Trabecular variation in the first metacarpal and manipulation in hominids

Abstract

Objectives: The dexterity of fossil hominins is often inferred by assessing the comparative manual anatomy and behaviors of extant hominids, with a focus on the thumb. The aim of this study is to test whether trabecular structure is consistent with what is currently known about habitually loaded thumb postures across extant hominids.Materials and methods: We analyze first metacarpal (Mc1) subarticular trabecular architecture in humans (Homo sapiens, n = 10), bonobos (Pan paniscus, n = 10), chimpanzees (Pan troglodytes, n = 11), as well as for the first time, gorillas (Gorilla gorilla gorilla, n = 10) and orangutans (Pongo sp., n = 1, Pongo abelii, n = 3 and Pongo pygmaeus, n = 5). Using a combination of subarticular and whole-epiphysis approaches, we test for significant differences in relative trabecular bone volume (RBV/TV) and degree of anisotropy (DA) between species. Results: Humans have significantly greater RBV/TV on the radiopalmar aspects of both the proximal and distal Mc1 subarticular surfaces and greater DA throughout the Mc1 head than other hominids. Nonhuman great apes have greatest RBV/TV on the ulnar aspect of the Mc1 head and the palmar aspect of the Mc1 base. Gorillas possessed significantly lower DA in the Mc1 head than any other taxon in our sample.Discussion: These results are consistent with abduction of the thumb during forceful "pad-to-pad" precision grips in humans and, in nonhuman great apes, a habitually adducted thumb that is typically used in precision and power grips. This comparative context will help infer habitual manipulative and locomotor grips in fossil hominins. K E Y W O R D Sfirst metacarpal, grip, hominid, manipulation, trabeculae

The position of Australopithecus sediba within fossil hominin hand use diversity

“…The trabecular distribution within the finger metacarpal heads suggests A. sediba loaded its finger joints in a flexed power grip posture, similar to that of Pongo. In contrast, the internal structure of the thumb metacarpal is human-like, otherwise only showing an affinity with one Gorilla specimen, a dexterous taxon 38,46 which has the closest thumb-tofinger length ratio to humans among great apes 4 . This trabecular distribution in MH2 suggests habitual use of an abducted opposed thumb that is typical of human forceful precision grips.…”

“…Segmented metacarpal images were processed with the whole-epiphysis method, as described in more detail in many studies 24,26,42 and subsequent geometric morphometric landmark analysis was performed in a similar manner to that described in 46,51 . In brief, a number of image filters, run via medtool 4.2 (Dr. Pahr Ingenieurs e.U.…”

“…The present study focuses on subchondral or subarticular BV/TV since forces at a joint must pass through part of the subchondral cortical bone 29,30 and trabecular structure, before reaching the deep trabecular structure or diaphyseal cortical bone 27 . Therefore, if bone functional adaption to habitually loaded joint positions exists, it should be found in the subchondral trabeculae 46,51 . To achieve this subchondral trabecular analysis the outer surface of the 3D trabecular mesh was isolated in Paraview and a Poisson surface reconstruction filter was used to smooth the surface in Meshlab.…”

The implications of thumb movements for Neanderthal and modern human manipulation

“…This characterization can elucidate which specific thumb movements and, by extension, which grip(s) would have been favored by this morphology; that is, would each TMc complex be better suited to precision (i.e., involvement of the pad of the fingers in opposition to the pad of the thumb) or power grips (i.e., involvement of the palm of the hand). Following previous studies of external and internal bone morphology, we predict that humans will demonstrate a TMc complex morphology that favors thumb abduction [44][45][46] as this movement, combined with axial pronation and flexion of the thumb, comprises thumb opposition. An opposed thumb is habitually used by modern humans in strong precision "pad-to-pad" grips 47 , in which the thumb pad opposes the index finger pad, and the joints of the TMc complex are oriented obliquely relative to the transverse plane.…”

Kinetics of stone tool production among novice and expert tool makers

“…More recently, experimental studies have generated evidence supporting these hypotheses (e.g., the presence of an independent flexor pollicis longus muscle (Hamrick et al, 1998; Marzke et al, 1998), a robust pollical metacarpal; Rolian et al, 2011; Key & Dunmore, 2015; Williams‐Hatala et al, 2018). Still other quantitative studies have reported osteological responses to stone tool and other manual manipulative behaviors (e.g., Dunmore et al, 2020; Dunmore, Bardo, Skinner, & Kivell, 2019; Niewoehner, Weaver, & Trinkaus, 1997; Stephens, Kivell, Pahr, Hublin, & Skinner, 2018; Tocheri, Razdan, Williams, & Marzke, 2005). Based on this reasoning, we propose that the second digit, particularly the metacarpal head, should be investigated further for possible osteological signatures of habitual and skilled stone tool behaviors.…”