing among non-climbing human populations . Not only can humans be surprisingly good climbers, but all extant apes can and do occasionally move bipedally on the ground (Rosen, Jones, & DeSilva, 2022); facultative bipedalism is one of a range of locomotor modes used by apes, including suspensory locomotion, arboreal bipedalism and quadrupedalism, brachiation and knuckle-walking (Napier & Napier, 1967). All primates walking bipedally need to maintain balance and stability while on one leg in the stance phase, propel that leg forward during the swing phase and deal with impact forces when the foot strikes the ground again, all as efficiently and 'cheaply' as possible. ...
... hylobatian model; Tuttle, 1974), but Rosen and colleagues explore whether or not it is arboreal individuals with these traits already who are most likely to move bipedally when on the ground. Among extant apes housed in zoos, they observed bipedal behaviour on the ground more often, and for longer per bout, among hylobatids than bonobos, chimpanzees, gorillas, orangutans or cercopithecines (Rosen et al., 2022). Of the 93 hylobatid individuals observed, bipedal bouts were documented among 83% of them, on average almost twice a day, and more than 35% of these bouts involved eight or more bipedal strides. ...
The form-function conceptual framework, which assumes a strong relationship between the structure of a particular trait and its function, has been crucial for understanding morphological variation and locomotion among extant and fossil species across many disciplines. In biological anthropology, it is the lens through which many important questions and hypotheses have been tackled with respect to relationships between morphology and locomotor kinematics, energetics, and performance. However, it is becoming increasingly evident that the morphologies of fossil hominins, apes, and humans can confer considerable locomotor diversity and flexibility, and can do so with a range of kinematics depending on soft tissue plasticity and environmental and cultural factors. This complexity is not built in to traditional biomechanical or mathematical models of relationships between structure and kinematics or energetics, limiting our interpretation of what bone structure is telling us about behaviour in the past. The nine papers presented in this Special Collection together address some of the challenges that variation in the relationship between form and function pose in evolutionary biomechanics, to better characterize the complexity linking structure and function and to provide tools through which we may begin to incorporate some of this complexity into our functional interpretations.
The dominant paradigm regarding human evolution since the split with Pan considers australopithecines as hominins, i.e., the closest relatives and/or direct ancestors of Homo. Historically, this paradigm started from the assumption that the Homo/Pan/Gorilla last common ancestor was a knuckle-walking ape that evolved into the fully upright (orthograde), obligate bipedal genus Homo, whereas Pan and Gorilla remained knuckle-walkers. Obligate terrestrial upright bipedalism, unique for our species, is an odd locomotor behaviour for a primate. Therefore, it had become generally accepted that a cooler and drier African climate had caused deforestation, which had forced our ancestors to develop upright bipedalism as an adaptation to living on open grassland savannah. This view, already held by Lamarck and Darwin, appeared most parsimonious in the almost complete absence of fossils. The discovery in the 20th century of australopithecine fossils, bipedal apes with small brains, in open country in southern and eastern Africa corroborated the savannah paradigm. Therefore, australopithecines are considered hominins. However, it is now recognized that most australopithecines instead lived in a mosaic of forests, grasslands and wetlands, and better knowledge of their fossils clearly indicates that they possessed several climbing adaptations. Moreover, none of the extinct ape species older than Australopithecus and Paranthropus for which postcranial remains have been described (e.g., Morotopithecus, Sahelanthropus, Orrorin, Ardipithecus) were knuckle-walking. On the other hand, upright posture/gait is already present to different degrees even in Miocene apes. Moreover, the notion that hominoid orthogrady is a primitive characteristic is corroborated by the growing consensus that knuckle-walking is not a primitive trait but has evolved in parallel, independently in both Pan and Gorilla. Consequently, it is possible that australopithecines are not transitional between a semi-erect ancestor and upright bipedal humans, but to the contrary, are intermediate between a more upright ancestor and extant semi-erect African apes. In summary, hypotheses that attempt to explain how a semi-erect Homo/Pan last common ancestor transitioned into the bipedal australopithecines as an adaptation to life on the savannah appear to be ill-conceived and moreover seem to have been superfluous from the very start. We review the numerous similarities between australopithecines and extant African apes, suggesting that they are possibly not hominins and therefore not our direct ancestors. We suggest that we may have been barking up the wrong ancestral tree, for almost a century.
Since the first discovery of human fossils in the mid‐19th century, two subjects—our phylogenetic relationship to living and fossil apes and the ancestral locomotor behaviors preceding bipedalism—have driven the majority of discourse in the study of human origins. With few fossils and thus limited comparative evidence available to inform or constrain them, morphologists of the 19th and early mid‐20th centuries posited a range of scenarios for the evolution of bipedalism. In contrast, there exists a rich hominin fossil record and the acceptance of Pan (chimpanzees and bonobos) as our closest living relatives is nearly universal, yet consensus about the ancestral condition from which hominins evolved remains elusive. Notably, while the earliest known hominins are generally congruent with parsimonious inferences of an African ape‐like last common ancestor, our more distantly related Miocene ape cousins are frequently invoked as evidence in favor of more complex scenarios that require substantial homoplasy. Debate over these alternatives suggests that how we infer ancestral nodes and weigh evidence to test their relative likelihoods remains a stumbling block. Here we argue that a key contributor to this impasse includes the history of terminology associated with positional behavior, which has become confused over the last century. We aim to clarify positional behavior concepts and contextualize knuckle‐walking and other forms of posture and locomotion chimpanzees and gorillas engage in, while arguing that the presence of homoplasy in ape evolution does not alter the weight of evidence in favor of an African ape‐like evolutionary history of hominins.
