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

Abstract

26Objectives: Extinct hominins can provide key insights into the development of tool use, 27with the morphological characteristics of the thumb of particular interest due to its 28 fundamental role in enhanced manipulation. This study quantifies the shape of the first 29 metacarpal's body in the extant Homininae and some fossil hominins to provide 30 insights about the possible anatomical correlates of manipulative capabilities. 31 Materials and methods: The extant sample includes MC1s of modern humans (n=42), 32 gorillas (n=27) and chimpanzees (n=30), whilst the fossil sample included Homo 33 neanderthalensis, Homo naledi and Australopithecus sediba. 3D geometric 34 morphometrics were used to characterize the overall shape of MC1's body. 35 Results: Humans differ significantly from extant great apes when comparing overall 36 shape. H. neanderthalensis mostly falls within the modern human range of variation 37 although also showing a more robust morphology. H. naledi varies from modern 38 human slightly, whereas A. sediba varies from humans to an even greater extent. 39 When classified using a linear discriminant analysis, the three fossils are categorized 40 within the Homo group. 41Discussion: The results are in general agreement with previous studies on the 42 morphology of the MC1. This study found that the modern human MC1 is 43 characterized by a distinct suite of traits, not present to the same extent in the great 44 apes, that are consistent with an ability to use forceful precision grip. This morphology 45 was also found to align very closely with that of H. neanderthalensis. H. naledi shows 46 a number of human-like adaptations consistent with an ability to employ enhanced 47 manipulation, whilst A. sediba apparently presents a mix of both derived and more 48 primitive traits. 49 3

Variation and covariation of external shape and cross‐sectional geometry in the human metacarpus

“…In support of our third hypothesis, we found no evidence to support an allometric relationship between metacarpal size (using either centroid size or maximum metacarpal length) and external shape (using Procrustes aligned coordinates). Previous studies of metacarpal shape have identified weak or no allometric relationships between shape and size in hominoids (e.g., Bardo et al, 2020;Bowland et al, 2021;Galletta et al, 2019: Morley et al, 2020Niewoehner, 2005), and suggests that variation in activity, among other factors such as genetics, may be one of the main factors driving variation in metacarpal shape in this study sample. However, without detailed profiles of individuals within our study sample, we must assume that differences in sex, age, hormones (in addition to activity), and the interaction between them all, may also be in part responsible for the morphological differences we find across our sample.…”

“…In this study, we use whole surface GM to capture the shape of both the proximal and distal articular surfaces and the diaphysis of MC1-5 and combine this with analyses of CSG. We test for allometry on metacarpal shape with the expectation that results will be consistent with previous GM studies that have used solely hominoid samples (Bardo et al, 2020;Bowland et al, 2021;Galletta et al, 2019;Morley et al, 2020;Niewoehner, 2005) and shown size to be a minor contributor to metacarpal shape variation.…”

“…Regarding the first objective, we test the null hypothesis that all three human groups will not be significantly different in terms of their whole bone shape. For our second objective, we hypothesize that there will be no allometric signal in external metacarpal shape in this intraspecific study given the results of previous interspecific studies of hominoid metacarpals (Bardo et al, 2020;Bowland et al, 2021;Galletta et al, 2019;Morley et al, 2020;Niewoehner, 2005). Based on bone functional adaptation, we hypothesize for our third objective that the Mary Rose and Pre-Industrial populations, which we assume engaged in intense manual daily activities, will have greater crosssectional properties than our Post-Industrial sample, but that the patterns of cross-sectional properties across the palm will be the same for all three groups (i.e., MC1 largest, followed by MC2 + 3, followed by the MC4 + 5).…”

Biomechanics of the human thumb and the evolution of dexterity

“…Several of these studies have focused on morphological characters with extensive functional significance, 4 , 8 , 9 , 21 , 22 providing novel insights into hominin behavior on the basis of variation in the three-dimensional (3D) form of the bone’s external aspects 18 , 9 , 10 , 22 or their underlying trabecular structures (e.g., Kivell 4 and Dunmore et al. 21 ).…”

“…Even though muscle forces comprise a central component of biomechanical efficiency, 22 , 71 past morphological research on fossil hand bones did not address the potential differences across hominin species in muscle force-generating abilities. However, considering that muscle force-producing capacities are known to vary greatly among great apes and even between humans and chimpanzees, 34 assessing the manual dexterity of fossil hominins entirely based on bone geometry is prone to severe misinterpretations regarding their manual dexterity.…”