Characterizing the body morphology of the first metacarpal in the Homininae using 3D geometric morphometrics

Abstract

Objectives: The morphological characteristics of the thumb are of particular interest due to its fundamental role in enhanced manipulation. Despite its possible importance regarding this issue, the body of the first metacarcapal (MC1) has not been fully characterized using morphometrics. This could provide further insights into its anatomy, as well as its relationship with manipulative capabilities. Hence, this study quantifies the shape of the MC1's body in the extant Homininae and some fossil hominins to provide a better characterization of its morphology. Materials and methods:The sample includes MC1s of modern humans (n = 42), gorillas (n = 27), and chimpanzees (n = 30), as well as Homo neanderthalensis, Homo naledi, and Australopithecus sediba. 3D geometric morphometrics were used to quantify the shape of MC1's body. Results:The results show a clear distinction among the three extant genera.H. neanderthalensis mostly falls within the modern human range of variation. H. naledi varies slightly from modern humans, although also showing some unique trait combination, whereas A. sediba varies to an even greater extent. When classified using a discriminant analysis, the three fossils are categorized within the Homo group. Conclusion:The modern human MC1 is characterized by a distinct suite of traits, not present to the same extent in the great apes, that are consistent with an ability to use forceful precision grip. This morphology was also found to align very closely with that of H. neanderthalensis. H. naledi shows a number of human-like adaptations, while A. sediba presents a mix of both derived and more primitive traits.

The deep trabecular structure of first metacarpals in extant hominids

“…Given the high external and muscular forces incurred by the modern H. sapiens Mc1 (Cooney & Chao, 1977; Rolian et al, 2011), it is surprising that H. sapiens do not also show a distal extension of trabeculae within the medullary cavity. However, the distinct absence of medullary trabeculae in humans may be due to the concentrated trabecular distribution in regions of habitual loading (D'Agostino et al, 2017; Dunmore, Bardo, et al, 2020; Figures 3–6), robust external shape and cortical structure (Bowland et al, 2021; Dunmore, Skinner, et al, 2020; Galletta et al, 2019; Marchi et al, 2017; Marzke et al, 2010; Morley et al, 2022; Wong et al, 2018; Figure S7), as well as other soft tissue adaptations for manipulation (Marzke, 2013; van Leeuwen et al, 2018). That is, other H. sapiens Mc1 morphology may make medullary trabeculae unnecessary for the efficient resistance of incurred loads.…”

“…As the present study is focused on internal bone structure within the volume of the Mc1, rather than the external morphology as in previous studies (Bowland et al, 2021; Galletta et al, 2019; Morley et al, 2022), boundaries between each of the three segments were defined on the canonical trabecular volume created by cHMA to ensure geometrically homologous volumes required for analysis (Bachmann et al, 2022). The boundary between the medullary cavity and the head was defined as a plane orthogonal to the proximo‐distal axis of the volume, where the edges of the trabecular volume are no longer convex in dorsal view and just beneath the most proximal aspect of the dorso‐radial epicondyle on the canonical trabecular volume.…”

“…The shape of the human Mc1 diaphysis has also been argued to aid in the dissipation of large forces incurred by the human thumb during manipulation or tool‐use, as it is more robust than in most other extant hominids (Hamrick & Inouye, 1995; Niewoehner, 2006; Rolian et al, 2011; Tocheri, Orr, Jacofsky, & Marzke, 2008b). The term robust, however, has been variously used to refer to the radio‐ulnar width of the Mc1, the development of the distal epicondyles, the development of the entheses, or some combination of these (Bowland et al, 2021; Hamrick & Inouye, 1995; Morley et al, 2022; Susman, 1994). Overlap in the radio‐ulnar width of the Mc1 between modern humans and gorillas, complicates our understanding of the potential manipulative or technological pressures that may have selected for it in human evolution (Bowland et al, 2021; Hamrick & Inouye, 1995; Morley et al, 2022; Susman, 1994).…”

“…The term robust, however, has been variously used to refer to the radio‐ulnar width of the Mc1, the development of the distal epicondyles, the development of the entheses, or some combination of these (Bowland et al, 2021; Hamrick & Inouye, 1995; Morley et al, 2022; Susman, 1994). Overlap in the radio‐ulnar width of the Mc1 between modern humans and gorillas, complicates our understanding of the potential manipulative or technological pressures that may have selected for it in human evolution (Bowland et al, 2021; Hamrick & Inouye, 1995; Morley et al, 2022; Susman, 1994). This radio‐ulnar width of the Mc1 diaphysis is also highly dependent on the disto‐radial flange (sometimes also referred to as a crest or ridge), which is a linear ridge of bone on the radial margin of the Mc1 diaphysis that is thought to provide greater surface area for the m. opponens pollicis (OP) insertion (Bowland et al, 2021;Karakostis et al, 2021; Maki & Trinkaus, 2011).…”

Morphological integration and shape covariation between the trapezium and first metacarpal among extant hominids

“…The concavo‐convex curvature of the TMc joint as well as the complexity of radiocarpal articulations, in which the trapezium articulates with the scaphoid (or os centrale), trapezoid, and capitate, makes this anatomical region challenging to quantify using traditional linear measurements. Three‐dimensional approaches have recently been applied to the TMc joint (Marzke et al, 2010; Tocheri, 2007), the radiocarpal articulations (Orr et al, 2010; Tocheri et al, 2003, 2005), the Mc1's trapezium surface alone (Marchi et al, 2017; Niewoehner, 2005), the non‐articular Mc1 body alone (Bowland et al, 2021; Morley et al, 2022), the distal (phalangeal) Mc1 surface alone (Galletta et al, 2019), and the overall shape of the Mc1 including a focus of its entheses (Kunze et al, 2022). For example, using mathematical modeling, Marzke et al (2010) quantified variation in the 3D curvature of both surfaces of the TMc saddle‐shaped joint, showing that the human joint surfaces are flatter, both dorsopalmarly and radioulnarly, than in other extant hominids.…”