Did modern human carpal morphology evolve from knuckle walking traits?

Abstract Hominin forelimbs have evolved from primarily locomotive to manipulative appendages over approximately 6 million years. As such, hand functions in fossil hominins and the Pan–Homo last common ancestor (LCA) are intensely debated, with carpal morphology central to this debate. However, owing to their irregular and challenging shapes, few studies have comprehensively quantified carpal morphology. We analyse the overall carpal morphology of anthropoids, including fossil hominins, using spherical harmonics and use classification methods to characterize fossil hominins within the context of extant taxa. Results show that hominins share with African apes derived carpal morphology possibly related to knuckle walking. Furthermore, unique modern human carpal morphology appears to have evolved from these possible knuckle-walking features and in a piecemeal manner, causing some hominin capitates to resemble those of palmigrade monkeys. Striking variation in biomechanically relevant carpal morphology and retention of potentially ancestral features persists as late as Homo naledi, suggesting that most hominins probably neither knuckle walked nor extensively used stone tools. These results indicate that the hominin carpus evolved from an African ape-like wrist, with radial-side reorganization related to manipulation occurring only recently. Although it remains unclear whether the LCA knuckle walked, our results suggest that this is the most likely existing hypothesis.