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.

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