Homoplasy in the evolution of modern human-like joint proportions in Australopithecus afarensis

Abstract

The evolution of bipedalism and reduced reliance on arboreality in hominins resulted in larger lower limb joints relative to the joints of the upper limb. The pattern and timing of this transition, however, remains unresolved. Here, we find the limb joint proportions of Australopithecus afarensis, Homo erectus, and Homo naledi to resemble those of modern humans, whereas those of A. africanus, Australopithecus sediba, Paranthropus robustus, Paranthropus boisei, Homo habilis, and Homo floresiensis are more ape-like. The homology of limb joint proportions in A. afarensis and modern humans can only be explained by a series of evolutionary reversals irrespective of differing phylogenetic hypotheses. Thus, the independent evolution of modern human-like limb joint proportions in A. afarensis is a more parsimonious explanation. Overall, these results support an emerging perspective in hominin paleobiology that A. afarensis was the most terrestrially adapted australopith despite the importance of arboreality throughout much of early hominin evolution.

Covariation between the shape and mineralized tissues of the rib cross section in Homo sapiens, Pan troglodytes and Sts 14

“…Alternatively, as stated for other post-cranial elements (Prabhat et al, 2021), Australopithecus species could demonstrate a wide range of costal features such as a chimpanzee-like rib cross-sectional anatomy and a modern human-like rib torsion and declination. This would indicate potentially opposed selective pressures acting over locomotion and breathing, such as seen in other post-cranial elements like the (meta)tarsus and pedal phalanges (Ward, 2013).…”

New fossils of Australopithecus sediba reveal a nearly complete lower back

“…Previous work has shown that the adult, presumed female individual from Malapa (MH2) demonstrates clear adaptations to bipedal locomotion ( Zipfel et al, 2011 ; DeSilva et al, 2013 ; DeSilva et al, 2018 ; Williams et al, 2013 ; Williams et al, 2018b ), as do other Australopithecus specimens, despite their retention of features linked to suspensory behavior and other arboreal proclivities ( Zipfel et al, 2011 ; Henry et al, 2012 ; Churchill et al, 2013 ; Churchill et al, 2018 ; DeSilva et al, 2013 ; DeSilva et al, 2018 ; Prang, 2015a ; Prang, 2015b ; Prang, 2016 ; Meyer et al, 2017 ; Rein et al, 2017 ; Holliday et al, 2018 ; Prabhat et al, 2021 ). The new fossils here reinforce these conclusions, signaling a lower back in MH2 as that of an upright biped equipped with powerful trunk musculature potentially used in both terrestrial and arboreal locomotion.…”

“…Evidence from the lower limb also suggests that A. sediba lacked a robust calcaneal tuber ( Prang, 2015a ) and a longitudinal arch ( Prang, 2015b ), both thought to be adaptations to obligate, human-like bipedalism, and demonstrates evidence for a mobile subtalar joint proposed to be adaptively significant for vertical climbing and other arboreal locomotor behaviors ( Prang, 2016 ; DeSilva et al, 2013 ; Zipfel et al, 2011 ; DeSilva et al, 2018 ). The upper thorax ( Schmid et al, 2013 ), scapula ( Churchill et al, 2013 ; Churchill et al, 2018 ), and cervical vertebrae ( Meyer et al, 2017 ) of A. sediba suggest shoulder and arm elevation indicative of arboreal positional behaviors requiring overhead arm positions, and the limb joint size proportions are ape-like ( Prabhat et al, 2021 ). Furthermore, analysis of dental calculus from Malapa Hominin 1 (MH1) indicates that this individual’s diet was high in C 3 plants like fruit and leaves, similar to savannah chimpanzees and Ardipithecus ramidus ( Henry et al, 2012 ).…”

African apes and the evolutionary history of orthogrady and bipedalism

“…In fact, A. afarensis had recently been discovered and was not particularly bonobo‐like (e.g., Kimbel et al, 2004; Latimer et al, 1981). Today, we can look back on this episode and readily point out that A. afarensis is not one of the earliest hominins, nor does it represent the primitive hominin condition for all features, particularly in that it appears to be independently derived in some ways (Berger, 2002; Haile‐Selassie, 2021; Haile‐Selassie et al, 2010, 2019; Kimbel et al, 2004; Prabhat et al, 2021; Prang, 2015a). However, the earliest putative hominins yet discovered— Sahelanthropus tchadensis , Orrorin tugenensis , Ardipithecus kadabba , and Ardipithecus ramidus —are also not specifically bonobo‐like in appearance (Brunet et al, 2002; Guy et al, 2005; Haile‐Selassie, 2001; Senut et al, 2001; White et al, 2015; White, Asfaw, et al, 2009).…”

“…It is also possible that early hominins retained knuckle‐walking features, either as part of an active locomotor repertoire that potentially also included facultative bipedalism, or as primitive retentions. Indeed, hominins seem to have retained adaptations to arboreality for several million years (Stern & Susman, 1983; Susman et al, 1984; Lovejoy, Latimer, et al 2009; Lovejoy, Simpson, et al 2009; Lovejoy, Suwa, Simpson, et al 2009; Lovejoy, Suwa, Spurlock, et al 2009; White, Asfaw, et al, 2009; White et al, 2015; Rein et al, 2017; Dunmore et al, 2020; Georgiou et al, 2020; Prabhat et al, 2021; Prang et al, 2021; Williams et al, 2021).…”