A possible locomotor bauplan for Australopithecus informed by an ecomorphology perspective applied to StW 573 Au. prometheus, Sterkfontein Member 2, ~3.67 Ma

2022 
The StW 573 skeleton of Australopithecus prometheus from Sterkfontein Member 2 is some 93% complete and thus by far the most complete member of that genus yet found. Firmly dated at 3.67 Ma it is one of the earliest of its genus. A crucial aspect of interpretation of locomotor behaviour from fossil remains, is an understanding of the palaeoenvironment in which individuals lived and the manner in which they would have used it. While the value of this ecomorphological approach is largely accepted, it has not been widely used, as a stable framework on which to build evolutionary biomechanical interpretations. Here, we collate the available evidence on StW 573’s anatomy in order to reconstruct what might have been this individual’s realised and potential niche. We explore the concept of a common australopithecus bauplan by comparing the morphology and ecological context of StW 573 to that of paenocontemporaneous australopiths including Au. anamensis and KSD-VP-1/1 Au. afarensis. Each was probably substantially arboreal and woodland-dwelling, relying substantially on arboreal resources. Using an hypothesis-driven approach, tested by virtual experiments, in the case of extinct species, biomechanical analyses of the locomotor behaviour of living great ape species, and analogical experiments with human subjects, we conclude that the habitual locomotor mode of all australopiths was upright bipedalism, whether on the ground or on branches. Some later australopiths such as Au. sediba undoubtedly became more terrestrial, allowing sacrifice of arboreal stability in favour of manual dexterity. Indeed, modern humans retain arboreal climbing skills but have further sacrificed arboreal effectiveness for enhanced ability to sustain striding terrestrial bipedalism over much greater distances. We compare StW573’s locomotor adaptations to those of living great apes and protohominins and agree with those earlier observers who suggest that the common panin-hominin Last Common Ancestor was postcranially more like Gorilla then Pan.
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