Reply to Sarmiento E. “Australopithecine Taxonomy and Phylogeny and the Savanna Hypothesis; Comment on Vaneechoutte et al. Have We Been Barking up the Wrong Ancestral Tree? Australopithecines Are Probably Not Our Ancestors. Nat. Anthropol. 2023, 2, 10007” (original) (raw)

Paleoanthropology's uses of the bipedal criterion

History and Philosophy of the Life Sciences, 2018

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,

Have We Been Barking up the Wrong Ancestral Tree? Australopithecines Are Probably Not Our Ancestors

Nature Anthropology, 2023

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.

The first hominins and the origins of bipedalism

Evolution: Education & Outreach

Molecular and paleontological evidence now point to the last common ancestor between chimpanzees and modern humans living between five and seven million years ago. Any species considered to be more closely related to humans than chimpanzees we call hominins. Traditionally, early hominins have been conspicuous by their absence in the fossil record, but discoveries in the last 20 years have finally provided us with a number of very important finds. We currently have three described genera, Ardipithecus, Orrorin and Sahelanthropus, of which Ardipithecus is extremely well represented by cranial, dental, and postcranial remains. All three genera are argued to be hominins based on reduced canine size and an increased capacity for bipedal locomotion. The evolutionary relationships between these taxa and both earlier hominoids and later hominins are somewhat disputed, but this is to be expected for any species thought to be close to the root of the hominin lineage.

The arboreal origins of human bipedalism

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,

A Wading Component in the Origin of Hominin Bipedalism

Was Man More Aquatic in the Past? Fifty Years after Alister Hardy Waterside Hypotheses of Human Evolution, 2012

For over 150 years the field of palaeo-anthropology has grappled with several problems of understanding human evolution, notably those explaining key differences between human beings and our most closely related species, the African great apes. The first difference to be explained, perhaps in terms of importance but certainly in terms of chronology, is our bipedality. This chapter will review the models of hominin bipedal origins published to date, and categorize them, as was done by Rose [1], by the adaptive mechanism being suggested. In addition, it will propose a new evaluative framework against which each model may be assessed and compared. In this evaluation, published wading models appear to be among the strongest although they are among the least well reported in university-level text books, a discrepancy attributed here to their association with the so-called 'aquatic ape hypothesis' (AAH). Despite their apparent strengths, published wading models do nevertheless contain weaknesses. This chapter addresses a few of those weaknesses either theoretically or through studies, such as one obtaining new empirical data comparing the energy efficiency of different bipedal gaits in water. Furthermore, a series of falsifiable predictions of the wading hypothesis are made about the postcranial anatomy of australopithecines. The chapter concludes by proposing a specific timescale and ecological niche where such wading behavior could have provided a stable evolutionary scenario in early hominins that is compatible with the fossil record and other models of human evolution.

Bioenergetics and the origin of hominid bipedalism

American Journal of Physical Anthropology, 1980

Compared to most quadrupedal mammals, humans are energetically inefficient when running a t high speeds. This fact can be taken to mean that human bipedalism'evolved for reasons other than to reduce relative energy cost during locomotion. Recalculation of the energy expended during human walking a t normal speeds shows that 1) human bipedalism is a t least as efficient as typical mammalian quadrupedalism and 2) human gait is much more efficient than bipedal or quadrupedal locomotion in the chimpanzee. We conclude that bipedalism bestowed an energetic advantage on the Miocene hominoid ancestors of the Hominidae.