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Poster De Conférence Année : 2021

Revisiting the functional morphology of the Homo habilis OH 7 hand

Résumé

The Olduvai Hominid (OH) 7 subadult hand fossils were used to define Homo habilis as a species, the genus Homo and our evolutionary understanding of hominin dexterity and tool use. Early functional interpretations focussed on strong, curved digits indicating power-grasping abilities, but were more cautious about inferring modern human-like precision grip. However, the presumed association of the OH 7 hand with stone tools and a H. habilis living site has perhaps overstated its manipulative function and overshadowed its relatively poor preservation. Here we assess the morphology of the OH 7 hand bones through 3D geometric morphometrics (GM), cross-sectional geometry, and data-driven modelling to reconstruct intrinsic hand proportions. We compare the morphology of the partial scaphoid, partial trapezium and phalanges to a sample of extant great apes (Pan, Gorilla, Pongo and modern humans) and other fossil hominins, including australopiths, Homo naledi, Homo floresiensis and Homo neanderthalensis. Curvature of the OH 7 intermediate phalanges falls within the range of adult and juvenile Gorilla, adult Pan, and A. sediba, but also within the highest extreme of humans. Cortical thickness maps (using Morphomap) reveal that cortical bone is thicker palmarly in great apes and dorsally in humans. OH 7 shows a great ape-like pattern, with thicker palmar cortex, especially just proximal to the trochlea. Cortical analysis of the OH7 proximal and intermediate phalanges show high relative bending strength (Zpol, J), most similar to African apes and higher than that of A. sediba. The principal components analysis (using Procrustes coordinates) for the trapezium shows that the OH 7 first metacarpal facet shape alone falls just outside the human range of variation, but when all facets are considered together, OH 7 falls within the human distribution. In comparison, the H. naledi trapezium clusters with African apes, and A. afarensis (AL 333) is intermediate between humans and African apes. The GM analysis of the entire scaphoid shape shows that all four reconstructions of OH 7 (which is missing its tubercle) fall out closest to humans, Neandertals and, less so, LB1, but is distinct in having a rectangular capitate facet. H. naledi and A. sediba scaphoid shape is intermediate between humans and African apes, while Australopithecus sp. StW 618 is Gorilla-like. Based on a diverse extant primate sample of associated hand skeletons, we determined which species best predict the lengths of the phalanges preserved in OH 7, and from these models, imputed the lengths of the elements that are not preserved. The best performing models predict a thumb/third digit ratio between 48.3-49.6% in OH 7, a range that reflects a thumb longer than Gorilla (mean 40%), falls within the lower range of variation in modern humans (49-59%, mean 54%) but below the thumb proportions found in A. afarensis (estimated 52%), A. sediba (60%), H. naledi (58%).Taken together, the OH 7 proximal and intermediate phalanges are African ape-like in their curvature and cortical robusticity, indicating powerful grasping with fingers that are capable -more so than A. sediba-of withstanding the high loads incurred during arboreal locomotion. In contrast, the OH 7 scaphoid is more human-like than previously appreciated, and both carpals are more human-like than all other fossil hominins in our sample, excluding Neandertals. The imputed hand proportions, albeit uncertain, show a relative thumb that is shorter than other fossil hominins and paired with an expansive trapezium-first metacarpal facet and broad distal pollical phalanx (but potentially lacking human-like flexor musculature). Revisiting the functional morphology of OH 7 within the context of recent, more complete hominin skeletons, suggests that the H. habilis hand likely accommodated both locomotion and enhanced manipulation but in a different way to that of A. sediba and H. naledi.
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Dates et versions

hal-03969783 , version 1 (02-02-2023)

Identifiants

Citer

Tracy L Kivell, Ameline Bardo, Samar M. Syeda, Nadine G. Steer, Christopher J Dunmore, et al.. Revisiting the functional morphology of the Homo habilis OH 7 hand. Virtual European Society for the study of Human Evolution (ESHE) 2021, Sep 2021, VIRTUAL CONFERENCE, France. PaleoAnthropology, 1, pp.200, ⟨10.48738/2021.iss1.75⟩. ⟨hal-03969783⟩
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