Chimpanzee carrying behaviour and the origins of human bipedality (original) (raw)

Almost a century and a half ago, Charles Darwin in The Descent of Man (1871: 141) highlighted the evolution of bipedalism as one of the key features of the human lineage, freeing the hands for carrying and for using and making tools. But how did it arise? The famous footprints from Laetoli in Tanzania show that hominin ancestors were walking upright by at least 3.65 million years ago. Recent work, however, suggests a much earlier origin for bipedalism, in a Miocene primate ancestor that was still predominantly tree-dwelling. Here Susannah Thorpe, Juliet McClymont and Robin Crompton set out the evidence for that hypothesis and reject the notion that the common ancestor of great apes and humans was a knuckle-walking terrestrial species, as are gorillas and chimpanzees today. The article is followed by a series of comments, rounded off by a reply from the authors. Theories regarding the origins of hominin bipedalism have spent some considerable time 'on the ground' as a result of the knuckle-walking hypothesis, which postulates that our earliest bipedal ancestor evolved from an ape that knuckle-walked on the ground in a way similar to modern chimpanzees or gorillas. By contrast, we argue that there is compelling and unequivocal evidence that bipedalism has arboreal origins. The concept of an arboreal origin for habitual human bipedalism was first proposed over a century ago. The arboreal behaviour that was considered to be exaptive (i.e. to have 'prepared' the body) for bipedalism has, however, changed fundamentally with the gradual discoveries of new fossil evidence, and with the development of new approaches to reconstructing the ecology and locomotion of extinct species. In particular, study of the ecology and biomechanics of living apes has transformed our understanding of how bipedalism could have evolved. Living apes offer broad models for how the dynamic between habitat and morphology may combine to influence locomotor behaviour. Sir Arthur Keith (1903) was the first to suggest that the arboreal locomotion of apes was important in understanding the process by which upright posture evolved in human ancestors. His studies of primate anatomy and behaviour led to the paradigm that an ape that moved by brachiating (arm-swinging) underneath branches (suspension) later evolved into a habitual biped (e.g. Morton 1922; Keith 1923). Morphological and locomotor observations continued to be proffered in support of this hypothesis for many decades (see Tuttle 1974 for a review). However,

In this article, I advance a novel hypothesis on the evolution of hominin bipedalism. I begin by arguing extensively for how the transition to bipedalism must have been problematic for hominins during the Neogene. Due to this and the fact that no other primate has made the unusual switch to bipedalism, it seems likely that the selection pressure towards bipedalism was unusually strong. With this in mind, I briefly lay out some of the most promising hypotheses on the evolutionary origin of hominin bipedalism and show how most, if not all, fail in the face of the need for an unusually strong selection pressure. For example, some hypotheses maintain that hominins became bipedal so they could use their hands for carrying infants, food, or other valuable objects. But extant apes are able to carry objects in one of their front limbs (while walking with the other three), and thus it does not seem plausible that our hominin ancestors went through the troublesome transition to bipedalism just so they could carry objects a little more efficiently. After I show that past hypotheses are wanting in the face of this challenge, I argue that there is only one selection pressure powerful enough to instigate a strange and problematic evolutionary adaptation like bipedalism, and that is sexual selection. Specifically, from the fact that bipedal locomotion is an important strategy for intimidating others and ascending the dominance hierarchy in extant apes, I argue that for no particular selective reason bipedal locomotion became a signal for high fitness (much as a large and intricate tail became a signal for high fitness for peahens), and this led to the trait being continuously reinforced in spite of all its deleterious fitness consequences.

SUMMARYAn understanding of the evolution of human bipedalism can provide valuable insights into the biomechanical and physiological characteristics of locomotion in modern humans. The walking gaits of humans, other bipeds and most quadrupedal mammals can best be described by using an inverted-pendulum model, in which there is minimal change in flexion of the limb joints during stance phase. As a result, it seems logical that the evolution of bipedalism in humans involved a simple transition from a relatively stiff-legged quadrupedalism in a terrestrial ancestor to relatively stiff-legged bipedalism in early humans. However, experimental studies of locomotion in humans and nonhuman primates have shown that the evolution of bipedalism involved a much more complex series of transitions, originating with a relatively compliant form of quadrupedalism. These studies show that relatively compliant walking gaits allow primates to achieve fast walking speeds using long strides, low stride fr...

Bipedalism is one of the criteria that paleoanthropologists use in order to interpret the fossil record and to determine if a specimen belongs to the human lineage. In the context of such interpretations, bipedalism is considered to be a unique characteristic of this lineage that also marks its origin. This conception has largely remained unchallenged over the last decades, in spite of fossil discoveries that led to the emergence of bipedalism in the human lineage being shifted back by several millions of years. In this paper, I analyze the uses of this criterion in paleoanthropology and demonstrate that interpretative biases (such as underdeter-mined inferences and circular reasoning) are at play in interpretations of hominin remains. By discussing Darwin's hypotheses about the evolution of bipedalism,