StW 573 <b><i>Australopithecus prometheus</i></b>: Its Significance for an Australopith Bauplan

Abstract

The StW 573 skeleton of &lt;i&gt;Australopithecus prometheus&lt;/i&gt; 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 specimens of its genus. A crucial aspect of interpretation of locomotor behaviour from fossil remains is an understanding of the palaeoenvironment in which the individual lived and the manner in which it 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, as far as currently possible, to reconstruct what might have been this individual’s realized and potential niche. We explore the concept of a common &lt;i&gt;Australopithecus&lt;/i&gt; “bauplan” by comparing the morphology and ecological context of StW 573 to that of paenocontemporaneous australopiths including &lt;i&gt;Australopithecus anamensis&lt;/i&gt; and KSD-VP-1/1 &lt;i&gt;Australopithecus afarensis&lt;/i&gt;. Each was probably substantially arboreal and woodland-dwelling, relying substantially on arboreal resources. We use a 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. From these, 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 &lt;i&gt;Australopithecus sediba&lt;/i&gt; 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 StW 573’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 &lt;i&gt;Gorilla&lt;/i&gt; than &lt;i&gt;Pan&lt;/i&gt;.

Form, function and evolution of the human hand

“…Phalangeal morphology is difficult to assess, but the isolated right third, fourth and, especially, fifth proximal phalanges show well-developed flexor sheath ridges and the dorsal curvature appears similar to that of other hominin phalanges from Sterkfontein (Kivell, Churchill, et al, 2018). Overall, this morphology is consistent with detailed morphological assessments of the StW 573 upper limb that indicate climbing and/or suspension were part of its locomotor repertoire (Carlson et al, 2021;Crompton et al, 2021;Heaton et al, 2019).…”

“…StW 573 preserves a complete (excluding one distal non‐pollical phalanx) articulated left hand including, remarkably, a pollical sesamoid, along with several elements from the right hand (Figure 7). The hand is described as combining modern human‐like intrinsic hand proportions with an African ape‐like “ridge” on the trapezium's first metacarpal facet that would have stabilized the pollex in abducted pinch grips (Clarke, 1999, 2002; Crompton et al, 2021). Although the hand remains articulated within the breccia (Figure 7), making morphological analyses challenging, our own study of the fossils based on caliper linear measurements (T.L.K.)…”