Abstract
Bipedalism is a defining trait of the hominin lineage, associated with a transition from a more arboreal to a more terrestrial environment. While there is debate about when modern human-like bipedalism first appeared in hominins, all known South African hominins show morphological adaptations to bipedalism, suggesting that this was their predominant mode of locomotion. Here we present evidence that hominins preserved in the Sterkfontein Caves practiced two different locomotor repertoires. The trabecular structure of a proximal femur (StW 522) attributed to Australopithecus africanus exhibits a modern human-like bipedal locomotor pattern, while that of a geologically younger specimen (StW 311) attributed to either Homo sp. or Paranthropus robustus exhibits a pattern more similar to nonhuman apes, potentially suggesting regular bouts of both climbing and terrestrial bipedalism. Our results demonstrate distinct morphological differences, linked to behavioral differences between Australopithecus and later hominins in South Africa and contribute to the increasing evidence of locomotor diversity within the hominin clade.
Highlights
Bipedalism is a defining trait of the hominin lineage, associated with a transition from a more arboreal to a more terrestrial environment
We test for evidence of committed terrestrial bipedalism and/or evidence for significant bouts of climbing in South African hominins, including Australopithecus africanus
It has been argued that some Pliocene hominins may exhibit functional divergence of the upper and lower limbs associated with selection for both arboreality and terrestrial bipedalism, respectively [13, 23].The discoveries of StW 573 [12, 13], A. sediba [24], Homo floresiensis [25], and Homo naledi [26] reveal additional unexpected
Summary
We conduct a comparative analysis of the three-dimensional (3D) trabecular bone distribution beneath the subchondral layer of the proximal femoral head in humans, other great apes and two fossil hominin specimens from the Sterkfontein Caves, South Africa: StW 311 and StW 522 (SI Appendix, Table S4). We predict that StW 522, attributed to A. africanus [36], will present a distinct trabecular pattern that shows similarities to both humans and great apes, given skeletal evidence suggesting that this taxon was a committed terrestrial biped that engaged in facultative arboreality [4, 7, 8, 13]. Members of this genus) we predict that, like StW 522, it will show similarities to both humans and great apes
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