Interpreting the posture and locomotion ofAustralopithecus afarensis: Where do we stand?

Abstract

Reconstructing the transition to bipedality is key to understanding early hominin evolution. Because it is the best-known early hominin species, Australopithecus afarensis forms a baseline for interpreting locomotion in all early hominins. While most researchers agree that A. afarensis individuals were habitual bipeds, they disagree over the importance of arboreality for them. There are two main reasons for the disagreement. First, there are divergent perspectives on how to interpret primitive characters. Primitive traits may be retained by stabilizing selection, pleiotropy, or other ontogenetic mechanisms. Alternately, they could be in the process of being reduced, or they simply could be selectively neutral. Second, researchers are asking fundamentally different questions about the fossils. Some are interested in reconstructing the history of selection that shaped A. afarensis, while others are interested in reconstructing A. afarensis behavior. By explicitly outlining whether we are interested in reconstructing selective history or behavior, we can develop testable hypotheses to govern our investigations of the fossils. To infer the selective history that shaped a taxon, we must first consider character polarity. Derived traits that enhance a particular function, are found to be associated with that function in extant homologs, and that epigenetically sensitive data indicate were actually being used for that function, can be interpreted as adaptations. The null hypothesis to explain the retention of primitive traits is that of selective neutrality, or nonaptation. Disproving this requires demonstration of active stabilizing or negative selection (disaptation). Stabilizing selection can be inferred when primitive traits compromise a derived function clearly of adaptive value. Prolonged stasis, continued use of the trait for a particular function, or no change in variability in the trait are evidence that can support a hypothesis of adaptation for primitive traits, but still do not falsify the null hypothesis. Disaptation, or negative selection, should result in a trait being reduced or lost. To infer the behaviors of a fossil species, we must first determine its adaptations, use this to make hypotheses about its behavior, and test these hypotheses using epigenetically sensitive traits that are modified by an individual's activity pattern. When the A. afarensis data are evaluated using this framework, it is clear that these hominins had undergone selection for habitual bipedality, but the null hypothesis of nonaptation to explain the retention of primitive, ape-like characters cannot be falsified at present. The apparent stasis in Australopithecus postcranial form is currently the strongest evidence for stabilizing selection maintaining its primitive features. Evidence from features affected by individual behaviors during ontogeny shows that A. afarensis individuals were habitually traveling bipedally, but evidence presented for arboreal behavior so far is not conclusive. By clearly identifying the ques...

Taphonomic analysis of skeletal remains from chimpanzee hunts at Ngogo, Kibale National Park, Uganda

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“…Therefore, to maximize comparability of behavioral and ecological contexts when examining collections for these taphonomic signals, we suggest focusing on assemblages prior to the anatomical evidence for hominin obligate terrestriality. Although the degree to which early hominins were arboreal is currently unresolved, it is reasonable to suggest from the anatomy of the earliest hominins that they were at least occasionally in the trees (Richmond, 1998;Hailie-Selassie, 2001;Senut et al, 2001;Ward, 2002). We also suggest investigating sites with environments reconstructed as more wooded and wet with the trees necessary for arboreal monkeys and at least partially arboreal hominins, and particularly those sites where hominins and colobine monkeys co-occur.…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…It is well known that there is considerable and often rancorous debate about the nature of locomotion in early hominins: was it like modern humans, like that of chimpanzees, or something in between? It is not the goal of this paper to revisit this debate; the details can be found in the original works [7,26,27,42], in more recent summaries [10,11,21,[43][44][45][46][47] and a myriad of articles in between. With the recent description of Ar.…”

Laetoli footprints reveal bipedal gait biomechanics different from those of modern humans and chimpanzees

“…Yet the Hadar fossils have elicited conflicting conclusions and persistent debates over whether or not the anatomy of A. afarensis offers the earliest evidence of a human-like form of bipedalism [3 -6]. Continued analyses of A. afarensis skeletal morphology are unlikely to resolve these debates because of difficulties, uncertainties and long-standing disagreements over how to deduce habitual locomotor patterns from fossils that are rare and fragmentary, and that lack modern functional analogues [3].…”